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.

Geologists prove early Tibetan Plateau was larger than previously thought

This is Syracuse University professor Gregory Hoke. -  Syracuse University
This is Syracuse University professor Gregory Hoke. – Syracuse University

Earth scientists in Syracuse University’s College of Arts and Sciences have determined that the Tibetan Plateau-the world’s largest, highest, and flattest plateau-had a larger initial extent than previously documented.

Their discovery is the subject of an article in the journal Earth and Planetary Science Letters (Elsevier, 2014).

Gregory Hoke, assistant professor of Earth sciences, and Gregory Wissink, a Ph.D. student in his lab, have co-authored the article with Jing Liu-Zeng, director of the Division of Neotectonics and Geomorphology at the Institute for Geology, part of the China Earthquake Administration; Michael Hren, assistant professor of chemistry at the University of Connecticut; and Carmala Garzione, professor and chair of Earth and environmental sciences at the University of Rochester.

“We’ve determined the elevation history of the southeast margin of the Tibetan Plateau,” says Hoke, who specializes in the interplay between the Earth’s tectonic and surface processes. “By the Eocene epoch (approximately 40 million years ago), the southern part of the plateau extended some 600 miles more to the east than previously documented. This discovery upends a popular model for plateau formation.”

Known as the “Roof of the World,” the Tibetan Plateau covers more than 970,000 square miles in Asia and India and reaches heights of over 15,000 feet. The plateau also contains a host of natural resources, including large mineral deposits and tens of thousands of glaciers, and is the headwaters of many major drainage basins.

Hoke says he was attracted to the topography of the plateau’s southeast margin because it presented an opportunity to use information from minerals formed at the Earth’s surface to infer what happened below them in the crust.

“The tectonic and topographic evolution of the southeast margin has been the subject of considerable controversy,” he says. “Our study provides the first quantitative estimate of the past elevation of the eastern portions of the plateau.”

Historically, geologists have thought that lower crustal flow- a process by which hot, ductile rock material flows from high- to low-pressure zones-helped elevate parts of the plateau about 20 million years ago. (This uplift model has also been used to explain watershed reorganization among some of the world’s largest rivers, including the Yangtze in China.)

But years of studying rock and water samples from the plateau have led Hoke to rethink the area’s history. For starters, his data indicates that the plateau has been at or near its present elevation since the Eocene epoch. Moreover, surface uplift in the southernmost part of the plateau-in and around southern China and northern Vietnam-has been historically small.

“Surface uplift, caused by lower crustal flow, doesn’t explain the evolution of regional river networks,” says Hoke, referring to the process by which a river drainage system is diverted, or captured, from its own bed into that of a neighboring bed. “Our study suggests that river capture and drainage reorganization must have been the result of a slip on the major faults bounding the southeast plateau margin.”

Hoke’s discovery not only makes the plateau larger than previously thought, but also suggests that some of the topography is millions of years younger.

“Our data provides the first direct documentation of the magnitude and geographic extent of elevation change on the southeast margin of the Tibetan Plateau, tens of millions years ago,” Hoke adds. “Constraining the age, spatial extent, and magnitude of ancient topography has a profound effect on how we understand the construction of mountain ranges and high plateaus, such as those in Tibet and the Altiplano region in Bolivia.”

First evidence that dust and sand deposits in China are controlled by rivers

Northern China holds some of the world's most significant wind-blown dust deposits, known as loess. The origin of this loess-forming dust and its relationship to sand has previously been the subject of considerable debate. -  Royal Holloway University
Northern China holds some of the world’s most significant wind-blown dust deposits, known as loess. The origin of this loess-forming dust and its relationship to sand has previously been the subject of considerable debate. – Royal Holloway University

New research published today in the journal Quaternary Science Reviews has found the first evidence that large rivers control desert sands and dust in Northern China.

Northern China holds some of the world’s most significant wind-blown dust deposits, known as loess. The origin of this loess-forming dust and its relationship to sand has previously been the subject of considerable debate.

The team of researchers led by Royal Holloway University, analysed individual grains of fine wind-blown dust deposited in the Chinese Loess Plateau that has formed thick deposits over the past 2.5 million years. As part of this, they also analysed the Mu Us desert in Inner Mongolia and the Yellow River, one of the world’s longest rivers, to identify links between the dust deposits and nearby deserts and rivers.

The results showed that the Yellow River transports large quantities of sediment from northern Tibet to the Mu Us desert and further suggests that the river contributes a significant volume of material to the Loess Plateau.

“The Yellow River drains the northeast Tibetan plateau and so the uplift of this region and the development of Yellow River drainage seems to control the large scale dust deposits and sand formation in this part of China,” said lead researcher Tom Stevens from the Department of Geography at Royal Holloway.

“Identifying how this dust is formed and controlled is important, since it drives climate change and ocean productivity and impacts human health. Its relationship to the river and Tibet implies strong links between tectonics and climate change. This suggests that global climate change caused by atmospheric dust may be influenced by the uplift of Tibet and changes in major river systems that drain this area.”

New research suggests strong Indian crust thrust beneath the Tibetan Plateau

Earthquake mechanisms and the style of faulting in the Himalaya-Tibet region show that the Himalayan range is under north-south compression, southern Tibet is in east-west extension, and northern Tibet is in both east-west extension and north-south compression. The study shows that this pattern can be explained if the strong Indian crust thrust under southern Tibet is transmitting the north-south push of India to northern Tibet. -  Caltech's Tectonics Observatory
Earthquake mechanisms and the style of faulting in the Himalaya-Tibet region show that the Himalayan range is under north-south compression, southern Tibet is in east-west extension, and northern Tibet is in both east-west extension and north-south compression. The study shows that this pattern can be explained if the strong Indian crust thrust under southern Tibet is transmitting the north-south push of India to northern Tibet. – Caltech’s Tectonics Observatory

For many years, most scientists studying Tibet have thought that a very hot and very weak lower and middle crust underlies its plateau, flowing like a fluid. Now, a team of researchers at the California Institute of Technology (Caltech) is questioning this long-held belief and proposing that an entirely different mechanism is at play.

“The idea that Tibet is more or less floating on a layer of partially molten crust is accepted in the research community. Our research proposes the opposite view: that there is actually a really strong lower crust that originates in India,” says Jean-Philippe Avouac, professor of geology and director of Caltech’s Tectonics Observatory.

These insights lead to a better understanding of the processes that have shaped the Himalaya Mountains and Tibet-the most tectonically active continental area in the world.

Alex Copley, a former postdoctoral scholar with Caltech’s Tectonics Observatory, along with Avouac and Brian Wernicke, the Chandler Family Professor of Geology, describe their work in a paper published in the April 7 issue of the journal Nature.

Tibet and the surrounding Himalaya Mountains are among the most dynamic regions on the planet. Avouac points out that underground plate collisions, which cause earthquakes and drive up the Himalaya and Tibet, are common geological processes that have happened repeatedly over the course of Earth’s history, but are presently happening with a vigor and energy only found in that area.

Even though the elevation is uniform across the Tibetan Plateau, the type of stress seen within the plateau appears to change along a line that stretches east-west across the plateau-dividing the region into two distinct areas (southern and northern Tibet, for the purposes of this research.)

The researchers propose that a contrast in tectonic style-primarily east-west extension due to normal faulting in southern Tibet and a combination of north-south compression and east-west extension due to strike-slip faulting in northern Tibet-is the result of the Indian crust thrusting strongly underneath the southern portion of the Tibetan Plateau and locking into the upper crust. Strike-slip fault surfaces are usually vertical, and the rocks slide horizontally past each other due to pressure build-up, whereas normal faulting occurs where the crust is being pulled apart. They believe that the locked Indian crust alters the state of stress in the southern Tibetan crust, which can explain the contrast in the type of faulting seen between southern Tibet and northern Tibet.

To test their theory, the team performed a series of numerical experiments, assigning different material properties to the Indian crust. The simulations revealed evidence for a strong Indian lower crust that couples, or locks in, with the upper crust. This suggests that the “channel flow” model proposed by many geophysicists and geologists-in which a low-viscosity magma oozes through weak zones in the middle crust-¬is not correct.

“We have been able to create a model that addresses two long-standing debates,” says Copley, who is now a research fellow at the University of Cambridge. “We have constrained the mechanical strength of the Indian crust as it plunges beneath the Tibetan Plateau, and by doing so have explained the variations in the types of earthquakes within the plateau. This is interesting because it gives us new insights into what controls the behavior of large mountain ranges, and the earthquakes that occur within them.”

According to Wernicke, the results have motivated the team to think of ways to test further the “weak crust” hypothesis, at least as it might apply to the active tectonic system. “One way we might be able to image an extensive interface at depth is through geodetic studies of southern Tibet, which are ongoing in our research group,” he says.

Soot packs a punch on Tibetan Plateau’s climate

In some cases, soot – the fine, black carbon silt that is released from stoves, cars and manufacturing plants – can pack more of a climatic punch than greenhouse gases, according to a paper published in the journal Atmospheric Chemistry and Physics.

Researchers at the Department of Energy’s Pacific Northwest National Laboratory, the University of Michigan and NOAA found that soot landing on snow on the massive Tibetan Plateau can do more to alter snowmelt and monsoon weather patterns in Asia than carbon dioxide and soot in the air. Soot on snow causes the plateau’s annual glacial melt to happen sooner each year, causing farmers below it to have less water for their crops in the summer. In a domino effect, the melting then prods two of the region’s monsoon systems to become stronger over India and China.

“On the global scale, greenhouse gases like carbon dioxide cause the most concern related to climate change,” said Yun Qian, the paper’s lead author and an atmospheric scientist at PNNL. “But our research shows that in some places like the Tibetan Plateau, soot can do more damage.”

Roof of the Earth

Qian and his colleagues focused their research on the Tibetan Plateau, a giant outcropping of land between China and India that’s nicknamed the “Roof of the Earth.” About five times the size of Texas and as much as 5 miles high in places, the Tibetan Plateau greatly influences the Asia’s weather, including the annual deluge of rain and strong winds that come with monsoons. It’s also home to the largest volume of ice outside of the north and south poles. Glaciers and snow on the plateau grow and melt as seasons change, providing runoff that feeds most of the region’s major rivers, including the Yangtze in China and the Ganges in India.

Soot has increasingly dirtied the Tibetan Plateau’s winter-white surfaces in the past two decades. A byproduct of the region’s rapid growth in industry and agriculture, soot leaves smokestacks and burning fields in developing Asian countries before it floats into the sky, where winds carry it toward the plateau. Soot is dark and absorbs far more heat from sunlight than pristine white snow. Soot’s ability to soak up more solar rays causes the snow it lands on to melt faster. The Tibetan Plateau also receives more direct sunlight than the distant north and south poles, meaning soot’s snow-melting powers are be more pronounced on the plateau.

To find out how much soot is affecting the Tibetan Plateau’s region, Qian and colleagues used a global climate computer model, the Community Atmosphere Model. The model allowed them to examine a mixture of possible scenarios, including if soot sat on the Tibetan Plateau’s snow, if soot was floating in the air above the plateau and if increased carbon dioxide was in the air as a result of industrialization.

More heat, melting

The model’s calculations showed that the average air temperature immediately above the plateau increased when all the scenarios were combined. Alone, both soot on snow and carbon dioxide increased temperatures about 2 degrees Fahrenheit. But while carbon dioxide increased temperatures fairly evenly throughout the region, including the ocean, soot on snow only significantly heated up the Tibetan Plateau and north Asia. Researchers concluded that soot on snow can increase the temperature differences between air over land and air over the ocean, which drive monsoons.

Soot on snow also stood out when the model investigated water runoff. Smaller changes were observed when just carbon dioxide or soot in the air were examined, but soot on snow by itself increased runoff substantially during the late winter and early spring and then decreased it during the late spring and early summer. With all three scenarios combined, the runoff increased by 0.44 millimeters (or nearly two-one hundredths of an inch) daily between February and April and then decreased by 0.57 millimeters daily between May and July. These changes provide more water in the winter, when it’s not particularly useful to farmers, but less in the summer when it’s needed to grow crops.

The researchers reasoned that soot on snow is more efficient in melting the plateau’s snowpack because of its close proximity to the snow. Like a warm blanket covering the plateau, soot on snow can almost immediately warm and melt the snow beneath it. But carbon dioxide and soot in the atmosphere have to transfer the heat they absorb way down to the plateau below, with some heat inevitably being lost.

Nature’s heat pump

Before this research, scientists knew that the Tibetan Plateau acted like a natural heat pump for the region’s weather. The plateau reaches 5 miles high in some places, allowing the air above it to be warmer than other air at the same elevation. The warm air strengthens air circulation around the plateau and causes the iconic, drenching monsoons that move through the region every year.

But with soot on snow causing more snowmelt on the plateau, the plateau is increasingly bare. Less snow covering to reflect solar heat means the Tibetan Plateau is absorbing more sunlight, which the researchers hypothesized was causing the atmosphere above the plateau to warm up even more. They used climate models to find out of this affects the area’s monsoons.

Stronger monsoons

The surface temperature above the plateau increased by more than 2 degrees Fahrenheit in May due to soot on snow alone. The researchers found that this warmer air above the plateau rises and air is drawn from India to replace it. In turn, moist air hanging above Arabian Sea and Indian Ocean blows in over India. Known as the South Asian Monsoon system, this southwest-northeast flow also brings in more soot from India to the Tibetan Plateau that perpetuates the cycle. As a result, the researchers found that the South Asian Monsoon system is starting earlier and bringing more rain to central and Northern India in May than it would without soot on the plateau’s snow.

The soot-on-snow effect lingers throughout the summer and causes another weather shift in the East Asian Monsoon system over China. By July, much of the plateau’s snow has already melted. The plateau’s bare soil is warmer and further heats the plateau’s air. Coupled with cool ocean air nearby, the plateau’s heat strengthens the East Asian Monsoon. The models showed that rain increases 1 to 3 millimeters per day over southern China and the South China Sea. The strengthened monsoon advances to northern China, which also receives more rain than it would otherwise, while the rains mostly skip central East China, including the Yangtze River Basin.

More work needs to be done to refine these findings, however. Qian and his co-authors noted that existing global climate models don’t allow for the close-up, detailed resolution needed to accurately portray the Tibetan Plateau’s many varying peaks. The model’s coarse resolution likely resulted in the plateaus’ snowpack being overestimated, meaning the researchers’ results represent the maximum amount that soot on snow could potentially impact hydrological and weather systems in the region.

Future research could also factor in dust, which blows throughout Asia with the wind. While soot is believed to have a larger impact on snowmelt than dust per unit mass, the region likely has more total dust than soot. However, dust is more challenging to represent in models, since its sources can’t be as easily measured as the polluting smokestacks and burning fields that cause soot.

“The Tibetan Plateau is an amazing, dynamic place where many things come together to develop large climate systems,” Qian said. “Our research indicates that soot on snow can be a large player in the region’s climate, but it’s not the only factor. Many other elements need to be studies before we can say for sure what is the leading cause of snowmelt – which also contributes to retreating glaciers – on the plateau.”

Black carbon deposits on Himalayan ice threaten Earth’s ‘Third Pole’

To better understand the role that black soot has on glaciers, researchers trekked high into the Himalayas to collect ice cores that contain a record of soot deposition that spans back to the 1950s. -  Institute of Tibetan Plateau Research, Chinese Academy of Sciences
To better understand the role that black soot has on glaciers, researchers trekked high into the Himalayas to collect ice cores that contain a record of soot deposition that spans back to the 1950s. – Institute of Tibetan Plateau Research, Chinese Academy of Sciences

Black soot deposited on Tibetan glaciers has contributed significantly to the retreat of the world’s largest non-polar ice masses, according to new research by scientists from NASA and the Chinese Academy of Sciences. Soot absorbs incoming solar radiation and can speed glacial melting when deposited on snow in sufficient quantities.

Temperatures on the Tibetan Plateau — sometimes called Earth’s “third pole” — have warmed by 0.3°C (0.5°F) per decade over the past 30 years, about twice the rate of observed global temperature increases. New field research and ongoing quantitative modeling suggests that soot’s warming influence on Tibetan glaciers could rival that of greenhouse gases.

“Tibet’s glaciers are retreating at an alarming rate,” said James Hansen, coauthor of the study and director of NASA’s Goddard Institute for Space Studies (GISS) in New York City. “Black soot is probably responsible for as much as half of the glacial melt, and greenhouse gases are responsible for the rest.”

“During the last 20 years, the black soot concentration has increased two- to three-fold relative to its concentration in 1975,” said Junji Cao, a researcher from the Chinese Academy of Sciences in Beijing and a coauthor of the paper.

The study was published December 7th in the Proceedings of the National Academy of Sciences.

“Fifty percent of the glaciers were retreating from 1950 to 1980 in the Tibetan region; that rose to 95 percent in the early 21st century,” said Tandong Yao, director of the Chinese Academy’s Institute of Tibetan Plateau Research. Some glaciers are retreating so quickly that they could disappear by mid-century if current trends continue, the researchers suggest.

Since melt water from Tibetan glaciers replenishes many of Asia’s major rivers-including the Indus, Ganges, Yellow, and Brahmaputra-such losses could have a profound impact on the billion people who rely on the rivers for fresh water. While rain and snow would still help replenish Asian rivers in the absence of glaciers, the change could hamper efforts to manage seasonal water resources by altering when fresh water supplies are available in areas already prone to water shortages.

Researchers led by Baiqing Xu of the Chinese Academy drilled and analyzed five ice cores from various locations across the Tibetan Plateau, looking for black carbon (a key component of soot) as well as organic carbon. The cores support the hypothesis that black soot amounts in the Himalayan glaciers correlate with black carbon emissions in Europe and South Asia.

At Zuoqiupu glacier — a bellwether site on the southern edge of the plateau and downwind from the Indian subcontinent — black soot deposition increased by 30 percent between 1990 and 2003. The rise in soot levels at Zuoqiupu follows a dip that followed the enacting of clean air regulations in Europe in the 1970s.

Most soot in the region comes from diesel engines, coal-fired power plants, and outdoor cooking stoves. Many industrial processes produce both black carbon and organic carbon, but often in different proportions. Burning diesel fuel produces mainly black carbon, for example, while burning wood produces mainly organic carbon. Since black carbon is darker and absorbs more radiation, it’s thought to have a stronger warming effect than organic carbon.

To refine this emerging understanding of soot’s impact on glaciers, scientists are striving to gather even more robust measurements. “We can’t expect this study to clarify the effect of black soot on the melting of Tibetan snow and glaciers entirely,” said Cao. “Additional work that looks at albedo measurements, melting rate, and other types of reconnaissance is also needed.”

For example, scientists are using satellite instruments such as the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the NASA satellites Terra and Aqua to enhance understanding of the region’s albedo. And a new NASA climate satellite called Glory, which will launch late in 2010, will carry a new type of aerosol sensor that should be able to distinguish between aerosol types more accurately than previous instruments.

“Reduced black soot emissions, in addition to reduced greenhouse gases, may be required to avoid demise of Himalayan glaciers and retain the benefits of glaciers for seasonal fresh water supplies,” Hansen said.