Each year, the sun shines down on the dark surface of the Indian Ocean, and moist, warm air rises and forms clouds. This rising heat and the moisture form a powerful weather system, a natural pump that pulls up water and moves it in vast quantities hundreds of miles to the mainland. This is the Indian monsoon, which deposits rainfall on thousands of square miles of farmland. About a billion people, most of them poor, depend for their daily bread on crops that depend in turn on the reliability and regularity of the Indian monsoons.
India is a rapidly developing country with hundreds of millions of citizens who want to move into the middle class, drive cars and cool their homes with air-conditioning. It is also a country of poor people, many who still rely on burning agricultural waste to heat their homes and cook their suppers. Smoke from household fires has been a big source of pollution in the subcontinent, and it could disrupt the monsoons, too. The soot from these fires and from automobiles and buses in the ever more crowded cities rises into the atmosphere and drifts out over the Indian Ocean, changing the atmospheric dynamics upon which the monsoons depend. Aerosols (soot) keep much of the sun's energy from reaching the surface, which means the monsoon doesn't get going with the same force and takes longer to gather up a head of steam. Less rain makes it to crops.
At the same time, the buildup of greenhouse gases, coming mainly from developed countries in the northern hemisphere, has a very different effect on the Indian summer monsoons: it acts to make them stronger.
These two opposite influences make the fate of the monsoon difficult to predict and subject to instability. A small influence—a bit more carbon dioxide in the atmosphere, and a bit more brown haze—could have an out- size effect. Lenton believes that the monsoons could flip from one state to another as quickly as one year. What happens then is not a question that Lenton can answer with certainty, but he foresees two possibilities.
One is that the monsoons grow in force and intensity, but come less frequently. We have already seen hints of this in the newspapers. In the last few years rains have grown erratic and less frequent, but when they do come, they tend to dump an enormous amount of water, and in places where they wouldn't normally do so. This is almost as bad for farmers as drought, since the rain falls on parched ground with extra force, and much of it runs off without soaking into the ground, and it causes damage to boot by washing away soil and plants. The flooding that devastated Pakistan in 2011 is a case in point. If this trend continued and strengthened in intensity, it would be bad news for the two thirds of the Indian workforce that depends on farming. It would be nasty for the Indian economy—agriculture accounts for 25 percent of GDP. A permanently erratic and harsh monsoon would depress crop yields, increase erosion on farms, and cause a rise in global food prices as India is forced to import more food.
The other possibility is even worse: the monsoons could shut down entirely. This would be an unmitigated catastrophe. A sudden stopping of monsoon rain, which accounts for 80 percent of rainfall in India, could throw a billion people into danger of starvation. It would change the Indian landscape, wiping out native species of plants and animals, force farms into bankruptcy, and exacerbate water shortages that are already creating conflict. The Indian government would almost certainly be unable to cope with a disaster of such proportions. Refugees by the hundreds of millions would stream into big cities such as Mumbai and Bangalore, looking for some hope of survival. It would create a humanitarian crisis of unprecedented proportions. Lenton foresees a similar danger of sudden change in the West African monsoon, the second tipping point.
Tipping point number three in Lenton's list is the sea ice of the north pole. For years the ice has been thinning and retreating more and more during the summer. Soon it may disappear completely during the summer months. We may already have reached this tipping point—a transition to a new state in which the north pole is ice-free during summer months is already at hand. Eventually the north pole may flip and be free of ice year-round. The knock-on effects of such a transition would be huge—they would cause marked increase of warming at the pole, since open water absorbs more of the sun's energy than ice-covered seas. The effect of a year-round ice-free north pole would be like heating Greenland on a skillet.