Aerosols

2009-12-08
PNAS: Black soot and the survival of Tibetan glaciers , doi:10.1073/pnas.0910444106 (pdf)
We find evidence that black soot aerosols deposited on Tibetan glaciers have been a significant contributing factor to observed rapid glacier retreat. Reduced black soot emissions, in addition to reduced greenhouse gases, may be required to avoid demise of Himalayan glaciers and retain the benefits of glaciers for seasonal fresh water supplies.


JGR: An observationally based energy balance for the Earth since 1950 , doi:10.1029/2009JD012105
Scientists have used a new approach to sharpen the understanding of one of the most uncertain of mankind’s influences on climate: the effects of aerosols (or atmospheric “haze”), the tiny airborne particles from pollution, biomass burning, and other sources.

Recognizing that simple analyses can reveal bulk trends in the Earth system without needing to use global climate models, Murphy et al. examined the Earth’s energy balance since 1950, focusing specifically on how this governs the warming Earth.

The new observations-based study confirms that aerosols have the net effect of cooling the planet—in agreement with previous understanding—but arrives at the answer in a completely new way that is more straightforward and narrows the uncertainties of the estimate.

In balancing the budget for the processes perturbing the heating and cooling of the Earth, Murphy and colleagues found that since 1950, the planet released about 20 percent of the warming influence of heat-trapping greenhouse gases to outer space as infrared energy. Volcanic emissions lingering in the stratosphere offset about 20 percent of the heating by bouncing solar radiation back to space before it reached the surface. Cooling from the lower-atmosphere aerosols produced by humans balanced 50 percent of the heating. Only the remaining 10 percent of greenhouse-gas warming actually went into heating the Earth, and almost all of it went into the ocean.


2009-06-18
ScienceDaily: Some Particles Cool Climate, Others Add To Global Warming
There is large scientific agreement that human made emissions of CO2 and other gasses give global warming. But human activity doesn’t just cause gas emissions. Burning of fossil fuels and biomass also causes emissions of the particle black carbon. Other kinds of particles are formed in the atmosphere as a cause of human made emissions.
Science: Consistency Between Satellite-Derived and Modeled Estimates of the Direct Aerosol Effect
DOI: 10.1126/science.1174461
The direct aerosol effect has a radiative forcing estimate of –0.5 Wm–2 in the IPCC AR4, offsetting the warming from CO2 by almost one-third. The uncertainty range, however, ranges from –0.9 to –0.1 Wm–2, largely due to differences between estimates from global aerosol models and observation-based estimates, with the latter tending to have stronger (more negative) radiative forcing. This study demonstrates consistency between a global aerosol model and adjustment to an observational-based method, giving a global and annual mean radiative forcing weaker than –0.5 Wm–2 with a best estimate of –0.3 Wm–2. The physical explanation for the earlier discrepancy is that the relative increase in anthropogenic black carbon (absorbing aerosols) is much larger than the overall increase in the anthropogenic abundance of aerosols.
From the Science Podcast: an interview with Gunnar Myhre on the consistency between satellite-derived and modeled estimates of the direct aerosol effect.
2009-04-08
NASA: Aerosols May Drive a Significant Portion of Arctic Warming
Though greenhouse gases are invariably at the center of discussions about global climate change, new NASA research suggests that much of the atmospheric warming observed in the Arctic since 1976 may be due to changes in tiny airborne particles called aerosols.

Emitted by natural and human sources, aerosols can directly influence climate by reflecting or absorbing the sun’s radiation. The small particles also affect climate indirectly by seeding clouds and changing cloud properties, such as reflectivity.

A new study, led by climate scientist Drew Shindell of the NASA Goddard Institute for Space Studies, New York, used a coupled ocean-atmosphere model to investigate how sensitive different regional climates are to changes in levels of carbon dioxide, ozone, and aerosols.

The researchers found that the mid and high latitudes are especially responsive to changes in the level of aerosols. Indeed, the model suggests aerosols likely account for 45 percent or more of the warming that has occurred in the Arctic during the last three decades. The results were published in the April issue of Nature Geoscience.
Nature Geoscience: Climate response to regional radiative forcing during the twentieth century doi:10.1038/ngeo473
Regional climate change can arise from three different effects: regional changes to the amount of radiative heating that reaches the Earth’s surface, an inhomogeneous response to globally uniform changes in radiative heating and variability without a specific forcing. The relative importance of these effects is not clear, particularly because neither the response to regional forcings nor the regional forcings themselves are well known for the twentieth century. Here we investigate the sensitivity of regional climate to changes in carbon dioxide, black carbon aerosols, sulphate aerosols and ozone in the tropics, mid-latitudes and polar regions, using a coupled ocean–atmosphere model. We find that mid- and high-latitude climate is quite sensitive to the location of the forcing. Using these relationships between forcing and response along with observations of twentieth century climate change, we reconstruct radiative forcing from aerosols in space and time. Our reconstructions broadly agree with historical emissions estimates, and can explain the differences between observed changes in Arctic temperatures and expectations from non-aerosol forcings plus unforced variability. We conclude that decreasing concentrations of sulphate aerosols and increasing concentrations of black carbon have substantially contributed to rapid Arctic warming during the past three decades.


Atmospheric Aerosol Properties and Climate Impacts (pdf)
U.S. Climate Change Science Program, Synthesis and Assessment Product 2.3, January 2009
Atmospheric aerosols are suspensions of solid and/or liquid particles in air. Aerosols are ubiquitous in air and are often observable as dust, smoke, and haze. Both natural and human processes contribute to aerosol concentrations. On a global basis, aerosol mass derives predominantly from natural sources, mainly sea salt and dust. However, anthropogenic (manmade) aerosols, arising primarily from a variety of combustion sources, can dominate in and downwind of highly populated and industrialized regions, and in areas of intense agricultural burning.

Aerosols affect Earth’s energy budget by scattering and absorbing radiation (the “direct effect”) and by modifying amounts and microphysical and radiative properties of clouds (the “indirect effects”). Aerosols influence cloud properties through their role as cloud condensation nuclei (CCN) and/or ice nuclei. Increases in aerosol particle concentrations may increase the ambient concentration of CCN and ice nuclei, affecting cloud properties. A CCN increase can lead to more cloud droplets so that, for fixed cloud liquid water content, the cloud droplet size will decrease. This effect leads to brighter clouds (the “cloud albedo effect”). Aerosols can also affect clouds by absorbing solar energy and altering the environment in which the cloud develops, thus changing cloud properties without actually serving as CCN. Such effects can change precipitation patterns as well as cloud extent and optical properties.

On a global average basis, the sum of direct and indirect forcing by anthropogenic aerosols at the top of the atmosphere is almost certainly negative (a cooling influence), and thus almost certainly offsets a fraction of the positive (warming) forcing due to anthropogenic greenhouse gases. However, because of the spatial and temporal non-uniformity of the aerosol RF, and likely differences in the effects of shortwave and longwave forcings, the net effect on Earth’s climate is not simply a fractional offset to the effects of forcing by anthropogenic greenhouse gases.


Aerosols: Volcanoes, Dust, Clouds and Climate
Haze from small particles surely affected climate, but how? Old speculations about the effects of smoke from volcanoes were brought to mind in the 1960s, when urban smog became a major research topic. Some tentative evidence suggested that aerosols emitted by human industry and agriculture could change the weather. A few scientists exclaimed that smoke and dust from human activities would cause a dangerous global cooling. Or would pollution warm the atmosphere? Theory and data were far too feeble to answer the question, and few people even tried to address it. Among these few, the uncertainties fueled vigorous debates, in particular over how adding aerosols might change the planet’s cloud cover. Finally, in the late 1970s, powerful computers got to work on the stupefyingly complex calculations, helped by data from volcanic eruptions. It became clear that overall, human production of aerosols was cooling the atmosphere. Pollution was significantly delaying, and concealing, the coming of greenhouse effect warming.


2009-01-16
NASA: Report Calls Aerosol Research Key to Improving Climate Predictions
WASHINGTON — Scientists need a more detailed understanding of how human-produced atmospheric particles, called aerosols, affect climate in order to produce better predictions of Earth’s future climate, according to a NASA-led report issued by the U.S. Climate Change Science Program on Friday.

Atmospheric Aerosol Properties and Climate Impacts (PDF)


2008-11-13
United Nations Environment Programme: Atmospheric Brown Clouds (pdf)
After more than a century of scientific studies on greenhouse gases (GHGs) and chlorofluorocarbons (CFCs), we have today a fair understanding of the ‘global warming’ and ‘ozone hole’ issues. Many studies as well as policies are under implementation to better understand and address these issues.
Recent scientific studies have revealed a new atmospheric issue: Atmospheric Brown Clouds (ABC). The brown haze is caused by air pollution, mainly the sub-micron size aerosol particles, emitted from a wide range of anthropogenic and natural sources. Through the studies initiated under ABC project, scientists now have an overall view of the major sources and the global scale nature of the brown cloud problem.


2009-03-09
PNAS: Toxicity of atmospheric aerosols on marine phytoplankton (pdf) doi: 10.1073/pnas.0811486106
Dust blown off the continents and deposited in the open ocean is an important source of nutrients for marine phytoplankton, the tiny algae that are the foundation of the ocean food web. But new findings show that some sources of dust also carry toxic elements that can kill marine phytoplankton.

The study has several implications for climate change researchers, Adina Paytan (first author) said. The amount of dust blowing off the land may change as a result of changes in terrestrial ecosystems. The effects of the dust on phytoplankton growth can be positive or negative, depending on the balance of nutrients and toxins. And as marine phytoplankton grow and photosynthesize, they draw carbon dioxide out of the atmosphere, reducing the atmospheric concentration of the greenhouse gas.

Abstract:
Atmospheric aerosol deposition is an important source of nutrients and trace metals to the open ocean that can enhance ocean productivity and carbon sequestration and thus influence atmospheric carbon dioxide concentrations and climate. Using aerosol samples from different back trajectories in incubation experiments with natural communities, we demonstrate that the response of phytoplankton growth to aerosol additions depends on specific components in aerosols and differs across phytoplankton species. Aerosol additions enhanced growth by releasing nitrogen and phosphorus, but not all aerosols stimulated growth. Toxic effects were observed with some aerosols, where the toxicity affected picoeukaryotes and Synechococcus but not Prochlorococcus. We suggest that the toxicity could be due to high copper concentrations in these aerosols and support this by laboratory copper toxicity tests preformed with Synechococcus cultures. However, it is possible that other elements present in the aerosols or unknown synergistic effects between these elements could have also contributed to the toxic effect. Anthropogenic emissions are increasing atmospheric copper deposition sharply, and based on coupled atmosphere–ocean calculations, we show that this deposition can potentially alter patterns of marine primary production and community structure in high aerosol, low chlorophyll areas, particularly in the Bay of Bengal and downwind of South and East Asia.


2009-02-12
CSIRO Australia : Aerosols – their part in our rainfall
Aerosols may have a greater impact on patterns of Australian rainfall and future climate change than previously thought, according to leading atmospheric scientist, CSIRO’s Dr Leon Rotstayn.

Fast Facts:

  • Aerosols may have a greater impact on patterns of Australian rainfall and future climate change than previously thought, according to leading atmospheric scientist
  • Aerosols are fine particles suspended in the atmosphere. Sources of human-generated aerosols include industry, motor vehicles and vegetation burning
  • Natural sources include volcanoes, dust storms and ocean plankton. Human-generated aerosols have long been known to exert a cooling effect on climate
  • As aerosol pollution is predicted to decrease over the next few decades, unmasking of the greenhouse effect may lead to accelerated global warming

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