Researchers resolve the Karakoram glacier anomaly, a cold case of climate science

The researchers found that low-resolution models and a lack of reliable observational data obscured the Karakoram's dramatic shifts in elevation over a small area and heavy winter snowfall. They created a higher-resolution model that showed the elevation and snow water equivalent for (inlaid boxes, from left to right) the Karakoram range and northwest Himalayas, the central Himalayas that include Mount Everest, and the southeast Himalayas and the Tibetan Plateau. For elevation (left), the high-resolution model showed the sharp variations between roughly 2,500 and 5,000 meters above sea level (yellow to brown) for the Karakoram, while other areas of the map have comparatively more consistent elevations. The model also showed that the Karakoram receive much more annual snowfall (right) than other Himalayan ranges (right), an average of 100 centimeters (brown). The researchers found that the main precipitation season in the Karakoram occurs during the winter and is influenced by cold winds coming from Central Asian countries, as opposed to the heavy summer monsoons that provide the majority of precipitation to the other Himalayan ranges. -  Image by Sarah Kapnick, Program in Atmospheric and Oceanic Sciences
The researchers found that low-resolution models and a lack of reliable observational data obscured the Karakoram’s dramatic shifts in elevation over a small area and heavy winter snowfall. They created a higher-resolution model that showed the elevation and snow water equivalent for (inlaid boxes, from left to right) the Karakoram range and northwest Himalayas, the central Himalayas that include Mount Everest, and the southeast Himalayas and the Tibetan Plateau. For elevation (left), the high-resolution model showed the sharp variations between roughly 2,500 and 5,000 meters above sea level (yellow to brown) for the Karakoram, while other areas of the map have comparatively more consistent elevations. The model also showed that the Karakoram receive much more annual snowfall (right) than other Himalayan ranges (right), an average of 100 centimeters (brown). The researchers found that the main precipitation season in the Karakoram occurs during the winter and is influenced by cold winds coming from Central Asian countries, as opposed to the heavy summer monsoons that provide the majority of precipitation to the other Himalayan ranges. – Image by Sarah Kapnick, Program in Atmospheric and Oceanic Sciences

Researchers from Princeton University and other institutions may have hit upon an answer to a climate-change puzzle that has eluded scientists for years, and that could help understand the future availability of water for hundreds of millions of people.

In a phenomenon known as the “Karakoram anomaly,” glaciers in the Karakoram mountains, a range within the Himalayas, have remained stable and even increased in mass while many glaciers nearby — and worldwide — have receded during the past 150 years, particularly in recent decades. Himalayan glaciers provide freshwater to a densely populated area that includes China, Pakistan and India, and are the source of the Ganges and Indus rivers, two of the world’s major waterways.

While there have been many attempts to explain the stability of the Karakoram glaciers, the researchers report in the journal Nature Geoscience that the ice is sustained by a unique and localized seasonal pattern that keeps the mountain range relatively cold and dry during the summer. Other Himalayan ranges and the Tibetan Plateau — where glaciers have increasingly receded as Earth’s climate has warmed — receive most of their precipitation from heavy summer monsoons out of hot South and Southeast Asian nations such as India. The main precipitation season in the Karakoram, however, occurs during the winter and is influenced by cold winds coming from Central Asian countries such as Afghanistan to the west, while the main Himalayan range blocks the warmer air from the southeast throughout the year.

The researchers determined that snowfall, which is critical to maintaining glacier mass, will remain stable and even increase in magnitude at elevations above 4,500 meters (14,764 feet) in the Karakoram through at least 2100. On the other hand, snowfall over much of the Himalayas and Tibet is projected to decline even as the Indian and Southeast Asian monsoons increase in intensity under climate change.

First author Sarah Kapnick, a postdoctoral research fellow in Princeton’s Program in Atmospheric and Oceanic Sciences, said that a shortage of reliable observational data and the use of low-resolution computer models had obscured the subtleties of the Karakoram seasonal cycle and prevented scientists from unraveling the causes of the anomaly.

For models, the complication is that the Karakoram features dramatic shifts in elevation over a small area, Kapnick said. The range boasts four mountains that are more than 8,000 meters (26,246 feet) high — including K2, the world’s second highest peak — and numerous summits that exceed 7,000 meters, all of which are packed into a length of about 500 kilometers (300 miles).

Kapnick and her co-authors overcame this obstacle with a high-resolution computer model that broke the Karakoram into 50-kilometer pieces, meaning that those sharp fluctuations in altitude were better represented.

In their study, the researchers compared their model with climate models from the United Nations’ Intergovernmental Panel on Climate Change (IPCC), which averages a resolution of 210-kilometer squares, Kapnick said. At that scale, the Karakoram is reduced to an average height that is too low and results in temperatures that are too warm to sustain sufficient levels of snowfall throughout the year, and too sensitive to future temperature increases.

Thus, by the IPCC’s models, it would appear that the Karakoram’s glaciers are imperiled by climate change due to reduced snowfall, Kapnick said. This region has been a great source of controversy ever since the IPCC’s last major report, in 2007, when the panel misreported that Himalayan glaciers would likely succumb to climate change by 2035. More recent papers using current IPCC models have similarly reported snowfall losses in this region because the models do not accurately portray the topography of the Karakoram, Kapnick said.

“The higher resolution allowed us to explore what happens at these higher elevations in a way that hasn’t been able to be done,” Kapnick said. “Something that climate scientists always have to keep in mind is that models are useful for certain types of questions and not necessarily for other types of questions. While the IPCC models can be particularly useful for other parts of the world, you need a higher resolution for this area.”

Jeff Dozier, a professor of snow hydrology, earth system science and remote sensing at the University of California-Santa Barbara, said that the research addresses existing shortcomings in how mountain climates are modeled and predicted, particularly in especially steep and compact ranges. Dozier, who was not involved in the research, conducts some of his research in the Hindu Kush mountains west of the Karakoram.

Crucial information regarding water availability is often lost in computer models, observational data and other tools that typically do not represent ranges such as Karakoram accurately enough, Dozier said. For instance, a severe 2011 drought in Northern Afghanistan was a surprise partly due to erroneous runoff forecasts based on insufficient models and surface data, he said. The high-resolution model Kapnick and her co-authors developed for Karakoram potentially resolves many of the modeling issues related to mountain ranges with similar terrain, he said.

“The Karakoram Anomaly has been a puzzle, and this paper gives a credible explanation,” Dozier said. “Climate in the mountains is obviously affected strongly by the elevation, but most global climate models don’t resolve the topography well enough. So, the higher-resolution model is appropriate. About a billion people worldwide get their water resources from melting snow and many of these billion get their water from High Mountain Asia.”

The researchers used the high-resolution global-climate model GFDL-CM2.5 at the Geophysical Fluid Dynamics Laboratory (GFDL), which is on Princeton’s Forrestal Campus and administered by the National Oceanic and Atmospheric Administration (NOAA). The researchers simulated the global climate — with a focus on the Karakoram — based on observational data from 1861 to 2005, and on the IPCC’s greenhouse-gas projections for 2006-2100, which will be included in its Fifth Assessment Report scheduled for release in November.

The 50-kilometer resolution revealed conditions in Karakoram on a monthly basis, Kapnick said. It was then that she and her colleagues could observe that the monsoon months in Karakoram are not only not characterized by heavy rainfall, but also include frigid westerly winds that keep conditions in the mountain range cold enough for nearly year-round snowfall.

“There is precipitation during the summer, it just doesn’t dominate the seasonal cycle. This region, even at the same elevation as the rest of the Himalayas, is just colder,” Kapnick said.

“The high-resolution model shows us that things don’t happen perfectly across seasons. You can have statistical variations in one month but not another,” she continued. “This allows us to piece out those significant changes from one month to the next.”

Kapnick, who received her bachelor’s degree in mathematics from Princeton in 2004, worked with Thomas Delworth, a NOAA scientist and Princeton lecturer of geosciences and atmospheric and oceanic sciences; Moestasim Ashfaq, a scientist at the Oak Ridge National Laboratory Climate Change Science Institute; Sergey Malyshev, a climate modeler in Princeton’s Department of Ecology and Evolutionary Biology based at GFDL; and P.C.D. “Chris” Milly, a research hydrologist for the U.S. Geological Survey based at GFDL who received his bachelor’s degree in civil engineering from Princeton in 1978.

While the researchers show that the Karakoram will receive consistent — and perhaps increased — snowfall through 2100, more modeling work is needed to understand how the existing glaciers may change over time as a result of melt, avalanches and other factors, Kapnick said.

“Our work is an important piece to understanding the Karakoram anomaly,” Kapnick said. “But that balance of what’s coming off the glacier versus what’s coming in also matters for understanding how the glacier will change in the future.”

The paper, “Snowfall less sensitive to warming in Karakoram than in Himalayas due to a unique seasonal cycle,” was published online in-advance-of-print Oct. 12 by Nature Geoscience.

Snail shells show high-rise plateau is much lower than it used to be

This is the Zhada Basin on the southwest Tibetan Plateau, with the Himalayas to the south. -  Joel Saylor
This is the Zhada Basin on the southwest Tibetan Plateau, with the Himalayas to the south. – Joel Saylor

The Tibetan Plateau in south-central Asia, because of its size, elevation and impact on climate, is one of the world’s greatest geological oddities.

At about 960,000 square miles it covers slightly more land area than Alaska, Texas and California combined, and its elevation is on the same scale as Mount Rainier in the Cascade Range of Washington state. Because it rises so high into the atmosphere, it helps bring monsoons over India and other nations to the south while the plateau itself remains generally arid.

For decades, geologists have debated when and how the plateau reached such lofty heights, some 14,000 feet above sea level, about half the elevation of the highest Himalayan peaks just south of the plateau.

But new research led by a University of Washington scientist appears to confirm an earlier improbable finding – at least one large area in southwest Tibet, the plateau’s Zhada Basin, actually lost 3,000 to 5,000 feet of elevation sometime in the Pliocene epoch.

“This basin is really high right now but we think it was a kilometer or more higher just 3 million to 4 million years ago,” said Katharine Huntington, a UW associate professor of Earth and space sciences and the lead author of a paper describing the research.

Co-authors are Joel Saylor of the University of Houston and Jay Quade and Adam Hudson, both of the University of Arizona. The paper was published online in August and will appear in a future print edition of the Geological Society of America Bulletin.

The Zhada Basin has rugged terrain, with exposed deposits of ancient lake and river sediments that make fossil shells of gastropods such as snails easily accessible, and determining their age is relatively straightforward. The researchers studied shells dating from millions of years ago and from a variety of aquatic environments. They also collected modern shell and water samples from a variety of environments for comparison.

The work confirms results of a previous study involving Saylor and Quade that examined the ratio of heavy isotope oxygen-18 to light isotope oxygen-16 in ancient snail shells from the Zhada Basin. They found the ratios were very low, which suggested the basin had a higher elevation in the past.

Oxygen-18 levels decrease in precipitation at higher elevations in comparison with oxygen-16, so shells formed in lakes and rivers that collect precipitation at higher elevations should have a lower heavy-to-light oxygen ratio. However, those lower ratios depend on a number of other factors, including temperature, evaporation and precipitation source, which made it difficult to say with certainty whether the low ratios found in the ancient snail shells meant a loss of elevation in the Zhada Basin.

So the scientists also employed a technique called clumped isotope thermometry, which Huntington has used and worked to refine for several years, to determine the temperature of shell growth and get an independent estimate of elevation change in the basin.

Bonding, or “clumping” together, of heavy carbon-13 and oxygen-18 isotopes in the carbonate of snail shells happens more readily at colder temperatures, and is measured using a tool called a mass spectrometer that provides data on the temperature of the lake or river water in which the snails lived.

The scientists found markedly greater “clumping,” as well as lower ratios of oxygen-18 to oxygen-16 in the ancient shells, indicating the shells formed at temperatures as much as 11 degrees Celsius (20 F) colder than average temperatures today, the equivalent of as much as 5,000 feet of elevation loss.

Just why the elevation decline happened is open to speculation. One possibility is that as faults in the region spread, the Zhada Basin lowered, Huntington said. It is unknown yet whether other parts of the southern plateau also lowered at the same time, but if elevation loss was widespread it could be because of broader fault spreading. It also is possible the crust thickened and forced large rock formations even deeper into the Earth, where they heated until they reached a consistency at which they could ooze out from beneath the crust, like toothpaste squeezed from the tube.

She noted that climate records from deep-sea fossils indicate Earth was significantly warmer when the cold Zhada Basin snail shells were formed.

“Our findings are a conservative estimate,” Huntington said. “No one can say this result is due to a colder climate, because if anything it should have been warmer.”

Asian monsoon much older than previously thought

University of Arizona geoscientist Alexis Licht (bottom left) and his colleagues from the French-Burmese Paleontological Team led by Jean-Jacques Jaeger of the University of Poitiers, France (center with hiking staff) used fossils they collected in Myanmar to figure out that the Asian monsoon started at least 40 million years ago. -  French-Burmese Paleontological Team 2012
University of Arizona geoscientist Alexis Licht (bottom left) and his colleagues from the French-Burmese Paleontological Team led by Jean-Jacques Jaeger of the University of Poitiers, France (center with hiking staff) used fossils they collected in Myanmar to figure out that the Asian monsoon started at least 40 million years ago. – French-Burmese Paleontological Team 2012

The Asian monsoon already existed 40 million years ago during a period of high atmospheric carbon dioxide and warmer temperatures, reports an international research team led by a University of Arizona geoscientist.

Scientists thought the climate pattern known as the Asian monsoon began 22-25 million years ago as a result of the uplift of the Tibetan Plateau and the Himalaya Mountains.

“It is surprising,” said lead author Alexis Licht, now a research associate in the UA department of geosciences. “People thought the monsoon started much later.”

The monsoon, the largest climate system in the world, governs the climate in much of mainland Asia, bringing torrential summer rains and dry winters.

Co-author Jay Quade, a UA professor of geosciences, said, “This research compellingly shows that a strong Asian monsoon system was in place at least by 35-40 million years ago.”

The research by Licht and his colleagues shows the earlier start of the monsoon occurred at a time when atmospheric CO2 was three to four times greater than it is now. The monsoon then weakened 34 million years ago when atmospheric CO2 then decreased by 50 percent and an ice age occurred.

Licht said the study is the first to show the rise of the monsoon is as much a result of global climate as it is a result of topography. The team’s paper is scheduled for early online publication in the journal Nature on Sept. 14.

“This finding has major consequences for the ongoing global warming,” he said. “It suggests increasing the atmospheric CO2 will increase the monsoonal precipitation significantly.”

Unraveling the monsoon’s origins required contributions from three different teams of scientists that were independently studying the environment of 40 million years ago.

All three investigations showed the monsoon climate pattern occurred 15 million years earlier than previously thought. Combining different lines of evidence from different places strengthened the group’s confidence in the finding, Licht said. The climate modeling team also linked the development of the monsoon to the increased CO2 of the time.

Licht and his colleagues at Poitiers and Nancy universities in France examined snail and mammal fossils in Myanmar. The group led by G. Dupont-Nivet and colleagues at Utrecht University in the Netherlands studied lake deposits in Xining Basin in central China. J.-B. Ladant and Y. Donnadieu of the Laboratory of Sciences of the Climate and Environment (LSCE) in Gif-sur-Yvette, France, created climate simulations of the Asian climate 40 million years ago.

A complete list of authors of the group’s publication, “Asian monsoons in a late Eocene greenhouse world,” is at the bottom of this release, as is a list of funding sources.

Licht didn’t set out to study the origin of the monsoon.

He chose his study site in Myanmar because the area was rich in mammal fossils, including some of the earliest ancestors of modern monkeys and apes. The research, part of his doctoral work at the University of Poitiers, focused on understanding the environments those early primates inhabited. Scientists thought those primates had a habitat like the current evergreen tropical rain forests of Borneo, which do not have pronounced differences between wet and dry seasons.

To learn about the past environment, Licht analyzed 40-million-year-old freshwater snail shells and teeth of mammals to see what types of oxygen they contained. The ratio of two different forms of oxygen, oxygen-18 and oxygen-16, shows whether the animal lived in a relatively wet climate or an arid one.

“One of the goals of the study was to document the pre-monsoonal conditions, but what we found were monsoonal conditions,” he said.

To his surprise, the oxygen ratios told an unexpected story: The region had a seasonal pattern very much like the current monsoon – dry winters and very rainy summers.

“The early primates of Myanmar lived under intense seasonal stress – aridity and then monsoons,” he said. “That was completely unexpected.”

The team of researchers working in China found another line of evidence pointing to the existence of the monsoon about 40 million years ago. The monsoon climate pattern generates winter winds that blow dust from central Asia and deposits it in thick piles in China. The researchers found deposits of such dust dating back 41 million years ago, indicating the monsoon had occurred that long ago.

The third team’s climate simulations indicated strong Asian monsoons 40 million years ago. The simulations showed the level of atmospheric CO2 was connected to the strength of the monsoon, which was stronger 40 million years ago when CO2 levels were higher and weakened 34 million years ago when CO2 levels dropped.

Licht’s next step is to investigate how geologically short-term increases of atmospheric CO2 known as hyperthermals affected the monsoon’s behavior 40 million years ago.

“The response of the monsoon to those hyperthermals could provide interesting analogs to the ongoing global warming,” he said.

Research provides new theory on cause of ice age 2.6 million years ago

New research published today (Friday 27th June 2014) in the journal Nature Scientific Reports has provided a major new theory on the cause of the ice age that covered large parts of the Northern Hemisphere 2.6 million years ago.

The study, co-authored by Dr Thomas Stevens, from the Department of Geography at Royal Holloway, University of London, found a previously unknown mechanism by which the joining of North and South America changed the salinity of the Pacific Ocean and caused major ice sheet growth across the Northern Hemisphere.

The change in salinity encouraged sea ice to form which in turn created a change in wind patterns, leading to intensified monsoons. These provided moisture that caused an increase in snowfall and the growth of major ice sheets, some of which reached 3km thick.

The team of researchers analysed deposits of wind-blown dust called red clay that accumulated between six million and two and a half million years ago in north central China, adjacent to the Tibetan plateau, and used them to reconstruct changing monsoon precipitation and temperature.

“Until now, the cause of the Quaternary ice age had been a hotly debated topic”, said Dr Stevens. “Our findings suggest a significant link between ice sheet growth, the monsoon and the closing of the Panama Seaway, as North and South America drifted closer together. This provides us with a major new theory on the origins of the ice age, and ultimately our current climate system.”

Surprisingly, the researchers found there was a strengthening of the monsoon during global cooling, instead of the intense rainfall normally associated with warmer climates.

Dr Stevens added: “This led us to discover a previously unknown interaction between plate tectonic movements in the Americas and dramatic changes in global temperature. The intensified monsoons created a positive feedback cycle, promoting more global cooling, more sea ice and even stronger precipitation, culminating in the spread of huge glaciers across the Northern Hemisphere.”

Heat and moisture from Himalayas could be a key cause of the South Asian monsoon

Harvard climate scientists suggest that the Tibetan Plateau-thought to be the primary source of heat that drives the South Asian monsoon-may have far less of an effect than the Himalayas and other surrounding mountains. As the monsoon brings needed rainfall and water to billions of people each year, understanding its proper origin, especially in the context of global climate change, is crucial for the future sustainability of the region.

The researchers say the their findings, published in the January 14th issue of Nature, have broad implications for how the Asian climate may have responded to mountain uplift in the past, and for how it might respond to surface changes in the coming decades.

Often called the “roof of the world,” the Tibetan Plateau is a vast area of 2.5 million square kilometers with an average elevation of more than 4,500 meters. Scientists have long theorized that the massive release of heat from the surface of the plateau-with air being heated to higher temperatures over the plateau than air at the same height over lower-level surfaces nearby-has been a major contributor to the strength of the monsoon.

“The South Asian monsoon supplies water to billions of people, many of whom live in developing nations and agricultural societies that are highly vulnerable to variations in this water supply,” explains co-author Zhiming Kuang, Assistant Professor of Climate Science in Harvard’s School of Engineering and Applied Sciences (SEAS) and Department of Earth and Planetary Sciences (EPS).

While the heating by the plateau does enhance rainfall along its southern edge, Kuang and his colleague William Boos, Daly Postdoctoral Fellow in EPS and an environmental fellow at the Harvard University Center for the Environment (HUCE), used an atmospheric circulation model to show that the large-scale South Asian summer monsoon circulation remains unaffected when the plateau is removed.

It turns out that the narrow geography of the Himalayas and other nearby mountain ranges can, in fact, produce an equally strong monsoon by insulating warm, moist air over continental India from the cold dry extratropics, the area between the subtropics and polar regions.

“Because heat from the plateau has been seen as the main contributor to the power of the monsoon, much attention has been given to changes in the plateau’s albedo, or its reflectivity level of the sun’s radiation,” says Kuang.

For example, a decrease in snow cover over the Tibetan Plateau resulting from an increase in global temperatures can affect reflectivity, and hence, the level of heat. The revised theory, emphasizing the important role the mountains play in trapping warm and moist air, suggests that climate scientists should pay as much attention to changes over the Indian subcontinent due to, for example, land use.

How the region’s natural environment is modified through activities such as building, mining, and agriculture, Zhang explains, can influence albedo and moisture, thus altering the temperature/humidity of the boundary layer air.

By considering the influence of both the plateau and the mountains on the strength of the monsoon, the Harvard researchers expect a clearer picture will emerge about the potential changes in the South Asian water supply in the coming decades.

“Ultimately, our revised view has implications for future projections of how the South Asian monsoon might be altered in a warmer world and can be used to infer aspects about the earth’s climate history,” says Boos.

Global monsoon drives long-term carbon cycles in the ocean

Monsoon is a global system, and many arrays of evidence indicate that it drives long-term cyclicity of the carbon reservoir in the global ocean. The new view is introduced in a substantial paper in Issue 7 (April 2009) of Chinese Science Bulletin.

For over 300 years, monsoon has been considered as a gigantic land-sea breeze of regional scale, but now it is considered as a global system over all continents but Antarctica. This new develoment in modern climatology, however, has not yet been responded by paleo-climatology.

Prof. Pinxian Wang from Tongji University, Shanghai, reviews the geological evolution of the global monsoon and its impact, showing that the global monsoon exists through all geological history since at least 600 million years ago. It covaries with various geological cycles including those caused by the geometric changes of the Earth’s orbits. The 20,000-year precessional cycle of the global monsoon, for example, is responsible for the collapse of several Asian and African ancient cultures at ~ 4000 years ago. The same cyclicity is seen in the chemical composition of the air, such as methane concentration and isotope composition of air-bubbles captured in ice cores.

Now Wang found that the long-term cycles in the oceanic carbon reservoir also has a global monsoon origin. This 400,000-year cyclicity related to “long eccentricity” of the Earth’s orbit, is best seen in carbon isotope compositions of calcite test of foraminifera, a single-cell animal in the ocean. The rhythmic changes in oceanic carbon reservoir were likened to “heartbeat” of the Earth system. This cyclicity becomes longer since 1.6 million years ago, displaying a kind of “arrhythmia” in the Earth system, probably resulting from the growth of the Arctic ice. Although the mechanism of how monsoon drives oceanic carbon cycle remains unclear, the monsoon-related long-term cyclicity should not be overlooked in carbon-cycle modeling for long-term climate prediction.

“It is an authoritative review”, said Prof. Andre Berger, University of Louvain, in his commentary, “and probably also the first in which the monsoon issues are reviewed in a global scale through a so long geological history?.I totally agree with Wang’s argumentation about paying more attention to the importance of the tropical forcing in modulating the Earth’s climate system”. The geological evolution of the global monsoon is a new topic attracting growing interest from both modern and paleo-climatologic communities. An international symposium on global monsoon was organized by the PAGES (Past Global Changes) project in Shanghai in 2008, and the next symposium is scheduled in 2010.

Dry spells spelled trouble in ancient China

Chinese history is replete with the rise and fall of dynasties, but researchers now have identified a natural phenomenon that may have been the last straw for some of them: a weakening of the summer Asian Monsoons.

Such weakening accompanied the fall of three dynasties and now could be lessening precipitation in northern China.

Results of the study, led by researchers from the University of Minnesota and Lanzhou University in China, appear in this week’s issue of the journal Science.

The work rests on climate records preserved in the layers of stone in a 118-millimeter-long stalagmite found in Wanxiang Cave in Gansu Province, China.

By measuring amounts of the elements uranium and thorium throughout the stalagmite, the researchers could tell the date each layer was formed. And by analyzing the “signatures” of two forms of oxygen in the stalagmite, they could match amounts of rainfall–a measure of summer monsoon strength–to those dates.

The stalagmite was formed over 1,810 years; stone at its base dates from A.D. 190, and stone at its tip was laid down in A.D. 2003, the year the stalagmite was collected.

“It was unexpected that a record of surface weather would be preserved in underground cave deposits,” said David Verardo, director of the National Science Foundation (NSF)’s Paleoclimatology Program, which funded the research. “These results illustrate the promise of paleoclimate science to look beyond the obvious and see new possibilities.”

“Summer monsoon winds originate in the Indian Ocean and sweep into China,” said Hai Cheng, author of the paper and a scientist at the University of Minnesota. “When the summer monsoon is stronger, it pushes farther northwest into China.”

These moisture-laden winds bring rain necessary for cultivating rice. But when the monsoon is weak, the rains stall farther south and east, depriving northern and western parts of China of summer rains.

A lack of rainfall could have contributed to social upheaval and the fall of dynasties.

The researchers discovered that periods of weak summer monsoons coincided with the last years of the Tang, Yuan and Ming dynasties, which are known to have been times of popular unrest.

Conversely, the scientists found that a strong summer monsoon prevailed during one of China’s “golden ages,” the Northern Song Dynasty.

The ample summer monsoon rains may have contributed to the rapid expansion of rice cultivation from southern China to the midsection of the country. During the Northern Song Dynasty, rice first became China’s main staple crop, and China’s population doubled.

“The waxing and waning of summer monsoon rains are just one piece of the puzzle of changing climate and culture around the world,” said Larry Edwards, geologist at the University of Minnesota and a co-author of the paper.

For example, the study showed that the dry period at the end of the Tang Dynasty coincided with a previously identified drought halfway around the world, in Meso-America, which has been linked to the fall of the Mayan civilization.

The study also showed that the ample summer rains of the Northern Song Dynasty coincided with the beginning of the well-known Medieval Warm Period in Europe and Greenland.

During this time–the late 10th century–Vikings colonized southern Greenland. Centuries later, a series of weak monsoons prevailed as Europe and Greenland shivered through what geologists call the Little Ice Age.

In the 14th and early 15th centuries, as the cold of the Little Ice Age settled into Greenland, the Vikings disappeared from there. At the same time, on the other side of the world, the weak monsoons of the 14th century coincided with the end of the Yuan Dynasty.

A second major finding concerns the relationship between temperature and the strength of the monsoons. For most of the last 1,810 years, as average temperatures rose, so, too, did the strength of the summer monsoon.

That relationship flipped, however, around 1960, a sign that the late 20th century weakening of the monsoon and drying in northwestern China was caused by human activity.

If carbon dioxide is the culprit, as some have proposed, the drying trend may well continue in Inner Mongolia, northern China and neighboring areas on the fringes of the monsoon’s reach.

If, however, the culprit is man-made soot, as others have proposed, the trend could be reversed, the researchers said, by reduction of soot emissions.