Are humans mostly to blame for temperatures rises on Earth since 1951?
A UN panel of global climate scientists are working to ensure their strongest case yet for manmade global warming will make sense to the widest possible audience.
Drafts show that the Intergovernmental Panel on Climate Change (IPCC) is set to pronounce that most of the warming of the Earth's surface since the 1950s is "extremely likely" - at least 95 per cent probable - to be manmade.
At its last meeting in 2007, it put the probability at 90 per cent, and in 2001 it was 66 per cent.
The 30-page summary that the IPCC will produce today at about 8pm, the first of four about global warming in the coming year, was intended to be the main point of reference on the science of climate change for governments trying to develop their response to global warming.
Delegates said the tone was constructive, with countries urging better explanation of scientific findings, not challenging them as the basis for cuts in greenhouse gas emissions.
Most of the discussions were about how best to describe a world set to suffer more heatwaves, downpours and floods as well as higher sea levels as temperatures rose, they said.
"The tone is surprisingly good," one delegate said.
"It's all about: 'Can't we write this sentence more clearly?'."
The document also sought to explain a slowdown in the pace of warming this century.
At one point on Thursday, originally meant to be the fourth and final day of the negotiations, a display at the meeting showed that 85 per cent of the time had elapsed, but only 55 per cent of the work had been done, one delegate said.
Some countries also wanted to stress that it was also "virtually certain", or at least 99 per cent probable, that natural variations in the climate were not the sole cause.
Still, sceptics have said a slowing of the pace of warming this century, after fast gains in the 1980s and 1990s, was a sign that global warming may not be as urgent a problem as previously believed.
The IPCC report slightly cuts the likely warming impact of a build-up of carbon dioxide on the atmosphere.
Scenarios in the drafts predicted the hiatus would not last, however, and that temperatures would rise by between 0.3 degrees Celsius and 4.8C this century.
The lower end of the range would only be possible with emissions cuts deeper than anything that major economies have said they were prepared to tolerate.
The report would face extra scrutiny after the IPCC made errors in its 2007 report, including an exaggeration of the melt rate of Himalayan glaciers.
An outside review of the IPCC found that the mistake did not affect its main conclusions.
Almost 200 governments have agreed in principle to limit global warming to a maximum rise of 2C above pre-industrial times and have promised to work out a UN deal to limit their emissions accordingly by the end of 2015.
Separately, an academic study said people reacted best to the challenge of climate change if it was not presented as doom and gloom.
"The best way to encourage environmentally friendly behaviour is to emphasise the long life expectancy of a nation, not its imminent downfall," according to the study of 131 nations led by NYU Stern Professor Hal Hershfield.
Over the next year, the IPCC would issue three more reports about the impacts of climate change around the world, the possible solutions, and finally a summary of all the findings.
TOP 10 IPCC NEED-TO-KNOWS
1. WHAT IS THE IPCC?
A scientific body with 195 member countries, the panel was established by the UN 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 didn't conduct its own research, but appointed hundreds of experts to review and summarise the latest scientific studies on climate change. More than 800 scientists contributed to the report due to be released today.
2. WHAT REPORT?
The IPCC was meeting in Stockholm this week to adopt the first of four parts of its fifth assessment report on climate change. This part dealt with the physical science basis of global warming.
A summary for policy-makers of about 30 pages would be published today, and the full 2000-page report on Monday.
Next year, the IPCC would present sections assessing the impact of climate change and strategies to fight it. A synthesis of the three reports would be adopted in October 2014.
3. WHAT'S HAPPENING IN STOCKHOLM?
Delegates from member countries were meeting behind closed doors with authors of the physical science report to hammer out the summary for policy-makers.
They were going through a draft line by line, which could be a frustratingly slow process because the text needed to be approved by consensus.
Governments might 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 US 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".
4. WHAT ARE THE ASSESSMENTS USED FOR?
They form the scientific basis for UN negotiations aimed at curbing global warming. The fifth assessment report would be a reference point for governments as they negotiated a new global climate agreement, which was supposed to be adopted by 2015 and to take effect in 2020.
5. WHAT DO CRITICS SAY ABOUT THE IPCC?
Some scientists said the IPCC process was so time-consuming and laborious that by the time the assessments were published they were already out-of-date.
Policy-makers sometimes complained that the language in the reports was 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 sceptics seized on those errors as evidence the IPCC process was flawed.
Supporters said the fact that such errors were so rare showed how solid the process was. There had also been criticism about the IPCC's lack of openness.
6. WHAT WILL THE NEW REPORT SAY?
The final version would be adopted today, so changes were still possible, but leaked drafts suggested the IPCC would say it's "extremely likely" that climate change was manmade. That would be an upgrade from "very likely" in the 2007 report and would mean that scientists were now 95 per cent certain of manmade warming.
The report was also expected to raise the projections of sea level rise this century and analyse 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 would rise by 0.3-4.8C this century, depending in part on whether and how much countries reduced their CO2 emissions.
7. HOW ACCURATE ARE THE IPCC'S PROJECTIONS?
The IPCC made long-term projections about how the climate system would respond to warming temperatures over the next decades so we wouldn't know just how accurate they were until the evidence was in at the end of this century.
Scientists generally agreed that the assessments offered the best available estimates of future warming, and the projections were based on a solid understanding of the factors at play in the climate system. But they also stressed that there were uncertainties involved, just like knowledge about financial markets didn't mean you would be able to predict the stock market.
8. WHAT 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 didn't pay much attention to shorter time scales in which temperatures could 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 sceptics claimed this "hiatus" showed that global warming had stopped, even though if you compared decadal averages, the previous decade was the warmest on record.
It's an area that needed more research, but many scientists said the purported slowdown reflected random climate fluctuations and an unusually hot year, 1998, picked as a starting point for charting temperatures. Another leading hypothesis was that heat was settling temporarily in the oceans.
9. WHO HEADS THE IPCC?
Rajendra Pachauri, a scientist from India, has been the chairman of the IPCC since 2002. He was set to step down when his term expired in 2015.
10. WHERE IS THE IPCC BASED?
The secretariat of the IPCC is headquartered in Geneva, Switzerland, in the offices of the World Meteorological Organization, the UN's weather agency.
- Reuters and AP
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