(2017), Smaller desert dust cooling effect estimated from analysis of desert dust size and abundance, Nature Geoscience,10,[274][275][276][277][278] Desert dust aerosols affect Earth's global energy balance through direct interactions with radiation, and through indirect interactions with clouds and ecosystems. But the magnitudes of these effects are so uncertain that it remains unclear whether atmospheric dust has a net warming or cooling effect on global climate. Consequently, it is still uncertain whether large changes in atmospheric dust loading over the past century have slowed or accelerated anthropogenic climate change, or what the effects of potential future changes in dust loading will be. Here we present an analysis of the size and abundance of dust aerosols to constrain the direct radiative effect of dust. Using observational data on dust abundance, in situ measurements of dust optical properties and size distribution, and climate and atmospheric chemical transport model simulations of dust lifetime, we find that the dust found in the atmosphere is substantially coarser than represented in current global climate models. Since coarse dust warms climate, the global dust direct radiative effect is likely to be less cooling than the ~-0.4 W/m 2 estimated by models in a current global aerosol model ensemble. Instead, we constrain the dust direct radiative effect to a range between -0.48 and +0.20 W/m 2 , which includes the possibility that dust causes a net warming of the planet.The direct radiative effect (DRE) of desert dust aerosols on global climate depends sensitively on both the size distribution and atmospheric abundance of dust 1-3 . However, current global model estimates of the atmospheric loading of dust with geometric diameter D ≤ 10 µm (PM10) vary widely from ~6 to 30 Tg [4][5][6][7] . Similarly, the size distribution of atmospheric dust varies substantially across models, with the fraction of dust in the clay size range (D ≤ 2 µm) varying by over a factor of three 8 . This uncertainty in dust size and abundance is partially driven by a critical limitation of global models: the need to prescribe poorly known attributes of dust particles. In particular, the assumed dust optical properties and size distribution at emission greatly affect the resultant size-resolved dust loading 1,6 . Each model parameterizes these properties differently, and in a manner not always consistent with experimental results [8][9][10] . This divergence in assumed dust properties contributes to a wide range of estimates of the sizeresolved global dust loading 6,8 . Because fine dust cools global climate whereas coarse dust (D ≥ 5 μm) likely warms it 3 , this uncertainty in size-resolved dust loading contributes to a wide spread in model estimates of the dust DRE 1,3,9,[11][12][13][14] . Since the use of global models alone is thus unlikely to substantially narrow the uncertainty on dust climate effects 15 , we develop an alternative approach to determine the size-resolved global dust loading, which we subsequently use ...

No abstract

Aerosol-climate interactions constitute one of the major sources of uncertainty in assessing changes in aerosol forcing in the anthropocene as well as understanding glacial-interglacial cycles. Here we focus on improving the representation of mineral dust in the Community Atmosphere Model and assessing the impacts of the improvements in terms of direct effects on the radiative balance of the atmosphere. We simulated the dust cycle using different parameterization sets for dust emission, size distribution, and optical properties. Comparing the results of these simulations with observations of concentration, deposition, and aerosol optical depth allows us to refine the representation of the dust cycle and its climate impacts. We propose a tuning method for dust parameterizations to allow the dust module to work across the wide variety of parameter settings which can be used within the Community Atmosphere Model. Our results include a better representation of the dust cycle, most notably for the improved size distribution. The estimated net top of atmosphere direct dust radiative forcing is 20.23 6 0.14 W/m 2 for present day and 20.32 6 0.20 W/m 2 at the Last Glacial Maximum. From our study and sensitivity tests, we also derive some general relevant findings, supporting the concept that the magnitude of the modeled dust cycle is sensitive to the observational data sets and size distribution chosen to constrain the model as well as the meteorological forcing data, even within the same modeling framework, and that the direct radiative forcing of dust is strongly sensitive to the optical properties and size distribution used.

Abstract. Desert dust perturbs climate by directly and indirectly interacting with incoming solar and outgoing long wave radiation, thereby changing precipitation and temperature, in addition to modifying ocean and land biogeochemistry. While we know that desert dust is sensitive to perturbations in climate and human land use, previous studies have been unable to determine whether humans were increasing or decreasing desert dust in the global average. Here we present observational estimates of desert dust based on paleodata proxies showing a doubling of desert dust during the 20th century over much, but not all the globe. Large uncertainties remain in estimates of desert dust variability over 20th century due to limited data. Using these observational estimates of desert dust change in combination with ocean, atmosphere and land models, we calculate the net radiative effect of these observed changes (top of atmosphere) over the 20th century to be −0.14 ± 0.11 W/m 2 (1990-1999 vs. 1905-1914). The estimated radiative changeCorrespondence to: N. M. Mahowald (mahowald@cornell.edu) due to dust is especially strong between the heavily loaded 1980-1989 and the less heavily loaded 1955-1964 time periods (−0.57 ± 0.46 W/m 2 ), which model simulations suggest may have reduced the rate of temperature increase between these time periods by 0.11 • C. Model simulations also indicate strong regional shifts in precipitation and temperature from desert dust changes, causing 6 ppm (12 PgC) reduction in model carbon uptake by the terrestrial biosphere over the 20th century. Desert dust carries iron, an important micronutrient for ocean biogeochemistry that can modulate ocean carbon storage; here we show that dust deposition trends increase ocean productivity by an estimated 6% over the 20th century, drawing down an additional 4 ppm (8 PgC) of carbon dioxide into the oceans. Thus, perturbations to desert dust over the 20th century inferred from observations are potentially important for climate and biogeochemistry, and our understanding of these changes and their impacts should continue to be refined.