Here are 10 things to know about the Intergovernmental Panel on Climate Change, the U.N.-sponsored scientific group that will present a landmark report on global warming Friday.
WHAT IS THE IPCC?
A scientific body with 195 member countries, the panel was established by the U.N. in 1988 to assess the causes and impacts of climate change. Since then, it has released four assessments, each stating the human link to global warming with more certainty than the previous one. The IPCC doesn't conduct its own research, but appoints hundreds of experts to review and summarize the latest scientific studies on climate change. More than 800 scientists contributed to a report set to be released Friday.
The IPCC is meeting in Stockholm this week to adopt the first of four parts of its fifth assessment report on climate change. This part deals with the physical science basis of global warming. A summary for policy-makers of about 30 pages will be published Friday, and the full 2,000-page report on Monday. Next year, the IPCC will present sections assessing the impact of climate change and strategies to fight it. A synthesis of the three reports will be adopted in October 2014.
SO WHAT EXACTLY IS THE IPCC DOING IN STOCKHOLM?
Delegates from member countries are meeting behind closed doors with authors of the physical science report to hammer out the summary for policy-makers. They are going through a draft line by line, which can be a frustratingly slow process because the text needs to be approved by consensus. Governments may have problems with the text being either too complex or too vague, or they may have non-scientific concerns about grammar or word choice. Commenting on a June draft, the U.S. wanted it to read more like a narrative. Underscoring the politics involved, China wanted to remove national borders from a world map used in the draft to "avoid unnecessary disputes."
WHAT ARE THE ASSESSMENTS USED FOR?
They form the scientific basis for U.N. negotiations aimed at curbing global warming. The fifth assessment report will be a reference point for governments as they negotiate a new global climate agreement, which is supposed to be adopted by 2015 and to take effect in 2020.
WHAT DO CRITICS SAY ABOUT THE IPCC?
Some scientists say the IPCC process is so time-consuming and laborious that by the time the assessments are published they are already out of date. Policy-makers sometimes complain that the language in the reports is too scientific and difficult for non-scientists to understand. A series of errors embarrassed the authors of the 2007 assessment, including the incorrect statement that the glaciers in the Himalayas would disappear by 2035. Climate skeptics seized on those errors as evidence the IPCC process is flawed. Supporters say the fact that such errors are so rare shows how solid the process is. There has also been criticism about the IPCC's lack of openness.
WHAT WILL THE NEW REPORT SAY?
The final version will be adopted Friday, so changes are still possible, but leaked drafts suggest the IPCC will say it's "extremely likely" that climate change is man-made. That would be an upgrade from "very likely" in the 2007 report and would mean that scientists are now 95 percent certain of man-made warming. The report is also expected to raise the projections of sea level rise this century and analyze the human contributions to the loss of Arctic sea ice, which hit a new record in summer last year, and the retreat of glaciers and ice sheets. The June draft projected that surface temperatures will rise by 0.3-4.8 degrees C (0.5-8.6 F) this century, depending in part on whether and how much countries reduce their CO2 emissions.
HOW ACCURATE ARE THE IPCC'S PROJECTIONS?
The IPCC makes long-term projections about how the climate system will respond to warming temperatures over the next decades so we won't know just how accurate they are until the evidence is in at the end of this century. Scientists generally agree that the assessments offer the best available estimates of future warming, and the projections are based on a solid understanding of the factors at play in the climate system. But they also stress that there are uncertainties involved, just like knowledge about financial markets doesn't mean you will be able to predict the stock market.
WHAT DOES THE IPCC SAY ABOUT THE RECENT 'HIATUS' IN WARMING?
When climate scientists talk about global warming they refer to the rise in global temperatures observed in the past 100 years or so. They typically don't pay much attention to shorter time scales in which temperatures can go up or down in natural climate fluctuations. But there's been so much media attention recently to a purported slowdown in the temperature rise in the past 15 years that many governments want the IPCC to address it in the report. Some skeptics claim this "hiatus" shows that global warming has stopped, even though if you compare decadal averages, the previous decade was the warmest on record. It's an area that needs more research, but many scientists say the purported slowdown reflects random climate fluctuations and an unusually hot year, 1998, picked as a starting point for charting temperatures. Another leading hypothesis is that heat is settling temporarily in the oceans.
WHO HEADS THE IPCC?
Rajendra Pachauri, a scientist from India, has been the chairman of the IPCC since 2002. He is set to step down when his term expires in 2015.
WHERE IS THE IPCC BASED?
The secretariat of the IPCC is headquartered in Geneva, Switzerland, in the offices of the World Meteorological Organization, the U.N.'s weather agency.
Explore further: UN climate experts stress solidity of new report (Update)