Impact of Dust Radiative Forcing upon Climate - Mineral Dust: A Key Player in the Earth System - page 323

resulting range of climate perturbations calculated by models remain a useful test of
our understanding of the mechanisms relating dust radiative forcing to the climate
response.

Keywords Aerosol radiative forcing ￿ Climate response ￿ Precipitation response ￿
Feedback upon dust mobilization

13.1 Introduction

Mineral dust from the Chinese deserts routinely shrouds western North America
(Kavouras et al. 2009 ), extending as far downwind as the French Alps (Grousset
et al. 2003 ), while African dust replenishes mineral nutrients within the Amazon
(Swap et al. 1992 ). Dust perturbs the radiative flux within the atmosphere, changing
transports of energy and moisture to alter temperature and precipitation thousands
of kilometers beyond the dust layer (Miller and Tegen 1998 ). Deposition measure-
ments suggest that the global mass of dust aerosols doubled during the twentieth
century (Mahowald et al. 2010 ). This demonstrates the potential importance of dust
radiative forcing to climate trends observed during the Anthropocene and the need
to anticipate future changes in the dust load.
Mineral dust (also referred to as “soil” dust due to its origin through wind
erosion of the land surface) alters climate through a number of mechanisms that are
described in this volume. However, robust inferences of the effect of dust remain
elusive, even in recent decades when the aerosol distribution is better observed due
to multiple satellite retrievals and an expanding network of surface measurements.
Scientists study dust aerosols because of their potential importance for climate, but
much of the activity within this large interdisciplinary community is directed toward
simply deriving more precise constraints upon the dust burden and its regional
distribution, along with the particle radiative properties needed to compute the
forcing. This remains a key unsolved problem as noted throughout this volume
(e.g., Chaps. 7 , 9 , and 11 ) .
During recent decades, large teams of scientists have built comprehensive
Earth system models (ESMs) that have been tested against a widening range of
observations (e.g., Randall et al. 2007 ). These models explicitly calculate the dust
cycle and the mechanisms by which it perturbs climate. However, the climate
response to direct radiative forcing by dust remains uncertain for several reasons.
First, the ESMs show varying sensitivity to radiative forcing as a result of their
different treatments of clouds, for example. Global average projections of the
twenty-first century warming in response to rising concentrations of greenhouse
gases vary by a factor of 2, and discrepancies among projections of regional climate
are even larger (e.g., Meehl et al. 2007 ). This is especially relevant for dust where
prolific sources are highly localized so that the aerosol concentration and forcing
show strong regional contrasts. In practice, models calculate a variety of temperature
and precipitation anomalies in response to dust radiative forcing. It is difficult to
know what behavior exhibited by the models is robust and likely to be corroborated