Air pollution helps plants blunt climate change: study

Marlowe Hood Yahoo News 22 Apr 09;

PARIS (AFP) – Cleaning up skies choked with smog and soot would sharply curtail the capacity of plants to absorb carbon dioxide and blunt global warming, according to a study released on Wednesday.

Plant life -- especially tropical forests -- soak up a quarter of all the CO2 humans spew into the atmosphere, and thus plays a critical role in keeping climate change in check.

Through photosynthesis, vegetation transforms sunlight, CO2 and water into sugar nutrients.

Common sense would suggest that air pollution in the form of microscopic particles that obstruct the Sun's rays -- a phenomenon called "global dimming" -- would hamper this process, but the new study shows the opposite is true.

"Surprisingly, the effects of atmospheric pollution seem to have enhanced global plant productivity by as much as a quarter from 1960 to 1999," said Linda Mercado, a researcher at the Met Office Hadley Centre in Britain, and the study's lead author.

"This resulted in a net ten percent increase in the amount of carbon stored by the land," she said in a statement.

Global dimming was especially strong from the 1950s up through the 1980s, corresponding to the period of enhanced plant growth, notes the study, published in the British journal Nature.

Research published last month found that dimming has since continued almost everywhere in the world except Europe.

The explanation for this botanical paradox lies in the way particle pollution reflects light.

Even if plants receive less direct sunshine, the presence of clouds and pollution scatter the light that does filter through such that fewer leaves -- which is where photosynthesis occurs -- wind up in total shade.

"Although many people believe that well-watered plants grow best on a bright sunny day, the reverse is true. Plants often thrive in hazy conditions," said colleague and co-author Stephen Sitch.

This process of diffuse radiation is well known. But the new study is the first to use a global model to calculate its impact on the ability of plants to absorb CO2.

The findings underline a cruel dilemma: to the extent we succeed in reducing aerosol pollution in coming decades, we will need to slash global carbon dioxide emissions even more than we would have otherwise.

"Aerosols offset approximately 50 percent of the greenhouse gas warming," Knut Alfsen, research director at the Centre for International Environmental Research in Oslo, Norway, said by phone.

Without this particle pollution, he said, average global surface temperatures would have increased by 1.0 to 1.1 Celsius (1.8 to 2.0 Fahrenheit) since the start of industrialisation, rather than 0.7 C (1.25 F).

The UN's Intergovernmental Panel on Climate Change (IPCC) has predicted that average global temperatures will rise before 2100 by 1.1 to 6.4 C (2.0 F to 11.5 F), depending on efforts to curb the gases that drive global warming.

Any increase above 2.0 C, the panel said, would unleash a maelstrom of human misery, including drought, famine, disease and forced migration.

To stay below that threshold, carbon dioxide concentrations in the atmosphere must be kept below 450 parts per million (ppm). The current level is about 385 ppm.

"As we continue to clean up the air -- which we must do for the sake of human health -- the challenge of avoiding dangerous climate change through reductions in CO2 emissions will be even harder," said Peter Cox, a researcher at Britain's University of Exeter and a co-author of the Nature study.

A major scientific review released last week at the United Nations showed that warming is itself limiting the capacity of plants to take up CO2, and that an increase in two degrees Celsius (3.6 degrees Fahrenheit) would transform forests from a sink into a net source of CO2.

When plants die, the carbon they store is released into the air.

Could Cleaning Up Air Pollution Actually Speed Up Global Warming?
Scientific American 23 Apr 09;

Taking the haze out of the atmosphere may make plants less efficient at absorbing and storing carbon dioxide

Do plants prefer the hazy skies brought on by pollution to the clean atmosphere envisioned by environmentalists, regulators and the public? That's the implication of a new study of exactly how much plants, ranging from broadleaf trees to grasses, have been benefiting from the pollution brought on by the particles—from soot to sulfur dioxide molecules—that burning fossil fuels leaves in the air.

That apparent benefit is because plants do their best photosynthesis—the chemical process that uses chlorophyll in their leaves to turn sunlight and carbon dioxide (CO2) into plant food and oxygen—under so-called diffuse radiation, or hazy skies, that scatters the sunlight, thereby distributing it more evenly. Ecosystem modeler Lina Mercado from the Center for Ecology and Hydrology headquartered in Wallingford, England, and her colleagues' study, published in Nature, found that plants stored 23.7 percent more CO2—the leading greenhouse gas causing climate change—between 1960 and 1999 thanks to more efficient photosynthesis brought on by air pollution scattering sunlight. Less CO2 storage in the plant "carbon sink" means more in the atmosphere, accompanied by more global warming.

"Plants often thrive in hazy conditions such as those that exist during periods of increased atmospheric pollution," Mercado says. But as the skies are cleaned up, "the contribution of diffuse radiation to the land carbon sink could disappear by the end of the 21st century."

The modeling results mimic the impact of the 1991 eruption of Mount Pinatubo in the Philippines that increased atmospheric haze and resulted in lower global atmospheric concentrations of CO2 in 1992 and 1993. All told, polluted skies resulted in the uptake of an extra 440 million metric tons of carbon per year between 1960 and 1980, declining to just 300 million metric tons of carbon per year between 1980 and 1999. Further declines are expected as skies clear going forward as U.S. and European Union regulations reduce sulfur dioxide and particulate emissions by attaching scrubbers to smokestacks, among other efforts. Although the scrubbers and other pollution controls are being added to combat aerosols, among other air pollution problems, few such measures are in place for CO2. "We conclude that steeper cuts in fossil-fuel emissions will be required to stabilize the climate if anthropogenic aerosols decline," the researchers wrote.

To be clear: there is no doubt that aerosols are bad for human health. The tiny particles contribute outsize health effects ranging from asthma to heart disease. That's why environmental regulators have focused on reducing and removing the aerosols from the atmosphere.

And aerosols have other environmental impacts, which this modeling study did not examine. "Aerosols and clouds affect not only diffuse and direct-beam forms of radiation but also other factors such as temperature and precipitation that are also important for the dynamics of the terrestrial carbon cycle," says environmental scientist Lianhong Gu of Oak Ridge National Laboratory, who first showed the effect of the Pinatubo eruption on photosynthesis. This "indicates that the two most uncertain factors in projecting climate change—clouds and aerosols—are more uncertain than we thought."

And that points up the complexity of trying to design policies to mitigate both air pollution and climate change. Hazy interactions can be important, both for carbon-sink reasons and the link between irrigated agriculture and declining rainfall, says climatologist Dev Niyogi of Purdue University. Further studies will be needed and the lifetimes of the various pollutants taken into account—aerosols last just a few weeks at most in the atmosphere, whereas CO2 can linger for a century—and atmospheric changes must be examined as a whole, he notes.

But it now seems clear that "global dimming" "has acted to increase the uptake of carbon by the land [plants], which has helped to slow the rate of increase in atmospheric CO2," Mercado notes. And her co-author climate modeler Peter Cox of the University of Exeter in England adds: "As we continue to clean up the air in the lower atmosphere, which we must do for the sake of human health, the challenge of avoiding dangerous climate change through reductions in CO2 emissions will be even harder."