Kohfeld – AAAS Symposium – Overview of Dust in the Earth System
Overview of Dust in the Earth System
Karen E. Kohfeld
School of Resource and Environmental Management, Simon Fraser University, 8888
University Drive, Burnaby, BC V5A 1S6, CANADA
Abstract.
Mineral dust plays an important role as a climatic and biogeochemical feedback within
the Earth system, linking land, air, and sea. Climatic and land-surface factors control the
production of atmospheric dust. Once in the atmosphere, mineral dust affects the global
radiative budget and the subsequent travel and deposition of dust delivers essential
nutrients for plant growth on land and in the ocean. These feedbacks of mineral dust have
played an important role in the geologic past, especially during the Quaternary period.
Knowledge of the impacts of dust in the atmosphere and ocean provides insights into
potential, future feedbacks of dust within the Earth system, especially as anthropogenic
perturbations increase.
Introduction
The purpose of this AAAS session is to bring forward some of the ground-breaking
research on the role of dust in the earth system, and to bring together an interdisciplinary
approach to studying its potential impacts on society into the future. The purpose of this
presentation is to provide an overview of what we already know about the role of dust
within the Earth system, and how it links processes on the land (where dust is produced),
in the air (where it travels, and either heats or cools the atmosphere), and in the ocean
(where it can serve as a fertilization for marine plants). In this presentation I will present
a summary of how we think dust1 has changed in the past, and what challenges our
knowledge of how dust is likely to change in the future.
Past Changes
The geologic record reveals to us that the activity of dust on Earth has varied
considerably through time, in response to large reorganizations of the climate system.
For example, major increases in the deposition of dust downwind of Asia occurred 6-2
million years ago, mostly likely associated with the uplift of Himalaya and the
subsequent evolution of the East Asian monsoon system (An et al., 2001; Rea et al.,
1998). We see a similar intensification of the dust production downwind of Africa with
the onset of Northern Hemisphere glaciations and cooling of North Atlantic surface
temperatures after 2.8 million years ago (deMenocal, 1995).
Global compilations of dust deposition suggest that when the earth enters an ice age, dust
deposition increases by 3-4 times globally, and by as much as 20 times in the polar
regions ((Kohfeld and Harrison, 2000; Kohfeld and Harrison, 2001; Kohfeld and Tegen,
2007; Mahowald et al., 2006a; Rea, 1994). Superb records of changes in dust,
The term ‘dust’ can encompass many types of materials ranging from living materials (such as bacteria
and viruses) to industrial particulates, I would like to emphasize that for this presentation I focus strictly on
soil dust, or clay mineral and mineral fragments ranging in size from mere one-hundredths up to several
hundred micrometers in diameter.
1
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Kohfeld – AAAS Symposium – Overview of Dust in the Earth System
atmospheric trace gases, and temperature over the last 800,000 from the Antarctic ice
cores demonstrate that the relationship between dust and climate is very cyclical
(Augustin et al., 2004; Lambert et al., 2008; Petit et al., 1999; Siegenthaler et al., 2005).
As the Earth enters an ice age, dust concentrations begin to rise only after temperature
and CO2 concentrations have begun to drop towards their full glacial conditions. In other
words, the dust cycle kicks in half-way through the descent into glaciation and thus
behaves as some form of feedback that perhaps amplifies glacial conditions (Kohfeld and
Ridgwell, 2009b). Well-constrained records of dust deposition from the equatorial North
Pacific Ocean confirm that these changes in the dust cycle are not just isolated to records
found at the poles. Cycles of dust deposition occur in a very similar fashion in other parts
of the world as well (Winckler et al., 2008).
When we examine the more recent history of the Holocene (approximately the last
10,000 years) and Anthropocene (here defined as the last 150 years), we still see large
changes in dust. (Neff et al., 2008) demonstrate a 500-fold increase in dust deposition in
the mid-continental USA, most likely in response to the expansion of human settlement
and livestock grazing. Recent work also suggests that recent climate changes have
affected dust production. Ice core samples from the Antarctic Peninsula demonstrate that
aluminosilicate dust fluxes have more than doubled during the 20th century. This change
is attributed to the combined effects of regional land use and increasing temperatures on
desertification in Patagonia and Argentina (McConnell et al., 2009).2 These past records
demonstrate that dust deposition has changed rather dramatically in the past and we can
easily anticipate that it will continue changing in the future, with natural and humaninduced perturbations to climate and the earth’s land surface. We can also anticipate that
future perturbations in the dust cycle will ripple through other parts of the earth system.
Dust in the Earth System: Land, Air, Sea
(Jickells et al., 2005) have presented an excellent overview of the biogeochemical
linkages of dust within the Earth system and its interactions between ‘climate and ‘land,’
‘air’ and ‘sea.’ In summary, dust is emitted from the land surface in response to changes
in land use, but also in response to climate, which controls changes in temperature,
precipitation and wind. Dust enters the atmosphere - air - where it affects the regional
and global radiative budget – and therefore climate – before it is deposited. When dust is
then deposited over the sea, it acts as a fertilizer to marine organisms, influencing ocean
carbon sequestration and the production of several trace gases and chemical constituents
that can affect climate. Thus, on land, in the air, and over the sea, the dust cycle
influences and is influenced by climate. In the rest of this presentation I will consider the
‘land’ ‘air’ and ‘sea’ components individually, as we have documented their changes in
the present and the past. I will provide the most recent speculations of how dust is likely
to change in future.
Land – Dust Emissions in the Present, Past, and Future
2
Although these locations demonstrate large increases in dust loadings since the mid-1800s, the one global
modeling assessment suggests that atmospheric dust loadings during the pre-industrial time period were
33% greater than today overall (Mahowald et al., 2006). This simulation considered the effects of climate
and CO2 fertilization alone (no changes in land use).
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Kohfeld – AAAS Symposium – Overview of Dust in the Earth System
Several factors influence the emission of dust from the land surface. These factors
include: (a) the characteristics of the soil surface including surface roughness, particle
size, soil mineralogy, (b) soil moisture, (c) soil availability, and (d) the lack of vegetative
cover. Once these basic conditions are met, dust emissions occur when the wind speeds
exceed a threshold velocity that allows soil particles to be mobilized. This happens today
mostly in arid and semi-arid regions around the world, mostly over lake, fluvial, and dune
deposits (Mahowald et al., 2009). (Engelstaedter and Washington, 2007)recently
identified 131 ‘hot spots’ of dust emissions using long-term mean Total Ozone Mapping
Spectrometer absorbing Aerosol Index (TOMS AI) averaged over 1984–1990.3 The
majority of these dust emission sources (63%) are located in North Africa and the Middle
East, with Australia and Asia holding 11% and 8% of these hotspots. A simple
qualitative analysis suggests that nearly half of them (47%) are associated with some
form of human agriculture (Mahowald et al., 2009). This fact implies some correlation
between human activity and dust production, although the relationship could simply be
that, in arid and semi-arid regions, human agriculture occurs in regions with at least
seasonal water and enriched sediment supply. The latest estimates of the human
contribution to global dust emissions via perturbations to the land surface range from less
than 15% (Tegen et al., 2004) to 0-50% (Mahowald et al., 2004).
Today dust emissions are on the order of 1790 Tg per year (Jickells et al., 2005). Given
that many of the factors controlling dust emissions are likely to change with climate (e.g.,
temperature, precipitation, and winds), it is not surprising that our estimates of dust
deposition have also changed in the past (e.g., (Rea, 1994). Several modeling studies
have demonstrated that the impact of glacial climate on soil moisture, vegetation, and
winds resulted in increases in dust emissions and deposition that were comparable to
those found in observational records (e.g., (Andersen et al., 1998; Mahowald et al., 1999;
Mahowald et al., 2006a; Reader et al., 1999; Reader et al., 2000; Werner et al., 2003).
During ice ages when large ice sheets covered the earth, it is also like that glacial ice
provided an important source of dust to the atmosphere ((Mahowald et al., 2006a). In
fact, inclusion of inferred, glaciogenic dust sources increased LGM emissions estimates
by 57% in a recent model simulation. The inclusion of these sources provides a better
match between model and data.
One might also anticipate that future changes in climate attributable to human activities is
likely to perturb dust emissions, although the climate-related changes in future
atmospheric dust loadings are much less certain. Several previous studies have predicted
that dust emissions may decrease by 20-60% from today’s levels by 2090 (Mahowald and
Luo, 2003) while others suggest that emissions may increase by as much as 200%
(Woodward et al., 2005). Other modeling studies have suggested smaller changes in dust
emissions of less than 20% by 2050, in which the sign of the change in dust emissions
depends on the model used (Tegen et al., 2004). A recent study re-emphasized these
3
It is worth noting that using TOMS AI to detect hotspots may result in some biases because of low
resolution of retrieval times and because dust may remain over some areas for several days, and therefore
be counted as an active source when it is not (Schepanski et al., 2007). Nevertheless, this analysis provides
a good first-order picture of the major dust-forming regions.
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Kohfeld – AAAS Symposium – Overview of Dust in the Earth System
differences by using 17 Intergovernmental Panel on Climate Change (IPCC) model
simulations to estimate changes in desert area (Mahowald, 2007). Individual models
predict increases and decreases of up to 50% in desert area, emphasizing that our
predictions of changes in desert areas (and therefore ultimately dust emissions) are rather
uncertain.
Air – The impact of dust on the radiative budget in the past, present and future
The radiative impact of dust depends on several factors including both direct and indirect
effects.4 Dust particles reflect incoming solar radiation back to space and therefore
reduce radiative fluxes at the surface. However, estimating a global impact of dust on the
radiative budget because both the sign and the magnitude of the impact of dust particles
on the radiative balance depends on the optical properties of dust particles as well as the
underlying albedo of clouds or the land surface. Dust can have a regional impact of more
than 100 Wm-2 (e.g. (Haywood et al., 2003).
The IPCC (2007) has recently provided a best estimate of -0.1 Wm-2 for the direct
radiative effect of anthropogenic dust, with values ranging from -0.3 to +0.1 Wm-2. This
range of values appears considerably smaller than those estimate for the Third
Assessment Report in which values ranged from -0.6 Wm-2 to +0.4 Wm-2. Between the
publication of these two reports, new constraints have been placed on the estimated
magnitude of anthropogenic emissions (dropping the value from 50% to 20% of the
global loading), but also on estimates of the optical properties of dust. In fact, when one
examines the summed natural and anthropogenic radiative forcing for dust, our newest
computed values range from -1.5 Wm-2 to +0.5 Wm-2. This range, although slightly more
negative, is not much different from the values published by the Third Assessment Report
(-1.3 to +0.8 Wm-2, IPCC, 2001). In other words, our uncertainty about the overall
radiative forcing of dust remains large.
Higher atmospheric dust loadings during the last ice age are likely to have impacted the
planet’s radiative budget as well as surface temperatures. (Claquin et al., 2003) estimated
that the overall, global impact of dust was likely to be small, less than 1 W m-2 at the top
of the atmosphere. This value is comparable to the estimate of -1.04 Wm-2 made by
(Mahowald et al., 2006b). Both studies suggest that the overall impact of radiative
forcing at high latitudes was small. (Mahowald et al., 2006b) showed a localized positive
impact of dust over the ice sheet edges, but when zonally averaged, the top-of-theatmosphere radiative forcing was dominated by the cooling effect over the oceans. The
largest impacts were observed in the tropics in both studies. (Claquin et al., 2003)
estimated that the negative radiative forcing due to dust reached values of -2.2 to -3.2
Wm-2 while (Mahowald et al., 2006b) saw values as great as -4 Wm-2 at the top of the
4
Dust also has the potential to produce indirect effects on climate, via its effects on cloud formation and
precipitation. For example, the rold of dust particles in providing nuclei for ice cloud formation may be
important. (DeMott et al., 2003). However, given the large uncertainties associated with the indirect
effects, I will only cover the direct effects for this presentation.
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Kohfeld – AAAS Symposium – Overview of Dust in the Earth System
atmosphere. Changes in the radiative forcing due to dust resulted in reduction of the
globally averaged surface temperatures of 0.85 ºC (Mahowald et al., 2006b).
Given our uncertainties of how dust emissions are likely to change in the future in
response to both climate and land use changes, it is not surprising that there are very few
estimates of future changes in the radiative forcing of dust. Two modeling studies that
have estimated the future impact of dust produce similar results, with an increase in the
globally averaged radiative forcing at the top of the atmosphere ranging from +0.14 Wm-2
(Mahowald et al., 2006b) to +0.17 Wm-2 (Woodward et al., 2005). However, a closer
examination of these studies shows that these results are reached for entirely different
reasons. (Woodward et al., 2005) estimated a fivefold increase in TOA forcing from
+0.04 to +0.21 Wm-2, as a result of increased dust emissions. In contrast, (Mahowald et
al., 2006b) estimate a less-negative TOA dust forcing in future in response to reduced
dust emissions, with a negative dust forcing estimate for modern conditions.
Furthermore, there are huge regional differences between these studies, with (Woodward
et al., 2005) predicting an enormous change in radiative forcing (+410 Wm-2) over the
Amazon basin. In conclusion, our estimates of the future impacts of dust on climate are
relatively small, but our level of uncertainty of these changes is large.
Sea – Dust as a fertilizer
Twenty years ago, (Martin, 1990) suggested that the effect of iron fertilization from
enhanced dust deposition to the oceans during ice ages would be large enough to reduce
atmospheric carbon dioxide concentrations to their glacial levels. This hypothesis has
been the subject of extensive modeling work, data studies, and multiple large-scale field
experiments over the past twenty years. On average, dust contains approximately 3.5
wt% of iron (Duce and Tindale, 1991). In many parts of the ocean the critical
concentrations of dissolved Fe are very low relative to other necessary nutrients for
phytoplankton production. As a result, we find many parts of the ocean where additional
aeolian inputs of Fe via dust would lead to an increase in phytoplankton activity and
growth (Boyd et al., 2007), and in some cases increased carbon export (Buesseler et al.,
2004; Pollard et al., 2009).
Modeling and observational studies that have focused on the impact of dust during the
last glacial period have and found the feedback of dust upon atmospheric CO2 is likely to
contribute in the range of 10-40 ppm to the glacial-interglacial change in CO2 (Archer et
al., 2000; Bopp et al., 2003; Kohfeld et al., 2005; Watson et al., 2000), with a best
estimate of 15 ppm (Kohfeld and Ridgwell, 2009a).
Estimating the future impact of dust on iron fertilization in the ocean is plagued with
some of the same uncertainties encountered with estimate the future radiative impacts. In
addition to uncertainties surrounding future land-use changes, our modeled estimates of
future dust emissions resulting from climate-induced changes in winds and vegetation
range from positive to negative changes. However, sensitivity studies investigating the
impact of aeolian iron inputs on carbon sequestration in the ocean have suggested that
even a 50% reduction in Aeolian inputs to the ocean would have a limited effect,
increasing atmospheric CO2 by 14 ppm (Parekh et al., 2008). Five-fold increases in
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Kohfeld – AAAS Symposium – Overview of Dust in the Earth System
aeolian Fe input would decrease atmospheric CO2 by approximately the same amount
(Parekh et al., 2008). These results taken together suggest aeolian inputs have a
substantial impact on the ocean carbon cycle, but that the sensitivity is somewhat limited.
It is important to note, however, that this assessment has not considered several key
points. Because we are only now beginning to understand the processes controlling the
solubility and bioavailability of iron (for review see e.g. (Mahowald et al., 2009), several
aspects of the iron cycle are greatly simplified in these modeling experiments. Iron
solubility, for example, is normally expressed as a constant 3.5% (Parekh et al., 2004)
whereas measured solubilities of atmospheric iron range from less than 1% to 81%
(Schroth et al., 2009). Furthermore, anthropogenic emissions of iron from combustion
sources tend to increase the solubility. Confounding this impact even further is the
impact of climate changes on ocean circulation, which may affect the ability of regions of
the ocean to utilize iron (Bowie et al., 2009).
Summary
In conclusion, this overview has examined the dust cycle and its linkages between the
land surface, the atmosphere, and the ocean. Several advances have been made in our
understanding of the dust cycle over the past few decades. Our ability to model dust
within the earth system has greatly improved, and our quantification of changes in dust
during the last glacial cycle has also led to understandings of some of the first-order
controls on dust at a global scale. We have a reasonable understanding of the
contribution of iron fertilization of dust to changes in atmospheric CO2 on glacialinterglacial timescales. And several challenges remain. Our understanding of the
radiative impact of dust today remains uncertain, hindered by uncertainties in our
understanding of the optical properties of dust as well as our quantification of emissions
and the anthropogenic contribution to the atmosphere dust loading. These uncertainties
increase as we attempt to predict what future changes in dust emissions, atmospheric dust
loadings, and dust impacts on global biogeochemical systems is likely to be.
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