Chapter IV. Way Forward 123 Authors:. D. Rind, NASA GISS; H. Yu, NASA GSFC/UMBC; S. E. Schwartz, DOE BNL; R. N. Halthore, NASA HQ/NRL. T ABLE OF CONTENTS 4.1 Introduction................................................................................123 4.2 Requirements for future research – observations ..........................124 4.2.1. In-situ measurements of aerosol properties and processes ..124 4.2.2. Laboratory studies of aerosol evolution and properties ......125 4.2.3. Surface- and satellite-based remote sensing ........................125 4.3 Requirements for future research - modeling ...............................129 4.3.1. Required modeling improvements ....................................129 4.3.2. Aerosol-climate modeling: the way forward ......................131 4.4 Concluding remarks ....................................................................132 References .........................................................................................133 4.1 Introduction e previous chapters have emphasized that while we have made progress in understanding aerosol forcing of the climate system, there are still many uncertainties. To put the work in perspective, seri- ous investigation of this issue has only been occurring for about the last 20 years. Given all the com- plexities, such as the varying aerosol types and emissions, uncertain refractive indices, great hetero- geneity, and the added issue of interactions with clouds, it is not hard to believe that work of at least that many more years will be necessary before we can define aerosol forcing with a sufficient degree of confidence. And without improved understanding of how much aerosols have offset the better known greenhouse gas forcing for the last 150 years, we cannot use the past temperature record to determine the climate sensitivity over that time, or indeed the likely magnitude of climate response to future greenhouse gas (and aerosol) increases. As discussed in Chapter 2, improved observations are already helping us to obtain an empirical estimate of the current direct effect of aerosols on climate, independent of models. Continued and even better observations are needed to refine this estimate. To be able to estimate climate forcing due to the indirect effect of aerosols, via their impact on cloud reflectivity and lifetime, will require much more extensive and coordinated campaigns. While choice of a target accuracy requirement for aerosol forcing is somewhat subjective (Schwartz, 2004), a possible target might be ± 0.3 W m -2 , comparable to the uncertainty associated with forcing by tropospheric ozone. Chapter IV. Way Forward