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IPCC climate report: the digested read

 
IPCC Chairman Rajendra Pachauri

The chairman of the IPCC, Rajendra Pachauri, during the presentation of the fifth assessment report in Stockholm. Photograph: Jonathan Nackstrand/AFP/Getty Images

 

Powered by Guardian.co.ukThis article titled “IPCC climate report: the digested read” was written by Damian Carrington and John Vidal, for theguardian.com on Friday 27th September 2013 16.29 UTC

Global change

The global climate has already changed in many ways that are unprecedented in the past hundreds or thousands of years, the world’s scientists and governments concluded in the new report from the Intergovernmental Panel on Climate Change. These changes have affected every region of the globe, on land and at sea. Continued carbon emissions will drive further heatwaves, sea level rise, melting ice and extreme weather. The changes will last for centuries and limiting the effects would require “substantial and sustained” cuts in carbon dioxide, the scientists report.

Temperatures

Scientists are now at least 66% certain that the last three decades are the warmest in 1400 years, with global temperature having risen by 0.9C in the last century. However, more than 90% of the heat trapped by greenhouse gases is being stored in the oceans.

By mid-century, scientists predict a further rise of 1.4-2.6C if carbon emissions continue to rise as they are today. If emissions were halted almost immediately and significant carbon was extracted from the atmosphere, the rise by mid-century would be 0.4-1.6C.

The scientists predict the average temperature between 2080 and 2100 will be 2.6-4.8C higher than today if emissions are unchecked. They are 90% certain that heatwaves will be more frequent and longer.

In the oceans, the strongest surface warming is expected in tropical and sub-tropical regions, up to 2C by 2100 and posing a grave threat to coral reefs which sustain much sealife. Scientists conclude that a collapse of the Gulf Stream that warms western Europe, as dramatised in the film The Day After Tomorrow, is very unlikely this century but cannot be ruled out afterwards.

Oceans

Global sea level has already risen by 20cm in the last century and scientists are now 90% certain that the rate of the rise will increase. The tide line is rising as warming glaciers and ice sheets pour hundreds of billions of tonnes of water into the oceans each year, but an equally big factor is the warming – and therefore expansion – of the seawater itself.

The new projections for the average sea level in the period 2080-2100 are greater than in the 2007 report, ranging from 45-82cm higher than now if nothing is done to curb emissions to 26-55cm if carbon emissions are halted and reversed. In the former case, sea level could have risen by a 98cm by the end of the century, seriously threatening cities from Shanghai to New York and meaning hurricanes and cyclones inflict far worse damage when they hit shorelines.

Sea level projections have been controversial because exactly how fast glaciers and ice sheets will slip into the sea is not well known. A collapse in ice sheets is therefore not included in the estimates and could add tens of centimetres more to the rise. Because the big Greenland and Antarctic ice sheets are slow to melt, scientists predict melting and sea level rise will continue for centuries. If a temperature rise of between 1C and 4C is sustained, the vast Greenland ice sheet will completely melt adding 7m to sea level, scientists predict, but over the course of a millennium.

The acidity of the ocean is also increasing, due the large amounts of carbon dioxide it is absorbing, and this will continue. This will harm shell-forming sealife but scientists are still determining to what extent.

Ice

The impact of warming is crystal clear in the faster rates of melting in virtually all the world’s glaciers and the huge ice sheets in Greenland and Antarctica. The ice sheets have been shedding at least five times more water in the 2000s than in the 1990s, the scientists report. Northern hemisphere snow cover has fallen by 11% a decade since 1967 and the temperature of the seasonally frozen ground, or permafrost, has increased by 2-3C in Russia and Alaska.

Arctic sea ice has been melting by 9-14% a decade since 1979, while sea ice around Antarctica has been increasing by 1-2%, probably due to current changes.

 

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