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James Hansen on Safe Climate Levels
Zero Emission Network Newsletter - April 2008 - by Adrian Whitehead

James Hansen (Jim) is the director of the NASA Goddard Institute for Space Studies and Adjunct Professor at the Columbia University Earth Institute.

Jim is one of the few scientists who speaks strongly about climate change and has released papers that shows the situation to be much worse than is suggested by the IPCC reports. For example, he recently showed that sea level rise could be as much as 4-5m by the end of the century rather than below 1m maximum as proposed by the IPCC. His most recent work has been released as a draft paper at Columbia University and is titled “Target Atmospheric CO2: Where Should Humanity Aim?” (Hansen 2008a). This paper has numerous critical findings including that we already have an additional 2 degrees temperature warming built into the system, not 0.6 degrees as commonly reported, and that 450 ppm CO2 is enough to trigger a 60-80m sea level rise.

However the most important information to come out of his work is the level at which of greenhouse gases in the atmosphere are “safe”. This information is summarised in a table taken from a presentation Hansen gave in January this year at the Royal College of Physicians, London Hansen 2008b).

table showing five key climate critical systems; arctic sea ice; ice sheets/sea level, shifting climate zones, alpine water supplies, ocean acidification, passing their tipping points between 300 and 350 ppm CO2 or 300-325 ppm CO2 in the case of arctic ice
This table shows that several critical systems pass their tipping points somewhere between 300 and 350 ppm CO2 and in the case of North Pole Summer Ice, between 300 and 325 ppm CO2.

Hansen makes reference to the retreating summer Arctic ice in his recent draft paper:

“Stabilization of Arctic sea ice cover requires, to first approximation, restoration of planetary energy balance. Climate models driven by known forcings yield a present planetary energy imbalance of +0.5-1 W/m2 (5), a result supported by observed increasing ocean heat content (48). CO2 amount must be reduced to 325-355 ppm to increase outgoing flux 0.5-1 W/m2, if other forcings are unchanged. A further reduced flux, by ~0.5 W/m2, and thus CO2 ~300-325 ppm, may be needed to restore sea ice to its area of 25 years ago.” (Hansen 2008a)

As we must have North Pole Summer Ice to have a “safe” climate system due to its ability to reflect large amounts of Sunlight, it means that “safe” levels of CO2 in the atmosphere lie somewhere between 300 and 325 ppm CO2. Unfortunately we are not sure exactly where that is, so the “safe” target / goal for CO2 concentrations is 300 ppm or below.

“Safe” Climate = 300 ppm CO2 or below

Now knowing this, you can see how ridiculous all the proposals to stabilise our Greenhouse gases at levels higher than today are.

James Hansen suggests that we have only decades to return to a “safe” level of greenhouse gases or we will run the risk of passing points of “no return” for a number of critical global warming positive feedback mechanisms.

James has proposed an interim target of 350 ppm CO2 and the closure of the coal industry, the latter of which he mentions in a recent letter to Kevin Rudd.

“Prime Minister Rudd, we cannot avert our eyes from the basic fossil fuel facts, or the consequences for life on our planet of ignoring these fossil fuel facts. If we continue to build coal-fired power plants without carbon capture, we will lock in future climate disasters associated with passing climate tipping points. We must solve the coal problem now.”
James Hansen letter to Kevin Rudd 27 March 2008

Unfortunately people have already begun to misinterpret Hansen's call for an interim target and are confusing this with a “safe” target. Please don't do the same, as 350 ppm CO2 is as much of a death sentence for the planet as 400, 450, 550, 650 etc.


Hansen, J 2008a. draft paper "Target Atmospheric CO2: Where Should Humanity Aim?", Columbia University, viewed 15 September 2008 http://www.columbia.edu/~jeh1/2008/TargetCO2_20080407.pdf

1. http://www.columbia.edu/~jeh1/2008/TargetCO2_20080407.pdf or arxiv.org/abs/0804.1126

Hansen, J 2008b Global Warming: The Perfect Storm. [slides] 29 Jan 2008 the Royal College of Physicians, London.

2. http://www.columbia.edu/~jeh1/RoyalCollPhyscns_Jan08.pdf

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300 ppm CO2 or below is the stabilisation target for a safe climate.

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