Cold water on IPCC monsoon forecast models

  • Date: 22/11/09
New Delhi: None of the multiple computer simulations used by a UN climate-change agency for assessments of global warming appears good enough to predict how India’s monsoon will behave, two Indian scientists have said.

The researchers examined 10 simulations of future climate scenarios used by the UN Intergovernmental Panel on Climate Change (IPCC) and found none could reproduce correctly the behaviour of even 20th-century rainfall.

Not a single model could simulate realistically key features of the Indian monsoon such as maximum activity over the Bay of Bengal and the Northeast and along the west coast, and minimum activity over the northwest, the researchers said. They have presented their analysis in a review paper in Current Trends in Science, a publication of the Indian Academy of Sciences.

In attempts to assess impacts of global warming, the IPCC considered 17 models of how climate would evolve as carbon dioxide levels in the atmosphere rose. Some models predict more rainfall over India, but with great uncertainty.

“The models have very serious problems in simulating even 20th century monsoon patterns,” said Madhavan Rajeevan, a senior scientist at the National Atmospheric Research Laboratory, Tirupati, and a co-author of the paper.

“When a model (computer simulation) cannot even show with reasonable accuracy monsoon behaviour in the past, there’s a big question mark over its ability to predict future patterns,” Rajeevan told The Telegraph.

Rajeevan and Ravi Nanjundiah, an atmospheric physicist at the Indian Institute of Science, Bangalore, and co-author of the paper, studied the capabilities of the 10 “best-performing” models developed by climate scientists in Europe, Japan and North America, ignoring seven models whose performance was worse.

All 10 models predicted less rainfall over the atmospheric feature called continental tropical convergence zone -— the rain belt whose fluctuating positions over India determine where and how much rainfall will occur — than what had actually been observed. The models also failed to simulate the connection between Indian Ocean surface temperatures and rainfall.

“But we have reason to believe that the monsoon is very sensitive to even slight changes in sea surface temperatures in the Indian Ocean region,” Rajeevan said.

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