nature-outlook for earth

The climatechange threat has not lost a jot of its urgency. Withoutdrastic emission reductions or controversial technical climatefixes, global warming is more than likely to continue throughoutthe twenty-first century and might severely alter our planet’snatural environments and the living conditions of billions ofpeople, the United Nation’s Intergovernmental Panel on ClimateChange (IPCC) warns in its latest report today.

Even if carbon-dioxide emissions were to cease overnight, thehalf a triillion tonnes of carbon that have been pumped into theatmosphere since major industrialization began in around 1850 willaffect Earth’s biosphere, glaciers and oceans for centuries tocome, the group says.

A summary for policymakers of the IPCC's latest report, on thephysical basis of climate change, was released in Stockholm afterfour days of marathon negotiations between lead authors andgovernment representatives from 195 countries, each of whom has toagree every line and figure in the final 36-page report.

In 19 headline messages, the summary lays out that observedchanges since 1950 unequivocally point to climate change that is"unprecedented over decades to millennia". Relative to the1986–2005 period, the global mean surface temperature is projectedto further increase by between 0.3 and 4.8 °C by the end of thecentury, depending on future economic and technologicaldevelopment.

As glacier melt in Greenland and in parts of Antarctica isaccelerating, sea-level rise — in the range of 26 to 82 centimetresby 2100 according to the latest IPCC projections — will increasethe risk of flooding at many coastlines. The report also warns ofincreasing ocean acidification — a stark threat to marinebiodiversity — and disruptions of the global water cycle and localfresh water availability owing to changing precipitationpatterns.

"Climate change is the greatest challenge of our time," saysThomas Stocker, a climate scientist at the University of Bern inSwitzerland who co-chaired the assessment. "I am proud that we havebeen able to convince politicians that what we have come up with isa robust assessment of climate change. The scientific essence ofprevious drafts has not changed and the main messages have all beenkept."

Government officials and scientists spent four long daysreviewing and editing the document, line by line. The marathonprocess extended through the night on Thursday and concluded at5:20 am today. Gerald Meehl, a senior scientist at the NationalCenter for Atmosopheric Research in Boulder, Colorado, saysgovernment delegates regularly sought to clarify the science butvery rarely questioned the underlying messages or how scientistsarrived at their conclusions. "It’s an exercise in language," Meehlsays, "and we’re here to make sure that the science doesn't getchanged."

The IPCC's assessment draws on real-world observations,such as of glacier and sea ice retreat, climate models based onfour different greenhouse gas emission scenarios and a sweepingreview of the scientific literature.

Overall, the report cites more than 9,200 scientific papers,two-thirds of which have been published since 2007. There is now anoverwhelming body of evidence, says Stocker, that the 1 °C or so ofglobal warming since the mid-nineteenth century is the result ofhuman activity.

To give the world a 66% chance of limiting global warming to 2°C, future emissions must be kept below 500 gigatonnes of carbon —a fraction of known untapped oil and gas resources — the reportconcludes.

"The IPCC has provided an even more robust scientific basis foraction on climate change," says Johan Rockstroem, the director ofthe Stockholm Resilience Centre. "I am glad that the scientificcommunity is standing very firm on the true scale of the threatwe're facing."

A slowdown in the rise of global average temperatures in recentyears suggests that global warming is proceeding moreintermittently, and less predictably, than it does in some climatemodels. But the 'hiatus' since the record hot year of 1998 —probably due to increased heat uptake by the oceans — is no sign that globalwarming has stopped, as some would like to hope.

"Comparing short-term observations with long-term modelprojections is inappropriate," says Stocker. "We know that there isa lot of natural fluctuation in the climate system. A 15-yearhiatus is not so unusual even though the jury is out as to whatexactly may have caused the pause."

Claims that there might be something fundamentally wrong withclimate models are unjustified unless "temperature were to remainconstant for the next 20 years", he says.

"Surface temperature warming is not the only signature ofclimate change," says Brian Hoskins, director of the GranthamInstitute for Climate Change at Imperial College London in theUnited Kingdom. "We're confronted with an interplay betweendifferent parts of a larger system which is clearly changing. Itspeaks well for the IPCC that it has tackled without fear thecomplexities involved in fulfilling its task."

A final draft of the full report, the fifth since 1990, will bereleased next week. The IPCC's reports on adaptation and mitigationof climate change, as well as a synthesis report, will follow nextyear. Its last full assessment led to the IPCC winning the 2007Nobel Peace Prize, together with former US vice-president AlGore.

   Nature

DOI:

doi:10.1038/nature.2013.13832

·        Jeff Tollefson contributed reporting.