Scientists are more certain than ever that humans are causing the majority of climate change, a major new report has shown.
The first part of the Intergovernmental Panel on Climate Change’s (IPCC) fifth assessment report shows that warming in the climate system is “unequivocal” and human influence on the climate is clear.
The report, which has been published after line-by-line scrutiny by scientists and policymakers, found it is “extremely likely”, or 95 per cent certain, that the majority of the warming since the 1950s is down to human activity.
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The likelihood is up from a 90 per cent certainty in the last IPCC study in 2007.
As a result of the warming, ice sheets are losing mass, glaciers are shrinking, sea ice cover has reduced in the Arctic and the permafrost is thawing in the northern hemisphere, the report - which draws on thousands of scientific papers - warns.
The study predicts that temperatures are set to rise by more than 1.5 degrees above pre-industrial levels by the end of the century without ambitious action to tackle emissions, and could rise by over 4 degrees if emissions continue to increase.
Storms will become more intense and frequent, sea levels will rise by between 26cm (10in) and 82cm (32in) by the end of the century and the oceans will become more acidic, the assessment projects.
One of the scientists leading the first section of the IPCC’s fifth assessment , which looks at the science of climate change and its causes, Thomas Stocker, said: “Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system.
“Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.”
The report considered a series of scenarios for greenhouse gas emissions and their impacts on the climate. “Global surface temperature change for the end of the 21st century is projected to be likely to exceed 1.5C relative to 1850 to 1900 in all but the lowest scenario considered, and likely to exceed 2C for the two high scenarios.
“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 report also suggests warming has been particularly marked since the 1970s, with each of the last three decades significantly warmer than all the previous decades since 1850.
In the run-up to the publication of the IPCC report, questions have been raised about the slowdown in temperature rises in the past 15 years, with climate “sceptics” claiming it undermines the theory of climate change.
UK researchers have said a temporary slowdown in temperature rises is not unexpected, due to natural variation, and that heat going into the deep ocean may partly be the cause of lower rises in surface temperatures.
A number of different indicators of climate change, as well as surface temperatures, provide evidence of the impact of greenhouse gases on the climate.
Questions have also been raised about the future of the IPCC, with critics claiming major high-level assessments of climate change were no longer helpful for governments having to make political decisions about tackling global warming.
The IPCC was established by the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO) in 1988 to provide the world with a clear scientific view on climate change.
Press Association