though this approach would enable comparisons with traditional radiative forcings, it would not convey fully the impacts of nonradiative forcings on societally relevant climate variables, such as precipitation or ecosystem functioning. Furthermore, quantifying nonradiative forcings in terms of their radiative effects is not straightforward. Another consideration in identifying potential metrics for nonradiative forcings is their significant regional variation; any new metrics will have to be able to characterize the regional structure in forcing and climate response. Further work is needed to quantify links between regional nonradiative forcing and climate response, whether the response occurs in the region, in a distant region through teleconnections, or globally.

RECOMMENDATIONS:

Improve understanding and parameterizations of aerosol-cloud thermodynamic interactions and land-atmosphere interactions in climate models in order to quantify the impacts of these nonradiative forcings on both regional and global scales.

Develop improved land-use and land-cover classifications at high resolution for the past and present, as well as scenarios for the future.

• Develop parameterizations of terrestrial and marine biogeochemistry to investigate the resulting nonradiative forcings.

• Identify suitable climate diagnostics, metrics, and monitoring procedures for specific nonradiative forcing processes and responses.

ADDRESSING KEY UNCERTAINTIES

Whereas the level of understanding associated with radiative forcing by well-mixed greenhouse gases is relatively high, there are major gaps in understanding for the other forcings, as well as for the links between forcings and climate response. Error bars remain large for current estimates of radiative forcing by ozone, and are even larger for estimates of radiative forcing by aerosols. Nonradiative forcings are even less well understood. The potential for large and abrupt climate change triggered by radiative and nonradiative forcings needs to be explored. The following recommendations identify critical research avenues for addressing these key uncertainties.

Reduce Uncertainties Associated with Indirect Aerosol Radiative Forcing

The interactions between aerosols and clouds can lead to a number of indirect radiative effects, which arguably represent the largest uncertainty in current radiative forcing assessments. In the so-called first indirect aerosol effect, the presence of aerosols leads to clouds with more, but smaller



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