Smaller desert dust cooling effect estimated from analysis of dust size and abundance

Abstract

(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 ...

Profiling of Saharan dust from the Caribbean to western Africa – Part 2: Shipborne lidar measurements versus forecasts

“…In their overview and review paper, the authors stated that the dust extinction coefficient and the corresponding dust AOT-related radiative effects are sensitively controlled by the amount of occurring fine dust particles due to their higher extinction efficiency, whereas coarse dust dominates the surface concentration, deposition, and removal. In a recent study, Kok et al (2017) emphasize in detail the consequences of a not-well-modeled dust size distribution and abundance for the global energy balance through direct interaction of dust with radiation. As mentioned, the indirect climate effect of dust through interaction with clouds is also affected if the dust size distribution and load, and thus estimates of CCN and INP concentrations are wrong in the forecast models.…”

“…Gravitational settling also sensitively depends on the simulated fine and coarse dust fractions. The same holds for dust radiative effects which are rather different for fine and coarse dust particles (Nabat et al, 2012;Ridley et al, 2016;Kok et al, 2017). Meanwhile, dust simulations are also used to estimate dust ice-nucleating particle (INP) concentrations (Hande et al, 2015;.…”

“…Polarization lidars which allow us to separate dust from non-dust aerosol profiles in terms of light extinction coefficient and mass concentration are most useful in this respect. Recently, it was shown that even fine dust and coarse dust can be distinguished Ansmann, 2014, 2017), so one of the most important modeling aspects which deals with the emitted and transported dust particle size spectrum (Kok, 2011a, b;Kok et al, 2017) can now be illuminated in model-observation comparisons.…”

East Asian dust storm in May 2017: observations, modelling, and its influence on the Asia-Pacific region

“…Atmospheric dust has been observed across continents and oceans, giving rise to its importance in both terrestrial and marine ecosystems (Huebert et al, 2003;Mahowald et al, 2009;IPCC, 2013;Kok et al, 2017). Mineral aerosols can influence air quality by reducing visibility and bolstering concentrations of inhalable particulate matter (Sharratt and Lauer, 2006;Huneeus et al, 2011;Goudie, 2014).…”

Mineralogy and mixing state of North African mineral dust by on-line single-particle mass spectrometry

“…In addition, laboratory studies have suggested that the internal mixing state, particularly metal oxide -clay mineral assemblages, has important influence on radiative properties of dust (Nousiainen et al, 2009;Jeong and Nousiainen, 2014;Kemppinen et al, 2015;Di Biagio et al, 2017;Caponi et al, 2017). This highlights the need for measurement of single particle composition, including both mineralogy and internal mixing state in order to 20 provide better constraints for global climate models (Kok et al, 2017). …”