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Climate forcings in the industrial era
1National Aeronautics and Space Administration Goddard Institute for Space Studies, New York, NY 10025, USA. jhansen@giss.nasa.gov
Summary
Future climate change projections are uncertain because climate forcings are poorly understood. Uncertainties in aerosol, cloud, and land-use forcings are now more critical than global climate sensitivity for long-term climate change predictions.
Area of Science:
- Climate Science
- Earth System Science
- Environmental Science
Background:
- Long-term climate change is driven by various forcings, with significant uncertainties in their precise measurement and impact.
- Anthropogenic greenhouse gases (GHGs) induce warming, but other poorly quantified anthropogenic forcings, like aerosols and land-use changes, exert a cooling effect, partially offsetting GHG-induced warming.
- The relative contribution of natural forcings, such as solar irradiance, may be underestimated due to uncertainties in anthropogenic forcings.
Purpose of the Study:
- To highlight the critical role of accurately quantifying climate forcings for reliable long-term climate change projections.
- To emphasize the shift in focus from global climate sensitivity to the uncertainties in climate forcings as the primary challenge in climate modeling.
- To re-evaluate the influence of natural forcings in the context of current understanding of anthropogenic forcings.
Main Methods:
- Analysis of current trends in greenhouse gas (GHG) climate forcings.
- Assessment of the accuracy and impact of poorly measured anthropogenic forcings (aerosols, clouds, land-use).
- Comparison of observed GHG forcing trends with standard emission scenarios (e.g., business as usual, 1% CO2 growth).
Main Results:
- Greenhouse gas forcing trends are currently lower than projected in many standard emission scenarios.
- Uncertainties in the magnitude and direction of anthropogenic aerosol, cloud, and land-use forcings are substantial.
- The partial offsetting effect of these negative forcings suggests solar irradiance changes might play a more significant role than previously inferred.
Conclusions:
- Uncertainties in climate forcings, not global climate sensitivity, are now the dominant factor limiting the accuracy of long-term climate change projections.
- A paradigm shift is needed in climate research, prioritizing the reduction of uncertainties in climate forcings.
- Accurate measurement and modeling of all climate forcings are essential for defining future climate change.
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