MADE IN THE SHADE: Plants perform photosynthesis more efficiently under hazy skies, which means cleaning up air pollution may exacerbate climate change.
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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."
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10 Comments
Add CommentGlobal warming we can deal with. It may be inconvenient, but it can be done.
Reply | Report Abuse | Link to thisPollution will kill us.
I'm not so sure Silvrhairdevil. Particulae pollution has been with us for several centuries, since the start of the industrial revolution. It harms us, but so do many things.
Reply | Report Abuse | Link to thisConsider - we, the species, have been dealing with global climate changes for millions of years.
Reply | Report Abuse | Link to thisPollution is a relatively new problem. With all the focus on GW, pollution (which we actually can do something about) is being left behind in the consciousness sweepstakes.
Pollution will poison us, sooner or later. Global Warming will only open new territory while closing some of the areas we use now.
global warming is a thoughtless sentient being sent by the god of thor to rape our children and pillage our villages without any moral thought for its actions.
Reply | Report Abuse | Link to thisAlthough this planet might become to polluted to use there are other planets that can be terraformed to suit the needs of our civilisation, so in retrospect i suggest that the governments of all the large nations in the world start to develop a plasma driven space shuttle large enough to transport a huge amount of Earth's population to the newly terraformed Red Planet. Ion is just to unstable and lives would be lost in the valiant struggle to keep our species alive.
Reply | Report Abuse | Link to thisDrastically increase the quantity of carbon sequestering, pollution mitigating trees...
Reply | Report Abuse | Link to thisfrom personal experience i know that its the people like twala that are ruining our planet.
Reply | Report Abuse | Link to thisIt is an interesting article that demonstrates the difficulties of modeling potential future climate changes.
Reply | Report Abuse | Link to thisMy personal view is that we should first control what we know is definitely harmful as well as clean up localized areas of toxicity from past dumping. Many chemicals and particulates are harmful to us and other animals and it is not a point that can be debated like GW. Yet we haven't taken proper care of the obvious.
Of course GW may just be a means to this ends. Plus maybe global warming will happen as some people predict. Obviously the human race will survive but the Earth reacts alot better to slow changes rather than fast ones.
On the flip side I don't know how we can possibly argue that countries like China and India can't burn up all the coal they can get ahold of. They deserve cheap power just like many in completely indutrialized nations already have. I actually don't see how we can stop the burning of all fossil fuels by the end of this century.
Perhaps it is time that everyone gave up smoking?
Reply | Report Abuse | Link to thiscleaning up air pollution & removing particulates out of the air would speed global warming because it would allow more insolation to reach the earth's surface. countless studies have been done by japanese climatologist atsumu ohmura since the 1980's which have shown that the amount & intensity of solar radiation hitting the earth's surface had significantly declined, up to about 35% net decline in insolation. this is because sulfate aerosols & particulates that are emitted as a result of our energy production create more surface area in the upper atmosphere on which water molecules can adhere to, adding additional reflectivity or "albedo" to the clouds, essentially turning them into massive mirrors & reflectors.
Reply | Report Abuse | Link to thisskeptics of global warming often fall back on the fact that increases in global temperature have been .6-.7 degrees centigrade since the dawn of the industrial era, saying those increases are minute, not worth worrying about & caused by something other than human behavior. however, climatically that amount & rate of temperature increase in as little as 150 years is dramatic, unusual & absent elsewhere in the earth's geographic timeline. that is to say, global dimming, the decrease in solar irradiance due to the massive emission of sulfate aerosols & particulates in the last 50 years, actually masks the actual amount of warming.
many may jump to the conclusion to say since these aerosols & particulates are keeping us from encountering a runaway global warming freak out that we should emit MORE into the atmosphere. however, the emission of these aerosols and particulates go hand in hand with the emission of man-produced greenhouse gases, the molecules responsible for climate change in the first place. this is why a push must be made politically to develop feasible, affordable & effective renewable AS WELL AS clean methods of energy production. natural gas is clean, but it still contributes enormous amounts of GHG. equally stringent focus must be put on renewables such as wind, solar & hydrokinetic turbines along with the further development of natural gas vehicles & more efficient power grids. geothermal & hydro-power in the form of dams is great in some places, but those methods present a whole other slew of issues regarding the loss of exorbitant amounts of water in the process (losses due to irreversible degradation of water in geotherm. & through evaporation in damming).