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Climate change: U.N. summary of latest science coming Friday

By Brandon Miller, CNN
updated 4:06 PM EDT, Thu September 26, 2013
A recent study published in the science journal "Nature" has concluded that many more trees are vulnerable to drought stress than previously thought. A majority of trees studied operate very close to their hydraulic threashold, taking on just enough water to survive. A recent study published in the science journal "Nature" has concluded that many more trees are vulnerable to drought stress than previously thought. A majority of trees studied operate very close to their hydraulic threashold, taking on just enough water to survive.
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Is climate change moving too fast for trees?
Drought and disease
Amazon fires threaten vital carbon sink
Forests ravaged by beetles
Hot spells spark forest fires
Endgame for old trees?
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STORY HIGHLIGHTS
  • The Intergovernmental Panel on Climate Change released a report every five or six years
  • A draft that leaked in August said there is more certainty that humans are playing a role
  • The report will lay out projections for climate change through the end of the century
  • The first section of the report is aimed at assisting policymakers

(CNN) -- How much of a role do humans play in climate change? A new report scheduled to come out on Friday will give scientists' latest take on the topic.

The United Nations report is expected to contain language that further identifies the role human activity is playing in increasing global temperatures through burning of fossil fuels and greenhouse gas emissions.

The last report, in 2007, indicated that it was "very likely" that humans were responsible for most of the observed warming -- a judgment that corresponds to a confidence level greater than 90%. A draft that leaked in August raised that confidence level to "extremely likely," or greater than 95% confidence that humans are responsible for the majority of global warming through carbon pollution.

The impacts of this warming are already being seen through increases in extreme weather events such as heat waves, droughts and extreme flooding events, which the assessment will highlight.

The report will lay out projections for climate change through the end of the century, based upon four different carbon emission scenarios. Those range from a low end which would require significant cutbacks in emissions, to a worst-case scenario in which carbon emissions would continue to climb unabated. The projections will provide more details about sea-level rise, for which observations show rates increasing, and should give a more precise estimate of the levels we will see at the end of the century.

Loss of sea ice will also be addressed, and the report will discuss the accelerating loss of sea ice in the Arctic and the slight increase of ice seen in the Antarctic.

Hundreds of experts weigh in

The latest Assessment Report from the Intergovernmental Panel on Climate Change brings together the latest research from top scientists around the field. It contains a "summary for policymakers" aimed at guiding politicians and lawmakers worldwide on decisions regarding the environment over the next several years.

The U.N. panel releases a report every five or six years. Friday's report is the culmination of work by over 250 authors from 39 countries and was subject to an extensive review process involving more than 1,000 experts.

More than 850 expert authors from 85 countries contributed research for the full report, which will be released in three stages through April. The first, on the physical science behind climate change, accompanies the summary for policymakers. The second, expected in March, will cover "impacts and vulnerabilities" of climate change; the third, on mitigation efforts, is set to go out in April.

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Critics of the report

Despite the overall breadth of the scientific expertise involved, and the extensive review and approval process, the IPCC Assessment Reports spark quite a few criticisms, from both climate change believers and skeptics.

Skeptics claim the IPCC exists only to produce further evidence supporting the idea of man-made climate change while ignoring opposing research. But climate change activists, and many climate scientists, believe that the IPCC's consensus-seeking policy produces conclusions and estimates that are too conservative.

Another often-cited critique of the report is that, due to its size and lengthy approval process, it is already outdated by the time it is released. Several important studies already have been published in the past year in the constantly evolving science of climate change that will not be included in this assessment.

Despite the critics, this week's document will serve as a major measuring stick for the current state of the world's climate and what type of change is in store.

The summary for policymakers will be available Friday at www.climatechange2013.org.

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