The Global Heat Engine

When viewing the 216 monthly false-color images consecutively in a time-series animation, distinct large-scale patterns of change become quickly obvious to the eye. The relationship between sea surface temperature and plant productivity becomes apparent too. 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.

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 then 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, says Collatz, 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)." (The NAO is an ongoing, long-distance relationship between a high-pressure system over the Azores Islands and a low-pressure system over Iceland. For more details, read Searching for Atlantic Rhythms.)
 

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, the team now has almost 20 years of global observations to give scientists a perspective they’ve never had before. With these new data the team can begin to examine in more detail the roles of the terrestrial biosphere in both the carbon and water cycles.

Collatz adds that the team is already looking ahead to the new NASA satellite sensors now in orbit that are much better calibrated than AVHRR, and they are 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.

• References
• Los, Sietse O., G. James Collatz, L. Bounoua, P.J. Sellers, and C.J. Tucker, 2001: "Global interannual variations in sea surface temperature and land surface vegetation, air temperature and precipitation." Journal of Climate, Vol. 14, pp. 1535-1549
• Myneni, Ranga B., C.D. Keeling, C.J. Tucker, G. Asrar, and R.R. Nemani, 1997: "Increased plant growth in the northern high latitudes from 1981-1991." Nature, Vol. 386, pp. 698-702.

 When Plants are Thriving