Blame for global warming placed firmly on humankind

The 2nd of February 2007 will one day hopefully be remembered as the day the question mark was removed from the debate on whether human activities are driving climate change, said the head of the UN Environment Programme at the launch of the most authoritative scientific report on climate change to date.

The new Intergovernmental Panel on Climate Change report says there is 90% certainty that the burning of fossil fuels and other human activities are driving climate change.

“The word unequivocal is the key message of this report,” said Achim Steiner, executive director of UNEP, adding that those who have doubts about the role of humans in driving the climate “can no longer ignore the evidence”.

The IPCC report says the rise in global temperatures could be as high as 6.4°C by 2100. The report also predicts sea level rises and increases in hurricanes. It is the work of 1200 climate experts from 40 countries, who have spent six years reviewing all the available climate research. It was released in Paris, France, on Friday (read the 21-page summary here, though the IPCC server may be struggling due to high demand). Listen to audio from today’s press conference.

The last IPCC report, issued in 2001, predicted that temperatures would rise by 1.4°C to 5.8°C by 2100, relative to 1990 temperatures.

But the new report says temperature rises by 2100 could, in the most extreme scenarios, range from 1.1°C and 6.4°C. The most likely range is 1.8°C to 4.0°C (see figure 1, right), with the report predicting that 4°C is most likely if the world continues to burn fossil-fuels at the same rate (read the The impacts of rising global temperatures).

Melting, moving ice

Rises in sea levels are predicted by the new report, threatening low-lying areas of land around the world. As the oceans warm, their waters expand, while rising temperatures also increase the melting of the ice sheets that cover Greenland and Antarctica.

In 2001, the IPCC predicted that sea levels would rise by between 9 and 88 centimetres by 2100, relative to 1990 levels. The new report says rises could range from 18 cm to 59 cm. The top end of the range corresponds to a fossil-fuel intensive future (see A1F1 scenario in Modelling the future climate: the baseline scenarios).

But predictions of sea level rise are one of the most contentious areas of the report – very recent research has suggested that rises of up to 140 cm are possible (see Shorelines may be in greater peril than thought.

The problem is that the understanding of how warming affects Greenland and Antarctic ice sheets remains limited, and they are predicted to be the most important contributors to change. Estimates of the straightforward melting of ice are incorporated in the IPCC report. But warming may also accelerate the movement of ice in glaciers into the ocean, perhaps by meltwater lubricating the undersides of ice streams.

Susan Solomon, one of the report’s lead authors, said there was no published research that quantified this effect, and so it was not included. But she added: “If temperatures exceed 1.9 to 4.6°C above pre-industrial temperatures, and were to be sustained for thousands of years, eventually we would expect the Greenland ice sheet to melt. That would raise sea level by 7 metres.”

Climate change is also expected to affect the frequency and strength of tropical storms and hurricanes. The latest IPCC report says the activity of tropical cyclones is “likely” to increase over the 21st century. It says “likely” indicates a probability of more than 66%. This is a bolder statement than the World Meteorology Organisation issued in January.

Precipitation patterns will change too by 2100, according to IPCC predictions (see figure 2, right). Mid- to high-latitude regions will see up to 20% more rain and snow, while the tropical regions will see less.

Humans to blame

Considering the human role in causing climate change, the IPCC report is damning: “The understanding of [human] influences on climate has improved since the [2001] report, leading to a very high confidence that human activities” are responsible for most of the warming seen since 1950, says the report’s summary for policymakers. “Very high confidence” is described as “at least a 9 out of 10 chance of being correct”.

Before the industrial revolution, human greenhouse gas emissions were small, and the atmospheric concentration of carbon dioxide – the main greenhouse gas – was about 280 parts per million (ppm).

Thanks largely to the burning of fossil fuels and changes in land use, such as agricultural exploitation and deforestation, the atmospheric concentration of carbon dioxide reached 379 ppm in 2005, says the IPCC.

Gold standard

The IPCC draws together the world’s leading climate experts to review and assess all available research, under the auspices of UN Environment Programme and the World Meteorology Organization.

The result of their assessment, which is done every five to six years, establishes what is considered the gold standard of consensus on climate change science.

The latest IPCC report was written by hundreds of experts and reviewed by hundreds more, from 113 countries. It is being released in stages during 2007. The first chapter, released on Friday, deals with the scientific basis for climate change.

The next two parts of the IPCC’s 2007 assessment, plus a synthesis, will be released throughout the year. Part 2, dealing with the impacts of climate change and our vulnerability to those impacts, will be released in April. Part 3, to be released in May, deals with how we might mitigate these impacts.

Check back for further updates and reaction from Paris during the day.

Climate Change – Want to know more about global warming – the science, impacts and political debate? Visit our continually updated special report.

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