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Tuesday, April 8, 2008

James Hansen et al.: Target Atmospheric CO2: Where Should Humanity Aim?

Target Atmospheric CO2: Where Should Humanity Aim?

by J. Hansen, M. Sato, P. Kharecha, D. Beerling, V. Masson-Delmotte, M. Pagani, M. Raymo, D. Royer, & J. C. Zachos

Paleoclimate data show that climate sensitivity is ~3°C for doubled CO2, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6°C for doubled CO2 for the range of climate states between glacial conditions and icefree Antarctica. Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, large scale glaciation occurring when CO2 fell to 425±75 ppm, a level that will be exceeded within decades, barring prompt policy changes. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm. The largest uncertainty in the target arises from possible changes of non-CO2 forcings. An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.

Human activities are altering Earth’s atmospheric composition. Concern about global warming due to long-lived human-made greenhouse gases (GHGs) led to the United Nations Framework Convention on Climate Change (1) with the objective of stabilizing GHGs in the atmosphere at a level preventing “dangerous anthropogenic interference with the climate system.”

The Intergovernmental Panel on Climate Change (IPCC, 2) and others (3) used several
“reasons for concern” to estimate that global warming of more than 2-3°C may be dangerous. The European Union adopted 2°C above pre-industrial global temperature as a goal to limit human-made warming (4). Hansen et al. (5) argued for a limit of 1°C global warming (relative to 2000, 1.7°C relative to pre-industrial time), aiming to avoid practically irreversible ice sheet and species loss. This 1°C limit, with nominal climate sensitivity of ¾°C per W/m2 and plausible control of other GHGs (6), implies maximum CO2 ~ 450 ppm (5).

Our current analysis suggests that humanity must aim for an even lower level of GHGs.
Paleoclimate data and ongoing global changes indicate that ‘slow’ climate feedback processes not included in most climate models, such as ice sheet disintegration, vegetation migration, and GHG release from soils, tundra or ocean sediments, may begin to come into play on time scales as short as centuries or less (7). Rapid on-going climate changes and realization that Earth is out of energy balance, implying that more warming is ‘in the pipeline’ (8), add urgency to investigation of the dangerous level of GHGs.

A probabilistic analysis (9) concluded that the long-term CO2 limit is in the range 300-500 ppm for 25 percent risk tolerance, depending on climate sensitivity and non-CO2 forcings. Stabilizing atmospheric CO2 and climate requires that net CO2 emissions approach zero (10), because of the long lifetime of CO2.

We use paleoclimate data to show that long-term climate has high sensitivity to climate forcings and that the present global mean CO2, 385 ppm, is already in the dangerous zone. Despite rapid current CO2 growth, ~2 ppm/year, we show that it is conceivable to lower CO2 this century to less than the current amount, but only via prompt policy changes.

Climate sensitivity
A global climate forcing, measured in W/m2 averaged over the planet, is an imposed perturbation of the planet’s energy balance. Increase of solar irradiance (So) by
2% and doubling of atmospheric CO2 are each forcings of about 4 W/m2 (11).

Charney (12) defined an idealized climate sensitivity problem, asking how much global
surface temperature would increase if atmospheric CO2 were instantly doubled, assuming that slowly-changing planetary surface conditions, such as ice sheets and forest cover, were fixed. Long-lived GHGs, except for the specified CO2 change, were also fixed, not responding to climate change. The Charney problem thus provides a measure of climate sensitivity including only the effect of ‘fast’ feedback processes, such as changes of water vapor, clouds and sea ice. Classification of climate change mechanisms into fast and slow feedbacks is useful, even though time scales of these changes may overlap. We include as fast feedbacks aerosol changes, e.g., of desert dust and marine dimethylsulfide, that occur in response to climate change (7). Charney (12) used climate models to estimate fast-feedback doubled CO2 sensitivity of 3 ± 1.5°C. Water vapor increase and sea ice decrease in response to global warming were both found to be strong positive feedbacks, amplifying the surface temperature response. Climate models in the current IPCC (2) assessment still agree with Charney’s estimate.

Climate models alone are unable to define climate sensitivity more precisely, because it is difficult to prove that models realistically incorporate all feedback processes. The Earth’s history, however, allows empirical inference of both fast feedback climate sensitivity and longterm sensitivity to specified GHG change including the slow ice sheet feedback.

Pleistocene Epoch
Atmospheric composition and surface properties in the late Pleistocene are known well
enough for accurate assessment of the fast-feedback (Charney) climate sensitivity. We first compare the pre-industrial Holocene with the last glacial maximum [LGM, 20 ky BP (before present)]. The planet was in energy balance in both periods within a small fraction of 1 W/m2, as shown by considering the contrary: an imbalance of 1 W/m2 maintained a few millennia would melt all ice on the planet or change ocean temperature an amount far outside measured variations (Table S1 of 8). The approximate equilibrium characterizing most of Earth’s history is unlike the current situation, in which GHGs are rising at a rate much faster than the coupled climate system can respond.

[The rest of this article may be found in pdf format at this link: ]

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