Looking ahead: Carbon sinks in 2100

Looking ahead: Carbon sinks in 2100
February 1, 2001

But let the ruffian Boreas once enrage
The gentle Thetis, and anon behold
The strong-ribbed bark through liquid mountains cut,
Bounding between the two moist elements
Like Perseus’ horse.

—Wm. Shakespeare
Troilus and Cressida

The latest climate models cast a baleful future abetted by plants and soil. In the models, future global warming upsets the ability of plants and soil to hold carbon—so much so that they will add to the air’s carbon dioxide beyond the direct emissions from human activities, thus worsening global warming. How?

So far less than half of the 6.5 billion tons of carbon emitted annually as carbon dioxide from human actions goes into the air. The larger fraction ends up in the ocean or on land (i.e., in plants and soil). Each year photosynthesis and respiration, mostly in the world’s forests and savannahs, take up and release about 60 billion tons of carbon. Uptake wins over release by about 2 billion tons annually, and plants and soil store the difference.

In sunlight, plants take up carbon dioxide and, through photosynthesis, hold carbon by building leaves, stems, and roots. Soil also harbors carbon, in the form of plant detritus such as dead leaves and wood, besides the microbes that feed on the plant matter. Soil releases carbon and thus can return carbon dioxide to the air when microbes decompose plant material.

Temperature and precipitation changes may vary the distribution of carbon among these reservoirs—plants, soil, and air—but the outcome is difficult to predict. Plant and soil respiration rates generally increase with temperature on a day-to-day basis. But when the air’s carbon dioxide content rises, plants grow more vigorously and so store more carbon, while soil continues to hold or even absorb more carbon.

What is the net effect in the instance of global warming from an increase of the air’s carbon dioxide content? P. Cox and colleagues project the climate effect of plants and soil out to the year 2100 under an assumed rise to an atmospheric carbon dioxide level of 700 parts per million (ppm)—almost twice the current level.

In their model, the Amazon area becomes warmer and drier and less able to soak up carbon in its soil. Indeed, so great is the decline in the ability of Amazonian soil to hold carbon that the net global effect is to turn land from a carbon sink to a source of atmospheric carbon dioxide by around 2050.

By 2100 plants and soil emit an extra 280 ppm of carbon dioxide, adding to the prescribed forcing of 700 ppm, for a total of 980 ppm in the air. Correspondingly, the air temperature rises by a dramatic 8°C over land compared with 5.5°C without the plant and soil effect.

Land switches from a sink to a source of carbon because the model assumes soil respiration rates increase exponentially as temperature rises over time. But, as Cox notes, that notion remains “a subject of debate.”

Does the increase of ecosystem respiration with rising temperature, a physiological effect that operates over days to weeks, persist over decades—the timeframe of the calculation? Does increased respiration overwhelm the growth in carbon storage by plants and soil through enhanced photosynthesis at elevated levels of atmospheric carbon dioxide?

Many field and lab experiments contradict that key model assumption. New findings indicate soil respiration is not very sensitive to changes in temperature over long periods.

J. Van Ginkel and colleagues find that over periods of months, soil around ryegrass holds more carbon when carbon dioxide air content and temperature both increase. C. Giardina and M. Ryan studied 82 sites over five continents and found little change in soil respiration as temperature changed. One reason is that, over decades, the enzyme action of microbes, which releases carbon dioxide from the soil, is not sensitive to temperature. It depends more on other factors, such as available nutrients.

J. Grace and M. Rayment model a European conifer forest out to the year 2100 in a globally warmed world. Case 1 assumes that over decades the ecosystem respiration is sensitive to temperature rise. For Case 1, the ability of the simulated forest to absorb carbon dioxide decreases slightly over time as temperature rises.

By contrast, Case 2 is based on the long-term physiology pointed out by Giardina and Ryan. Here, the forest uptake of carbon doubles by the year 2100, owing to the strong fertilization effect of the air’s elevated carbon dioxide content on plants and soil. According to Grace and Rayment, it may be “misleading” to use the short-period, rather than the relevant long-period, process of carbon release by soil microbes to calculate the vegetation effect on climate.

“Does this mean,” they conclude, “that the doomsday view of runaway global warming now seems unlikely? We hope so.”


Sallie Baliunas, Ph.D., and Willie Soon, Ph.D., are colleagues at the Harvard-Smithsonian Center for Astrophysics.


References

Cox, P. M., et al., 2000. Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model. Nature, 408, 184-187.

Giardina, C.P., and M.G. Ryan, 2000. Evidence that decomposition rates of organic carbon in the mineral soil do not vary with temperature. Nature, 404, 858-861.

Grace, J., and M. Rayment, 2000. Respiration in the balance. Nature, 404, 819-820.

Van Ginkel, J. H., A. Gorissen, and D. Polci, 2000. Elevated atmospheric carbon dioxide concentration: Effects of increased carbon input in a Lolium perenne soil on microorganisms and decomposition. Soil Biology and Biochemistry, 32, 449-456.