Cracks in the theory
Global warming, warn scientists, could unleash its destructive wrath on the world through frequent disasters, such as increased flooding and droughts, and rapid spread of infectious diseases. But while the phenomenon - a result of heavy burning of fossil fuels - is linked to significant alterations of ecological systems, experts remain divided on its effects.
On June 6, a farmer walks across the cracking rice field in rural Mianyang city of Southwest China's Sichuan Province. Some 770,000 local people are suffering from the drought. Cheng Heping |
Predicting changes in rainfall is one of the most controversial topics. The latest research based on satellite analysis has found the increase in global precipitation may be three times greater than currently predicted by climate models.
US scientist Frank Wentz's alarming conclusion, as reported by Science two weeks ago, was based on an analysis of satellite data between 1987 and 2006. Wentz and his colleagues at California's Remote Sensing Systems observed that with a rise of 1C in temperature, global precipitation and total atmospheric water have increased at about the same rate of 6.5 percent over the last two decades.
The result challenges the prediction of climate models that the increase will be at a much slower rate of 1-3 percent.
"With respect to severe weather events like hurricanes, I am not sure what the implications are. But this much more rain worldwide could certainly pose one of the most serious risks associated with climate change," Wentz told China Daily in an email interview.
But where this additional rain would fall remains an "unanswered question", he says.
There is already consensus that the increase in greenhouse gas concentrations is likely to alter the planet's hydrologic cycle, but in which way and how require further research. Predicting precipitation for specific regions is even harder, in consideration of related factors such as topographic conditions, upward movement and density of aerosols.
Though Wentz could not forecast weather for a given place, his work would still prove valuable for other scientists, including those in China. Ding Yihui, an academic with the Chinese Academy of Engineering, said Wentz' report improves people's understanding of the impact of global warming on the hydrologic cycle.
"It encourages scientists to use more satellite data to correct their models for a more precise prediction," he says.
But, according to the prestigious meteorologist, China's precipitation trend is actually more in line with climate models, which indicate a slower increase rate.
"Since 1956, the total precipitation in China has seen a slight rise at a slower speed. But the increase of moisture in the atmosphere is obvious," Ding says.
Ding and other Chinese scientists hold that China's thirsty northern land may welcome more rains after 2020, by then the average annual precipitation in China will increase by 2-3 percent and the temperature is expected to rise by 1.3-2.1C, due to the impact of climate change.
Given the theory that global warming will bring more rains to lands at middle and high latitudes, parts of northern China above 40 degrees in latitude should see more rain, Ding says.
Between 1951 and 1978, China's rain belt was in the north, but had since moved southward. Ding says it is unclear why this shift occurred, though it "may not necessarily relate to climate change".
"So we predict that the rain belt might move northward again after 2020, with the effect of global warming," Ding says.
Zhai Panmao, a senior official with the China Meteorological Administration, says changed rain belt or total precipitation did not equate to the relief of floods or droughts in some places. In the May issue of the Chinese journal Advances in Climate Change Research, Zhai and his colleagues reported that precipitation intensity has increased significantly during the past 50 years, while the number of precipitation days has decreased considerably except in Northwest China.
On June 7, a flood brought by heavy rain ravaged Yao'an Township, Lianzhou city of South China's Guangdong Province. Cao Jing |
Zhai, also a renowned meteorologist, says in the paper that the reduction of precipitation days indicates a tendency of drought across the country. He says the relationship between extreme precipitation and total precipitation is complicated under the impact of global warming.
Some climate models predict that wet areas of the planet will become wetter and dry areas will become drier. Within Chinese academia, it is also a popular view that in the future, North China will experience a drier climate while the south will become wetter.
Wentz, the US researcher, says there is a hint of this theory in satellite data with the Western Pacific, an area of heavy rain, whose rainfall shows the largest increase of any region during the last 20 years.
"However, given the discrepancy we have observed between the observations and models, I have some doubts about the wet-getting-wetter, dry-getting-drier prediction by the models," Wentz says.
Though saying China currently goes right along the model prediction of "wet-getting-wetter, dry-getting-drier", Ding admits that present computer models require more adjustments. "We specially lack data on oceans," he says.
(China Daily 06/13/2007 page20)
