As I report today in The Times, the South Asian monsoon is critical to sustaining plants, animals and 1.6 billion people on the subcontinent. When it brings too little rain between June and October, shortages of food and drinking water can develop. When its bounty is too great, floods can displace millions and cause hundreds of deaths.
Scientists who have devoted their careers to studying the monsoon and predicting its dimensions say that the prognostications can be incredibly difficult. Adding to the complexity is global warming, which could potentially cause monsoon patterns to change.
But in a recent paper in Nature Climate Change, researchers write that they are beginning to understand more about the systems driving the monsoon and that they hope to improve their projections in years to come. We discussed the challenges recently with Andrew Turner, a researcher in the Department of Meteorology at the University of Reading in England who was a co-author of the article with H. Annamalai. Following are excerpts from the e-mail conversation, edited for brevity and clarity.
Q.
In recent years, has global warming had a discernible impact on the monsoon and on median temperatures? Can the below-average rainfall seen in the last decade be attributed partly to climate change?
A.
Certainly, as with many dry-land regions in the tropics (e.g. northern India prior to the monsoon), the daily mean temperatures have increased in the late 20th century, consistent with climate warming.
It is more difficult to say whether the below-average rainfall in the last decade can be attributed in part to climate change.?What we do know is that in this period there were several central El Ni?o events in the Pacific (warmer-than-normal sea surface temperatures, commencing during summer), which are known to be related to monsoon drought.?These occurred in 2002 (when the monsoon brought only 81 percent of the normal rainfall), 2004 (87 percent) and 2009 (77 percent).?
The current year is already facing a deficit in the monsoon of around 14.4 percent (85.6 percent of normal rainfall), with one month to run, and again, a central Pacific El Ni?o may be the cause.?However, this is not to say that global warming could not be involved.?
What we do know from future model projections is that increases in greenhouse gas concentrations alone generally cause increases in the seasonal average monsoon rainfall over the South Asia region.?This is mainly related to an increase in the supply of moisture to India as the monsoon winds pass over the now-warmer Indian Ocean.
What I haven?t mentioned yet, and which we discussed in the Nature Climate Change paper, is the possible role of anthropogenic aerosol emissions. We know that with increases in population, there has been increased industrialization of the region (leading mainly to emissions of sulfates) and increased used of cooking fires (emitting black carbon).
We know that sulfates can have a negative impact on monsoon rainfall, although to what magnitude we are not sure.?Certainly, in future climate change experiments the positive impacts on mean rainfall of increased greenhouse gas (carbon dioxide) concentrations tend to outweigh the negative impacts of sulfates.?The effects of black carbon on the monsoon are much more uncertain, and we not even sure of the sign of their impact on monsoon rainfall!
Q.
Is there reason to think that greater climate variability would result in a greater incidence of high-intensity rainfall and longer dry spells?
A.
It is generally accepted that climate warming brings about an intensification of the hydrological cycle and the so-called ? rich get richer? mechanism.?Because a warmer atmosphere is able to hold more moisture, the result is heavier bursts when it does rain.? There are also longer dry spells between these events.?(Such results have been shown in theory, in observations and in future projections of the monsoon using computer modeling.)
Q.
Some studies have shown that the Indian monsoon may become harder to predict in the future because of climate change. What is the consensus on this? Has there been enough research on this issue?
A.
Since the rain may be coming in heavier bursts less often, one could suggest that monsoon rainfall will become more difficult to predict?at the short ?weather? time scale.? At the seasonal scale (i.e., predicting total seasonal rainfall for June to September), we are not sure. We need to know what will happen to large-scale drivers like the El Ni?o Southern Oscillation, or ENSO, and what will happen to the mechanism that connects El Ni?o warming with the monsoon.?
At present there is no reason to suggest this mechanism will change in the future, and our model projections of what will happen to ENSO are uncertain.? Much more research could be undertaken on these aspects of how the monsoon may change and has changed.
Q.
Would certain parts of India be more severely affected than others? If so, which?
A.
In model projections of future monsoon rainfall, there is general agreement that in an area-average sense, it will increase in the future. However, looking at individual models, the picture is hugely uncertain, so it is not currently possible to say which areas will be affected most.
Q.
What impact will the average rise in surface temperatures in coming decades have on India?
A.
Rising surface temperatures (particularly just prior to the monsoon onset, and in the north of the country) will exacerbate existing problems ? health-related, water shortages (problematic every year near the capital), agriculture, etc.
Q.
What kind of mitigation efforts should Indian policymakers pursue to prepare for these climatic changes?
A.
Due to the large uncertainties, it is difficult to suggest options other than to choose ones that are ?win-win? ? those that will bring benefits no matter how the climate changes in India.?These could include better water storage and management practices, better controls on anthropogenic aerosol pollution.? Certainly there needs to be an effort to better communicate climate changes and the uncertainties to the end-user level ? farmers etc. ? so they do not expend effort or capital on trying to adapt to a change that is unlikely.
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