The really short version of the climate change story hasn’t changed much for the last ten years or more, of course: we know we are cooking the earth and things are going to get very uncomfortable, if not catstrophic, unless we stop pumping CO2 into the atmosphere. (NASA’s version is one of the simplest, clearest and most authoritative if you need to direct anyone to such a resource.) However, the scientific work continues to improve our knowledge every year. Two significant recent publications are discussed in not-too-technical terms in two highly regarded blogs, RealClimate and ThinkProgress.
Should we trust the bloggers? These bloggers, yes. Stefan Rahmstorf, who discusses Marcott’s paper in Science, teaches physics of the oceans as a professor at Potsdam University, is a member of the Advisory Council on Global Change of the German government and is a lead author of the paleoclimate chapter of the 4th assessment report of the IPCC. Joe Romm, who discusses Hansen’s paper, worked at the highest levels of the US Department of Energy in the 1990s before moving into full-time environmental campaigning. (For more about them, see Wikipedia: Rahmstorf, Romm.) From here on, this summary uses their words, drastically condensed; follow the links at the end for the full versions.
Marcott study: The End of the Holocene
by Stefan Rahmstorf
Recently a group of researchers from Harvard and Oregon State University has published the first global temperature reconstruction for the last 11,000 years – that’s the whole Holocene (Marcott et al. 2013). The results are striking.
Over the last decades, numerous researchers have painstakingly collected, analyzed, dated, and calibrated many data series that allow us to reconstruct climate before the age of direct measurements. Such data come e.g. from sediment drilling in the deep sea, from corals, ice cores and other sources. Shaun Marcott and colleagues for the first time assembled 73 such data sets from around the world into a global temperature reconstruction for the Holocene [i.e. the 11,700 years since the last Ice Age].
Global temperature reconstruction from proxy data by Marcott et al, Science 2013
The climate curve looks like a “hump”. At the beginning of the Holocene global temperature increased, and subsequently it decreased again by 0.7 ° C over the past 5000 years. The well-known transition from the relatively warm Medieval into the “little ice age” turns out to be part of a much longer-term cooling, which ended abruptly with the rapid warming of the 20th Century. Within a hundred years, the cooling of the previous 5000 years was undone. (One result of this is, for example, that the famous iceman ‘Ötzi’, who disappeared under ice 5000 years ago, reappeared in 1991.)
The curve of Marcott et al. will not be the last word on the global temperature history during the Holocene; like Mann et al. in 1998 [the famous “Hockey Stick” which was a similar temperature record for the last 1,000 years] it is the opening of the scientific discussion. There will certainly be some corrections and improvements. However, I believe that (as was the case with Mann et al.) the basic shape will prove correct: a relatively smooth curve with slow cooling trend lasting millennia from the Holocene optimum to the “little ice age”, mainly driven by the orbital cycles. At the end this cooling trend is abruptly reversed by the modern anthropogenic warming. …
Just looking at the known drivers [factors affecting climate change] and the actual temperature history shows it directly, without need for a climate model: without the increase in greenhouse gases caused by humans, the slow cooling trend would have continued. Thus virtually the entire warming of the 20th Century is due to man. This May, for the first time in at least a million years, the concentration of carbon dioxide in our atmosphere has exceeded the threshold of 400 ppm. If we do not stop this trend very soon, we will not recognize our Earth by the end of this century.
Hansen study: Climate Sensitivity is High, Burning All Fossil Fuels Would Make Most of Planet ‘Uninhabitable’
by Joe Romm, Sept 17, 2013
James Hansen, the country’s most prescient climatologist, is out with another must-read paper, “Climate sensitivity, sea level and atmospheric carbon dioxide,” … co-authored by a number of Hansen’s former colleagues at NASA. … The key findings are:
• The Earth’s actual sensitivity to a doubling of CO2 levels from preindustrial levels (to 550 ppm) — including slow feedbacks — is likely to be larger than 3–4°C (5.4-7.2°F).
• Given that we are headed towards a tripling (820 ppm) or quadrupling (1100 ppm) of atmospheric CO2 levels, inaction is untenable.
• “Burning all fossil fuels” would warm land areas on average about 20°C (36°F) and warm the poles a stunning 30°C (54°F). This “would make most of the planet uninhabitable by humans, thus calling into question strategies that emphasize adaptation to climate change.”
Burning all or even most fossil fuels would be a true scorched Earth policy.
Given that James Hansen has been right about global warming for more than 3 decades, his climate warnings need to be taken seriously. [The] whole paper is worth reading. The authors conclude:
Most of the remaining fossil fuel carbon is in coal and unconventional oil and gas. Thus, it seems, humanity stands at a fork in the road. As conventional oil and gas are depleted, will we move to carbon-free energy and efficiency—or to unconventional fossil fuels and coal?
If fossil fuels were made to pay their costs to society, costs of pollution and climate change, carbon-free alternatives might supplant fossil fuels over a period of decades. However, if governments force the public to bear the external costs and even subsidize fossil fuels, carbon emissions are likely to continue to grow, with deleterious consequences for young people and future generations.
It seems implausible that humanity will not alter its energy course as consequences of burning all fossil fuels become clearer. Yet strong evidence about the dangers of human-made climate change have so far had little effect. Whether governments continue to be so foolhardy as to allow or encourage development of all fossil fuels may determine the fate of humanity.
That’s the end of my quote from Romm’s blog, and now you know the considered opinion of the experts. But we will be all right here in Oz, of course:
Realclimate post on Marcott: http://www.realclimate.org/index.php/archives/2013/09/paleoclimate-the-end-of-the-holocene/
Marcott’s paper on Science: http://www.sciencemag.org/content/339/6124/1198.abstract
Science 8 March 2013
Shaun A. Marcott1, Jeremy D. Shakun, Peter U. Clark, Alan C. Mix. A Reconstruction of Regional and Global Temperature for the Past 11,300 Years
ThinkProgress post on Hansen: http://thinkprogress.org/climate/2013/09/17/1892241/hansen-climate-sensitivity-uninhabitable/