New report presents strongest evidence yet of human link to global warming

“We are now seeing, not merely predicting, effects of greenhouse warming on a scale and in ways that were not observable before,” said Gabriele Hegerl, associate research professor at Duke's Nicholas School of the Environment and Earth Sciences, who also co-authored a summary of the report for policymakers.

“When you look at the changes in temperature, circulation, ocean warming, arctic sea ice reduction and glacial retreat together, it paints a much clearer picture that external drivers, particularly greenhouse gases, are playing a key role,” she said. “As a result, we can be much more confident that 20th century climate changes were not just linked to natural variability.”

Hegerl was a coordinating lead author of the IPCC report's chapter on “Understanding and Attributing Climate Change.” Francis Zwiers of the Canadian Centre of Climate Modeling and Analysis was also a coordinating lead author of the chapter.

IPCC assessment reports are issued every five to six years to provide a comprehensive review of the current state of knowledge on climate change. The 2007 report will be issued in four phases during the year. The first phase, released today in Paris, focuses on the physical evidence of global change.

THE IPCC operates under the auspices of the United Nations Environmental Programme and the World Meteorological Organization and draws on the expertise of about 2,500 scientists worldwide.

Hegerl and her chapter's team of co-authors were charged with reviewing the evidence of changes observed so far and assessing which changes can be attributed to greenhouse gas increases and other external influences on climate. In the chapter, they look at the actual measurements of climate and weather changes and compare them with predictions made for the 20th century by sophisticated computer models.

“We've studied improved observations from land, sea and space, as well as better temperature reconstructions covering the last 1,000 years,” Hegerl said. By comparing observation against modeled projections, she says scientists are gaining a better sense of which external climate influences have been important.

“Understanding the observations is really what this all is about. For instance, looking at the patterns of change in 20th-century temperatures, we can now distinguish between changes caused by greenhouse gases, man-made aerosols, variability in solar radiation and major volcanic eruptions,” Hegerl said. “We can also better understand which changes in the more distant past were caused by external influences of climate, such as volcanic eruptions, and how strong the variability of the climate system is.

“One of the most fascinating things is that we see that changes have already happened or are happening now in more climate variables than just temperature,” Hegerl added. “For instance, there have been observed changes in ocean temperatures, global rainfall and in circulation of the atmosphere. We now are beginning to understand that these changes occur at least partly in response to anthropogenic influences on climate. This allows us to better evaluate model simulations, which do simulate aspects of these changes, although not as successfully as they simulate changes in temperature,” she said.

“There are still things, like ice-sheet melting, that the models don't do very well yet. But overall, the predictions and uncertainty ranges of future climate change are becoming much better understood and much more credible,” Hegerl said.

The IPCC report “hits the nail squarely on the head,” she said. “It gives a very balanced view of the evidence for climate change, predictions of future change, and the remaining uncertainties, and it draws input from very large number of scientists worldwide.”

The report went through several phases of review, giving individual experts and governments opportunity to comment. “There were many steps that ensure that the report is both scientifically rigorous and balanced,” Hegerl said.

“The information in the report will be very important to develop effective policies to address global climate change and to prepare for the change that is coming our way,” she said.