UN climate report: Key findings

27 September 2013

A UN panel has released part one of its six-yearly update on the state of the Earth's climate. The much-anticipated report by the Intergovernmental Panel on Climate Change (IPCC) says that with 95% certainty, humans are the dominant cause of global warming since the 1950s. The BBC News website's science editor Paul Rincon breaks down some of the document's key findings.

"Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased."

Part one of the IPCC's fifth assessment report (AR5) on the Earth's climate opens with the message that we are seeing changes in the climate system unprecedented in records spanning hundreds of years.

With this scene-setting out of the way, the report says the period from 1983-2012 in the Northern Hemisphere was likely the warmest 30-year period of the last 1,400 years. Each of the last three decades has got successively warmer, and these decades have all been warmer than any of the preceding decades since 1850.

The combined average land and ocean surface data show a temperature rise of 0.85C over the period 1880-2012, the authors go on to say.

In addition, it is "virtually certain" that the upper 700m of the Earth's oceans have warmed during the period from 1971 and 2010. The deep ocean, below 3,000m in depth, "likely" warmed between 1992 and 2005, says the report, with the largest effect observed in the Southern Ocean.

The report says that the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink, and Arctic sea-ice as well as Northern Hemisphere spring snow cover have continued to fall in extent.

Sea level rise will proceed more quickly than it has done over the past 40 years. Global mean sea level rise for 2081−2100 is projected to be between 26cm (at the low end) and 82cm (at the high end), depending on the greenhouse emissions path this century.

"It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."

One of the key findings in the IPCC report is the attribution of more than half the increase in global surface temperatures from 1951-2010 to human activities, underlining the dominant role of fossil fuel burning as a driver for climate change.

The atmospheric concentration of the greenhouse gas carbon dioxide (CO2) has increased by 40% since pre-industrial times. And the mean rates of rise in concentrations of CO2, methane (CH4) and nitrous oxide (N20) in the last 100 years are "with very high confidence" unprecedented in the last 22,000 years, the authors say.

"The rate of warming over the past 15 years... which begins with a strong El Nino, is smaller than the rate calculated since 1951."

Addressing the pause, or slow-down, in global temperature rise since 1998 had been described as "central" to this report. While acknowledging this hiatus, the final analysis downplays it and points out that this period began with a very hot El Nino year. El Nino is a cyclical weather and climatic pattern that affects the Pacific Ocean, but which has knock-on impacts for conditions felt over the rest of the globe.

The report's authors ultimately conclude that 15 years is still not a long enough timescale to draw firm conclusions about the pause. Scientific studies on the slow-down have cited uptake of heat by the upper oceans as a possible cause, along with the properties of particulate matter in the atmosphere which can reflect solar energy back into space. But published research is still relatively sparse.

A favourite of climate "sceptics", the Medieval Warm Period, is also referenced in the report. In some places this period, from 950-1250 AD, was comparable in its warming with the late 20th Century, but the report says this did not occur as coherently across seasons and regions as the current phase of warming.

"Models reproduce observed continental-scale surface temperature patterns and trends over many decades, including the more rapid warming since the mid-20th century and the cooling immediately following large volcanic eruptions."

Climate sceptics have previously targeted the reliability of future projections based on computer simulations, or climate models. But those who work with such models will often cite the phrase: "All models are wrong, but some are useful". The IPCC says these simulations are indeed useful, faithfully reproducing long-term temperature trends, the rapid global warming in the second part of the 20th Century and the cooling observed following large volcanic eruptions.

But there remain inconsistencies between observed changes in the climate system and the conditions simulated by computers. An obvious one is the slowdown in warming since 1998. The report says this could be due to unpredictable variability in the climate and over-sensitive responses to greenhouse gases in some climate models.

The report adds that there has been improvement in the way these models simulate changes in continental rainfall patterns, but regional-scale rainfall is not well reproduced.

"Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system."

Global surface temperature change for the end of the 21st Century is "likely" to exceed 1.5C relative to the period 1850-1900 for all but one of the greenhouse gas emissions scenarios proposed in the IPCC's computer climate simulations.

The equilibrium climate sensitivity (ECS) - the response of the climate to a doubling of CO2 in the atmosphere - is likely to result in a change of between 1.5C and 4.5C. The lower bound of this range has fallen compared with the one the IPCC used in its last report, published in 2007. Six years ago, it found ECS to be 2.0C to 4.5C.

But it is "virtually certain" that most places will see more hot and fewer cold temperature extremes as temperatures rise. And it is very likely that heat waves will occur more frequently, though occasional winter extremes will continue to occur.

"Most aspects of climate change will persist for many centuries even if emissions of CO2 are stopped. This represents a substantial multi-century climate change commitment created by past, present and future emissions of CO2."

Limiting the global average temperature rise to 2C (above the pre-industrial average) is a commonly agreed means of avoiding "dangerous climate change". But a significant amount of the maximum quantity of CO2 that can be emitted while still keeping the mercury under 2C had already been released by 2011. However the data are sliced, the report says, the rationale for cutting greenhouse emissions is powerfully underlined.

Some 15%-40% of released CO2 will remain in the atmosphere longer than 1,000 years after those emissions have ended, raising the prospect that some fraction of climate change will be irreversible. It is "virtually certain" that sea level rise will continue beyond 2100 and sustained warming above some unknown threshold would lead the Greenland Ice Sheet to melt in a millennium or more, causing a global mean sea level rise of up to 7m.

The various methods and technologies proposed to counter climate change - a field known as geoengineering - have some potential, say the report's authors. But they also carry with them unintended side effects, and long-term consequences. Either way, limited scientific evaluation of these techniques prevented the panel from offering a more detailed assessment of them.

Paul.Rincon-INTERNET@bbc.co.uk and follow me on Twitter, external