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Clouds' role in warming more certain, report shows
It is more certain than ever that human civilisation is the main cause of global warming, putting the world on track for dangerous temperature rises, the latest major UN assessment of climate change science has found.
The Intergovernmental Panel on Climate Change says it is "extremely likely" that humans are the dominant cause of observed warming since the mid-20th century, with carbon dioxide emissions the main factor.
If emissions remain high, by 2100 temperatures are likely to rise by more than 2 degrees - and up to 4.8 degrees - breaching a threshold agreed by governments as limiting the worst impacts of climate change.
Heatwaves will be more frequent and last longer, the report found. Most wet regions will get more rainfall, and most dry regions less.
Glaciers and ice sheets will continue to shrink, and the sea level will rise more quickly.
UN secretary-general Ban Ki-moon said political commitment was needed to keep global temperature rise below the 2-degrees threshold. "The heat is on, now we must act," he said.
On Friday in Stockholm the UN's Intergovernmental Panel on Climate Change released a summary of its fifth major assessment of climate science after a week of debate, and years of work.
"The atmosphere and ocean have warmed, the amount of snow and ice has diminished, the global mean sea level has risen and the concentrations of greenhouse gases have increased," said Qin Dahe, co-chair of the IPCC working group that compiled the report.
These findings were based on multiple lines of independent evidence, he said, much of it new since the IPCC's previous report in 2007.
Co-chair Thomas Stocker said "substantial and sustained" reductions of greenhouse gas emissions would be necessary to prevent further warming and climate change.
The report found that:
Each of the last three decades has been warmer than any preceding decade since 1850, and the last 30 years have been the warmest since 600 AD. Combined land and ocean temperatures rose on average by 0.85 degrees since 1880.
From 1901 to 2010, the sea level rose by 19cm – more quickly than the average for the last 2000 years. It is very likely to rise even more quickly during the 21st century.
Greenhouse gases have reached levels unseen in at least 800,000 years, from fossil fuel emissions and land use. A third of the extra carbon dioxide has been absorbed by the oceans, making them more acidic.
It is very likely that Arctic sea cover will continue to shrink and thin, and spring snow cover will continue to decrease through the 21st century.
It is more likely than not that there will be more intense tropical cyclones.
Evidence for human influence in climate change has grown in the last five years, the report found.
It says that by 2011 the world had emitted more than half the carbon gases that could be released if the world was to have a good chance of keeping warming below two degrees. Only the strictest of emissions cuts considered by the IPCC would ensure the world did not exceed the 2-degree threshold.
The IPCC report has been six years in the making and has involved more than 800 scientists from around the world to pull it together. The panel was established by the UN to provide scientific assessments of climate change to governments, who get the final sign off on its reports.
The working group went right down to the wire, finishing the substantive parts of the report only hours ahead of its planned release after an all-night debate.
The group has been wrestling with figures that showed a slower rise in global temperatures than expected in the past decade.
The report concludes "with high confidence" that more than 90 per cent of the extra heat generated in between 1971 and 2010 has been stored in the world's oceans.
Australian Environment Minister Greg Hunt said: "The report's findings reinforce the government's bi-partisan support for the science and the targets set for emissions reductions.
United States Secretary of State John Kerry said: "This is yet another wake up call. Those who deny the science or choose excuses over action are playing with fire. This is science, these are facts, and action is our only option."
Australian Academy of Science president Suzanne Cory said the world could be more certain than ever that human-induced climate change was real and a serious threat to the planet.
"Overwhelmingly, the scientific evidence suggests the world should take action to limit the dangers posed by climate change for societies and ecosystems and to adapt to the changes that are already inevitable," Professor Cory said.
The 36-page 'summary for policymakers' released on Friday covers the first part of its assessment looking at the physical science, with the full version to be released on Monday.
It is based on 9200 scientific studies, more than three quarters of which were published since 2007.
Two more reports – looking at impacts and mitigation of climate change – will be released next year.
ABOUT THE INTERGOVERNMENTAL PANEL ON CLIMATE CHANGE
Formed by the UN in 1988 to inform governments on climate science.
Every 5 or 6 years releases a major assessment of the science. This is its fifth.
Does not carry out its own research, instead draws together and summarises thousands of sources of existing data.
Assessments prepared by hundreds of climate scientists from around the world who volunteer their time.
Representatives from the governments of 195 countries get final sign off.
Each major assessment is broken into three sections. Parts 2 and 3 will be released next year.
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