UW Today

September 30, 2013

UW researchers helped draft international assessment of climate change

News and Information

The United Nations Intergovernmental Panel on Climate Change late last week released its summary for policy-makers, the Cliffs Notes version of the massive international assessment released about every six years.

The full text of the fifth IPCC report was released today, and University of Washington atmospheric science professors Dennis Hartmann and Chris Bretherton were among 209 researchers from 39 countries who were lead authors on the 900-page report.

“Warming is unequivocal,” Hartmann said Friday at a news conference in Sweden. He was a coordinating lead author for Chapter 2, Observations of Atmosphere and Surface, which reviews the evidence for global warming in temperature records. Hartmann also helped draft the technical summary and the summary for policymakers, and was in Stockholm last week for the final line-by-line reviews of the 36-page summary document.

“For the most part the conclusions of previous IPCC assessments can be given with even more certainty,” Hartmann commented by e-mail. “The surface of Earth is warming and humans are responsible.”

The new report moves from asking whether warming has occurred, he said, to determining exactly how much warming has taken place since 1750, around the beginning of the industrial age. This assessment also makes more forward-looking statements for policymakers.

“Simpler messages with more clarity can be given about how greenhouse gas release is related to future climate change,” Hartmann wrote.

The report recommends that, to keep temperature change within 3.6 degrees F (2 degrees C) of preindustrial levels, carbon emissions should not exceed 1 trillion metric tons. Without changes that level would be reached by 2040, Hartmann said.

Hartmann and Bretherton

UW professors Dennis Hartmann and Chris Bretherton were lead authors on the new report.

While much of the report fills in the details of previous versions, that’s not a bad thing, Bretherton said.

“I think it is important that the basic conclusions of the assessment, about how much warming (will occur) and patterns of rainfall change in a warmer climate, are essentially identical to the previous IPCC assessments,” Bretherton said. He is a lead author on Chapter 7: Clouds and Aerosols, which for the first time was the subject of a separate chapter instead of being discussed in other sections.

“Clouds and aerosols are the single largest source of uncertainty in simulating the climate change of the next 50 to 100 years,” Bretherton said. Models have a hard time simulating clouds, he said, and it’s not well understood how clouds interact with human-produced aerosols such as pollution haze.

Chapter 7 authors also were asked to take a first IPCC look at geoengineering, a controversial idea to start trying to bury carbon dioxide or reflect sunlight by spraying aerosol particles into the top of the atmosphere. The summary text warns of technological limitations and possible side effects from implementing either of these techniques on a global scale.

This IPCC report was an even bigger undertaking than usual, with almost twice as many scientists contributing as last time.

“Maybe the next IPCC assessment, in 2020 or so, will be a much shorter update requiring a lot less effort from the global climate science community,” Bretherton said. “But I won’t bet on it.”

UW faculty members who were not involved in drafting the report first saw the document Friday, when the summary was released to the public. Most agreed that this assessment reaffirms the science while providing some updates on sea-level rise and ocean changes.

Graphic of IPCC report depicts temperatures at the end of the 21st century.

A graphic in the new report depicts temperature changes at the end of the 21st century.IPCC

“The biggest difference (between this report and the last one) is our confidence in the results,” said LuAnne Thompson, an oceanography professor and director of the UW’s interdisciplinary Program on Climate Change. “It contains new information about how ice sheets at the poles are contributing to sea-level rise, changes in the chemistry of the ocean through ocean acidification. Also new are discussions of the long-term changes – of a thousand years or more – that we are already committed to” from long-lived carbon emitted since the beginning of the industrial age.

The full text of Working Group I, on the physical basis for climate change, was released Sept. 30. The reports of the other two IPCC working groups, on the effects of climate change and possible mitigation responses, will follow in 2014.

“The IPCC results emphasize the need to get serious about avoiding dangerous interference with the climate system and on preparing – globally, nationally and locally – for the changes already set in motion,” commented Amy Snover, director of the UW’s Climate Impacts Group and co-author of an upcoming report on climate change impacts on the Pacific Northwest.

A full list of UW-affiliated authors on the report is available here.

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For more information, contact Bretherton at 206-685-7414 or breth@atmos.washington.edu, Hartmann at 206-543-7460 or dhartm@uw.edu and Snover at 206-221-0222 or aksnover@uw.edu. Hartmann will be in Europe until Oct. 7.

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