Burmese pythons like a meal they can really get their fangs around, especially since the snakes are known to go half a year or more between meals. That gustatory pause is merely one of pythons' more remarkable adaptations.
New research shows that when the reptiles swallow whole rats, birds, and other prey, the pythons' hearts temporarily grow bigger.Scientists in California say the snakes experience a 40 percent increase in heart muscle mass within 48 hours of feeding. The change enables the pythons to meet the metabolic demands of digesting a meal.
What's more, the process is fully reversible, with the snakes' hearts shrinking back to their original size once feeding ends.
Pythons can offer new insights to understanding heart growth in other species, including humans, according to researchers behind the discovery, which is reported in the current issue of the science journal Nature.
One of the world's largest snakes, the Burmese python can grow as long as 25 feet (7.6 meters) and weigh as much as 200 pounds (90 kilograms). Native to Southeast Asia, it preys on mammals, birds, and other animals, which the reptile swallows whole. But python meals are few and far between.
"These animals have a remarkable ability to shut down their metabolism between meals," said James Hicks, a professor of ecology and evolutionary biology at the University of California, Irvine.
"We currently have 1.5-kilogram [3.3-pound] pythons in the lab that have not eaten for three months and have only lost one to ten grams [four to thirty-five hundredths of an ounce] of weight," noted Hicks, who is also the study's lead author.
But when these reptiles do feed, Hicks added, they often tackle prey that is 50 to 100 percent the size of their own body mass. Such meals require a considerable digestive effort.
"Some investigators have reported as much as a 44-fold increase in metabolism during digestion," Hicks said.
Metabolic Demands
Hicks and his colleagues investigated how Burmese pythons meet the metabolic demands of digestion.
They found that oxygen consumption rose sevenfold in lab pythons after feeding. This was accompanied by an extraordinarily rapid growth in heart size. The snakes' heart ventricle muscle mass (ventricles are the heart's pumping chambers) increased 40 percent in just two days.
The study team was able to link this sudden growth to increased production of a cardiac protein. The protein is associated with cells that enlarge the heart and boost its pumping capacity, a condition known as cardiac hypertrophy.
The researchers say feeding-induced cardiac hypertrophy likely explains why Burmese pythons pump 50 percent more blood per heartbeat while quietly digesting a meal than when slithering at full speed.
Previous studies point to why python hearts need to go into overdrive when these animals digest food. Researchers report livers growing to three times their normal size, intestines doubling in mass, and pancreatic enzyme activity increasing threefold. Such changes within the snake significantly raise the demand for oxygenated blood.
Stephen Secor, a biologist at the University of Alabama in Birmingham, is among those to have studied digestion in pythons. While most carnivores are able chew, tear up, or crush their prey first, snakes "swallow only intact prey and must delegate to the stomach the whole job of breaking [it] down," Secor said.
Yet once a python has finished its meal, its heart quickly returns to its usual size.
Heart Remodeling
Hicks, the University of California ecologist and evolutionary biologist, said that by quickly remodeling their hearts depending on whether they are feeding or fasting, Burmese pythons are able to match their metabolism to their bodily needs.
Hicks said he is unaware of any other animal that is able to do this with such speed.
His lab is currently investigating other reptiles that feed intermittently, including lizards and crocodiles. American alligators, for instance, exhibited a two- to threefold increase in metabolism during digestion. But, Hicks added, "So far, we haven't seen cardiovascular remodeling."
Nevertheless, hearts are known for their ability to adapt to the physiological demands of their owners. Human athletes, for example, often develop cardiac hypertrophy in response to vigorous training routines. Benefits of the condition include lowered heart rates and improved blood circulation.
The difficulty, Hicks said, is in understanding the mechanisms that lead to heart remodeling in humans and other mammals. Such investigations involve complex and highly invasive surgical procedures that could easily result in death.
Hicks and his colleagues propose the Burmese python as an ideal investigative model instead.
August Krogh, the 20th-century Danish physiologist, once wrote, "For a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied."
Krogh's approach has been a guiding principle for comparative physiology ever since.
Hicks said if we want to better understand how the human heart is able to remodel itself, we should look no further than the Burmese python.
After all, the reptile can grow its heart in the time it takes to eat its lunch
Monday, June 22, 2009
Global warming not linked to sun
Cyclical changes in the sun's energy output are not responsible for Earth's recent warming, a new study asserts.
The findings put the blame for climate change squarely on human-created carbon dioxide and other greenhouse gases, reinforcing the beliefs of most climate scientists.
The sun's output waxes and wanes due to a variety of mechanisms. Its power rose during much of the 20th century, but it has declined. “Up until 1985, you could argue that the sun was (trending) in a direction that could have contributed to Earth's rising temperatures,” said study author A. Mike Lockwood of the University of Southampton in Britain.
Two decades ago, “it did a U-turn. If the sun had been warming the Earth, that should have come to an end, and we should have seen temperatures start to go the other way,” Lockwood said.
Yet temperatures have continued to climb since that date, making a strong solar role in warming appear unlikely.
read more >
Global Warring: Climate Change Could Be The Root Of Armed Conflicts
Source - www.sciencedaily.com Tuesday, July 10, 2007
Climate change, and the resulting shortage of ecological resources, could be to blame for armed conflicts in the future, according to David Zhang from the University of Hong Kong and colleagues. Their research, which highlights how temperature fluctuations and reduced agricultural production explain warfare frequency in eastern China in the past, has been published online in Springer’s journal Human Ecology.
Zhang and his team looked at the impact of climate change on warfare frequency over the last millennium in eastern China. The agricultural production in the region supports the majority of the Chinese population. The authors reviewed warfare data from 899 wars in eastern China between 1000 and 1911, documented in the Tabulation of Wars in Ancient China. They cross-referenced these data with Northern Hemispheric climate series temperature data for the same period.
They found that warfare frequency in eastern China, and the southern part in particular, significantly correlated with temperature oscillations. Almost all peaks of warfare and dynastic changes coincided with cold phases.
Temperature fluctuations directly impact agriculture and horticulture and, in societies with limited technology such as pre-industrial China, cooling temperatures hugely impact the availability of crops and herds. In times of such ecological stress, warfare could be the ultimate means of redistributing resources, according to Zhang and his team.
The authors conclude that "it was the oscillations of agricultural production brought by long-term climate change that drove China’s historical war-peace cycles." They recommend that researchers consider climate change part of the equation when they consider the reasons behind wars in our history.
Looking to the future and applying their findings, Zhang and colleagues suggest that shortages of essential resources, such as fresh water, agricultural land, energy sources and minerals may trigger more armed conflicts among human societies.
The findings put the blame for climate change squarely on human-created carbon dioxide and other greenhouse gases, reinforcing the beliefs of most climate scientists.
The sun's output waxes and wanes due to a variety of mechanisms. Its power rose during much of the 20th century, but it has declined. “Up until 1985, you could argue that the sun was (trending) in a direction that could have contributed to Earth's rising temperatures,” said study author A. Mike Lockwood of the University of Southampton in Britain.
Two decades ago, “it did a U-turn. If the sun had been warming the Earth, that should have come to an end, and we should have seen temperatures start to go the other way,” Lockwood said.
Yet temperatures have continued to climb since that date, making a strong solar role in warming appear unlikely.
read more >
Global Warring: Climate Change Could Be The Root Of Armed Conflicts
Source - www.sciencedaily.com Tuesday, July 10, 2007
Climate change, and the resulting shortage of ecological resources, could be to blame for armed conflicts in the future, according to David Zhang from the University of Hong Kong and colleagues. Their research, which highlights how temperature fluctuations and reduced agricultural production explain warfare frequency in eastern China in the past, has been published online in Springer’s journal Human Ecology.
Zhang and his team looked at the impact of climate change on warfare frequency over the last millennium in eastern China. The agricultural production in the region supports the majority of the Chinese population. The authors reviewed warfare data from 899 wars in eastern China between 1000 and 1911, documented in the Tabulation of Wars in Ancient China. They cross-referenced these data with Northern Hemispheric climate series temperature data for the same period.
They found that warfare frequency in eastern China, and the southern part in particular, significantly correlated with temperature oscillations. Almost all peaks of warfare and dynastic changes coincided with cold phases.
Temperature fluctuations directly impact agriculture and horticulture and, in societies with limited technology such as pre-industrial China, cooling temperatures hugely impact the availability of crops and herds. In times of such ecological stress, warfare could be the ultimate means of redistributing resources, according to Zhang and his team.
The authors conclude that "it was the oscillations of agricultural production brought by long-term climate change that drove China’s historical war-peace cycles." They recommend that researchers consider climate change part of the equation when they consider the reasons behind wars in our history.
Looking to the future and applying their findings, Zhang and colleagues suggest that shortages of essential resources, such as fresh water, agricultural land, energy sources and minerals may trigger more armed conflicts among human societies.
GLOBAL WARMING:Early warning signs
This map illustrates the local consequences of global warming.
FINGERPRINTS: Direct manifestations of a widespread and long-term trend toward warmer global temperatures
Heat waves and periods of unusually warm weather
Ocean warming, sea-level rise and coastal flooding
Glaciers melting
Arctic and Antarctic warming
HARBINGERS: Events that foreshadow the types of impacts likely to become more frequent and widespread with continued warming.
Spreading disease
Earlier spring arrival
Plant and animal range shifts and population changes
Coral reef bleaching
Downpours, heavy snowfalls, and flooding
Droughts and fires
The map of early warning signs clearly illustrates the global nature of climate changes. In its 2001 assessment, the Intergovernmental Panel on Climate Change (IPCC) concluded that, ?an increasing body of observations gives a collective picture of a warming world and other changes in the climate system."
While North America and Europe—where the science is strongest—exhibit the highest density of indicators, scientists have made a great effort in recent years to document the early impacts of global warming on other continents. Our map update reflects this emerging knowledge from all parts of the world.
Although factors other than climate may have intensified the severity of some of the events on the map, scientists predict such problems will increase if emissions of heat-trapping gases are not brought under control.
You can purchase a copy of the map as a 3 feet by 2 feet display poster. Please note that the hard-copy versions of the map do not contain the recently added map points (points 90 - 156).-->
The following organizations produced GLOBAL WARMING: Early Warning Signs:
Environmental DefenseNatural Resources Defense CouncilSierra ClubUnion of Concerned Scientists
U.S. Public Interest Research GroupWorld Resources InstituteWorld Wildlife Fund
FINGERPRINTS: Direct manifestations of a widespread and long-term trend toward warmer global temperatures
Heat waves and periods of unusually warm weather
Ocean warming, sea-level rise and coastal flooding
Glaciers melting
Arctic and Antarctic warming
HARBINGERS: Events that foreshadow the types of impacts likely to become more frequent and widespread with continued warming.
Spreading disease
Earlier spring arrival
Plant and animal range shifts and population changes
Coral reef bleaching
Downpours, heavy snowfalls, and flooding
Droughts and fires
The map of early warning signs clearly illustrates the global nature of climate changes. In its 2001 assessment, the Intergovernmental Panel on Climate Change (IPCC) concluded that, ?an increasing body of observations gives a collective picture of a warming world and other changes in the climate system."
While North America and Europe—where the science is strongest—exhibit the highest density of indicators, scientists have made a great effort in recent years to document the early impacts of global warming on other continents. Our map update reflects this emerging knowledge from all parts of the world.
Although factors other than climate may have intensified the severity of some of the events on the map, scientists predict such problems will increase if emissions of heat-trapping gases are not brought under control.
You can purchase a copy of the map as a 3 feet by 2 feet display poster. Please note that the hard-copy versions of the map do not contain the recently added map points (points 90 - 156).-->
The following organizations produced GLOBAL WARMING: Early Warning Signs:
Environmental DefenseNatural Resources Defense CouncilSierra ClubUnion of Concerned Scientists
U.S. Public Interest Research GroupWorld Resources InstituteWorld Wildlife Fund
Scientists: Global warming has already changed oceans
In Washington state, oysters in some areas haven't reproduced for four years, and preliminary evidence suggests that the increasing acidity of the ocean could be the cause. In the Gulf of Mexico, falling oxygen levels in the water have forced shrimp to migrate elsewhere. Though two marine-derived drugs, one for treating cancer and the other for pain control, are on the market and 25 others are under development, the fungus growing on seaweed, bacteria in deep sea mud and sea fans...
Global warming: Want to see Northwest impacts? Just look around
Living in a corner of America powered, irrigated and inspired by water, we ought to treat Tuesday's report released by the White House, Global Climate Change Impacts in the United States, as a wake-up call and cold shower.
"We are the alpha and the omega of global warming," said Rep. Jay Inslee, D-Wash., who helped write a flawed -- but needed -- bill to change national energy policy. It's pending in the House.
Want to know how climate change is changing America? Read the report. Want its bottom line: "Global warming is unequivocal and primarily human-induced." Changes "are expected to increase."
Want to see impacts on the Northwest? Just look around, something that global-warming skeptics resolutely refuse to do.
Global warming is shrinking the winter snowpack. A smaller snowpack means reduction in the runoff that sustains our river flows, makes the desert bloom, allows salmon to reach and return from the ocean, and powers the world's greatest hydroelectric system.
The consequences don't end when our rivers reach salt water.
"Climate change and ocean acidification are already having major impacts on Washington: Our $100 million shellfish industry is in crisis after four years of oyster reproductive failure from ocean acidification," said Sen. Maria Cantwell, D-Wash.
If oyster beds are in peril, so are salmon-spawning streams. One third of current habitat for Northwest salmon and other cold-water fish will be lost in this century, or so finds the report.
What critters will most likely be conflicted? Us. Just look at the legal and political battles that have broken out in years of low stream flow on the Columbia, Snake and Klamath rivers.
Irrigators in the Klamath Basin cried one season that they lacked water to grow crops. A year later, the Bush administration tipped scales in irrigators' favor, and caused a massive salmon kill in a warm, low-flowing Klamath River.
Climate change is going to require a lot of hard thinking, which better begin right now.
"The worst response for all the user/sectors is, given the certainty of intensified conflict, to hunker down to defend 'my slice of the shrinking pie,'" opined Pat Ford of Save Our Wild Salmon.
"Global warming's accumulated impacts on our waters are best viewed as a vise, steadily tightening on all water users regardless of past ideology, who's right, and past power relations. A shift is needed, away from old Western 'water is for fighting' lens, to a shared solutions, shared sacrifice, shared shortage lens."
The Northwest shares climate impacts with its neighbors in the West.
Alaska has warmed at more than twice the rate of the rest of the country, its annual average temperature up 3.4 degrees Fahrenheit: Winters have warmed 6.3 degrees.One result is the largest outbreak of tree-killing spruce beetles in the world. In the report's words, rising temperatures "allowed the beetle to survive the winter and to complete its life cycle in half the usual time."
The same has happened with the pine bark beetle in Canada. It has killed forests over a Sweden-sized area of British Columbia, has crossed the Continental Divide and threatens to munch its way across the Great White North.
In our inland West, more than 50 percent of whitebark pine forests in the Northern Rockies has been lost since 1970 -- largely due to beetle infestations. The whitebark pine anchors the soil at high elevations. Its fatty cones are a key pre-hibernation food for grizzly bears.
Hiking in the Wind River Mountains of Wyoming last summer, retired Forest Service scientist Dr. Jesse Logan showed us tiny holes bored by beetles.
"These trees are dead," he said. "They don't know it yet, though. I guess they are zombie trees."
A few hours earlier, down in a park at Dubois, oil industry workers told us that global warming was a "hoax." But the hoax is killing the forests above them and melting glaciers that sustain flow of the Wind River.
The global-warming report, and its White House release, is welcome on one front: As science struggles to keep up with impacts of global warming, politics is at last trying to keep up with science.
"Finally, the U.S. government is leveling with the American people about the threat we all face," said Dr. Jeffrey Short, a former government scientist who now works for Oceana.
The U.S. House of Representatives will soon vote on what's known as the Waxman-Markey Bill. It makes concessions to polluters. It sets what Denis Hayes of The Bullitt Foundation calls "a wimpy 17 percent reduction in carbon emissions" as a goal for 2020.
Yet, Hayes is urging lawmakers to hold their noses and vote for the bill -- to give the Obama administration credibility and needed momentum in the global effort to curb global warming.
"Climate legislation has to pass a Senate in which the oil, coal and electric utility industries wield fearsome power," Hayes said.
Some will deny this. Such is their right. It's dangerous, however, to stick your head in the sand when sea levels are rising.
"We are the alpha and the omega of global warming," said Rep. Jay Inslee, D-Wash., who helped write a flawed -- but needed -- bill to change national energy policy. It's pending in the House.
Want to know how climate change is changing America? Read the report. Want its bottom line: "Global warming is unequivocal and primarily human-induced." Changes "are expected to increase."
Want to see impacts on the Northwest? Just look around, something that global-warming skeptics resolutely refuse to do.
Global warming is shrinking the winter snowpack. A smaller snowpack means reduction in the runoff that sustains our river flows, makes the desert bloom, allows salmon to reach and return from the ocean, and powers the world's greatest hydroelectric system.
The consequences don't end when our rivers reach salt water.
"Climate change and ocean acidification are already having major impacts on Washington: Our $100 million shellfish industry is in crisis after four years of oyster reproductive failure from ocean acidification," said Sen. Maria Cantwell, D-Wash.
If oyster beds are in peril, so are salmon-spawning streams. One third of current habitat for Northwest salmon and other cold-water fish will be lost in this century, or so finds the report.
What critters will most likely be conflicted? Us. Just look at the legal and political battles that have broken out in years of low stream flow on the Columbia, Snake and Klamath rivers.
Irrigators in the Klamath Basin cried one season that they lacked water to grow crops. A year later, the Bush administration tipped scales in irrigators' favor, and caused a massive salmon kill in a warm, low-flowing Klamath River.
Climate change is going to require a lot of hard thinking, which better begin right now.
"The worst response for all the user/sectors is, given the certainty of intensified conflict, to hunker down to defend 'my slice of the shrinking pie,'" opined Pat Ford of Save Our Wild Salmon.
"Global warming's accumulated impacts on our waters are best viewed as a vise, steadily tightening on all water users regardless of past ideology, who's right, and past power relations. A shift is needed, away from old Western 'water is for fighting' lens, to a shared solutions, shared sacrifice, shared shortage lens."
The Northwest shares climate impacts with its neighbors in the West.
Alaska has warmed at more than twice the rate of the rest of the country, its annual average temperature up 3.4 degrees Fahrenheit: Winters have warmed 6.3 degrees.One result is the largest outbreak of tree-killing spruce beetles in the world. In the report's words, rising temperatures "allowed the beetle to survive the winter and to complete its life cycle in half the usual time."
The same has happened with the pine bark beetle in Canada. It has killed forests over a Sweden-sized area of British Columbia, has crossed the Continental Divide and threatens to munch its way across the Great White North.
In our inland West, more than 50 percent of whitebark pine forests in the Northern Rockies has been lost since 1970 -- largely due to beetle infestations. The whitebark pine anchors the soil at high elevations. Its fatty cones are a key pre-hibernation food for grizzly bears.
Hiking in the Wind River Mountains of Wyoming last summer, retired Forest Service scientist Dr. Jesse Logan showed us tiny holes bored by beetles.
"These trees are dead," he said. "They don't know it yet, though. I guess they are zombie trees."
A few hours earlier, down in a park at Dubois, oil industry workers told us that global warming was a "hoax." But the hoax is killing the forests above them and melting glaciers that sustain flow of the Wind River.
The global-warming report, and its White House release, is welcome on one front: As science struggles to keep up with impacts of global warming, politics is at last trying to keep up with science.
"Finally, the U.S. government is leveling with the American people about the threat we all face," said Dr. Jeffrey Short, a former government scientist who now works for Oceana.
The U.S. House of Representatives will soon vote on what's known as the Waxman-Markey Bill. It makes concessions to polluters. It sets what Denis Hayes of The Bullitt Foundation calls "a wimpy 17 percent reduction in carbon emissions" as a goal for 2020.
Yet, Hayes is urging lawmakers to hold their noses and vote for the bill -- to give the Obama administration credibility and needed momentum in the global effort to curb global warming.
"Climate legislation has to pass a Senate in which the oil, coal and electric utility industries wield fearsome power," Hayes said.
Some will deny this. Such is their right. It's dangerous, however, to stick your head in the sand when sea levels are rising.
Reconstructing Climatic and Environmental Changes of the Past 1000 Years: A Reappraisal
The 1000-year climatic and environmental history of the Earth contained in various proxy records is examined. As indicators, the proxies duly represent or record aspects of local climate. Questions on the relevance and validity of the locality paradigm for climatological research become sharper as studies of climatic changes on timescales of 50–100 years or longer are pursued. This is because thermal and dynamical constraints imposed by local geography become increasingly important as the air-sea-land interaction and coupling timescales increase. Because the nature of the various proxy climate indicators are so different, the results cannot be combined into a simple hemispheric or global quantitative composite. However, considered as an ensemble of individual observations, an assemblage of the local representations of climate establishes the reality of both the Little Ice Age and the Medieval Warm Period as climatic anomalies with world-wide imprints, extending earlier results by Bryson et al. (1963), Lamb (1965), and numerous other research efforts. Furthermore, these individual proxies are used to determine whether the 20th century is the warmest century of the 2nd Millennium at a variety of globally dispersed locations. Many records reveal that the 20th century is likely not the warmest nor a uniquely extreme climatic period of the last millennium, although it is clear that human activity has significantly impacted some local environments.
1. INTRODUCTION
Are the Little Ice Age and Medieval Warm Period widespread climatic anomalies? Nearly four decades ago, H. H. Lamb (1965, pp. 14–15) wrote,
“[M]ultifarious evidence of a meteorological nature from historical records, as well as archaeological, botanical and glaciological evidence in various parts of the world from the Arctic to New Zealand… has been found to suggest a warmer epoch lasting several centuries between about A.D. 900 or 1000 and about 1200 or 1300… Both the “Little Optimum” in the early Middle Ages and the cold epochs [i.e., “Little Ice Age”], now known to have reached its culminating stages between 1550 and 1700, can today be substantiated by enough data to repay meteorological investigation… It is high time therefore to marshal the climatic evidence and attempt a quantitative evidence.” …
Thirty-three years later, however, Jones et al. (1998) tentatively concluded that,
“[w]hile the ‘Little Ice Age’ cooling (with the seventeenth century being more severe over Eurasia and the nineteenth century more severe over North America) is clearly evident … we can only concur… that there is little evidence for the ‘Medieval Warm Period’… although the fact that we have only four series before 1400 and the timescale limitations described earlier [i.e., not resolving timescales of multidecades to century with tree ring proxies used in their study] caution against dismissing the feature.” …
These results are but a few of the many that have become available since Lamb’s pioneering analysis. Given advancements in retrieval of information from climate proxies, as well as their extensive surface coverage, we review the accumulated evidence on climatic anomalies over the last 1000 years. ..What are the regional and global patterns of climatic change over the last 1000 years? Accurate answers to these questions are important, both as benchmarks for the 20th century global average warming exhibited by surface thermometer records and as physical constraints for theories or mechanisms of climate change on timescales of decades to centuries.
To make progress towards this understanding, we address three questions of many individual climate proxies that differ too widely to be quantitatively averaged or compared:
(1) Is there an objectively discernible climatic anomaly occurring during the Little Ice Age, defined as 1300–1900 A.D.? This broad period in our definition derives from historical sea-ice, glaciological and geomorphological studies synthesized in Grove (2001a, 2001b) and Ogilvie and Jónsson (2001).
(2) Is there an objectively discernible climatic anomaly occurring during the Medieval Warm Period, defined as 800–1300 A.D.? This definition is motivated by Pfister et al. (1998) and Broecker (2001) and is slightly modified from Lamb’s original study (1965).
(3) Is there an objectively discernible climatic anomaly occurring within the 20th century that may validly be considered the most extreme (i.e., the warmest) period in the record? An important consideration in answering this question is to distinguish the case in which the 20th century warming began early in the century versus after the 1970s, as recorded by surface thermometers. This criterion is necessary in order to judge the influence of 20th century warming by anthropogenic forcing inputs such as increased atmospheric carbon dioxide content.
Anomaly, in our context, is simply defined as a period of 50 or more years of sustained warmth, wetness, or dryness within the Medieval Warm Period, or a 50-year or longer period of cold, dryness, or wetness during the Little Ice Age. …
4. RESULTS
… For questions 1 and 2, we find the answer to be ‘Yes’ when the proxy record shows a period of 50 years or longer of cooling, dryness or wetness during the Little Ice Age and a period of 50 years or longer of warming, wetness or dryness during the Medieval Warm Period. …
… most of the proxy records do not suggest the 20th century to be the warmest or the most extreme in its local representations, which seems surprising until one realizes the more limited and contrary view was drawn primarily from familiar instrumental thermometer records that yield no information on centennial-scale climate variability. … Another interesting feature of the result is that the warmest or most extreme climatic anomalies in the proxy indicators often occurred in the early-to-mid 20th century, rather than throughout the century.
4.1. Glaciers – Worldwide
Broadly, glaciers retreated all over the world during the Medieval Warm Period, with a notable but minor re-advance between 1050 and 1150 A.D. (Grove and Switsur 1994). Large portions of the world’s glaciers, both in the Northern and Southern Hemispheres, advanced during the 1300–1900 A.D. period (Grove 2001b; see also Winkler 2000). The world’s small glaciers and tropical glaciers have simultaneously retreated since the 19th century, but some glaciers have advanced (Kaser 1999; Dyurgerov and Meier 2000; D. Evans 2000). …
Additional proxy records used here reveal that the climatic anomaly patterns known as the Medieval Warm Period (circa 800–1300 A.D.) and the Little Ice Age (1300–1900 A.D.) occurred across the world. …
4.2.2. North Atlantic and other oceans
The colonization of Greenland’s coastal area by the Vikings starting in 986 A.D. is well documented; and the generally mild and benign climatic conditions from about 800–1200 A.D. that helped to sustain the settlement are also well supported by ice core and borehole proxy information (Dansgaard et al. 1975; Dahl-Jensen et al. 1998). The Norsemen’s ‘Western Settlement’ (around the Godthab district) was mysteriously abandoned sometime between 1341 and 1362 A.D., while the ‘Eastern Settlement’ (actually near the southernmost tip of west Greenland, around the Narssaq and Julianehab districts) died out between 1450 and 1500 A.D. (Grove 1996; Ogilvie et al. 2000). The timing of the abandonment of the settlements coincided with a general cooling over Greenland, as established by both ice-core isotopic and borehole thermometry (Dansgaard et al. 1975; Stuiver et al. 1995; Dahl-Jensen et al. 1998). …
Over the equatorial Central Pacific, around the NINO3.4 (5N–5ºS; 160ºE–150ºW) region, Evans et al. (2000), in their skillful reconstruction of the ENSO-like decadal variability of the NINO3.4 sea surface temperature (SST), found an apparent sustained cool phase of the proxy NINO3.4 SST variability from about 1550 A.D. to approximately 1895 A.D., thereby extending the geographical area covered by the Little Ice Age Climate Anomaly. …
4.2.3. Asia and Eastern Europe
From 49 radiocarbon-dated subfossil wood samples, Hiller et al. (2001) determined that the alpine tree-limit on the Khibiny low mountains of the Kola Peninsula was located at least 100–140 meters above the current tree-limit elevation during the relatively warmer time between 1000 A.D. and 1300 A.D. The summer temperatures corresponding to the tree-line shift during this warm time are estimated to have been at least 0.8ºC warmer than today. …
Middle Russia (around 50–60ºN and 30–50ºE) seems to have experienced its coolest winters around 1620–1680 A.D., its coolest summers and springs around 1860–1900 A.D., and distinctively warm conditions during the first half of the 16th century, similar to conditions for western Europe described above. …
Based on less precise climate proxies like cherry-blossom-viewing dates, lake freezing dates and historical documentation of climate hazards and unusual weather, Tagami (1993, 1996) found that a warm period prevailed between the 10th and 14th centuries, and a cold period between the late 15th and 19th centuries, over large parts of southern Japan. …
4.2.4. North America
From an extensive collection of multiproxy evidence, Stine (1998) concluded that during the Medieval Warm Period prolonged intervals of extreme drought affected California, the northwestern Great Basin, and the northern Rocky Mountains/Great Plains, while markedly wetter regimes persisted over the Upper Midwest/sub-arctic Canada and Southern Alaska/British Columbia regions. …
Graumlich’s (1993) reconstruction of summer temperature and winter precipitation from trees in the Sierra Nevada confirmed the overall warm and dry conditions for California during Medieval times, when two of the warmest and driest 50-year intervals occurred – at 1118–1167, 1245–1294 A.D. and 1250–1299, 1315–1364 A.D., respectively. …
Hu et al. (2001), based on their high-resolution (multidecadal) geochemical analysis of sediments from Farewell Lake by the northwestern foothills of the Alaska Range, also found pronounced signatures of the Medieval Warm Period around 850–1200 A.D. During the Little Ice Age, the surface water temperature of Farewell Lake fell to a low in 1700 A.D. that was estimated to be about 1.75ºC cooler than at present. …
4.3. Southern Hemisphere
4.3.1. New Zealand
In New Zealand, the O-18 concentration in a stalagmite record from a cave in northwest Nelson shows the coldest times during the Little Ice Age to be around 1600–1700 A.D., while exceptionally warm temperatures occurred around 1200–1400 A.D., in association with the general phenomenology of the Medieval Warm Period (Wilson et al. 1979). …
4.3.4. Antarctica
The last important source of geographical information for conditions during the Medieval Warm Period and the Little Ice Age in the Southern Hemisphere is obtained from glaciers, ice cores and sea sediments on and around Antarctica. …
For the Little Ice Age, advances of glaciers on South Georgia Island (which is half-covered by glaciers) began after the late 13th century, with a peak advancement around the 18th–20th centuries (Clapperton et al. 1989). Glacier retreats occurred after about 1000 A.D., which corresponds to the timing for the Medieval Warm Period. Baroni and Orombelli (1994) noted a similar scenario for glacier advances and retreats during the Little Ice Age and Medieval Warm Period for the Edmonson Point glacier at the Terra Nova Bay area of Victoria Land on the Antarctic continent (East Antarctica). The Edmonson Point glacier retreated in two distinct phases, around 920–1020 A.D. and 1270–1400 A.D., and then advanced at least 150 meters after the 15th century. …
5. DISCUSSION
The widespread, but not truly global, geographical evidence assembled here argues for the reality of both the Little Ice Age and the Medieval Warm Period, and should serve as a useful validation target for any reconstruction of global climate history over the last 1000 years. …
Another significant problem is the indication that an anthropogenic influence may have already left its fingerprint on the recent growth of trees across the Northern Hemisphere. If this anthropogenic effect were present in tree ring data, then the calibration and verification procedure designed for extended paleoclimatic reconstructions would be significantly corrupted by further uncertainties (Idso 1989). …
Karlén (2001), for example, notes that according to the Vostok ice core record of atmospheric carbon dioxide, the present concentration of atmospheric CO2 is about 100 ppmv higher than it was during any previous interglacial during the last 400,000 years. Thus, if climate were to respond sensitively to carbon dioxide, global temperatures, or at least Vostok temperature, today ought to be considerably higher than previous interglacials. Yet evidence exists to suggest that the “present interglacial [at least for conditions around Vostok] has been about 2ºC cooler than the previous one and the climate is now, in spite of the recent warming, cooler than it was at the beginning of this interglacial” (Karlén 2001). …
6. CONCLUSIONS
This paper presents a survey of site-specific paleoclimatic reconstructions, then considers whether they indicate that the Medieval Warm Period and the Little Ice Age were observed on broad area of the globe. We conclude that the Medieval Warm Period and Little Ice Age are widespread climatic anomalies, although we emphasize the complex nature of translating the proxy changes into convenient measures like temperature and precipitation as well as confirming their spatio-temporal representation and resolution. …
The picture emerges from many localities that both the Little Ice Age and Medieval Warm Period are widespread and perhaps not precisely timed or synchronous phenomena, easily within the margin of viewpoints conceived by Bryson et al. (1963), Lamb (1965) and numerous other researchers like J. Grove (1996, 2001a, 2001b). Our many local answers confirm that both the Medieval Climatic Anomaly and the Little Ice Age Climatic Anomaly are worthy of their respective labels. Furthermore, thermometer warming of the 20th century across the world seems neither unusual nor unprecedented within the more extended view of the last 1000 years. Overall, the 20th century does not contain the warmest or most extreme anomaly of the past millennium in most of the proxy records. …
However, it is also clear that human activity has shaped almost every aspect of past environmental and climatic changes on local and regional spatial scales …
It might seem surprising or frustrating that paleoclimatic reconstruction research has not yet provided confident and applicable answers to the role of anthropogenic forcing on climate change. This point is particularly sharp when considering the fact that even though some proxy records (e.g., those from Overpeck et al. 1997) show unprecedented 20th century warmth with most of the increase occurring in the early to mid-decades of the 20th century, when the amount of anthropogenic CO2 in the air was less than 20–30% of the total amount there now. Unless there are serious flaws in the timing of the early-to-middle 20th century surface thermometer warming, or unknown anthropogenic mechanisms that caused a large amplification of surface temperature of the then-small increase in anthropogenic atmospheric CO2, then the early part of the 20th century warming must be largely dissociated from anthropogenic CO2 emissions. Other anthropogenic factors still need to be studied on a case by case basis. …
1. INTRODUCTION
Are the Little Ice Age and Medieval Warm Period widespread climatic anomalies? Nearly four decades ago, H. H. Lamb (1965, pp. 14–15) wrote,
“[M]ultifarious evidence of a meteorological nature from historical records, as well as archaeological, botanical and glaciological evidence in various parts of the world from the Arctic to New Zealand… has been found to suggest a warmer epoch lasting several centuries between about A.D. 900 or 1000 and about 1200 or 1300… Both the “Little Optimum” in the early Middle Ages and the cold epochs [i.e., “Little Ice Age”], now known to have reached its culminating stages between 1550 and 1700, can today be substantiated by enough data to repay meteorological investigation… It is high time therefore to marshal the climatic evidence and attempt a quantitative evidence.” …
Thirty-three years later, however, Jones et al. (1998) tentatively concluded that,
“[w]hile the ‘Little Ice Age’ cooling (with the seventeenth century being more severe over Eurasia and the nineteenth century more severe over North America) is clearly evident … we can only concur… that there is little evidence for the ‘Medieval Warm Period’… although the fact that we have only four series before 1400 and the timescale limitations described earlier [i.e., not resolving timescales of multidecades to century with tree ring proxies used in their study] caution against dismissing the feature.” …
These results are but a few of the many that have become available since Lamb’s pioneering analysis. Given advancements in retrieval of information from climate proxies, as well as their extensive surface coverage, we review the accumulated evidence on climatic anomalies over the last 1000 years. ..What are the regional and global patterns of climatic change over the last 1000 years? Accurate answers to these questions are important, both as benchmarks for the 20th century global average warming exhibited by surface thermometer records and as physical constraints for theories or mechanisms of climate change on timescales of decades to centuries.
To make progress towards this understanding, we address three questions of many individual climate proxies that differ too widely to be quantitatively averaged or compared:
(1) Is there an objectively discernible climatic anomaly occurring during the Little Ice Age, defined as 1300–1900 A.D.? This broad period in our definition derives from historical sea-ice, glaciological and geomorphological studies synthesized in Grove (2001a, 2001b) and Ogilvie and Jónsson (2001).
(2) Is there an objectively discernible climatic anomaly occurring during the Medieval Warm Period, defined as 800–1300 A.D.? This definition is motivated by Pfister et al. (1998) and Broecker (2001) and is slightly modified from Lamb’s original study (1965).
(3) Is there an objectively discernible climatic anomaly occurring within the 20th century that may validly be considered the most extreme (i.e., the warmest) period in the record? An important consideration in answering this question is to distinguish the case in which the 20th century warming began early in the century versus after the 1970s, as recorded by surface thermometers. This criterion is necessary in order to judge the influence of 20th century warming by anthropogenic forcing inputs such as increased atmospheric carbon dioxide content.
Anomaly, in our context, is simply defined as a period of 50 or more years of sustained warmth, wetness, or dryness within the Medieval Warm Period, or a 50-year or longer period of cold, dryness, or wetness during the Little Ice Age. …
4. RESULTS
… For questions 1 and 2, we find the answer to be ‘Yes’ when the proxy record shows a period of 50 years or longer of cooling, dryness or wetness during the Little Ice Age and a period of 50 years or longer of warming, wetness or dryness during the Medieval Warm Period. …
… most of the proxy records do not suggest the 20th century to be the warmest or the most extreme in its local representations, which seems surprising until one realizes the more limited and contrary view was drawn primarily from familiar instrumental thermometer records that yield no information on centennial-scale climate variability. … Another interesting feature of the result is that the warmest or most extreme climatic anomalies in the proxy indicators often occurred in the early-to-mid 20th century, rather than throughout the century.
4.1. Glaciers – Worldwide
Broadly, glaciers retreated all over the world during the Medieval Warm Period, with a notable but minor re-advance between 1050 and 1150 A.D. (Grove and Switsur 1994). Large portions of the world’s glaciers, both in the Northern and Southern Hemispheres, advanced during the 1300–1900 A.D. period (Grove 2001b; see also Winkler 2000). The world’s small glaciers and tropical glaciers have simultaneously retreated since the 19th century, but some glaciers have advanced (Kaser 1999; Dyurgerov and Meier 2000; D. Evans 2000). …
Additional proxy records used here reveal that the climatic anomaly patterns known as the Medieval Warm Period (circa 800–1300 A.D.) and the Little Ice Age (1300–1900 A.D.) occurred across the world. …
4.2.2. North Atlantic and other oceans
The colonization of Greenland’s coastal area by the Vikings starting in 986 A.D. is well documented; and the generally mild and benign climatic conditions from about 800–1200 A.D. that helped to sustain the settlement are also well supported by ice core and borehole proxy information (Dansgaard et al. 1975; Dahl-Jensen et al. 1998). The Norsemen’s ‘Western Settlement’ (around the Godthab district) was mysteriously abandoned sometime between 1341 and 1362 A.D., while the ‘Eastern Settlement’ (actually near the southernmost tip of west Greenland, around the Narssaq and Julianehab districts) died out between 1450 and 1500 A.D. (Grove 1996; Ogilvie et al. 2000). The timing of the abandonment of the settlements coincided with a general cooling over Greenland, as established by both ice-core isotopic and borehole thermometry (Dansgaard et al. 1975; Stuiver et al. 1995; Dahl-Jensen et al. 1998). …
Over the equatorial Central Pacific, around the NINO3.4 (5N–5ºS; 160ºE–150ºW) region, Evans et al. (2000), in their skillful reconstruction of the ENSO-like decadal variability of the NINO3.4 sea surface temperature (SST), found an apparent sustained cool phase of the proxy NINO3.4 SST variability from about 1550 A.D. to approximately 1895 A.D., thereby extending the geographical area covered by the Little Ice Age Climate Anomaly. …
4.2.3. Asia and Eastern Europe
From 49 radiocarbon-dated subfossil wood samples, Hiller et al. (2001) determined that the alpine tree-limit on the Khibiny low mountains of the Kola Peninsula was located at least 100–140 meters above the current tree-limit elevation during the relatively warmer time between 1000 A.D. and 1300 A.D. The summer temperatures corresponding to the tree-line shift during this warm time are estimated to have been at least 0.8ºC warmer than today. …
Middle Russia (around 50–60ºN and 30–50ºE) seems to have experienced its coolest winters around 1620–1680 A.D., its coolest summers and springs around 1860–1900 A.D., and distinctively warm conditions during the first half of the 16th century, similar to conditions for western Europe described above. …
Based on less precise climate proxies like cherry-blossom-viewing dates, lake freezing dates and historical documentation of climate hazards and unusual weather, Tagami (1993, 1996) found that a warm period prevailed between the 10th and 14th centuries, and a cold period between the late 15th and 19th centuries, over large parts of southern Japan. …
4.2.4. North America
From an extensive collection of multiproxy evidence, Stine (1998) concluded that during the Medieval Warm Period prolonged intervals of extreme drought affected California, the northwestern Great Basin, and the northern Rocky Mountains/Great Plains, while markedly wetter regimes persisted over the Upper Midwest/sub-arctic Canada and Southern Alaska/British Columbia regions. …
Graumlich’s (1993) reconstruction of summer temperature and winter precipitation from trees in the Sierra Nevada confirmed the overall warm and dry conditions for California during Medieval times, when two of the warmest and driest 50-year intervals occurred – at 1118–1167, 1245–1294 A.D. and 1250–1299, 1315–1364 A.D., respectively. …
Hu et al. (2001), based on their high-resolution (multidecadal) geochemical analysis of sediments from Farewell Lake by the northwestern foothills of the Alaska Range, also found pronounced signatures of the Medieval Warm Period around 850–1200 A.D. During the Little Ice Age, the surface water temperature of Farewell Lake fell to a low in 1700 A.D. that was estimated to be about 1.75ºC cooler than at present. …
4.3. Southern Hemisphere
4.3.1. New Zealand
In New Zealand, the O-18 concentration in a stalagmite record from a cave in northwest Nelson shows the coldest times during the Little Ice Age to be around 1600–1700 A.D., while exceptionally warm temperatures occurred around 1200–1400 A.D., in association with the general phenomenology of the Medieval Warm Period (Wilson et al. 1979). …
4.3.4. Antarctica
The last important source of geographical information for conditions during the Medieval Warm Period and the Little Ice Age in the Southern Hemisphere is obtained from glaciers, ice cores and sea sediments on and around Antarctica. …
For the Little Ice Age, advances of glaciers on South Georgia Island (which is half-covered by glaciers) began after the late 13th century, with a peak advancement around the 18th–20th centuries (Clapperton et al. 1989). Glacier retreats occurred after about 1000 A.D., which corresponds to the timing for the Medieval Warm Period. Baroni and Orombelli (1994) noted a similar scenario for glacier advances and retreats during the Little Ice Age and Medieval Warm Period for the Edmonson Point glacier at the Terra Nova Bay area of Victoria Land on the Antarctic continent (East Antarctica). The Edmonson Point glacier retreated in two distinct phases, around 920–1020 A.D. and 1270–1400 A.D., and then advanced at least 150 meters after the 15th century. …
5. DISCUSSION
The widespread, but not truly global, geographical evidence assembled here argues for the reality of both the Little Ice Age and the Medieval Warm Period, and should serve as a useful validation target for any reconstruction of global climate history over the last 1000 years. …
Another significant problem is the indication that an anthropogenic influence may have already left its fingerprint on the recent growth of trees across the Northern Hemisphere. If this anthropogenic effect were present in tree ring data, then the calibration and verification procedure designed for extended paleoclimatic reconstructions would be significantly corrupted by further uncertainties (Idso 1989). …
Karlén (2001), for example, notes that according to the Vostok ice core record of atmospheric carbon dioxide, the present concentration of atmospheric CO2 is about 100 ppmv higher than it was during any previous interglacial during the last 400,000 years. Thus, if climate were to respond sensitively to carbon dioxide, global temperatures, or at least Vostok temperature, today ought to be considerably higher than previous interglacials. Yet evidence exists to suggest that the “present interglacial [at least for conditions around Vostok] has been about 2ºC cooler than the previous one and the climate is now, in spite of the recent warming, cooler than it was at the beginning of this interglacial” (Karlén 2001). …
6. CONCLUSIONS
This paper presents a survey of site-specific paleoclimatic reconstructions, then considers whether they indicate that the Medieval Warm Period and the Little Ice Age were observed on broad area of the globe. We conclude that the Medieval Warm Period and Little Ice Age are widespread climatic anomalies, although we emphasize the complex nature of translating the proxy changes into convenient measures like temperature and precipitation as well as confirming their spatio-temporal representation and resolution. …
The picture emerges from many localities that both the Little Ice Age and Medieval Warm Period are widespread and perhaps not precisely timed or synchronous phenomena, easily within the margin of viewpoints conceived by Bryson et al. (1963), Lamb (1965) and numerous other researchers like J. Grove (1996, 2001a, 2001b). Our many local answers confirm that both the Medieval Climatic Anomaly and the Little Ice Age Climatic Anomaly are worthy of their respective labels. Furthermore, thermometer warming of the 20th century across the world seems neither unusual nor unprecedented within the more extended view of the last 1000 years. Overall, the 20th century does not contain the warmest or most extreme anomaly of the past millennium in most of the proxy records. …
However, it is also clear that human activity has shaped almost every aspect of past environmental and climatic changes on local and regional spatial scales …
It might seem surprising or frustrating that paleoclimatic reconstruction research has not yet provided confident and applicable answers to the role of anthropogenic forcing on climate change. This point is particularly sharp when considering the fact that even though some proxy records (e.g., those from Overpeck et al. 1997) show unprecedented 20th century warmth with most of the increase occurring in the early to mid-decades of the 20th century, when the amount of anthropogenic CO2 in the air was less than 20–30% of the total amount there now. Unless there are serious flaws in the timing of the early-to-middle 20th century surface thermometer warming, or unknown anthropogenic mechanisms that caused a large amplification of surface temperature of the then-small increase in anthropogenic atmospheric CO2, then the early part of the 20th century warming must be largely dissociated from anthropogenic CO2 emissions. Other anthropogenic factors still need to be studied on a case by case basis. …
CLIMATIC CHANGES THAT MAKE THE WORLD FLIP
Global warming’s impact on the environment is not necessarily a drawn-out affair. Recent evidence shows that dramatic changes or ‘climatic flips’ could happen virtually overnight.The once-green land of Ireland turned into a frozen wilderness. Harp seals swimming among ice-floes off the coast of France. Polar bears prowling the streets of Amsterdam. These are the images conjured up by the latest research into global warming.Yes, you read that correctly: global warming–the rise in the world’s average temperature caused by the trapping of the sun’s heat by pollution in the atmosphere.If you are baffled by that, then prepare to be shocked. For the same research is now suggesting that such dramatic changes in the climate of northern Europe could take place in as few as 10 years. Again, this figure is not a misprint: no zero has gone missing. Scientists have recently uncovered compelling evidence that global warming can have a devastating impact on timescales far shorter than anyone believed possible. Not centuries, not even decades, but years, in what are being called “climatic flips”. One leading expert has recently gone on record to warn that some north Atlantic countries could find themselves plunged into Arctic conditions over the space of just 10 years.Risk of sudden upheavalIn geological terms, that is as fast as the blink of an eye. But even in human terms, such a rate of climatic change is incredibly–and quite probably intolerably–rapid. It is far from clear whether any economy or agricultural system could cope with such sudden upheaval.Yet evidence is now mounting that such “climatic flips” not only can happen, but have happened in the past. It is evidence that adds new urgency to the global warming debate, which has lost much of its momentum in recent years. It also highlights the frightening complexity of the task facing scientists trying to predict the earth’s response to human activity.Arguments about climatic change typically focus on how increasing levels of so-called greenhouse gases–principally carbon dioxide from burning fossil fuels–in the earth’s atmosphere trap ever more of the sun’s heat. Huge efforts have been put into predicting the likely global temperature rise caused by the extra greenhouse gases, and current best estimates point to a rise of 1.5 degrees celsius or so over the next century.But while scientists warn that even so apparently small a rise in temperature could cause upheaval in everything from agricultural practices to the spread of disease, the rate of change hardly seems terrifying. Surely we can cope and have coped with events that change over several generations?Such arguments are buttressed by another, apparently compelling, argument against rapid climate change. The earth’s oceans have colossal thermal inertia, and would surely iron out any sudden upheaval: weight for weight, it takes ten times more energy to heat water than it does solid iron. Small wonder, then, that scientists were unsurprised when they failed to find any signs of rapid climatic changes when they first studied ancient ocean sediments, the isotope levels of which retain a record of past temperatures.The end of the Ice Age: a puzzling discoveryBut this apparently comforting confluence of theory and data is now known to contain two huge loopholes. The first reared its head in the early 1980s, when a joint U.S.-European team of scientists working in Greenland made a puzzling discovery. They had extracted an ice-core from a site in the southern part of the country, and had measured isotope levels in the gas trapped at different depths in an attempt to gauge the temperature in the region over thousands of years.Because the ice builds up relatively rapidly, the ice-core was expected to give the researchers the most fine-detailed picture yet of temperature changes in the region. Plotting out the corresponding temperatures, the researchers discovered something puzzling–and disturbing.As expected, the core showed the rise in temperature corresponding to the end of the last Ice Age around 11,000 years ago. But it also showed that the bulk of that warm-up had taken place in the space of just 40 years.At the time, no one knew what to make of the result, which flew in the face of everything scientists then knew about climate change–or thought they knew. In the years that followed, however, further ice-cores were extracted, and they revealed an even more dramatic story: a 5- to 10-degree increase in temperature and doubling of precipitation over Greenland in the space of just 20 years.Nothing in the earlier ocean sediment core data had prepared scientists for such a finding–nor could it. For this was the first loophole in the argument against sudden climatic flips: the absence of evidence from the original ocean sediment cores simply reflected the very broad-brush picture they gave of temperature change. They lacked the detail offered by ice-cores.Prompted by the Greenland findings, scientists have since tracked down locations where ocean sediment builds up fast enough to give a record of temperature comparable in detail to that from the ice-cores. And, sure enough, they reveal the same story of rapid climatic change in locations as far apart as California and India.The history of science shows that finding evidence for some astonishing phenomenon is often only part of the story. To convince the scientific community at large, the evidence has to be backed up by a more comprehensive explanation. And for many years the standard explanation for why Ice Ages begin and end provided yet more reasons for thinking all climatic change must be slow and graceful. That explanation rests on work by a Serbian scientist named Milutin Milankovitch, who in 1920 linked Ice Ages to changes in the shape of the earth’s orbit. Caused by the push and pull of the other planets, these orbital changes altered the concentration of sunlight reaching the planet. Such changes would naturally take place very gradually, on timescales of many thousands of years–a recipe for climatic change that is anything but abrupt.A global heat transporterYet, once again, there is a loophole in this comforting argument–as Wallace Broecker of Columbia University, New York State, realized around the time climate experts were puzzling over the ice-core data.This loophole centres on a very specific feature of the earth’s oceans: their circulation patterns. Ocean currents transport heat around the globe like a vast conveyor belt. In the Atlantic, for example, warm water travels northwards from the Gulf of Mexico, passing its heat to the air by evaporation as it goes. This makes the current progressively cooler, saltier and denser until eventually, near Iceland, the water is so heavy that it sinks, and begins a long journey southward, along the ocean floor.Broecker realized that this complex, subtle process–which he called “The Conveyor”–could be the Achilles heel of the earth’s climate, allowing subtle changes to be turned into dramatic upheaval. For instead of having to alter the whole body of the oceans, just a small change in temperature might be enough to alter the behaviour of the Conveyor–and trigger radical and rapid climatic change over a large area.For example, gradually melting ice from the Arctic could dilute the saltiness of the Conveyor to a critical density where it no longer sinks and begins its journey southward to pick up more heat. The Conveyor would, in effect, be switched off, isolating the north Atlantic from the warming waters of the tropics. The result would then be distinctly paradoxical, with a slight warming of the Arctic causing temperatures of north Atlantic countries to plunge.Broecker’s explanation is now widely believed to lie at the heart of rapid climate change in the past. Worryingly, however, global warming is predicted to have precisely the type of warming effect on the Arctic ice that threatens the existence of the Conveyor. Computer projections of the effect of pollution on global temperatures predict an inflow of cold, fresh water into the northern Atlantic–water that could dilute the Conveyor enough to switch it off.The Achilles heel of the earth’s climateIf that happened, says Broecker, winter temperatures in the north Atlantic region would fall by 10 or more degrees Celsius within 10 years, giving places like Dublin the climate of Spitsbergen, 400 km north of the Arctic Circle. “The consequences could be devastating,” he says.It is a scenario that gains credibility from ice-core data, according to climate expert Kendrick Taylor of the Desert Research Institute in Reno, Nevada. He says that many cores suggest that around 8,000 years ago there was a sudden plunge back to a “mini Ice Age” which lasted around 400 years. The most likely cause, says Taylor, was the release of melted ice-water from lakes in Canada into the Atlantic, which switched off the heat-transporting Conveyor.“The change in freshwater flux to the oceans was large, but not that much different from what greenhouse-induced changes may produce in the future,” he said in a recent paper in American Scientist. “It is ironic that greenhouse warming may lead to rapid cooling in eastern Northern America, Europe and Scandinavia.”So just how close is the Conveyor to switching off once again? The short answer is: no one knows. Computer models have still to identify the critical density of sea-water at which the Conveyor will switch off, or the greenhouse gas concentrations needed to release the requisite amount of melt-water.Cutting pollution buys timeWhat computer models have shown, says Taylor, is that reducing pollution emissions buys time–both by slowing the rate of global warming, and also by driving the climate more gently, which seems to increase its stability against rapid change.But while scientists struggle to capture the full complexity of the climate on their supercomputers, evidence of other causes of dramatic climate change is beginning to emerge.Last July, Professor Martin Claussen and his colleagues at the Potsdam Institute for Climate Science, Germany, reported evidence that today’s Sahara desert was created in a sudden climatic “flip” that took place just 5,500 years ago, turning vast areas of lush grassland into an arid wilderness and devastating ancient civilizations.Using a sophisticated computer model of the land, sea and atmosphere, the team has discovered just how subtle are some of the effects that can turn Milankovitch-style changes in the earth’s orbit into major climate upheaval.The Sahara’s quick-change actThey found that over the last 9,000 years the gravitational pull of the planets has altered the tilt of the earth’s axis by about half a degree, and changed the timing of earth’s closest approach to the sun by around five months.By themselves, such subtle changes should not cause major climatic effects. But when Claussen and his colleagues included the effect of vegetation in their computer model, they found that it caused rainfall levels to plummet over the Sahara region.They traced the cause to “feedback” effects, in which a slight drop in vegetation level makes the earth’s surface slightly better at reflecting sunlight, which causes rainfall levels to drop–prompting more vegetation loss, and so on.According to Claussen, these feedback effects turned the vast, once-green Sahara into a brown wasteland within just 300 years. “It was the largest change in land cover during the last 6,000 years,” he says. “It was very severe, ruining ancient civilizations.”The discovery is likely to force historians to rethink their explanations of events in the region. For according to Claussen, it contradicts the long-held belief that the collapse of agriculture in the region was caused by ancient farmers exhausting the soil: “Although humans lived in the Sahara and used the land to some extent, we think that ancient land use played only a negligibly small role.”The findings are also being seen as another warning of just how unstable even today’s climate may be. “It is capable of changing very abruptly,” says climate expert Andrew Goudie of Oxford University. “We’ve known that the extent of the Sahara has yo-yoed back and forwards for millions of years, and that about 8,000 years ago it was much wetter than today, with big rivers feeding into the Nile. But I hadn’t realized just how rapid the changeover had been. It is salutary.”Temperature nose-divesAlso in July, a team of researchers from the universities of Illinois and Minnesota reported the discovery of another climatic “flip” in the northern hemisphere around 9,000 years ago, which temporarily plunged the region back into an Ice Age.Using lake sediments from Minnesota, the team confirmed the existence of the cold snap around 8,200 years ago, as revealed by the ice core data. But they also found evidence for another dive in temperatures around 8,300 to 8,900 years ago. The team thinks this older cold snap was linked to the release of melted ice from lakes into the Atlantic–which may have switched off the Conveyor. But the researchers now think that the more recent flip most likely had another–and as yet unknown–origin.What is clear is that until we know much more about the complexity of climate change, all bets about how much time we have to take action are definitely off. What evidence we do have increasingly points to the stark possibility that we may have far less time than we thought.“I used to believe that change in climate happened slowly and would never affect me,” admits Taylor. “Now I know that our climate could change significantly in my lifetime
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