A report requested by the US Congress has found that there is enough scientific evidence to conclude that the past few decades have been the warmest in the past 400 years.
It is less certain, though 鈥減robable鈥�, that the past 25 years have also been the warmest since 900AD, say the authors of the report from the National Academies of Science (NAS), the non-profit body that advises the US government.
They also conclude that humans may have influenced the recent warming. While some people 鈥渉ave attempted to promote the idea that the recent warming is a natural phenomenon鈥� unconnected with human activity, 鈥渢his report renders that notion much less plausible鈥�, comments Martin Rees, president of the Royal Society in London, UK, on the findings.
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The report paid particular attention to the controversial findings of two papers published by palaeoclimatologist Michael Mann and his colleagues in the late 1990s (Geophysical Research Letters, vol 26, p 759-762 and Nature, vol 392, p 779-787). These seminal studies used a wide range of evidence 鈥� including tree rings and historical records 鈥� to reconstruct average temperatures of the northern hemisphere over past centuries.
The 鈥渉ockey stick鈥� graph from Mann鈥檚 team, showing the average temperature over the past 1000 years, gained wide attention. For the first 900 years there was little variation 鈥� like the shaft of a hockey stick 鈥� and then there has been a massive warming over recent decades 鈥� a spike that formed the 鈥渂lade鈥� of the graph. Sherwood Boehlert, chairman of the House science committee, requested the NAS report in November 2005 in response to the political debate around Mann鈥檚 work.
Birth of a debate
The NAS report finds that conclusions drawn from Mann鈥檚 reconstructions of warming over the past 400 years bear weight, but it stresses that there is too little evidence to be statistically certain that a given year or single decade stands out as unusually warm.
Mann鈥檚 team had suggested that the 1990s were the warmest decade of the millennium and 1998 the warmest year.
鈥淚t was really the first analysis of its type,鈥� panel member Kurt Cuffey of the University of California, Berkeley, US, said at a news conference on Thursday.
He added that it was the first time anyone has done such a large-scale and continual analysis of temperature over time. 鈥淪o it鈥檚 not surprising that they could have probably done some detailed aspects of it better.
鈥淏ut it was a remarkable contribution and gave birth to a debate that鈥檚 ongoing, that鈥檚 teaching us a lot about how climate has changed.鈥�
The NAS document says that humans may have influenced the recent warming but notes: 鈥淪urface temperature reconstructions for periods prior to the industrial era are only one of multiple lines of evidence supporting the conclusion that climatic warming is occurring in response to human activities, and they are not the primary evidence.鈥�
First case
The publication of the NAS report comes too late to influence whether or not the Supreme Court will hear its first case on climate change (see Climate change: Let the Supreme Court decide).
Prominent climate change scientists have been urging the US Supreme Court to hear a case regarding whether the US federal Clean Air Act compels the Environmental Protection Agency (EPA) to regulate greenhouse gas emissions from motor vehicles.
But James Milkey, a lead attorney hoping to argue the case for stronger regulation of carbon dioxide pollution from cars, sees the report as a boost nonetheless. 鈥淲e certainly think that the report confirms what we鈥檝e been saying about the science,鈥� he says.
鈥淚t will definitely have weight and could definitely be brought to bear on a future case,鈥� says John Harte of the University of California, Berkeley, who along with other scientists has urged the Supreme Court to hear the case.
