Impact of Dust Radiative Forcing upon Climate - Mineral Dust: A Key Player in the Earth System - page 324

by future studies. Here, mechanistic arguments relating the forcing to the response
may provide guidance, although these arguments are typically based upon simple
models lacking the complexity that may be necessary to simulate the regional
climate anomalies resulting from dust.
In this chapter, we discuss the influence of dust radiative forcing upon climate.
Our discussion of the climate response emphasizes variables like surface air
temperature and precipitation that directly impact our society. The climate effect
of dust through its influence upon clouds and the carbon cycle is discussed
elsewhere in this volume (Chaps. 12 and 14 , respectively). Our method is to
summarize calculations by ESMs, attempting to identify robust behavior and turning
to mechanistic arguments when available. In Sect. 13.2 , we summarize the physical
and environmental factors contributing to varying estimates of the forcing among
models. We discuss the climate response to this forcing in Sect. 13.3 . In Sect. 13.4 ,
we consider the feedback of the climate perturbation upon the process by which
dust enters the atmosphere. In particular, we discuss the relation of dust radiative
forcing to surface wind speed and vegetation. Our concluding remarks are presented
in Sect. 13.5 , where we emphasize outstanding questions.

13.2 Radiative Forcing by Dust Aerosols

Direct radiative forcing is defined as the change in the radiative flux by dust particles
prior to any response by the climate (e.g., Hansen et al. 2005 ). Forcing is typically
calculated at the top of the atmosphere (TOA), but because the aerosol perturbation
to the flux at the surface can be markedly different and modify the hydrologic
cycle, the forcing is characterized at this level as well. Within a few months of
the forcing onset, the stratosphere returns to approximate radiative equilibrium,
and the radiative flux at TOA approaches the value at the tropopause. The initial
tropopause forcing is often a better indicator of the radiative perturbation at TOA
following stratospheric adjustment (Hansen et al. 1997 ). It is this adjusted flux
at TOA that perturbs the climate within the troposphere, but here we neglect the
distinction between the initial forcing at TOA and the tropopause that is small
for dust (Miller et al. 2004b ). Direct radiative forcing by dust is described more
fully in Chap. 11 . Here, we summarize the physical properties that contribute to
differences in the forcing among the models because these differences ultimately
result in uncertainties in the effect of dust radiative forcing upon climate.
Dust particles scatter and absorb both solar and thermal (or “longwave”)
radiation (Tegen and Lacis 1996 ). At TOA, net insolation is reduced by scattering
but increased by absorption of sunlight that otherwise would be reflected back to
space. The dust layer also acts like a greenhouse gas, reducing outgoing longwave
radiation (OLR) at TOA, while increasing thermal emission toward the surface.
One model calculation of dust radiative forcing for boreal summer is shown
in Fig. 13.1 . The dust distribution is prescribed from Miller et al. ( 2006 ), while
the forcing is calculated using a development version of the NASA Goddard
Institute for Space Studies (GISS) ModelE: an ESM that is intermediary between the