SCIENCE FOCUS: SeaWiFS AND GLOBAL WARMING, PART 2

The diagram above provides a particularly good perspective of all the processes that affect the cycling of carbon in the Earth's biosphere. (The diagram is from Schimel, D.S., I. Enting, M. Heimann, T.M. Wigley, D. Raynaud, D. Alves, and U. Siegenthaler, "CO₂ and the carbon cycle", in the book Climate Change 1994: Radiative Forcing of Climate Change and An Evaluation of the IPCC IS92 Emission Scenarios, published by Cambridge University Press.) In the diagram, carbon reservoirs (where carbon is stored) are represented by boxes, and those units are the gigatons of carbon contained in each reservoir. Carbon fluxes (transfers of carbon between reservoirs) are represented by arrows, and their units are gigatons of carbon per year. A gigaton is the same quantity as a petagram, which is 10¹⁵ grams.

The main sector of the carbon cycle where SeaWiFS data lends insight is in the reservoir called "marine biota". One of the remarkable things about the marine biota reservoir is that the size of the reservoir is much smaller than the fluxes in and out the reservoir. Everywhere else in the carbon cycle, the reservoirs are much larger than the fluxes. What this means is that the marine biota reservoir is very dynamic, and any changes in the activity of this reservoir can mean substantial changes in the fluxes to related reservoirs, i.e., in the ocean and also in the atmosphere.

The 1997-1998 El Niño caused a significant change in the activity of the marine biota, as shown by the two images that began this feature. In fact, a recent article in the journal Science quantified the difference in net global primary productivity between the El Niño year and the following year, when the Pacific Ocean switched to the La Niña state. Primary productivity simply means the production of organic carbon by the process of photosynthesis, either by plants on land or in the ocean (the latter called phytoplankton). In the article, Biospheric Primary Production During an ENSO Transition, the authors determined that net primary productivity increased by 6 petagrams (a petagram is 1 billion metric tonnes) in the year following the 1997-1998 El Niño. Although most of the change occurred in the oceans, the authors also used SeaWiFS data to estimate primary productivity on land using the NDVI data.

[Note: NDVI was originally calculated from data acquired by an instrument that orbits on National Oceanic and Atmospheric Admnistration (NOAA) satellites. This instrument, the Advanced Very High Resolution Radiometer or AVHRR, has been observing the Earth for almost 20 years. SeaWiFS data can also be used to calculate NDVI, and thus SeaWiFS is the first instrument whose data can be used to estimate primary productivity both on land and in the oceans, an estimate of global primary productivity.]

So SeaWiFS data provides a very good way of estimating how much carbon is being cycled through the oceans and the plants living on land. How does that help us understand global warming?

The data allows scientists to distinguish between natural fluctuations in the carbon cycle and man-made fluctuations in the carbon cycle. Despite the changes in global primary productivity that occurred over the 1997-1999 period, the CO₂ concentration in the atmosphere continued to increase, as shown in this close-up view of the Mauna Loa CO₂ data over that period of time:

This image is one frame from an animated view of the data that can be seen here: Colors of Life: The Carbon Cycle. The image shows that despite the increase in global primary productivity that occurred at the end of the El Niño event, which resulted in the removal of more CO₂ from the atmosphere (and the minimum CO₂ concentration that occurred near the end of 1998), the atmospheric concentration of CO₂ continued to increase.

And there's a good reason for that. In the diagram at the top of the page, there's a small building with a smokestack. That building, and the automobile parked next to it, represents the addition of CO₂ to the atmosphere by the burning of fossil fuels (oil, coal, and natural gas) to produce energy. This anthropogenic (human-related) process is the main change to the Earth's carbon cycle that has occurred in the last 200 years. Additional CO₂, and also methane, may be added to the atmosphere due to changes in the way land is used, such as the conversion of forests to agricultural areas.

The main question facing scientists who study Earth's climate is how the increasing amount of CO₂ in the atmosphere will ultimately affect the average temperature of the Earth. While almost all scientists expect the temperature of the Earth to increase, predicting the exact amount of temperature increase is very difficult. The Intergovernmental Panel on Climate Change recently predicted on the basis of climate modeling that the temperature of the Earth could increase by as little as 1.5 degrees Centigrade to as much as 5.8 degrees Centigrade by the end of this century. The majority of the models indicated a 2-3 degree Centigrade increase in global temperature.

There are still a lot of uncertainties, and those uncertainties make decisions on what can be done about global warming difficult indeed. SeaWiFS has actually provided several views of processes that can affect global warming. One process is the burning of forests. Near the end of the 1997-1998 El Niño, SeaWiFS captured this view of smoke from burning rain forests in th Yucatan Peninsula (click on the image for a full-size view):

More recently, the lack of rain in Florida has led to several large fires. In an image obtained on May 18, 2000, two such fires can be seen burning southwest of Lake Okeechobee, in the Big Cypress National Preserve:

Fires such as these in the Big Cypress Swamp affect wetlands, where methane is produced due to the decay of organic matter by anaerobic microbes (bacteria that function without oxygen). The rate of increase of methane concentration in the atmosphere has slowed recently, and this may be due to a loss of wetland area. Global warming might increase precipitation in some areas, leading to more wetlands, but it can also decrease precipitation in other areas, so the net effect is not known.

In early January 2000, SeaWiFS viewed China when a dense haze, from both coal-burning power plants and industrial emissions, covered much of the country. (Note: The image linked here is fairly large.)

Haze over China, January 3, 2000

Smoke and soot may be important to global warming, and it may be more feasible to control smoke and soot than to control emissions of CO₂, which has been the main goal of international political agreements on global warming. In fact, the head of the Goddard Institute of Space Studies (GISS), Dr. James Hansen, has proposed an alternative scenario that he and his co-authors believe would be more effective in mitigating global warming than the current international treaties. The primary points of this alternative scenario are: reduction in "black carbon" aerosols that are released by coal and wood burning (which may also provide a health benefit for respiratory disease); reduction in methane emissions, primarily via changes in agricultural practices (which may also produce a health benefit for infectious disease transmission); and reductions in CFC production and tropospheric ozone. Reduction of tropospheric ozone also provides benefits to health and agriculture. The alternative scenario expects that reductions in CO₂ emissions can be best achieved by increasing use of renewable energy sources, more use of natural gas as opposed to coal and oil for energy production, and improvements in energy efficiency, such as more efficient "hybrid" automobile engines.

So, while SeaWiFS provides a much better quantification of one part of the global carbon cycle, this improved insight doesn't answer the critical question of global warming: how much will the average global temperature increase in the next century? However, the answer to this question depends on improved climate models, and SeaWiFS data aids the improvement of our understanding of the global carbon cycle, which is a critical element in these models.

We thank Dr. Michael Behrenfeld for providing a review of this Science Focus! feature.

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