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    Written by: LEUNG Wing-mo    December 2009

Climate change as a result of human activities is very often coined the greatest challenge of our generation. With the increasing efforts of the government, local educators and green groups to launch public education of various scales, as well as the larger coverage by the media on climate change issues, almost every man-in-the-street knows that the culprit of man-made climate change and global warming is the rapidly increasing concentration of greenhouse gas (GHG), of which carbon dioxide (CO2) is the main constituent. This is clearly summarized by the "radiative forcing" figure (Fig.1 below) in the 4th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) in 2007.

figure 1
Fig.1   Global average radiative forcing (RF) estimates and ranges in 2005 for anthropogenic carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and other important agents and mechanisms. IPCC 2007.

The term radiative forcing can loosely be explained as how efficient a particular agent, CO2 for example, can heat up the atmosphere. Of these agents, Fig.1 indicates that total aerosol has a negative radiative forcing, suggesting that instead of warming up the atmosphere, it actually cools the atmosphere. But we also note that the level of scientific understanding (LOSU) of some aerosol is low, meaning that a lot more scientific research is needed to understand the complete picture. 

Fortunately for us at the Observatory, Dr William K M Lau, Chief of Laboratory for Atmospheres of NASA's Goddard Space Flight Center, and one of the Observatory's Scientific Advisors, visited us in November and explained to us the results of his latest study on the effects of aerosol to climate in Asia. Dr Lau summarized the results of his computer simulation of the effect of aerosol to the Indian summer monsoon in a conceptual model, which is depicted in one of his diagrams reproduced here (Fig.2) below:

figure 2 
Fig.2   Graphical explanation of the elevated heat pump (EHP) hypothesis and its effects on the Indian monsoon. Courtesy: Dr K M Lau, GSFC, NASA

The upper part of the figure explains the normal Indian monsoon. The figure is a north-south cross section, with the Indian Ocean in the far left, the Indian subcontinent to its immediate right and then the Himalayas/Tibetan Plateau. In the normal Indian monsoon, the Indian subcontinent begins to warm up in mid May to mid June, drawing in moisture from the Indian Ocean and results in abundance of rain over India. 

The lower part of the figure tries to postulate the effects of aerosols.  Aerosols in India can be blown in from neighboring countries as well as from far away places like the Middle East. These aerosols couple with black carbon (arising from incomplete combustion of fossil fuels, bio fuels burning, forest fire etc) and accumulate before the monsoon in late spring against the northern and southern slopes of the Tibetan Plateau. They absorb the sun's radiation, heating the surface air above the mountainous slopes of the region. The heated air rises and draws warm, moist air to northern India from the Indian Ocean, hence, Dr Lau dubbed it as the "elevated heat pump, (EHP)". With more moisture in northern India, the EHP gives more rainfall there.

On the contrary, the aerosols over central India warm the middle troposphere while the land surface becomes cooler, creating a thermodynamic condition not favourable for convective rain. Hence there is a reduction in monsoon rain in central and southern India. 

The net effect is therefore a shift of the path of the monsoon toward the foothills of the Himalayas. More rain will fall earlier in the season in northern India and less over southern India. 

This is one of the typical consequences of climate change a re-distribution of precious fresh water resource. People living in the region will have a hard time adapting to such a change. In the worst case scenario, it might lead to conflict between countries in the region as they fight over the much needed resources. 

Another effect of the EHP and the enhanced rain in Himalayas is the increased mountain glacier melt, which in the long term will lead to a shortage of fresh water to billions of people in the region.  We should not take this phenomenon lightly. 

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  1. Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) (IPCC, 2007).
  2. Lau, K. M., K. M. Kim, C. Hsu and B. Holben, 2009: Possible influences of air pollution, dust and sandstorms on the Indian monsoon. WMO Bulletin, 58 (1), 22-30.

Last revision date: <22 Jan 2013>