Observed 20th century desert dust variability: impact on climate and biogeochemistry

Abstract

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.

An ice‐core proxy for northerly air mass incursions into West Antarctica

“…Because change in the extent of these ice-free areas has not been reported and they offer a minimal source region, we do not consider the coastal West Antarctic ice-free areas to be a significant contributor to dust loading over the region encompassing the majority of cores used in this study. The primary SH dust source regions feeding West Antarctica, in order of importance, are Australia, South America, and South Africa (Prospero et al, 2002;Li et al, 2008;De Deckker et al, 2010;Mahowald et al, in press) We calculated 30-day forward trajectories (FT) originating from the southernmost dust source region on each of these SH continents, except Antarctica (Figure 2) using the NOAA Hysplit Model v.4.8 (Draxler and Rolph, 2003) in conjunction with the NCEP/NCAR global atmospheric reanalysis datasets archived on the READY website (ftp://www.ready.noaa.gov/pub/ archives/reanalysis/). The Hysplit model control file included: (1) starting locations = 7 (each location with a slightly different latitude and longitude, up to ±2°f rom the centre of each dust source area), (2) altitude of each starting location = 100 m, (3) top of the model = 30 000 m (agl), and (4) vertical motion = data (default value, uses meteorological model's vertical velocity fields).…”

Climate engineering by mimicking natural dust climate control: the iron salt aerosol method

“…The current state of knowledge of iron in the oceans is lower than that of carbon, although numerous scientific publications deal with this topic (Archer and Johnson, 2000;Boyd and Ellwood, 2010;Johnson et al, 2002a, b;Misumi et al, 2014;Moore and Braucher, 2008;Moore et al, 2013;Tagliabue et al, 2015;Turner and Hunter, 2001); however, the iron biogeochemical cycle in the atmosphere is described by fewer authors (Mahowald et al, 2005(Mahowald et al, , 2010. This is in contrast to the iron biogeochemical cycle in soil and land, as almost no recent publications details the current knowledge about iron in soils and over the landscape (Anderson, 1982;Lindsay and Schwab, 1982;Mengel and Geurtzen, 1986), a task we attempt in this review.…”

Desert dust and anthropogenic aerosol interactions in the Community Climate System Model coupled-carbon-climate model

“…Mahowald and Luo, 2003;Tegen et al, 2004;Woodward et al, 2005). Recent estimates suggest roughly a doubling in dust over the last 100 years (Mahowald et al, 2010;Mulitza et al, 2010). Therefore we include sensitivity studies with different changes in desert dust sources.…”