Saharan desert dust feeds deep ocean life

by Tim Radford, Climate News Network, July 14, 2014

US scientists have found that dust from the Sahara desert provides most of the iron found in the Atlantic ocean.

LONDON − Marine scientists have measured levels of iron dissolved in the Atlantic ocean, and at the same time worked out where it came from.

In the course of doing so, they have helped explain in more detail why the deep ocean is blue while coastal waters are usually green, and at the same time helped answer more complex questions about the ocean’s role in the great carbon dioxide question.

And in the course of settling these points, they have also answered questions about North Africa’s importance to the rest of the world. It keeps the oceans and the Amazon supplied with valuable dust.

Tim Conway and Seth John of the University of South Carolina report in Nature that they devised a way to sample large volumes of seawater to identify the content of dissolved iron in the water, and then to distinguish the ratio between different isotopes of that iron.

An isotope is a natural variant of an element, and often indicates a different source of origin. Iron is a vital trace element: without it, mammals cannot make haemoglobin to transport oxygen around the bloodstream and plants cannot make chlorophyll to photosynthesize tissue from air and sunlight.

Missing iron

The deep oceans have everything needed for plant growth – sunlight, carbon, nitrogen and water – but they don’t have iron. That is one reason why they tend to be blue while nutrient-rich coastal waters are green.

Estuaries and deltas are rich in iron and other nutrients and good for algal growth. Because ocean phytoplankton (microscopic plants which sustain the marine food web) cannot get enough iron, there is a limit to the carbon dioxide they can absorb from the atmosphere. So iron is an element in the great carbon cycle. And it doesn’t need to be available in huge quantities.

“I did a calculation once on a ton of sea water. The amount of iron in that ton of water would weigh about as much as a single eyelash,” says Dr John. “The key reason that everybody cares about iron is because it limits the growth of phytoplankton such as algae, in maybe a fifth of the ocean.”

The researchers collected 600 samples of sea water during a cruise across the North Atlantic on a research ship, and set to work trying to identify the origin of the few billionths of a gram of iron in every litre of the water collected.

Saharan source

They found that a measurable proportion of oceanic iron seeped up from deep within the crust through hydrothermal vents along the mid-ocean ridge. A fraction came from sediments on the African coast, and more than 10% came from oxygenated muds on the American coast.

But they also found that the answer had been blowing in the wind. Somewhere between 71% and 87% was delivered by dust storms from the Sahara desert.  That is, life in the deep ocean depended on an annual delivery of fertiliser from one of the world’s emptiest and most parched regions.

The play between dust and life has fascinated scientists for more than a decade. In 2006, Israeli researchers found that more than half the dust needed to fertilise the Brazilian rainforest blew in from just one desiccated valley in Chad.

Two years later a team in Liverpool in the UK confirmed the role of Saharan dust as a mineral source for the Atlantic ocean and in 2007 Swiss and German microbiologists analysed dust samples collected by Charles Darwin.

They found that wind-blown dust could transport microbes from West Africa all the way to the Caribbean. An estimated 50 million tons of Saharan dust is blown across the Atlantic to the Amazon every year.

Explaining the past

So the South Carolina research is just another example of science in action; a painstaking increment to human knowledge rather than a breakthrough. It adds quantifiable figures to a picture already taking shape. It is a reminder that intercontinental migration is as old as life itself. And it also helps explain a little bit more about the global climate machine.

Researchers have already theorised that airborne dust must play a role in cloud formation – and therefore in rainfall and drought – and even that dust storms may play a role in damping down hurricanes.

If more dust in the oceans and the forests means more carbon uptake from the atmosphere, then cycles of superstorms of dust could also help tweak the global thermostat. “It could help us understand past climate change, like glacial-interglacial cycles,” Dr John says.

“There would have been huge changes in dust fluxes to the ocean in glacial times, and so understanding how much iron comes from dust in the modern day helps us figure out whether that was an important driver of glacial–interglacial cycles.”

Hansen, Kharecha & Sato, ERL (2013), Climate forcing growth rates: doubling down on our Faustian bargain

Environmental Research Letters, 7 (2012) 044035.

Perspective

Rahmstorf et al. 's (2012) conclusion that observed climate change is comparable to projections, and in some cases exceeds projections, allows further inferences if we can quantify changing climate forcings and compare those with projections. The largest climate forcing is caused by well-mixed, long-lived greenhouse gases. Here we illustrate trends of these gases and their climate forcings, and we discuss implications. We focus on quantities that are accurately measured, and we include comparison with fixed scenarios, which helps reduce common misimpressions about how climate forcings are changing.

Annual fossil fuel CO₂ emissions have shot up in the past decade at about 3%/yr, double the rate of the prior three decades (Figure 1). The growth rate falls above the range of the IPCC (2001) 'Marker' scenarios, although emissions are still within the entire range considered by the IPCC SRES (2000). The surge in emissions is due to increased coal use (blue curve in Figure 1), which now accounts for more than 40% of fossil fuel CO₂ emissions.

Figure 1. CO₂ annual emissions from fossil fuel use and cement manufacture, an update of Figure 16 of Hansen (2003) using data of British Petroleum (BP 2012) concatenated with data of Boden et al. (2012).

The resulting annual increase of atmospheric CO₂ (12-month running mean) has grown from less than 1 ppm/yr in the early 1960s to an average ~2 ppm/yr in the past decade (Figure 2). Although CO₂ measurements were not made at sufficient locations prior to the early 1980s to calculate the global mean change, the close match of global and Mauna Loa data for later years suggests that Mauna Loa data provide a good approximation of global change (Figure 2), thus allowing a useful estimate of annual global change beginning with the initiation of Mauna Loa measurements in 1958 by Keeling et al. (1973).

Figure 2. Annual increase of CO₂ based on data from the NOAA Earth System Research Laboratory (ESRL 2012). CO₂ change and global temperature change are 12-month running means of differences for the same month of consecutive years. Nino index (Nino3.4 area) is 12-month running mean. Both temperature indices use data from Hansen et al. (2010). Annual mean CO₂ amount in 1958 was 315 ppm (Mauna Loa) and in 2012 was 394 ppm (Mauna Loa) and 393 ppm (Global).

Interannual variability of CO₂ growth is correlated with ENSO (El Nino–Southern Oscillation) variations of tropical temperatures (Figure 2). Ocean–atmosphere CO₂ exchange is affected by ENSO (Chavez et al. 1999), but ENSO seems to have a greater impact on atmospheric CO₂ via the terrestrial carbon cycle through effects on the water cycle, temperature, and fire, as discussed in a large body of literature (referenced, e.g., by Schwalm et al. 2011). In addition, volcanoes, such as the 1991 Mount Pinatubo eruption, slow the increase of atmospheric CO₂ (Rothenberg et al. 2012), at least in part because photosynthesis is enhanced by the increased proportion of diffuse sunlight (Gu et al. 2003, Mercado et al. 2009). Watson (1997) suggests that volcanic dust deposited on the ocean surface may also contribute to CO₂ uptake by increasing ocean productivity.

An important question is whether ocean and terrestrial carbon sinks will tend to saturate as human-made CO₂ emissions continue. Piao et al. (2008) and Zhao and Running (2010) suggest that there already may be a reduction of terrestrial carbon uptake, while Le Quéré et al. (2007) and Schuster and Watson (2007) find evidence of decreased carbon uptake in the Southern Ocean and North Atlantic Ocean, respectively. However, others (Knorr 2009, Sarmiento et al. 2010, Ballantyne et al. 2012) either cast doubt on the reality of a reduced uptake strength or find evidence for increased uptake.

An informative presentation of CO₂ observations is the ratio of annual CO₂ increase in the air divided by annual fossil fuel CO₂emissions (Keeling et al. 1973), the 'airborne fraction' (Figure 3, right scale). An alternative definition of airborne fraction includes in the denominator of this ratio an estimated net anthropogenic CO₂ source from changes in land use, but this latter term is much more uncertain than the two terms involved in the Keeling et al. (1973) definition. For example, analysis by Harris et al. (2012) reveals a range as high as a factor of 2–4 in estimates of recent land use emissions; see also the discussion by Sarmiento et al. (2010). However, note that the airborne fraction becomes smaller when estimated land use emissions are included, with the uptake fraction (one minus airborne fraction) typically greater than 0.5.

Figure 3. Fossil fuel CO₂ emissions (left scale) and airborne fraction, i.e., the ratio of observed atmospheric CO₂ increase to fossil fuel CO₂ emissions. Final three points are 5-, 3- and 1-year means.

The simple Keeling airborne fraction, clearly, is not increasing (Figure 3). Thus the net ocean plus terrestrial sink for carbon emissions has increased by a factor of 3–4 since 1958, accommodating the emissions increase by that factor.

Remarkably, and we will argue importantly, the airborne fraction has declined since 2000 (Figure 3) during a period without any large volcanic eruptions. The 7-year running mean of the airborne fraction had remained close to 60% up to 2000, except for the period affected by Pinatubo. The airborne fraction is affected by factors other than the efficiency of carbon sinks, most notably by changes in the rate of fossil fuel emissions (Gloor et al. 2010). However, it is the dependence of the airborne fraction on fossil fuel emission rate that makes the post-2000 downturn of the airborne fraction particularly striking. The change of emission rate in 2000 from 1.5%  to 3.1% per year (Figure 1), other things being equal, would have caused a sharp increase of the airborne fraction (the simple reason being that a rapid source increase provides less time for carbon to be moved downward out of the ocean's upper layers).

A decrease in land use emissions during the past decade (Harris et al. 2012) could contribute to the decreasing airborne fraction in Figure 3, although Malhi (2010) presents evidence that tropical forest deforestation and regrowth are approximately in balance, within uncertainties. Land use change can be only a partial explanation for the decrease of the airborne fraction; something more than land use change seems to be occurring.

We suggest that the huge post-2000 increase of uptake by the carbon sinks implied by Figure 3 is related to the simultaneous sharp increase in coal use (Figure 1). Increased coal use occurred primarily in China and India (Boden et al. 2012; BP 2012; see graphs at www.columbia.edu/~mhs119/Emissions/Emis_moreFigs/). Satellite radiance measurements for July–December, months when desert dust does not dominate aerosol amount, yield an increase of aerosol optical depth in East Asia of about 4%/yr during 2000–2006 (van Donkelaar et al. 2008). Associated gaseous and particulate emissions increased rapidly after 2000 in China and India (Lu et al. 2011, Tian et al. 2010). Some decrease of the sulfur component of emissions occurred in China after 2006 as wide application of flue-gas desulfurization began to be initiated (Lu et al. 2010), but this was largely offset by continuing emission increases from India (Lu et al. 2011).

We suggest that the surge of fossil fuel use, mainly coal, since 2000 is a basic cause of the large increase of carbon uptake by the combined terrestrial and ocean carbon sinks. One mechanism by which fossil fuel emissions increase carbon uptake is by fertilizing the biosphere via provision of nutrients essential for tissue building, especially nitrogen, which plays a critical role in controlling net primary productivity and is limited in many ecosystems (Gruber & Galloway 2008). Modeling (e.g., Thornton et al. 2009) and field studies (Magnani et al. 2007) confirm a major role of nitrogen deposition, working in concert with CO₂ fertilization, in causing a large increase in net primary productivity of temperate and boreal forests. Sulfate aerosols from coal burning also might increase carbon uptake by increasing the proportion of diffuse insolation, as noted above for Pinatubo aerosols, even though the total solar radiation reaching the surface is reduced.

Thus we see the decreased CO₂ airborne fraction since 2000 as sharing some of the same causes as the decreased airborne fraction after the Pinatubo eruption (Figure 3). CO₂ fertilization is likely the major effect, as a plausible addition of 5 Tg N per year from fossil fuels and net ecosystem productivity of 200 kg C per kg N (Magnani et al. 2007, 2008) yields an annual carbon drawdown of 1 Gt C per year, which is of the order of what is needed to explain the post-2000 anomaly in airborne CO₂. However, an aerosol-induced increase of diffuse radiation might also contribute. Although tropospheric aerosol properties are not accurately monitored, there are suggestions of an upward trend of stratospheric background aerosols since 2000 (Hofmann et al. 2009, Solomon et al. 2011), which could be a consequence of more tropospheric aerosols at low latitudes where injection of tropospheric air into the stratosphere occurs (Holton et al. 1995). We discuss climate implications of the reduced CO₂ airborne fraction after presenting data for other greenhouse gases.

Atmospheric CH₄ is increasing more slowly than in IPCC scenarios (Figure 4), which were defined more than a decade ago (IPCC 2001). However, after remaining nearly constant for several years, CH₄ has increased during the past 5 years, pushing slightly above the level that was envisaged in the Alternative Scenario of Hansen et al. (2000). Reduction of CH₄, besides slowdown in CO₂ growth in the 21st century and a decline of CO₂ in the 22nd century, is a principal requirement to achieve a low climate forcing that stabilizes climate, in part because CH₄ also affects tropospheric ozone and stratospheric water vapor. The Alternative Scenario, defined in detail by Hansen and Sato (2004), keeps maximum global warming at ~1.5 °C relative to 1880–1920, under the assumption that fast-feedback climate sensitivity is ~3 °C for doubled CO₂ (Hansen et al. 2007). The Alternative Scenario allows CO₂ to reach 475 ppm in 2100 before declining slowly; this scenario assumes that reductions of non-CO₂ greenhouse gases and black carbon aerosols can be achieved sufficient to balance the warming effect of likely future decreases of reflective aerosols.

Figure 4. Observed atmospheric CH₄ amount and scenarios for the 21st century. Alternative scenario (Hansen et al. 2000, Hansen & Sato 2004) yields maximum global warming ~1.5 °C above 1880–1920. Other scenarios are from IPCC (2001). Forcing on right hand scale is adjusted forcing, Fa, relative to values in 2000 (Hansen et al. 2007).

There are anthropogenic sources of CH₄ that potentially could be reduced; indeed, the leveling off of CH₄ amount during the past 20 years seems to have been caused by decreased venting in oil fields (Simpson et al. 2012), but the feasibility of overall CH₄ reduction also depends on limiting global warming itself, because of the potential for amplifying climate-CH₄ feedbacks (Archer et al. 2009, Koven et al. 2011). Furthermore, reduction of atmospheric CH₄ might become problematic if unconventional mining of gas, such as 'hydro-fracking', expands widely (Cipolla 2009), as discussed further below.

The growth rate for the total climate forcing by well-mixed greenhouse gases has remained below the peak values reached in the 1970s and early 1980s, has been relatively stable for about 20 years, and is falling below IPCC (2001) scenarios (Figure 5). However, the greenhouse gas forcing is growing faster than in the Alternative Scenario. MPTGs and OTGs in Figure 5 are Montreal Protocol Trace Gases and Other Trace Gases (Hansen & Sato 2004).

Figure 5. Five-year mean of the growth rate of climate forcing by well-mixed greenhouse gases, an update of Figure 4 of Hansen and Sato (2004). Forcing calculations use equations of Hansen et al. (2000). The moderate uncertainties in radiative calculations affect the scenarios and actual greenhouse gas results equally and thus do not alter the conclusion that the actual forcing falls below that of the IPCC scenarios.

If greenhouse gases were the only climate forcing, we would be tempted to infer from Rahmstorf's conclusion (that actual climate change has exceeded IPCC projections) and our conclusion (that actual greenhouse gas forcings are slightly smaller than IPCC scenarios) that actual climate sensitivity is on the high side of what has generally been assumed. Although that may be a valid inference, the evidence is weakened by the fact that other climate forcings are not negligible in comparison to the greenhouse gases and must be accounted for.

Natural forcings, by changing solar irradiance and volcanic aerosols, are well-measured since the late 1970s and included in most IPCC (2007) climate simulations. The difficulty is human-made aerosols. Aerosols are readily detected in satellite observations, but determination of their climate forcing requires accurate knowledge of changes in aerosol amount, size distribution, absorption and vertical distribution on a global basis—as well as simultaneous data on changes in cloud properties to allow inference of the indirect aerosol forcing via induced cloud changes. Unfortunately, the first satellite mission capable of measuring the needed aerosol characteristics (Aerosol Polarimetry Sensor on the Glory satellite (Mishchenko et al. 2007)) suffered a launch failure, and as yet there are no concrete plans for a replacement mission.

The human-made aerosol climate forcing thus remains uncertain. IPCC (2007) concludes that aerosols are a negative (cooling) forcing, probably between -0.5 and -2.5 W m^-2. Hansen et al. (2011), based mainly on analysis of Earth's energy imbalance, derive an aerosol forcing -1.6 ± 0.3 W m^-2, consistent with an analysis of Murphy et al. (2009) that suggests an aerosol forcing about -1.5 W m^-2 (see discussion in Hansen et al. 2011). This large negative aerosol forcing reduces the net climate forcing of the past century by about half (IPCC 2007; Figure 1 of Hansen et al. 2011). Coincidentally, this leaves net climate forcing comparable to the CO₂ forcing alone.

Reduction of the net human-made climate forcing by aerosols has been described as a 'Faustian bargain' (Hansen & Lacis 1990, Hansen 2009), because the aerosols constitute deleterious particulate air pollution. Reduction of the net climate forcing by half will continue only if we allow air pollution to build up to greater and greater amounts. More likely, humanity will demand and achieve a reduction of particulate air pollution, whereupon, because the CO₂ from fossil fuel burning remains in the surface climate system for millennia, the 'devil's payment' will be extracted from humanity via increased global warming.

So is the new data we present here good news or bad news, and how does it alter the 'Faustian bargain'? At first glance there seems to be some good news. First, if our interpretation of the data is correct, the surge of fossil fuel emissions, especially from coal burning, along with the increasing atmospheric CO₂ level is 'fertilizing' the biosphere, and thus limiting the growth of atmospheric CO₂. Also, despite the absence of accurate global aerosol measurements, it seems that the aerosol cooling effect is probably increasing based on evidence of aerosol increases in the Far East and increasing 'background' stratospheric aerosols.

Both effects work to limit global warming and thus help explain why the rate of global warming seems to be less this decade than it has been during the prior quarter century. This data interpretation also helps explain why multiple warnings that some carbon sinks are 'drying up' and could even become carbon sources, e.g., boreal forests infested by pine bark beetles (Kurz et al. 2008) and the Amazon rain forest suffering from drought (Lewis et al. 2011), have not produced an obvious impact on atmospheric CO₂.

However, increased CO₂ uptake does not necessarily mean that the biosphere is healthier or that the increased carbon uptake will continue indefinitely (Matson et al. 2002, Galloway et al. 2002, Heimann & Reichstein 2008, Gruber & Galloway 2008). Nor does it change the basic facts about the potential magnitude of the fossil fuel carbon source (Figure 6) and the long lifetime of the CO₂ in the surface carbon reservoirs (atmosphere, ocean, soil, biosphere) once the fossil fuels are burned (Archer 2005). Fertilization of the biosphere affects the distribution of the fossil fuel carbon among these reservoirs, at least on the short run, but it does not alter the fact that the fossil carbon will remain in these reservoirs for millennia.

Figure 6. Fossil fuel CO₂ emissions and carbon content (1 ppm atmospheric CO₂~2.12 GtC). Historical emissions are from Boden et al. (2012). Estimated reserves and potentially recoverable resources are based on energy content values of Energy Information Administration (EIA 2011), German Advisory Council (GAC 2011), and Global Energy Assessment (GEA 2012). We convert energy content to carbon content using emission factors of Table 4.2 of IPCC (2007) for coal, gas, and conventional oil, and, following IPCC, we use an emission factor of unconventional oil the same as that for coal.

Humanity, so far, has burned only a small portion (purple area in Figure 6) of total fossil fuel reserves and resources. Yet deleterious effects of warming are apparent (IPCC 2007), even though only about half of the warming due to gases now in the air has appeared, the remainder still 'in the pipeline' due to the inertia of the climate system (Hansen et al. 2011). Already it seems difficult to avoid passing the 'guardrail' of no more than 2 °C global warming that was agreed in the Copenhagen Accord of the United Nations Framework Convention on Climate Change (UNFCCC 2010). And Hansen et al. (2008), based primarily on paleoclimate data and evidence of deleterious climate impacts already at 385 ppm CO₂, concluded that an appropriate initial target for CO₂ was 350 ppm, which implied a global temperature limit, relative to 1880–1920 of about 1 °C. What is clear is that most of the remaining fossil fuels must be left in the ground if we are to avoid dangerous human-made interference with climate.

The principal implication of our present analysis probably relates to the Faustian bargain. Increased short-term masking of greenhouse gas warming by fossil fuel particulate and nitrogen pollution represents a 'doubling down' of the Faustian bargain, an increase in the stakes. The more we allow the Faustian debt to build, the more unmanageable the eventual consequences will be. Yet globally there are plans to build more than 1,000 coal-fired power plants (Yang & Cui 2012) and plans to develop some of the dirtiest oil sources on the planet (EIA 2011). These plans should be vigorously resisted. We are already in a deep hole—it is time to stop digging.

http://iopscience.iop.org/1748-9326/8/1/011006

pdf file of original article: http://iopscience.iop.org/1748-9326/8/1/011006/pdf/1748-9326_8_1_011006.pdf

Jasper F. Kok, PNAS (2010), A scaling theory for the size distribution of emitted dust aerosols suggests climate models underestimate the size of the global dust cycle

Proceedings of the National Academy of Sciences, published online before print December 28, 2010; doi: 10.1073/pnas.1014798108

A scaling theory for the size distribution of emitted dust aerosols suggests climate models underestimate the size of the global dust cycle

Jasper F. Kok


Advanced Study Program, National Center for Atmospheric Research, Boulder, CO 80307, U.S.A.

Abstract

Mineral dust aerosols impact Earth’s radiation budget through interactions with clouds, ecosystems, and radiation, which constitutes a substantial uncertainty in understanding past and predicting future climate changes. One of the causes of this large uncertainty is that the size distribution of emitted dust aerosols is poorly understood. The present study shows that regional and global circulation models (GCMs) overestimate the emitted fraction of clay aerosols (< 2 μm diameter) by a factor of ∼2–8 relative to measurements. This discrepancy is resolved by deriving a simple theoretical expression of the emitted dust size distribution that is in excellent agreement with measurements. This expression is based on the physics of the scale-invariant fragmentation of brittle materials, which is shown to be applicable to dust emission. Because clay aerosols produce a strong radiative cooling, the overestimation of the clay fraction causes GCMs to also overestimate the radiative cooling of a given quantity of emitted dust. On local and regional scales, this affects the magnitude and possibly the sign of the dust radiative forcing, with implications for numerical weather forecasting and regional climate predictions in dusty regions. On a global scale, the dust cycle in most GCMs is tuned to match radiative measurements, such that the overestimation of the radiative cooling of a given quantity of emitted dust has likely caused GCMs to underestimate the global dust emission rate. This implies that the deposition flux of dust and its fertilizing effects on ecosystems may be substantially larger than thought.

Link:  http://www.pnas.org/content/early/2010/12/23/1014798108.abstract

NASA Study Shows Desert Dust Cuts Colorado River Flow

Dust-covered snow in the San Juan Mountains. Image credit: NASA/JPL-Snow Optics Laboratory. Full image and caption

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NASA, JPL, September 20, 2010

PASADENA, Calif. -- Snowmelt in the Colorado River basin is occurring earlier, reducing runoff and the amount of crucial water available downstream. A new study shows this is due to increased dust caused by human activities in the region during the past 150 years.

The study, led by a NASA scientist and funded by the agency and the National Science Foundation, showed peak spring runoff now comes three weeks earlier than before the region was settled and soils were disturbed. Annual runoff is lower by more than five percent on average compared to pre-settlement levels.

The findings have major implications for the 27 million people in the seven U.S. states and Mexico who rely on the Colorado River for drinking, agricultural and industrial water. The results were published in this week's Proceedings of the National Academy of Sciences.

The research team was led by Tom Painter, a snow hydrologist at both NASA's Jet Propulsion Laboratory in Pasadena, Calif., and UCLA. The team examined the impact of human-produced dust deposits on mountain snowpacks over the Upper Colorado River basin between 1915 and 2003. Studies of lake sediment cores showed the amount of dust falling in the Rocky Mountains increased by 500 to 600 percent since the mid-to-late 1800s, when grazing and agriculture began to disturb fragile but stable desert soils.

The team used an advanced hydrology model to simulate the balance of water flowing into and out of the river basin under current dusty conditions, and those that existed before soil was disturbed. Hydrologic data gathered from field studies funded by NASA and the National Science Foundation, and measurements of the absorption of sunlight by dust in snow, were combined with the modeling.

More than 80 percent of sunlight falling on fresh snow is typically reflected back into space. In the semi-arid regions of the Colorado Plateau and Great Basin, winds blow desert dust east, triggering dust-on-snow events. When dark dust particles fall on snow, they reduce its ability to reflect sunlight. The snow also absorbs more of the sun's energy. This darker snow cover melts earlier, with some water evaporating into the atmosphere.

Earlier melt seasons expose vegetation sooner, and plants lose water to the atmosphere through the exhalation of vapor. The study shows an annual average of approximately 35-billion cubic feet of water is lost from this exhalation and the overall evaporation that would otherwise feed the Colorado River. This is enough water to supply Los Angeles for 18 months.

"The compressed mountain runoff period makes water management more difficult than a slower runoff," Painter said. "With the more rapid runoff under dust-accelerated melt, costly errors are more likely to be made when water is released from and captured in Colorado River reservoirs."

Prior to the study, scientists and water managers had a poor understanding of dust-on-snow events. Scientists knew from theory and modeling studies that dust could be changing the way snowfields reflect and absorb sunlight, but no one had measured its full impact on snowmelt rates and runoff over the river basin. The team addressed these uncertainties by making systematic measurements of the sources, frequency and snowmelt impact of dust-on-snow events.

"These researchers brought together their collective expertise to provide a historical context for how the Colorado River and its runoff respond to dust deposition on snow," said Anjuli Bamzai, program director in the National Science Foundation's Division of Atmospheric and Geospace Sciences in Arlington, Va. "The work lays the foundation for future sound water resource management."

Painter believes steps can be taken to reduce the severity of dust-on-snow events in the Colorado River basin. He points to the impact of the Taylor Grazing Act of 1934 for potential guidance on how dust loads can be reduced. The act regulated grazing on public lands to improve rangeland conditions. Lake sediment studies show it decreased the amount of dust falling in the Rocky Mountains by about one quarter.

"Restoration of desert soils could increase the duration of snow cover, simplifying water management, increasing water supplies and reducing the need for additional reservoir storage of water. Peak runoff under cleaner conditions would then come later in summer, when agricultural and other water demands are greater," Painter said.

"It could also at least partially mitigate the expected regional impacts of climate change, which include reduced Colorado River flows, increased year-to-year variability in its flow rate, and more severe and longer droughts," he added. "Climate models project a seven to 20 percent reduction in Colorado River basin runoff in this century due to climate change."

Other institutions participating in the study include the National Snow and Ice Center in Boulder, Colo.; U.S. Geological Survey Southwest Biological Center in Moab, Utah; University of Washington in Seattle; Center for Snow and Avalanche Studies in Silverton, Colo.; and the University of Colorado-NOAA Western Water Assessment in Boulder.

For more information about NASA and agency programs, visit: http://www.nasa.gov . JPL is managed for NASA by the California Institute of Technology in Pasadena.

Alan Buis, Jet Propulsion Laboratory, Pasadena, Calif. Tel.  (818) 354-0474. e-mail: Alan.buis@jpl.nasa.gov ; Steve Cole, NASA Headquarters, Washington, DC. Tel. (202) 358-0918. e-mail: Stephen.e.cole@nasa.gov 

Release 2010-306; link:  http://www.jpl.nasa.gov/news/news.cfm?release=2010-306

Thomas H. Painter et al., PNAS (2010), Response of Colorado River runoff to dust radiative forcing in snow

Proceedings of the National Academy of Sciences, published online before print September 20, 2010; doi:10.1073/pnas.0913139107

Response of Colorado River runoff to dust radiative forcing in snow

1. Thomas H. Painter^a,^b,*, 
2. Jeffrey S. Deems^c,^d, 
3. Jayne Belnap^e,
4. Alan F. Hamlet^f, 
5. Christopher C. Landry^g and 
6. Bradley Udall^d

+

1. ^aJet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109;
2. ^bJoint Institute for Regional Earth System Science and Engineering, University of California, Los Angeles, CA 90095;
3. ^cNational Snow and Ice Data Center, Boulder, CO 80309;
4. ^dNational Oceanic and Atmospheric Administration Western Water Assessment, Boulder, CO 80309;
5. ^eUnited States Geological Survey, Southwest Biological Center, Moab, UT 84532;
6. ^fUniversity of Washington, Department of Civil and Environmental Engineering, Seattle, WA 98195; and
7. ^gCenter for Snow and Avalanche Studies, Silverton, CO 81433

1. Edited by Peter H. Gleick, Pacific Institute for Studies in Development, Environment, and Security, Oakland, CA, and approved August 3, 2010 (received for review November 12, 2009)

Abstract

The waters of the Colorado River serve 27 million people in seven states and two countries but are overallocated by more than 10% of the river’s historical mean. Climate models project runoff losses of 7–20% from the basin in this century due to human-induced climate change. Recent work has shown however that by the late 1800s, decades prior to allocation of the river’s runoff in the 1920s, a five-fold increase in dust loading from anthropogenically disturbed soils in the southwest United States was already decreasing snow albedo and shortening the duration of snow cover by several weeks. The degree to which this increase in radiative forcing by dust in snow has affected timing and magnitude of runoff from the Upper Colorado River Basin (UCRB) is unknown. Here we use the Variable Infiltration Capacity model with postdisturbance and predisturbance impacts of dust on albedo to estimate the impact on runoff from the UCRB across 1916–2003. We find that peak runoff at Lees Ferry, Arizona, has occurred on average 3 wk earlier under heavier dust loading and that increases in evapotranspiration from earlier exposure of vegetation and soils decreases annual runoff by more than 1.0 billion cubic meters or ∼5% of the annual average. The potential to reduce dust loading through surface stabilization in the deserts and restore more persistent snow cover, slow runoff, and increase water resources in the UCRB may represent an important mitigation opportunity to reduce system management tensions and regional impacts of climate change.

• *Correspondence e-mail: Thomas.Painter@jpl.nasa.gov.

Link:  http://www.pnas.org/content/early/2010/09/14/0913139107.abstract