Tree studies link climate change to calamity

by Martin Richardson

SINGAPORE — Extreme weather, from heat waves and drought to snowstorms and floods, is nothing new. The big question is what are the causes. Are they natural or man-made, or a combination of both?

If the causes are natural, there is not much we can do except prepare and recover as quickly as possible after disaster strikes. If human activity is the main cause, it should be possible to take remedial action, provided countries can agree how to apportion responsibility and share costs.

Recent advances in the study of tree rings in Asia confirm connections between climate change and the collapse of past civilizations. Teams of scientists from Japan and other countries spent more than 15 years traveling across the region to locate living trees old enough to provide long-term records. Prize finds included a Vietnamese cypress tree over 1,000 years old and a hemlock of similar age in the mountains of Nepal.

The researchers looked for certain tree species, those for which rainfall determines the width of annual growth rings inside their trunks. By extracting tiny cores of wood from the trunks of hundreds of trees in both lowland and highland forests, they were able to reconstruct a record of year-to-year moisture levels stretching back centuries.

Their conclusions, published in March and July, shed some new light on history and underscored the vital role of seasonal monsoon rains in Asia that feed nearly half the world’s population.

The tree ring evidence reveals that at least five prolonged droughts have shaken Asia in the last millennium, playing significant roles in the 1644 fall of China’s Ming dynasty, the collapse of 18th century kingdoms in what are now Myanmar, Thailand and Vietnam, and an 1876-1878 famine that killed up to 30 million people in South Asia, Southeast Asia and China.

A succession of monarchs ruled the Angkor empire that stretched across much of mainland Southeast Asia between the ninth and 14th centuries, producing the architectural masterpieces and stone carving now preserved in Cambodia as a World Heritage site and magnet for international tourists.

The tree ring study provides the strongest evidence yet that two severe droughts, punctuated by several unusually intense rainy seasons, weakened the empire, first by cutting water supplies for drinking and agriculture and then by damaging Angkor’s vast irrigation system that was central to its economy. The kingdom is thought to have collapsed in 1431 after an attack by the Siamese from present-day Thailand.

The tree ring record used by the Angkor research team stretches back for 760 years and they were able to correlate dry years with El Nino, a natural warming of the tropical Pacific Ocean that occurs periodically and continues to affect weather patterns in Asia and the Americas. A similar warming and cooling cycle in the Indian Ocean influences rainfall in South Asia and Australia.

The authors of the Angkor study and other researchers suggest that El Nino may have played an important role in stopping monsoon rains and causing withering droughts. Indeed, some scientists suspect that human-induced global warming may amplify the natural cycles of warming and cooling around the Pacific and Indian oceans, raising the possibility of alternating Angkor-like droughts and destructive floods that could affect vast numbers of people.

The Hadley Center in Britain’s weather forecasting agency prepared a report for the latest round of international negotiations on climate change in Cancun, Mexico this month. The report said it is clear from objective observations across a wide range of indicators that the world is gradually heating up and that the evidence for man-made warming from emissions of greenhouse gases has grown even stronger in the last year.

Measurable signs of warming include rising surface temperature above both the land and sea, increases in water temperature from the sea surface down to a depth of several hundred meters, higher humidity as a warmer atmosphere holds more moisture, rising sea level as warmer waters expand and land ice melts, and the shrinking of Arctic sea ice, glaciers in various parts of the world and spring snow cover in the Northern Hemisphere.

The World Meteorological Organization, a United Nations agency, presented similar conclusions to the Cancun conference while the head of the scientific panel advising the U.N. on climate change warned that warming would be accompanied by more extreme weather.

One of the indicators of most concern to Asia is the warming of the upper layer in the ocean, not just because it is a direct cause of sea level rise but also because it may intensify the El Nino cycle.

Drawing data from a global network of more than 3,000 automated floats and other devices to measure sea temperature, U.S. and British scientists have found that the upper layer of the world’s ocean down to a depth of about 610 meters has warmed steadily since 1993. The ocean is the biggest reservoir for heat in the climate system. As the planet warms, as much as 80 to 90 percent of the increased heat ends up in the ocean.

Meanwhile, marine scientists are calling for control of global greenhouse gas emissions after one of the most severe episodes of coral stress hit reefs in the Indian Ocean and Southeast Asia earlier this year. The bleaching was caused by much hotter than usual water from the eastern Indian Ocean. By July, local divers recorded water temperatures of 32 degrees Celsius, over four degrees higher than the normal average.

Earth’s climate system appears to be putting out distress signals. But many scientists fear that the response of governments will be too little, too late.

Michael Richardson is a visiting senior research fellow at the Institute of Southeast Asian Studies in Singapore.