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Atmospheric Sciences & Global Change
Research Highlights

May 2004

The Consequences of Uncertainty: Addressing Climate Sensitivity and Climate Stabilization

Current estimates of climate sensitivity - the equilibrium global mean surface temperature rise that would arise from a doubling of pre-industrial CO2 concentrations in the atmosphere - range from 1.5°C to 4.5°C, with a median estimate of 2.5°C. Pacific Northwest National Laboratory researchers at the Joint Global Change Research Institute at the University of Maryland have examined the implications of this uncertainty for climate policy.

In conducting this analysis, researchers assumed an increase in global mean surface temperature of 2.5°C above what the pre-industrial value might represent as a possible climate stabilization target. Three global CO2 emission pathways, consistent with a "minimum cost" approach to meeting this target, were evaluated. These pathways were compared with an estimate of future global emissions assuming no climate policies are adopted. The pathways are the reference case (B2 Reference in red), 1) the case for a climate sensitivity of 2.5°C (in blue), 2) the case for a sensitivity of 1.5°C (in brown), and 3) the case for a sensitivity of 4.5°C (in orange).

The differences are astounding. If climate sensitivity is as high as 4.5°C, there is no realistic way of meeting the target as global CO2 emissions would have to go to zero within 15 years. If climate sensitivity is as low as 1.5°C, emissions would not have to deviate from the reference case until the next century.

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