Calculating the impact of aerosols

by Michael Richardson

SINGAPORE — Scientists have developed an extensive understanding of the impact that carbon dioxide, methane, nitrous oxide and other global warming gases have on Earth’s climate.

These are large-scale emissions from human activity such as burning coal, oil and natural gas, and clearing forests for farming, urban growth and industry.

However, much less is known about the impact of aerosols on the climate. Aerosols are tiny liquid and solid particles suspended in the atmosphere. They come in many different forms, from both man-made and natural sources.

As sunlight passes through the atmosphere, some aerosols scatter light back into space, cooling Earth’s surface. Other aerosols absorb some of the light and have a warming effect. Among them are soot from incomplete burning of fossil fuels, wood, farm residue and village cow dung fires in Asia.

Last month, a study sponsored by two United Nations agencies and the Stockholm Environment Institute found that strict curbs on black carbon soot, and ozone in the lower atmosphere, could quite quickly reduce future global warming by 0.5 degree C. This would give more time to agree on costly steps to reduce mainstream greenhouse gases, especially carbon dioxide, which persists for a long time in the atmosphere.

Since the average land and sea surface temperature has risen by nearly 0.8 C since the start of the industrial revolution, a 0.5 C offset would be significant.

In December the international community agreed in Cancun, Mexico, to hold the rise in world temperature to below 2 C above pre-industrial levels. Many scientific studies have warned that to go higher would risk dangerous climate change from extreme weather and rising sea levels.

Ozone is a key component of urban smog. The U.N.-backed study said that the measures it was recommending on ozone and soot control would clear the air, cut illness and premature human deaths from pollution, and improve crop yields, especially in Asia.

Of the 25 computer climate models included in a report in 2007 by scientists advising the U.N. on climate change, only a handful considered the role of aerosols other than sulphates, a well-studied category from volcanic eruptions and coal burning.

Less than a third of the panel’s climate models included aerosol impacts on clouds, even in a limited way. Those that did only considered sulphates. Clouds that form in the presence of aerosols can be more reflective when human-produced aerosols are present.

The need for better measurement of aerosols is obvious. Almost all remote sensing of aerosols from satellites in space is limited to detecting the total depth of the particles. It cannot differentiate between soot, nitrates from the spread of farm fertilizer, dust whipped up by desert winds, salt from sea spray, and sulphates from volcanoes or plankton plants in the ocean.

As a result, it is difficult to distinguish precisely how much of the aerosol layer is natural or from human activity. In fact, the range of uncertainty associated with the climate impact of aerosols may be as much as three or four times that of mainstream greenhouse gases.

So the loss in a launch failure earlier this month of a U.S. Earth observation satellite, known (ironically) as Glory, is big blow to climate science. The Earth-facing side of the spacecraft was carrying the world’s most advanced aerosol sensor. It would have enabled researchers to distinguish between the types and amounts of different aerosols in the atmosphere.

The side of the satellite facing the sun was carrying an instrument to more accurately measure the intensity of solar radiation at the top of Earth’s atmosphere. This is another major area of uncertainty — and controversy — in climate science. Researchers used to believe that incoming energy from the sun, the foundation of Earth’s climate, was constant.

However, since modern satellite instruments started monitoring total solar irradiance in 1978, they have shown that solar heat fluctuates by about 0.1 percent over an 11-year cycle as the sun passes through periods of more and less intense electromagnetic activity.

These fluctuations are not nearly big enough to explain the global warming of the last few decades as man-made greenhouse gas emissions have intensified. But scientists say that over the longer-term solar cycles may have an impact on climate and need to be better understood.

The U.S. program to observe the Earth and sun from space has not only suffered from satellite losses in the past few years. It is also being crimped by budget cuts as pressure grows to reduce America’s huge deficit.

In an ideal world, all major space-faring nations, including China, Japan and India, would cooperate to advance scientific knowledge and close the gaps in the U.S. satellite program, since Asia would be hit hard by any serious climate change.

This is not happening at present. In the meantime, the U.S. must try to sustain continuity in its earth and solar observation satellite network. One way could be to work more closely with European and Japanese space and science agencies.

The satellites the U.S., Europe and Japan operate provide increasingly accurate insights into the way Earth’s complex climate works. Without them, we would be groping in the dark.

Michael Richardson is a visiting senior research at the Institute of South East Asian Studies in Singapore.