Exceptional century. Temperatures recorded in tree rings and elsewhere (purple) reveal that the 20th century (red is instrumental record) was unique in the millennium.

U.N. to Blame Global Warming on Humans

Dick writes about Earth and planetary science for Science magazine.

Global warming has stirred up political and scientific debate between those who blame humans for the heat and those who point to natural causes. After testing alternative explanations, a United Nations-sponsored panel concludes in a draft of an upcoming report that "there has been a discernible human influence on global climate." If these words make it into the final report, they would be the strongest official pronouncement yet that human-induced warming is real.

The same group, the Intergovernmental Panel on Climate Change (IPCC), warned in 1995 that "the balance of the evidence suggests a discernible human influence" on climate change. The newest report should be nearly finalized later this year, in time to inform negotiations that culminate this fall on the implementation of the Kyoto Protocol for the reduction of greenhouse gas emissions, an international treaty that focuses on a probable major cause of global warming.

Over the past several years, the panel of climate scientists gained confidence in human culpability for the past century's warming--by 0.6°C--through a process of elimination. Researchers ran improved climate models repeatedly and longer to examine alternatives--the natural ups and downs of temperature, solar variability, or volcanic emissions. None seems to suffice. But model simulations of the past century that incorporate greenhouse gas emissions bear a strong resemblance to the observed warming.

Climate researcher Tim P. Barnett of the Scripps Institution of Oceanography in La Jolla, California, says, "we have a change we can't explain with natural variations. There aren't many other options" aside from human activity. Even greenhouse contrarians are tacitly going along with the IPCC's confident conclusion that humans are impacting global climate. Rather than dispute the reality of the warming or its cause, they have lately emphasized its modest size and say mild warming won't do much damage.

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