2ºC rise will be a disaster, say leading scientists
Dec. 3, 2013
Countries 'round the world have pledged to limit the average global temperature rise to 2ºC above pre-industrial figures. That's way too high and risks major dislocations for civilization, say a group of prominent scientists.
By Tim Radford
Climate News Network
LONDON – Governments have set the wrong target to limit climate change. The goal at present – to limit global warming to a maximum of 2 degrees Celsius higher than the average for most of human history – "would have consequences that can be described as disastrous," say 18 scientists in an analysis published in the journal PLOS One.
With a 2°C increase, "sea level rise of several meters could be expected," they say. "Increased climate extremes, already apparent at 0.8°C warming, would be more severe. Coral reefs and associated species, already stressed with current conditions, would be decimated by increased acidification, temperature and sea level rise.
The paper's lead author is James Hansen, now at Columbia University, New York, and the former NASA scientist who in 1988 put global warming on the world's front pages by telling a US Congress committee "it's time to stop waffling so much and say the evidence is pretty strong that the greenhouse effect is here."
Hansen's fellow authors include the economist Jeffrey Sachs of Columbia University and the biologist Camille Parmesan, of the University of Plymouth in the UK and the University of Texas at Austin in the United States.
Their argument is that humanity and nature – "the modern world as we know it" – is adapted to what scientists call the Holocene climate that has existed for more than 10,000 years – since the end of the Ice Age, the beginnings of agriculture and the first settlement of the cities.
Warming of 1°C relative to 1880–1920 keeps global temperature close to the Holocene range, but warming of 2°C, could cause "major dislocations for civilization."
The scientists' study, "Assessing 'dangerous climate change': Required reduction of carbon emissions to protect young people, future generations and nature," differs from many similar climate analyses because it sets out its argument with remarkable directness and clarity, and it serves as a useful briefing document for anyone – politicians, journalists and lay audiences – anxious to better understand the machinery of climate and forces dictating climate change.
Critics will point out that it is also remarkably short on the usual circumlocutions, caveats, disclaimers and equivocations that tend to characterize most scientific papers. Hansen and his co-authors are quite open about the major areas of uncertainty. Their implicit argument: If the worst outcomes turn out to be true, the consequences for humankind could be catastrophic.
The scientists' case is that most political debate addresses the questions of reducing greenhouse gas emissions. But it does not and perhaps cannot factor in the all potentially dangerous unknowns – the slow feedbacks that will follow the thawing of the Arctic, the release of frozen reserves of methane and carbon dioxide in the permafrost, and the melting of polar ice into the oceans. They point out that 170 nations have agreed on the need to limit fossil fuel emissions to avoid dangerous human-made climate change.
"The stark reality is that global emissions have accelerated, and new efforts are underway to massively expand fossil fuel extractions by drilling to increasing ocean depths and into the Arctic, squeezing oil from tar sands and tar shale, hydro-fracking to expand extraction of natural gas, developing exploitation of methane hydrates and mining of coal via mountain-top removal and mechanized long wall-mining."
The scientists argue that swift and drastic action to limit global greenhouse gas emissions and contain warming to around 1°C would have two useful consequences. One is that it would not be far from the climate variations experienced as normal during the last 10,000 years, and secondly that it would make it more likely that the biosphere, and the soil, would be able to sequester a substantial proportion of the carbon dioxide released by human industrial civilization.
Trees are, in essence, captive carbon dioxide. But the warmer the world becomes, the more likely it is that existing forests – the Amazon, for example – will start to release more carbon dioxide than they absorb, making the planet progressively even warmer.
Therefore the scientists make a case for limiting overall global carbon emissions to 500 gigatonnes rather than the 1,000 billion tonnes in the 2°C rise scenario.
"Although there is merit in simply chronicling what is happening, there is still opportunity for humanity to exercise free will," says Hansen.
Tim Radford is an editor at Climate News Network, a journalism news service delivering news and commentary about climate change for free to media outlets worldwide.
The Daily Climate is an independent, foundation-funded news service that covers climate change. Find us on Twitter @TheDailyClimate or email editor Douglas Fischer at dfischer [at] DailyClimate.org
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