THEGREENGROK    Planetary Watch

As the World Warms II: How Goes It With the Land’s Biosphere?

by Bill Chameides | August 26th, 2010
posted by Wendy Graber (Researcher)

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Large-scale regional droughts (like the one in Australia) have contributed to a net decline in global NPP over the last ten years. Is this a decadal variation or the start of new trend?

Yesterday we looked at rice in a warming world, today the subject is land plants in toto – the terrestrial biosphere.

The Basics: Plants and Animals Inhaling and Exhaling

Remember learning about the carbon cycle in elementary school? You know, green plants take carbon dioxide (CO2) out of the air via photosynthesis (add oxygen to make sugars, etc.), and respirers reverse the process by burning the sugars (or their by-products) produced during photosynthesis to put CO2 back in the air and take oxygen out.  While photosynthesis is limited to green plants, the respirers include virtually every living thing, including the plants themselves that burn some of the stuff they make to support their own metabolic processes. It also includes creatures like ourselves who eat plants or eat creatures that eat plants. And most importantly it includes microbes that eat all the plant material that ends up in the soil.

Subtract Respiration From Photosynthesis and What Do You Get?

Typically, photosynthesis and respiration are very nearly balanced and so the whole the process leads to no net change in CO2.  But the two processes are not always exactly balanced. We call the difference between photosynthesis and respiration, net primary productivity or NPP. When photosynthesis is larger than respiration, for example in a forest that is in its growing phase, NPP is positive and there is a net uptake of CO2 and a net storage of carbon in the forest. (In a forest that is dying back, respiration is larger than photosynthesis and NPP is negative.)

For climate, we are most interested in the global NPP. A positive value of NPP means that the sum total of all life on land (the terrestrial biosphere) is taking CO2 out of the atmosphere and storing it on the land. That would be a good thing because it would be applying a much needed brake to the amount of CO2 we add to the atmosphere each year from burning fossil fuels and forests.

And as it turns out NPP is doing just that. We’ve known for quite some time, that only a fraction (~ 45 percent) of the CO2 we put into the atmosphere each year stays in the atmosphere. Where does the rest go? Some into the ocean, and some to the terrestrial biosphere – this latter amount being equivalent to the NPP of the terrestrial biosphere.

Inquiring Climate Scientists Want to Know: Whither NPP?

What is not known, and is a source of uncertainty in predicting the future course of global climate is how the terrestrial biosphere will respond to global warming. For example, we are pretty sure that increasing CO2 concentrations tend to enhance photosynthesis (we call that CO2 fertilization) and so tend to increase NPP and take a little more CO2 out of the atmosphere. The net effect is a negative feedback that dampens the increase in atmospheric CO2 from fossil fuel burning and deforestation. That’s good because it gives us more time to work on mitigating climate change. On the other hand, increasing temperatures make microbes work faster, putting more CO2 into the atmosphere and providing a positive feedback.

The critical question is then: what is the net feedback of all these types of processes?

To help answer that question, in 1981, we began putting instruments onto space-borne platforms capable of providing global data on the state of vegetation and amount of solar radiation they were absorbing. By combining these data with relevant climatic data, like local rainfall rates and temperatures, scientists can estimate the NPP in ecosystems around the globe and then by summing them up get the total global NPP for the terrestrial biosphere.

The Good Old ’80s and ’90s: Plants Doing Fine Thank You Very Much

One of the first comprehensive studies of terrestrial NPP using satellite data was carried out by Ramakrishna Nemani then of the University of Montana at Missoula and colleagues and published in Science in 2003. Their results were surprising and reassuring; it suggested that the terrestrial biosphere was doing just fine with global warming. They estimated that over an 18-year period of rapid warming from 1982 to 1999, NPP had increased by 6%.  Nemani et al. found that several factors including longer growing seasons in the Northern Hemisphere, wetter conditions in the Southern Hemisphere and increasing solar radiation due to a decrease in cloud cover over the tropics (especially over the Amazon) and western Europe contributed to the increase in NPP.

But Things Have Changed

Now a new study published last week in Science by Maosheng Zhao and Steven Running of the University of Montana at Missoula updates Nemani et al.’s work by looking at the most recent and warmest decade (2000 – 2009) on record. Their work points to a very different result.  They estimate that from 2000 to 2009, global terrestrial NPP did not increase, but instead decreased by about one percent. The decrease occurred primarily in the Southern Hemisphere (the Northern Hemisphere actually saw a net increase over the 10-year period) and was especially intense over large parts of South America, Africa, and Australia.

The reason? Climatic data indicate that in general the Southern Hemisphere has seen a slow drying trend over the past decade, punctuated by some very intense regional droughts, and their calculations suggest that this is what has driven the NPP decline. (Interestingly, NPP for Asian forests was more strongly influenced by reduced solar radiation.)

How significant was the NPP decrease? It’s equivalent to 0.55 petagrams of C over the 10-year period. (One petagram (Pg) is equal to one billion metric tons of carbon.) By comparison the amount of CO2 we put into the atmosphere each year just from burning fossil fuels is about 8 Pg of C per year. So the change is small, but it is of concern. These new results suggest that the terrestrial biosphere may be at a turning point.  One where it no longer acts to slow global warming, but to hasten it.

Another Cliffhanger?  Stay Tuned

From a negative feedback to a positive feedback. Is this a random, temporary variation or a permanent change as the world warms. Is this like just another day in our lives or a shift to another world? Hard to say. I guess you’ll have to wait for the next episode…er…study.

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