NASA Demonstrates How Earth's Global Heat Engine Drives Plant Growth

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Scientists at NASA's Goddard Space Flight Center have assembled the first long-term global data set that demonstrates the connection between changing patterns of sea surface temperature and patterns of plant growth across the Earth's landscapes. The results of their new study appear in the April 2001 issue of the Journal of Climate.

"For the first time, we can see patterns of climate variability reflected in land vegetation growth, globally, which was not possible before," states Sietse Los, the paper's lead author. "Until now, we haven't had a good data set to show us how vegetation changes over long periods of time."

Since land vegetation absorbs carbon dioxide from the atmosphere through the process of photosynthesis, which is ultimately released back into the atmosphere through decomposition and fires, the authors wanted to gain new insights into where there are large variations in plant growth. Such variations have implications for the spatial distribution of carbon sources and sinks, and how they change over time. Although seasonal variations in plant growth can be large, growth can also vary widely from one year to the next. Moreover, recent studies suggest that due to global warming the growing season is getting longer at higher latitudes, thereby increasing the ability of terrestrial plants to serve as a carbon sink.

As part of Compton Tucker's (a co-author) satellite data processing effort, the team reprocessed nine years of NOAA Advanced Very High Resolution Radiometer (AVHRR) data--from January 1982 through December 1990--into a series of one-month global composite images of sea surface temperature and plant productivity (indicated by the normalized difference vegetation index, or NDVI). The authors note that AVHRR is a broadband remote sensor designed primarily to look at snow and clouds, not vegetation. Because the sensor did not have strong calibration and orbital requirements, as compared to today's satellite technologies for measuring vegetation, the authors had to painstakingly fine-tune each image to correct for errors that interfere with its interpretation, such as aerosol particles in the atmosphere.

"Using various analysis techniques, we can now extract signals from the vegetation data that relate to the climate system," Los states. "And we can now correlate vegetative response to climate change in three dimensions--through time and space."

These global-scale, false-color images represent data collected by the NOAA AVHRR sensor and composited for the months of April 1983 and April 1989, respectively. Over the oceans, the colors represent sea surface temperature anomaly--reds show higher than average temperatures, blues are lower than average, and white is average. On land, the colors represent anomalously high or low production of green foliage (also known as vegetation index)--greens represent anomalously high productivity and browns show areas of anomalously low productivity. These images demonstrate that there is a relationship between sea surface temperature and vegetation productivity.

Note that in April 1983, there was a large El Niño in the equatorial Pacific Ocean, whereas the Southern Atlantic was cooler than average. Much of the continent of South America was experiencing drought or drier than normal conditions, reflected by the low vegetation index values (brown pixels) at that time. Along the equator, winds tend to blow from east to west, helping transport moisture and rain clouds inland. Yet, because water evaporates much less readily from a cooler sea surface than from a warmer one, the trade winds brought very little precipitation from the Southern Atlantic to South America at this time. Conversely, at higher latitudes in both the Northern and Southern Hemispheres, winds tend to blow from west to east. Notice the effects of the anomalously warm sea surface on both North America and Australia during this month. The large pocket of warm water off the west coast of North America contributed to unusually high rainfall over the arid regions of Southern California and New Mexico, helping to spawn temporary grasslands where there is typically desert. Notice also how the large region of anomalously warm water in the Indian Ocean contributed to extensive greening across the typically dry Australian outback.

In contrast, April 1989 was a La Niña year in the equatorial Pacific and the Southern Atlantic was warmer than average. Many of the signals described in the previous paragraph appear to be reversed in this later image. For instance, the warm Atlantic waters are feeding more moisture than usual into the overlying atmosphere, which was eventually precipitated across South America, hence the greener than average landscape. However, the large pocket of cool water off the west coast of North America contributed to drought in the Great Plains region. There was also extensive drying across northern Australia as much of the Indian Ocean was cooler than normal.

Images courtesy Sietse Los, James Collatz, and Jim Tucker, NASA's Goddard Space Flight Center

When viewing the 108 monthly false-color images consecutively in a time-series animation, distinct large-scale patterns of change become quickly obvious to the eye. Reds representing unusually warm waters wax and wane across patches of ocean while the greens of vigorous plant growth, or the browns of drought, roll across landscapes in response.

Co-author James Collatz points to the recurring cycles of the El Niño-Southern Oscillation in the equatorial Pacific and Southern Atlantic during the 1980s. Then he notes the subsequent patterns of drought and vigorous growth that sweep back and forth across South America, as if the continent were the ball in an ongoing ping-pong match between the two mighty oceans.

"What it shows is what you might expect," he observes. "Sea surface temperatures have an impact on the climate (temperature and precipitation) over land and this affects growth of vegetation."

Dubbed the "global heat engine," Earth scientists have long since recognized that as the ocean releases warmth and moisture into the overlying atmosphere it dramatically influences weather patterns. Anomalously high sea surface temperature, as seen in the equatorial Pacific during El Niño, can drive weather patterns to extremes--producing torrential rains and flooding in some parts of the world and severe drought in others.

But, say the paper's authors, you cannot expect El Niño to always have the same effects on plant growth across a given region. The impacts of some El Niños are more intense than others.

"Climate oscillations can sometimes interact with one another," explains Collatz. "For instance, the effects of El Niño are sometimes magnified and at other times almost completely cancelled out by the North Atlantic Oscillation (NAO)."

Ultimately, say the authors, this new data set strengthens scientists' ability to forecast the effects of climate change on vegetation on a global scale. But in order to improve their predictions of what impacts El Niño might have, they need to know what other climate oscillations might affect the strength of El Niño.

"Natural resources, food--lots of things depend upon the healthy growth of vegetation," concludes Collatz. "It is important for us to understand and be able to predict how forests and crops will respond to climate cycles like El Niño."

Toward that objective, scientists now have almost 20 years of global observations to give them a perspective they've never had before. With this new data they can begin to examine in more detail the roles of the terrestrial biosphere in both the carbon and water cycles.

There are new NASA satellite sensors now in orbit that are much better calibrated than AVHRR and specifically designed to measure the Earth's vegetation. Even as they improve upon the quality of the measurements, these sensors--such as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), flying aboard OrbView-2, and the Moderate-resolution Imaging Spectroradiometer (MODIS), flying aboard Terra--will extend the heritage of the AVHRR data set well into the new millennium.