SCIENCE FOCUS: SeaWiFS AND GLOBAL WARMING, PART 1

Take a look at the two SeaWiFS images shown above. Do you see a difference?

These special Level 3 composite images show the Pacific Ocean and the continents of North and South America for two periods of time: September to November 1997 (on the left), and June to August 1998 (on the right). The images are "special" for two reasons. One, they show both the chlorophyll concentration in the oceans and the vegetation cover on land. (The vegetation cover is expressed by a quantity called the Normalized Differential Vegetation Index, NDVI for short. It basically measures the amount of "greenness" on the surface. Thus, both deserts, mountains, and ice-covered areas are brown, while savannahs and forests will be increasingly green.) Two, the images combine data over three-month periods. The standard products from the SeaWiFS Project that are available to scientists combine data for extended periods of eight days, a month, and a year.

Hopefully, the major difference between these two images is fairly obvious. In the image on the right, there is a large area of elevated chlorophyll concentration in the Pacific Ocean that is nearly absent in the image on the left. This is an area called the Pacific Equatorial Upwelling.

When the Pacific Ocean is in what oceanographers consider a "normal" state, wind/water interactions along the Equator result in the world's largest upwelling zone, which brings nutrient-rich subsurface waters to the surface. These nutrients sustain the growth of phytoplankton. However, when the Pacific Ocean is experiencing the phenomenon called El Niño, warmer water at the surface of the ocean suppresses upwelling, and phytoplankton growth is severely diminished.

In the autumn of 1997, the Pacific Ocean was in the grip of a strong El Niño event, one of the strongest ever observed. The El Niño state persisted into the early summer of 1998. Then, and remarkably fast, oceanic conditions converted to La Niña (which means, generally speaking, that the conditions are reversed compared to El Niño) and the Pacific Equatorial Upwelling reappeared. Just as El Niño conditions suppress upwelling along the Equator in the Pacific, La Niña conditions actually enhance upwelling, which helps explain the rapid reappearance of the Pacific Equatorial Upwelling zone.

There are numerous World Wide Web links that describe El Niño in detail; selected links appear at the bottom of this Science Focus! feature. The purpose of this Science Focus! feature is not to describe El Niño; actually, as the title indicates, it will examine how SeaWiFS data is related to the important issue called "global warming".

A quick summary of global warming basics

I. The Earth receives energy from the Sun (shortwave radiation), and this energy is absorbed by the ground surface, ocean, and atmosphere. The absorbed energy is re-radiated as longwave radiation. Some of the re-radiated energy returns to space, and some is trapped by "greenhouse gases", the most important of which is water vapor. Carbon dioxide (CO₂), methane (CH₄), and chlorofluorocarbons (CFCs) are also greenhouse gases. Aerosols, both natural and man-made, may also affect energy absorption, and they can also reflect some incoming solar radiation.

If it were possible to examine a theoretical state of affairs in which nothing involved in Earth's climate system changed, the Earth would eventually achieve radiative energy balance, which means that the incoming energy would be exactly balanced by the outgoing energy. If, however, either the incoming or outgoing energy changes by some amount (which is what is always happening) then the system has to compensate. For example, when the Earth receives less solar energy due to changes in its orbit or rotational axis, the mean temperature of the Earth decreases, which can lead to Ice Ages if the temperature decreases enough. (The time periods for changes in the orbit and rotational axis are well-known, and their effects have a cyclic effect on Earth's climate. These cycles are called "Milankovitch cycles" after the scientist who first described them.)

In the schematic diagram above, W m^-2 stands for "watts per square meter", the amount of energy received or released by the various components of the climate system. All of the numbers are in these units.

II. The activities of mankind have apparently contributed to increasing amounts of CO₂, CH₄, and CFCs in the atmosphere. The most notable increase has been in the concentration of CO₂, as demonstrated by measurements taken at the Mauna Loa volcano in Hawaii since 1950. Other records, such as ice cores obtained from Greenland and Antarctica, indicate that CO₂ has been steadily increasing since about 1850.

III. The increasing concentrations of greenhouse gases will affect the radiative balance of the Earth by trapping more longwave radiation, and this process should cause the mean temperature of the Earth to increase.

Point III is where the global warming issue gets complicated, because Earth's climate system is very complex.

Take another look at the Mauna Loa CO₂ curve above. Obviously, it isn't a smooth curve. Although the concentration of CO₂ is clearly increasing, every year the concentration increases and decreases. That seasonal cycle is due, primarily, to the growing season in the Northern Hemisphere. As deciduous trees spread their leaves (and also as the North Atlantic blooms) in the spring, a large amount of CO₂ is removed from the atmosphere. During the Northern Hemisphere winter, the concentration of CO₂ rises again.

Now look closely at the Mauna Loa curve about 1991. A slight "flattening" of the curve should be evident. That flattening may be due to the massive eruption of Mount Pinatubo in the Phillipines. The eruption injected a large amount of sulfur dioxide (SO₂) aerosols into the stratosphere. This cloud of SO₂ reflected incoming solar radiation, which actually reduced the Earth's temperature (temporarily) by about 1 degree Centigrade. The lower temperature appears to have caused sea surface temperatures in the Northern Hemisphere to be slightly lower than normal, which caused the absorption of more CO₂ from the atmosphere. In general, where ocean waters are cold and windy, CO₂ will be absorbed, and where ocean waters are warm and calm, CO₂ will be released. Overall, the oceans absorb CO₂ from the atmosphere.

The above discussion leads us to the Earth's carbon cycle, which is where SeaWiFS data is particularly valuable.

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