Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties

Climate Forcing Metric	Strengths	Limitations
Regional radiative forcing (direct and indirect)	May provide a better measure of regional climate response than global radiative forcing Allows characterization of teleconnected response to a regionally isolated forcing	Further work is needed to quantify links of regional radiative forcing to regional and global climate response
Regional nonradiative forcing (hydrological, land use, biogeochemical)	Recognizes additional nonradiative climate forcing components Allows characterization of teleconnected response to a regionally isolated forcing Nonradiative forcing alters radiative forcing and thus provides a more complete characterization of radi	No widely accepted metrics for quantifying regional nonradiative forcing Further work is needed to quantify links of regional nonradiative forcing to regional and global climate response Some types of nonradiative forcing are not easily quantified in watts per square meter, thus it is not clear how to compare them to radiative forcing
Ocean heat content	Can be used to calculate the net radiative imbalance of the Earth Offers a valuable constraint on the performance of climate models	Observations may have insufficient frequency and spatial coverage to accurately determine the radiative imbalance at the necessary resolution
NOTE: TAR = IPCC Third Assessment Report (IPCC, 2001).

wide range of changes in well-mixed greenhouse gases, solar irradiance, surface albedo, and nonabsorbing aerosols (IPCC, 2001). By assuming a constant climate sensitivity parameter, forcing can be translated directly into a temperature response. Because calculating radiative forcing is straightforward, many factors that may influence radiative forcing can be investigated in climate models, simpler versions of those models, and chemical transport models. For example, the effects of different estimates of past forcings can be compared to each other. Likewise, comparisons can be made of multiple possible future forcing scenarios. Furthermore, the radia-

Table of Contents

Front Matter R1-R14

Executive Summary 1-10

1 Introduction 11-27

2 State of Scientific Understanding 28-62

3 Radiative Forcing Over Earths History 63-82

4 Rethinking the Global Radiative Forcing Concept 83-99

5 Uncertainties Associated with Future Climate Forcings 100-116

6 Research Approaches to Furthering Understanding 117-143

7 Recommendations 144-158

References 159-190

Appendix A: Biographical Sketches of Committee Members and Staff 191-196

Appendix B: Statement of Task 197-197

Appendix C: Glossary and Acronyms 198-208