If you watched a movie of the Earths climate history, you would see the ocean frequently plays the role of leading actor. If you observed sea surface temperature across the entire globe, over a span of years you would begin to detect patterns. Of course, since you cannot see heat, you would have to assign colors to represent temperature. What would become immediately obvious to your eye is that the temperature anomalies of El Niño and La Niña occur roughly every 3 to 7 years. These would appear as vast red and blue spikes, respectively, appearing off the west coast of South America and extending westward along the equator across most of the Pacific Ocean.
Watching the movie a little longer, your eye might pick up the fact that there is a long-distance relationship between the equatorial Pacific and the equatorial Atlantic Oceans. Interestingly, and for reasons scientists dont fully understand, when the southern Pacific is warmer than average the southern Atlantic is cooler than average, and vice versa. These temperature patterns swing back and forth between the two oceans like a pendulum. You might also notice that there is an oscillation in the Northern and Southern Atlantic Ocean, on either side of the equator. One side is cooler than average for a year or two while the other side is warmer than average, then they flip-flop and this pattern continues.
If you watched for a decade or longer, many more recurring patterns would begin to reveal themselves. For instance, you would see that the body of water that extends from the western equatorial Pacific into the eastern Indian Ocean, called the Indo-Pacific warm pool, seems to pulsate. That is, the warm pool expands and contracts in size while its average annual temperatures rise and fall over cycles of about two decades.
The movie has a surprise ending. Perhaps you didnt notice, but there was another actor onscreenon the land. Turn your attention to the continents and you will see green waves of vigorous plant growth and creeping brown hues of drought wax and wane across the landscapes as if the worlds vegetation dances in response to the rhythms of the ocean. How can this be? Is there a connection?
Yes, say a team of scientists at NASAs Goddard Space Flight Center. Led by Sietse Los, terrestrial biologist, the team recently assembled the first long-term global data set that demonstrates there is a connection between changing patterns of sea surface temperature and patterns of plant growth across the Earths landscapes. The results of their new study appeared in the April 2001 issue of the Journal of Climate (Los et al. 2001).
The above series of images shows changes in sea surface temperature and land plant growth in every January from 1983 to 1989. In the ocean, blue indicates where temperatures are cooler than normal and red is warmer than normal; on land, yellow indicates less vigorous than normal vegetation growth and green shows more vigorous growth than normal. As sea surface temperatures rise and fall, the vegetation in adjacent areas responds. In general, cool ocean water upwind leads to drought and reduced vegetation growth, while warm ocean waters produce excess rainfall and vigorous plant growth. Notice how the vegetation in northern South America responds to water temperatures in the Atlantic Ocean. The corresponding animation shows the effect dramatically.
Images courtesy Marit Jentoft-Nilsen, NASA GSFC Visualization and Analysis Lab, based on data from Sietse Los, University of Wales.
Once the movie loads and begins playing you may click anywhere on it to stop it. Use your keyboard arrows to manually move forward and backward through the animation at a controlled rate. Double click on the movie to play it again at its normal rate.
Click to download the high quality animation (26 MB).
When Plants are Thriving
When it is thriving, land
vegetation can absorb vast amounts of carbon dioxide from the atmosphere
through the process of photosynthesis. Over a period ranging from
months to decades, however, the carbon stored in plants is released back
into the atmosphere through the processes of respiration, decomposition,
and fires, thus completing the carbon cycle. With their new data set,
the team wanted to gain new insights into where there are large
variations in plant growth because such variations have implications for
where and when vegetation serves as a source for carbon dioxide
(releasing it into the atmosphere) and when it is a sink (or absorbing
it). Additionally, they wanted to find out how these sources and sinks
change over time. Seasonal variations in plant growth can be quite
large, and plant growth can 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 (Myneni et
|Read "Measuring Vegetation (NDVI & EVI)" to learn how scientists use satellite data to monitor vegetation growth.|
To determine where and when plants are thriving, the team used AVHRR to measure Normalized Difference Vegetation Index (NDVI), which is basically an indication of how green a patch of land is. To derive NDVI, researchers must observe the distinct colors (wavelengths) of visible and near-infrared sunlight reflected by plants. As can be seen through a prism, many different wavelengths make up the spectrum of sunlight. When sunlight strikes an object, certain wavelengths of this spectrum are absorbed and other wavelengths are reflected. The pigment in plant leaveschlorophyllstrongly absorbs visible light (from 0.4 to 0.7 µm) for use in photosynthesis. The cell structure of the leaves, on the other hand, strongly reflects near-infrared light (from 0.7 to 1.1 µm). The more leaves a plant has, the more these wavelengths of light are reflected and absorbed, respectively. (Click for more details on NDVI.)
Los' team processed eighteen years of AVHRR data into a series of one-month global composite images of NDVI. From there, they were able to calculate the average greenness value for a given patch of land for a particular time of the year. Any significant departure from the average greenness value would then be an anomaly. Similarly, they used AVHRR data to calculate average global sea surface temperatures for any given patch of ocean for every month over the same 18-year time period. Again, any significant departure from average is termed an anomaly.
When they put the two sets of measurements together into one continuous movie, Los' theory was confirmedthere is a clear and obvious relationship between sea surface temperature trends and terrestrial plant growth across the continents. "For the first time, we can see patterns of climate variability reflected in land vegetation growth, globally, which was not possible before," Los states. "Until now, we haven't had a good data set to show us how vegetation changes over long periods of time."
Carbon atoms move from the atmosphere to the biosphere (plant and animal life) and the lithosphere (the solid Earth) in a cycle that spans from months to decades. Plants "breathe" carbon dioxide, using the carbon to grow. When a plant dies (or its leaves fall off) the carbon dioxide is released back into the air as the organic material decays. Alternatively, the carbon may be buried and eventually become peat, coal, or oil.
Image by Robert Simmon, NASA GSFC
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
A very strong El Niño brought drought to northern South America in 1983, while a large La Niña brought excess rain in 1989. The vegetation responded by growing poorly in 1983 and vigorously in 1989.
Image by Marit Jentoft-Nilsen, NASA GSFC Visualization and Analysis Lab, based on data from Sietse Los, University of Wales.
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 extremesproducing 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.)
Trade winds blow from east to west along the equator, carrrying moisture over South America. Evaporation is slowed if the sea surface is cooler than normal, leading to decreased rainfall over adjacent land. Conversely, more evaporation leads to excess rainfall when the sea surface temperature is higher than normal. The image at left shows winds over the Atlantic on June 3, 2001. Arrows correspond to direction, color to velocity.
Image courtesy Seaflux, NASA Jet Propulsion Lab.
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, foodlots 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 theyve 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 Earths vegetation. Even as they improve upon the quality of the measurements, these sensorssuch as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS), flying aboard OrbView-2, and the Moderate-resolution Imaging Spectroradiometer (MODIS), flying aboard Terrawill extend the heritage of the AVHRR data set well into the new millennium.
Graph by Robert Simmon, based on data provided by Sietse Los, University of Wales.