Leading climate scientists said on Friday they were more certain than ever before that mankind was the main culprit for global warming and warned the impact of greenhouse gas emissions would linger for centuries.
A report, by the Intergovernmental Panel on Climate Change (IPCC), played down the fact temperatures have risen more slowly in the past 15 years, saying there were substantial natural variations that masked a long-term warming trend.
It said the Earth was set for further warming and more heat waves, floods, droughts and rising sea levels as greenhouse gases built up in the atmosphere. The oceans would become more acidic in a threat to some marine life.
"It is extremely likely that human influence has been the dominant cause of observed warming since the mid-20th century," according to the summary issued after a weeklong meeting in Stockholm and meant to guide policymakers in shifting towards greener energies from fossil fuels. "Extremely likely" means a probability of at least 95 per cent, up from 90 per cent in the panel's last report in 2007 and 66 per cent in 2001.
The report, compiled from the work of hundreds of scientists, will face extra scrutiny this year after its 2007 report included an error that exaggerated the rate of melting of Himalayan glaciers. An outside review later found that the mistake did not affect its main conclusions.
Sceptics who challenge evidence for man-made climate change and question the need for urgent action have become emboldened by the fact that temperatures have risen more slowly recently, despite rising greenhouse gas emissions.
The IPCC reiterated from the 2007 report that a warming trend is "unequivocal". And some effects would last far beyond the lifetimes of people now alive. "As a result of our past, present and expected future emissions of carbon dioxide, we are committed to climate change and effects will persist for many centuries even if emissions of carbon dioxide stop," co-chair Thomas Stocker said.
The UN's top climate official, Christiana Figueres, said the report underscored a need for urgent action to combat global warming. Governments have promised to agree a UN deal by the end of 2015 to restrict emissions. "To steer humanity out of the high danger zone, governments must step up immediate climate action and craft an agreement in 2015 that helps to scale up and speed up the global response," she said.
UN panel report on climate change
A UN panel said on Friday it was more certain than ever that humans were causing global warming and predicted temperatures would rise by 0.3 to 4.8 degrees Celsius (0.5-8.6 degrees Fahrenheit) this century.
The Intergovernmental Panel on Climate Change (IPCC) also projected sea levels would rise by between 26 and 82 centimetres (10.4 and 32.8 inches) by 2100, according to a summary of the first volume in a long-awaited review.
The Nobel-winning group said it was “extremely likely” - a term meaning it was 95-per cent convinced - that humans caused more than half of the warming observed over the past 60 years.
In its last report in 2007, the panel had rated its conviction at 90 per cent.
The new document is the first volume in a trilogy that will seek to summarise the status of global warming and its impacts.
The IPCC has delivered four previous assessment reports in its 25-year history.
Each edition has pounded out an ever-louder drumbeat to warn that temperatures are rising and the risk to the climate system - in drought, floods, storms and rising seas - is accentuating.
The panel’s projections for 2100 are based on computer models of trends in heat-trapping greenhouse gas emissions, especially from coal, oil and gas, which provide the backbone of energy supply today.
The most optimistic of four scenarios for warming sees an average temperature rise of 1.0 C (1.8 F) by 2100 over 2000 levels, ranging from 0.3 to 1.7 C (0.5-3.1 F). This is the only scenario that can safely meet a UN target of 2 C (3.6 F) which also factors in warming from the start of the Industrial Revolution to 2000.
By comparison, the highest IPCC scenario has an average additional warming this century of 3.7 C (6.7 F), ranging from 2.6 C (4.7 F) to a 4.8 C (8.6 F) - a figure that many experts consider catastrophic.
“Global surface temperature change for the 21st century is projected to be likely to exceed 1.5 C relative to 1850-1900 in all but the lowest scenario considered,” said Thomas Stocker, co-chairman of the IPCC working group that authored the report.
He warned: “Limiting climate change will require substantial and sustained reductions of greenhouse-gas emissions.”
‘Unbiased assessments’ of climate change
Stocker said there would be an impact on the risk for heatwaves, floods and drought.
“Heatwaves are very likely to occur more frequently and last longer. As the Earth warms, we expect to see currently wet regions receiving more rainfall, and dry regions receiving less, although there will be exceptions,” he said.
The work released in Stockholm comprises a 2,000-page report authored by 257 scientists, plus a 36-page Summary for Policymakers.
Two more volumes will be released next year, touching on the impacts from climate change and options for dealing with the problem, followed by a synthesis of all three.
In the past, the massive overviews delivered a jolt to the troubled negotiations for a UN pact on curbing greenhouse gas emissions, and initiated grassroots campaigns on climate change in many countries.
In a video statement at a close-of-meeting press conference, UN Secretary General Ban Ki-moon praised the IPCC - targeted by climate sceptics as biased or flawed - for delivering “regular, unbiased assessments of the mounting impacts of a warming planet.”
“This new report will be essential for governments as they work to finalize an ambitious, legal agreement on climate change in 2015,” he said.
Ban recalled his plan to convene a climate summit in September 2014 “to generate the political commitment to keep global temperature rise below the agreed two-degree Celsius threshold.”
Political interest in dealing with climate change fell back after a nearly disastrous UN summit in Copenhagen in 2009, and willingness for concessions has fallen back in developed countries still struggling with the 2008 financial crisis.