Global warming is “unequivocal”, according to the fifth IPCC report which, after six years in preparation, delivers a detailed picture of the science behind climate change.
In the strongest language yet deployed in the fight against increasing temperatures, the report concluded that the 30 years until 2012 were probably the warmest in 1,400 years for the northern hemisphere, driven by “unprecedented” levels of greenhouse gases – these are now at levels not seen for 80,000 years.
The conclusion of hundreds of authors is that it is “extremely likely” that human activity has been the “dominant cause” of the rising temperatures witnessed during the 20th century.
The report is from the IPCC Working Group I, which examines the physical basis of climate change. Introducing the report’s Summary for Policymakers, which will be read by governments and form the basis of policy worldwide, co-chair Dr Thomas Stocker said “the human influence on the climate system is clear”.
“This is an assessment of a string of assessments, and we have confirmed again that warming in the climate system is unequivocal, a conclusion re-confirmed since the last report with the help of new evidence from the atmosphere, oceans, ice and land,” he said.
Michel Jarraud, Secretary-General of the World Meteorological Organization, one of the founding organisations of the IPCC, said the report will be essential for forming the basis of the international UN climate agreement in 2015. “This should come as yet another wake up call that our activities have an impact on the world,” he said.
Temperatures between 2001-2010 were the highest on record, a decade that saw more records than ever broken. “It would have been even higher were it not for the role of the deep oceans in absorbing heat,” he added. “But this does not mean that the oceans will save us from global warming.”
One of the questions raised since the previous report in 2007 was the so-called “hiatus” in global warming that was not predicted by models. Having led the delegation of scientists and government representatives that thrashed out the final wording, Dr Stocker explained it was an emerging scientific question that had been looked at very carefully. He said that while too few measurements were available in the deep ocean, a large amount of the recent “pause” in global warming was due to natural variability, including: a series of recent volcanic eruptions, natural Pacific cooling cycle, and absorption of heat in the deep oceans.
He added: “At current levels we are facing rates of warming higher than those assessed in the report’s lowest emissions scenarios. But this is dependent on our carbon emissions each year, so humankind has a choice on which emissions path they will follow.”
Other findings highlighted in the report are that global sea level rise will very likely exceed that recorded between 1971-2010, fuelled by the loss of glaciers and Arctic sea ice. Oceans will continue to absorb CO2 which will increase acidification. Perhaps most worrying is that most aspects of climate change – fuelled by already committed historical, present, and future emissions – will persist for centuries, even if CO2 emissions are stopped.
Ban Ki-moon, UN Secretary General, speaking from Geneva, said: “The world’s eyes are on Stockholm today, you have used the best science to fight the worlds biggest problem.”
“The heat is on; now we must act.”
This article has been updated to clarify that it is the northern hemisphere that has experienced the warmest 30-year period in 1,400 years.