The torrential rains of monsoons are essential to agriculture in Asia and Africa and any delay or change to their pattern is extremely bad news. Of great concern at the moment is the monsoons being delayed or weakened due to global warming.
Monsoons are basically winds that reverse direction in summer and winter. Generally in late summer they bring water-saturated air in from the oceans to the land and in the winter they reverse course.
Wind systems depend on a myriad of factors and change in any one of these can upset the entire system. The monsoon system works because land and sea cool down and heat up at different rates. This eventually creates pressure differences, which change depending on the season.
Warm, humid air rises from the oceans, forming clouds. The differences in air pressure between the land and the sea means that during the monsoon season winds blow inland, bringing clouds and rain with them.
A delayed monsoon can be devastating when your crops depend on rain coming at a certain time. It can lead to famine, livelihoods lost, waters shortages, and economic collapse.
In recent years the monsoon season has been unusually unpredictable. Some areas were experiencing especially heavy rainfall, it was delayed in others and weakened elsewhere.
This might be due in part to the El Nino effect. This is a periodic change in the weather that occurs due to altered wind systems over the Pacific ocean. Roughly every five years unusual weather occurs, with delayed rainfall, droughts, flooding and storms being more likely than normal.
However anything that affects temperatures on land and sea is going to change wind systems. Monsoons are effectively wind systems and very sensitive to short or long term changes in temperature and the resultant pressure changes.
Global warming will almost certainly affect monsoons in one way or another. Some areas will have monsoons later than usual, others will see them reduced, and others will see them increased.
Increased monsoons are not the godsend they might appear. You can have too much rain as well as too little. Flooding and mudslides from increased rainfall take thousands of lives.
With delayed, reduced, or even exceptionally heavy monsoons, being more likely than ever there are efforts being made throughout the world to mitigate, to at least some extent, the devastation this can cause.
Conserving water, especially on a local scale, together with changes to farming methods can reduce the yearly uncertainty that many farmers have of not knowing whether or not they can grow enough food to survive.
Further reading
http://news.bbc.co.uk/1/hi/sci/tech/8178463.stm
http://ngm.nationalgeographic.com/print/2009/11/india-rain/corbett-text
