The increase of green-house gases in the atmosphere reduces outgoing radiation and thus causes global warming. About 93% extra energy trapped by the greenhouse gases is stored in oceans and only 1% is used to heat the atmosphere. As a main reservoir of heat sink, Oceans are slowing down the global warming magnitude by absorbing the extra energy.

Global warming hiatus was observed since 1998, but the Tibetan Plateau (TP) (also known as the Third Pole) is still undergoing significant warming. Meanwhile, the thermal forcing over the TP, which is a well-known driver of the Asian summer monsoon, has shown a declined trend during last 30 years.

The study conducted by researchers from Institute of Tibetan Plateau Research, Chinese Academy of Sciences, found that the thermal weakening is attributed mainly to the enhancement of the TOA (top of atmosphere) outgoing radiation. Actually, both the TOA outgoing shortwave radiation and longwave radiation were enhanced during this period.

"There are two paradoxes. One is between the thermal weakening and the continuous warming over the TP; the other is between the enhancement of TOA outgoing radiation over the TP and the expected green-house-gas-induced weakening of the TOA outgoing radiation", said Dr. Kun Yang, a co-author of this research. The relevant article was published in 2015(3) issue of Science China: Earth Sciences.

Using surface observations and satellite radiation data, they found that the outgoing shortwave radiation enhancement is highly correlated with the increase of low-level cloud cover, which has a strong reflection to shortwave radiation, especially in summer. On the other hand, the strong warming Plateau surface emits more longwave radiation to outer space, and the decrease of total cloud cover further favorites the escape of longwave radiation over the TP.

Therefore, the radiative cooling enhancement at TOA and thus the thermal weakening over the TP is a response of the earth-atmosphere system to the unique change of cloud cover configuration and the rapid warming of the land surface. This energy budget change, in turn, slows down the warming rate of the climate system.

In summary, the enhancement of TOA outgoing radiation removes a part of excessive energy, which shines some light on the complexity of climate change over the TP. However, current major atmospheric reanalysis products are not able to reflect these trends in cloud cover and TOA outgoing radiation, suggesting some substantial deficits in these analysis models for the highest region in the world.

Wu H, Yang K, Niu X L, et al. 2015. The role of cloud height and warming in the decadal weakening of atmospheric heat source over the Tibetan Plateau. Science China: Earth Sciences, 58: 395-403, doi: 10.1007/s11430-014-4973-6