Climate Forcing by Aerosols--a Hazy Picture

he global average surface temperature has risen by 0.6 K since the late 19th century. Ocean heat content has increased, and other climate indices also point to a warming world. Many studies have attributed this warming largely to top-of-atmosphere radiative forcing—a change in planetary heat balance between incoming solar radiation and outgoing infrared radiation—by anthropogenic greenhouse gases (GHGs) (1, 2). Such attribution studies compare temperature observations to climate model simulations forced by various industrial-era agents. Among these agents, GHGs have well-constrained positive forcings (creating a warming influence) (3). In contrast, the mostly negative forcings (cooling) as-sociated with anthropogenic aerosols are highly uncertain (3, 4). Different forcings have different spatiotemporal patterns; however, model studies indicate that climate sensitivity (the ratio of global mean equilibrium temperature response to global mean forcing) is approximately equal for almost all of the major forcing agents (3). Thus, total forcing (the global mean sum of all industrial-era forcings) is a widely used diagnostic parameter. Here we argue that the magnitude and uncertainty of aerosol forcing may affect