Global warming is caused by humans, says UN body

The UN’s Intergovernmental Panel on Climate Change (IPCC) is more than 95 per cent certain that human influence has been the “dominant cause” of the “unequivocal” warming of Earth’s climate since 1950, now evident in most parts of the world.

The first volume of the IPCC's Fifth Assessment, dealing with the physical science basis of climate change, says many of the observed changes "are unprecedented over decades or millennia", with each of the last three decades progressively warmer than any since 1850.

Released yesterday in Stockholm, the report found both the atmosphere and oceans have warmed, the amount of snow and ice has diminished and global mean sea levels have risen, with higher concentrations of carbon dioxide (CO2) and other greenhouse gases.

Heatwaves

Projections of how the climate could change in the 21st century are based on a new set of four possible scenarios of future greenhouse gas concentrations, in a 2,500-page report.

“Global surface temperature change for the end of the 21st century is likely to exceed 1.5 degrees Celsius relative to 1850-1900 in all but the lowest scenario considered, and likely to exceed 2 degrees for the two high scenarios,” said the working group co-chair Dr Thomas Stocker.

“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,” he told a press briefing in Stockholm.

“Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions,” Dr Stocker said. But because of cumulative concentrations in the atmosphere, he warned the effects “will persist for many centuries even if emissions of CO2 stop”.

Qin Dahe, Chinese co-chair of the working group, warned that global mean sea levels would continue to rise "at a faster rate than we have experienced over the past 40 years" as a result of the warming ocean and the melting of glaciers and ice-sheets.

Sea level increase

Dr Stocker noted the oceans were absorbing 93 per cent of the energy released by global warming; otherwise, its effects on land would be “much more powerful” than we had already seen.

As there was a “linear relationship between warming and cumulative carbon emissions”, he warned sea levels could increase by up to 30cm by 2050 and up to 63cm by 2100, depending on the choices made. “In other words, it threatens the planet, our only home,” he said.

Because of a "large amount of natural variability", Dr Stocker stressed climate trends "should not be calculated for periods of less than 30 years" – a rejoinder to climate change sceptics who maintain that there has been no increase in surface temperatures since 1998.

Scenarios for future

Dr Stocker said it was “inappropriate to compare a short-term period of observations with climate model performance”.

Scenarios for the future “depend crucially” on greenhouse gas emissions – now at record levels, Dr Stocker said. “If we are to to stay below 2 degrees Celsius, we cannot emit more than 1,000 billion tonnes of carbon, of which already 54 per cent has been emitted.”

Where the world would be in 10 to 15 years “depends on the decisions we make today”, he added.