What happens if the monsoon falters ?
More than 3 billion people today are fed and watered thanks to the Asian monsoon. It is the greatest rainmaking machine on the planet—and possibly one of the most sensitive to climate change. Its mechanism is extremely simple. It is like a giant sea breeze operating over the world's largest continent. In winter, the vast Asian landmass becomes cold—extremely cold on the high ice caps of Tibet, the largest area of ice outside the polar regions. It cools the air above. That air descends, forcing cold, dry winds to blow off the land and out across the Indian and Pacific Oceans. Asia is mostly rainless for nine long months. But come summer, the land warms up much faster than the oceans. Warm air rises, and as it does, the winds reverse and moist winds blow in off the oceans. For about a hundred days, monsoon rains fall across Asia. The rains burst rivers, fill irrigation canals and water fields. Across the continent, rice farmers take their opportunity to grow the food that sustains half the world's population.
A failed monsoon has devastating consequences. They happened repeatedly in the nineteenth century. British colonial administrators in India watched in bemusement as tens of millions died in the famine of 1837-1838, and again in 1860-1861, 1876-1878, and 1896—1902. The Asian monsoon remained an unruly beast through the twentieth century. But despite tenfold increases in the populations of most monsoon countries, the death toll from famine has fallen. There are many reasons for this, one of which is that the rains proved more reliable in the twentieth century than in the nineteenth. That was a good news story. The question is: Can it last? The Asian monsoon has appeared for the past century to be self-contained and invulnerable. But, like other big features in the global climate system, it may have an Achilles heel. If the monsoon proves less reliable in the twenty-first century, there could be real trouble ahead—for about 3 billion people.
The monsoon's vulnerability in past centuries seems to lie in its links to two other parts of the global climate system. One is El Nino. Strong El Ninos often seem to switch off the Asian monsoon. British imperial scientists discovered more than a century ago that most of the great Indian famines coincided with marked climatic fluctuations in the Pacific. El Ninos seemed to draw heat away from Asia, and so to drain the monsoon's strength. But the argument has become a little academic in the past thirty years, because El Nino has intensified without any widespread weakening of the Asian monsoon. The break in the old link has been both a scientific surprise and a humanitarian godsend. But nobody knows what has caused it and whether it will last. If the Pacific climate system does what many predict, and in the twenty-first century leans heavily toward a permanent El Nino—like state, and if the monsoon resumes its former relationship, then the rains may soon fail over Asia more often than they succeed.
The second link is with the Atlantic. This was dramatically established in 2003, when Indian and U.S. researchers assembled a 10,000-year record of the strength of the Indian monsoon. They did it by counting fossilized plankton found in ancient marine sediments off the Indian coast. The plankton thrive when strong monsoon winds cause an upwelling of the nutrients that provide their food. The study found huge variability in the monsoon's strength over the centuries. And it confirmed that, over time-scales longer than individual El Nino years, "weak summer monsoons coincide with cold spells thousands of miles away in the North Atlantic," according to Anil Gupta, of the Indian Institute of Technology, in Kharagpur, who worked on the project. Strong monsoons go hand in hand with warm waters off Europe and North America.
It had been known for a while that the Indian monsoon turned off during the last ice age but probably flickered on briefly during the warm episodes that punctuated the glaciation. The new study showed that the strength of the monsoon also shadowed the flutters of the Atlantic system during the postglacial era, faltering during the Younger Dryas and the chill of 8,200 years ago, for instance. The changes clearly followed Bond's 1,500-year solar pulse. Thus the last faltering of the monsoon came during Europe's little ice age, which ended in the final decades of the nineteenth century. Soon, as colonial records confirm, the monsoon was regaining its reliability.
But this pattern, impressive though it is, does not explain how the link with the Atlantic works. Does the Atlantic tell the monsoon what to do? Does the monsoon tell the Atlantic what to do? Does Bond's solar pulse independently determine both? Or is there another element not taken into account? Where does El Nino fit in, for instance?
Jonathan Overpeck, of the University of Arizona, one of the authors of the monsoon history, holds that the Atlantic has the whip hand. He says that a warm North Atlantic sends heat east on the winds, warming Asia in spring, and allowing a rapid melt of the Tibetan plateau and an early start to the rain-giving monsoon winds. But when the Atlantic is cold, he says, "more snow on the Tibetan plateau in spring and early summer uses up all the sun's heating, because it has to be melted and evaporated before the land can warm." If he is right, then should the ocean conveyor falter in the coming years, the effects for Asia could be even more grievous than for Europe. "There could be a weakened monsoon and less water for all the people who depend on it," says Overpeck.
The tropical school disagrees with this analysis. It holds that both the cooling of the Atlantic and the weakening of the monsoon are likely to be triggered by changes in the heating of the tropics. According to this theory, a cooling of the tropics will weaken monsoon winds and rains, while at the same time sending less warm water north in the Gulf Stream. The theories of the polar and tropical schools are on this occasion not mutually exclusive. In fact, they are mutually reinforcing.
But right now, neither theory offers much enlightenment about what might happen to the Asian monsoon in the coming decades. Global warming driven by accumulating greenhouse gases without a solar component may have different features and different outcomes from the solar-dominated scenarios of the past. The situation is further complicated because across much of monsoon Asia, warming is itself severely compromised and sometimes extinguished by the aerosols in the Asian brown haze. As we have seen, the haze's biggest impact is on the radiation balance between the land surface and the air aloft—a vital parameter in determining the strength of the monsoon. The fear is that the haze may break the seasonal heating cycles between land and ocean, and turn off the monsoon. It hasn't yet, but it may. And, valuable though reconstructed histories of the Asian monsoon may be, it is unlikely that they will ever be able to provide a firm prognosis for the monsoon.
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