Plants, Temperature and CO2 Uptake
- Carbon Sink Or Carbon Source? Aerosols Play Significant Role In Shifts
Raleigh NC (SPX) Dec 09, 2004
Researchers at North Carolina State University have shown that the
amount of aerosols - dust particles, soot from automobile emissions
and factories, and other airborne particles - in the atmosphere has a
significant impact on whether the surface area below either absorbs
or emits more carbon dioxide (CO2).
The researchers discovered that changes in the levels of airborne
aerosols resulted in changes to the terrestrial carbon cycle, or the
cycle in which CO2 is absorbed by plant photosynthesis and then
emitted by the soil.
Besides documenting the effects of aerosols on the carbon cycle, the
research also showed that the type of landscape also influenced
whether a surface area served as a carbon sink, an area that absorbs
more CO2 than it emits, or as a carbon source, an area that emits
more CO2 than it absorbs.
[Caption] Researchers used data from six sites within NASA's AERONET
(AErosol RObotic NETwork). Sites represented a wide variety of
landscapes, including forests, cropland and grassland. This site in
Walker Branch, Tenn., shows a sun photometer over a broadleaf
deciduous forest. The sun photometer measures radiation and aerosol
properties that impact light.
In the research project, six locations across the United States -
encompassing forests, croplands and grasslands - were studied.
Increased amounts of aerosols over forests and croplands resulted in
surface areas below becoming carbon sinks, but increased amounts of
aerosols over grasslands resulted in surface areas becoming carbon
Dr. Dev Niyogi, research assistant professor of marine, earth and
atmospheric sciences at NC State and lead author of the study,
hypothesizes that the differences among landscapes can be attributed
to the amount of shade provided by tree and plant leaves in forests
The lack of shading in grasslands changes the ground surface
temperature, which alters the rate of photosynthesis in plants and
the CO2 emissions by soil. Since plants want to take in CO2 but also
preserve water at the same time, Niyogi believes the lack of shade
and increased temperatures may cause plants to slow the rate of
photosynthesis, causing less CO2 to be absorbed and thus more CO2 to
be effectively emitted. That would make the surface area a carbon
The research was published in Geophysical Research Letters, a journal
of the American Geophysical Union. Niyogi's co-authors on the
research paper include NC State graduate student Hsin-I Chang; Dr.
Vinod Saxena, professor of marine, earth and atmospheric sciences at
NC State; Dr. Randy Wells, professor of crop science at NC State; Dr.
Fitzgerald Booker, associate professor of crop science at NC State
and USDA-ARS plant physiologist; Dr. Teddy Holt, adjunct professor of
marine, earth and atmospheric sciences at NC State and a scientist at
Naval Research Laboratory-Monterey; and colleagues from across the
Aerosols have been known to affect the climate by changing the
radiation that reaches the earth surface. Increase in aerosols is
often considered one possible reason that the earth's surface has not
seen as much warming as previously projected by climate models.
Previous studies have shown that many factors affect the carbon
cycle, including rainfall and changes in land cover. But this study
is believed to be the first multisite, observational analysis
demonstrating that aerosols affect the carbon cycle. The study shows
aerosols affect the earth's regional climate in an even more profound
manner by affecting its biological and chemical exchanges of the
The study examined six sites across the United States in the
summertime; these locations were chosen because data on aerosols and
carbon fluxes, or the changes in the carbon absorption and emission
rates, were readily available. Sites ranged from grassland in Alaska
to mixed forestland in Wisconsin to cropland in Oklahoma.
Before showing the effects of aerosols on the carbon cycle, the paper
first showed the effects of diffuse radiation - radiation that is not
direct sunlight but radiation scattered by clouds, haze, or something
else - on carbon fluxes. The research showed that higher levels of
diffuse radiation resulted in higher rates of carbon sink.
Although common sense would suggest that areas with plants receiving
more constant direct sunlight would result in a surface becoming a
carbon sink, that is not necessarily the case, Niyogi says. In fact,
more radiation means plants more quickly reach a level of
As Niyogi explains it, "Plants absorb CO2 very efficiently. At very
high levels of radiation, as is the case with direct radiation,
additional increases do not necessarily cause increased
photosynthesis. It doesn't matter how much more radiation you add,
the plant is not going to absorb more CO2. But at lower levels of
radiation, as is the case with diffuse radiation, any increase in
radiation translates to additional photosynthesis."
The study then examined the effects of cloudiness on the carbon
cycle. Cloudiness, which increased the amount of diffuse radiation,
resulted in a greater amount of carbon sink in surface areas.
The study team then linked aerosols and diffuse radiation, and showed
strong relationships between high amounts of aerosols and high
amounts of diffuse radiation and between low amounts of aerosols and
low amounts of diffuse radiation.
Finally, the study yielded its most important findings: Aerosols
affect the carbon cycle in different types of landscapes, with
forests and croplands serving as carbon sinks while grasslands served
as carbon sources.
"When you have more carbon being absorbed, it means that plants and
forests there are going to grow faster," Niyogi said. "And so it has
the potential to alter the landscape. And when you have a change in
landscape, or a change in the biogeochemical properties - like the
carbon cycle - you have a landscape that is actively vulnerable to
"Studies like these can really start putting forward the right
processes in trying to quantify the carbon sink more accurately. Once
we start introducing these reality-based processes into our models,
we'll get better estimates" of carbon budget, Niyogi said.
Niyogi now plans to add other variables to studying the carbon cycle,
such as the effects of different types of aerosols, and factors like
soil moisture. He is also planning regional and global analyses -
using satellite remote sensing and models - to see if results square
with the field studies.
The research was funded by NASA, the National Science Foundation, the
Office of Naval Research, and an NC State Faculty Research and
Professional Development Award.