|May 13, 2013|
The climate of Tibet: Pole-land.
|OF ALL the transitions brought about on the Earth's surface by temperature change, the melting of ice into water is the starkest. It is binary. And for the land beneath, the air above and the life around, it changes everything.|
That is the main reason climatologists are interested in the Earth's north and south poles. The waxing and waning of the ice provides an unambiguous signal of what is going on---and it is a signal which can be read in rocks a billion years old almost as easily as it can be observed today. But the poles are only two examples. Another would be welcome. And there is one.
Though the amount of ice on the plateau of Tibet and its surrounding mountains, such as the Himalayas, Karakoram and Pamirs, is a lot smaller than that at the poles, it is still huge. The area's 46,000 glaciers cover 100,000 square kilometres (40,000 square miles)---about 6% of the area of the Greenland ice cap. Another 1.7m square kilometres is permafrost, which can be up to 130 metres deep. That is equivalent to 7% of the Arctic's permafrost. Unlike the ice at the poles, the fate of this ice affects a lot of people directly. The area is known by some as Asia's water tower, because it is the source of ten of the continent's biggest rivers. About 1.5 billion people, in 12 countries, live in the basins of those rivers. Welcome, then, to the Earth's "Third Pole".
Until recently studies of the Third Pole were piecemeal---not surprising, given its remoteness, the altitude, the harsh weather and the fact that little love is lost between the countries among which it is divided. In 2009, however, Yao Tandong of the Institute of Tibetan Plateau Research, in Beijing, Lonnie Thompson of the Ohio State University and Volker Mosbrugger of the Senckenberg World of Biodiversity, in Frankfurt, started an international programme involving these countries, called the Third Pole Environment (TPE). Last month, its fourth workshop met in Dehradun, India.
No longer poles apart
One question on everyone's mind is whether the glaciers are retreating, as is happening in parts of the real polar regions. The Intergovernmental Panel on Climate Change's report in 2007 foolishly suggested that the Himalayas' glaciers could disappear as early as 2035. Given the amount of ice they contain, it would take weather gods armed with blow torches to melt them that quickly, and this suggestion was rapidly discredited. Last year a study published in Nature by Thomas Jacob of the University of Colorado, in Boulder, showed that glaciers in the Himalayas and Karakoram had lost little ice between 2003 and 2010, and that those on the Tibetan plateau itself were growing.
Many glaciologists, however, take issue with this conclusion. As Tobias Bolch of the University of Zurich explained to the workshop, Dr Jacob's article was based on seven years of measurements by a satellite mission called the Gravity Recovery and Climate Experiment (GRACE). This used orbiting gravimeters to try to measure changes in ice cover from effects on the local gravitational field. According to Dr Bolch that approach suffered from two problems. One was the coarse resolving power of the satellites' instruments. These could not detect changes in features less than 200km across. This is enough to study large regions with homogenous surfaces, such as the Arctic and the Antarctic (which GRACE did in fact do). But mountainous terrain has complex topography.
The second, more serious problem is that the satellites cannot tell the difference between solid and liquid water. If a glacier melts, but the water stays put as a lake, GRACE will see no change. Since the Tibetan plateau contains a lot of "closed" catchments, from which meltwater cannot easily escape, large amounts of melting could happen without GRACE detecting them.
Indeed, a survey by Dr Yao and his colleagues shows the area of the glacial lakes on the plateau has increased by about 26% since the 1970s. Dr Bolch suspects that GRACE has mistaken these expanding lakes for growing glaciers. Using another satellite, called ICESat, which employs lasers to measure not only the areas of glaciers, but also the elevations of their surfaces, Dr Bolch and his colleagues conclude that, far from advancing, many of Tibet's glaciers are in headlong retreat.
But not all of them. What he saw supports work by Dr Yao and Dr Thompson, who have studied field reports and satellite photographs of more than 7,100 glaciers, collected over the past 30 years---not just the seven covered by GRACE. This study suggests a lot of regional variation.
Dr Yao and Dr Thompson found that some glaciers are indeed advancing. Most of these are in the Karakoram and the Pamirs, in the region's west. But glaciers in the eastern Himalayas and the east of the Tibetan plateau are retreating fast. Those in the middle of the plateau are shrinking too, though less rapidly. The net effect is a big loss of ice over the period in question.
To try to work out what is going on, Dr Yao and Dr Thompson looked at weather records. Over the decades the Indian monsoon, which brings snow to the southern part of the plateau and the eastern and central Himalayas, has been getting weaker---though no one is sure why. The westerlies that bring snow to the Karakoram and the Pamirs have, however, been getting stronger. Westerlies are caused by hot air rising from the oceans and moving north (because heat travels from warm regions to cold ones) and east (because of the Coriolis force caused by the Earth's rotation). Global warming means there is more hot air to rise, hence stronger westerlies.
The effects on glacier growth of these changes in wind strength are amplified by the season. The monsoon arrives in summer. The westerlies arrive in winter. A warming climate is more likely to stop summer snow accumulating than it is winter snow. Taken together, changes in wind strength and air temperature neatly account for what is going on. And it is not only glaciers that are melting. According to Wu Qingbai of the Cold and Arid Regions Environmental and Engineering Research Institute, in Lanzhou, Tibet's permafrost has been disintegrating rapidly for the past two decades.
A Himalayan task
One outcome of the workshop, then, has been to establish that the overall ice cover of the Third Pole, like that of the two real poles, is shrinking. Another is to show how precarious and piecemeal data about the area are. Its role as the source of so many rivers means that absence of data matters. The Chinese Academy of Sciences, of which both Dr Yao's and Dr Wu's institutes are part, has therefore set up a fund of 400m yuan ($65m) for research on the Third Pole and, crucially, a quarter of this is earmarked for work outside China.
The TPE's researchers will now monitor a set of bellwether glaciers every six months. They will set up observatories to measure solar radiation, snowfall, meltwater and changes in the soil, as well as air temperature, pressure, humidity and wind. And they plan to take cores from the ice on the Tibetan plateau. These will let them reconstruct the area's climate over the past few hundred thousand years. Together, these data will give them a better grip on how much---and why---the Third Pole is changing.