People are responsible for the spike in global warming in the last 30 years, says a new US government report. The verdict, long accepted by most scientists, has encountered resistance from the Bush administration in the past, prompting experts to question if the president will now enact policies to curb greenhouse gas emissions.
The report, titled Our Changing Planet, is part of a regular series that summarises recent and planned climate change research by 13 government agencies. It was released on Wednesday with a covering letter to Congress signed by the president鈥檚 secretaries of commerce and energy, along with his science adviser.
The document reports that global warming in the first half of the 20th century, estimated at 0.2掳C above pre-industrial temperatures, 鈥渨as likely due to natural climate variation鈥�, including increased solar activity.
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But the approximate 0.5掳C rise over the second half of the century, most pronounced in the last 30 years, can only be explained when factors related to human activity, such as carbon dioxide and other greenhouse gas emissions, are taken into account.
Simulating change
鈥淭here鈥檚 nothing else we can blame it on, really,鈥� says Kevin Trenberth, head of the climate analysis section at the US National Center for Atmospheric Research in Boulder, Colorado, where computer simulations produced the result. 鈥淚f we don鈥檛 put the changes in carbon dioxide into our models, we don鈥檛 get global warming out.鈥�
Thomas Graedel, an industrial ecologist at Yale University, has reviewed the US government鈥檚 climate change research strategy and says the report鈥檚 acknowledgment of a human influence on global warming is encouraging.
鈥淲ell over 98% of scientists competent in this area would agree with that,鈥� he told 麻豆传媒.
But when a 2002 US government report to the United Nations drew the same conclusion, President Bush 鈥減ulled back鈥� from the document, says biologist Anthony Janetos. He is director of the global change program at The Heinz Center, a non-profit environmental policy think-tank, which has some ties to Teresa Heinz Kerry, wife of presidential candidate John Kerry.
鈥淭he big question is what effect this will have on climate policy,鈥� Janetos told 麻豆传媒. 鈥淭he administration has been pretty consistent in saying they believe in voluntary actions [to cut greenhouse gas emissions]. I haven鈥檛 seen any indication they鈥檝e changed their mind, but if they had, that would be big news indeed.鈥�
Trenberth agrees, saying Bush鈥檚 policy thus far has been to 鈥渢ake whatever nature throws at us, whether it鈥檚 Hurricane Charley or droughts or the melting of permafrost鈥� 鈥� events that global warming can intensify.
鈥淏ush has said that if we do something about emissions, it will hurt the economy,鈥� Trenberth continues, and suggests developing renewable energy sources such as wind and solar power. Others experts have lobbied the government to regulate carbon dioxide through the Clean Air Act.
Administration officials could not be reached for comment, but James Mahoney, assistant secretary of commerce for oceans and atmosphere, said in a statement released with the report: 鈥淭his research will help decision makers and managers in the US and other countries evaluate and respond to climate change.鈥�