Introduction

Earth’s 4.5-billion-year history is a

study in change. Natural physical forceshave been rearranging the surface featuresand climatic conditions of our planet sinceits beginning. There is scientific evidence thatsome of these natural changes have not onlyled to mass extinctions of species like thedinosaurs, but have also severely impacted humancivilizations. For instance, there is evidence that a relatively sudden climate change caused a 300-year droughtthat contributed to the downfall of Akkadia, one of the most powerful empires in the Middle-East region around 2200 BC. More recently, the“little ice age” from 1200-1400 AD forced the Vikings to abandonGreenland when average temperatures there dropped by about 1.5°C, making it too difficult to grow enough crops to sustain the population. Today,there is compelling scientific evidence that human activity is impacting global climate conditions. Carbon dioxide levels in the atmosphere have risen 25percent since the industrial revolution, and about 40 percent of the world’sland surface has been transformed by human activity (see Figure 1). 

Scientists have been monitoring individual climate phenomena using

information from Earth-viewing satellites for several years, but don’t yet comprehensively understand the cause-and-effect relationships among land, ocean,and atmospheric conditions. Consistent measurements are needed globallyover several years in order to understand the interactions among the Earth’secosystems, and to accurately predict climate change and its impact on ourdaily lives. 

NASA’s Earth Observing System (EOS) has begun a long-term, comprehensive study of the Earth that is comprised of a series of satellites

specially designed to study the complexities of global change, anadvanced computer network for distributing scientific data, and international teams of scientists who will study the data. In 1999, NASAlaunches the EOS satellite Terra to collect a new 15-year global datarecord on which to base future scientific investigations about our complex planet.

Figure 1. Atmospheric carbon dioxide monthly mean mixing

ratios as observed by Tans and Keeling at Mauna Loa, Hawaii.

This globe shows

data collected

from multiple

sensors and

integrated into

one image.

Notice the

three-dimensional cloud

measurements; these

will be collected by ASTER

and MISR,

while MODIS

measures total

cloud cover on a

daily basis. The

1997-98 El Niño temperature anomaly is visible as red in the Pacific

Ocean, while the red dots on

land show the locations of forest

fires. The ASTER, MISR, MODIS, and

MOPITT instruments are all uniquely designed to observe

fires and help us measure the smoke and gases they

release. Together with CERES, these instruments will help

us understand the Earth as a whole, integrated system.

(Image by R.B. Husar, Washington U.; the land layer from

the SeaWiFS Project; fire maps from ESA; the sea surface

temperature from the Naval Oceanographic Office’s

Visualization Laboratory; and the cloud layer from SSEC,

U. of Wisconsin)
carbon dioxide accelerates, and temperatures rise, land plants could begin regulating their evapotranspiration (water loss due toevaporation) rates in order to conserve water. If this plant response occurs on a large scale, scientists predict that greenhouse effectwarming will be amplified over tropical land areas by as much as 50 percent above the current greenhouse warming trend.

The Terra spacecraft will enable scientists to compare plant productivity with carbon dioxide and

other important greenhouse gases, as well as global temperature trends. This information willbetter enable scientists to predict how changes in the climate will impact Earth’s ecosystems. The new data will also allow us to measure how certain human activities, such asbiomass burning and deforestation, may be contributing to climate change. The Oceans

The ocean also harbors abundant plant life, including microscopic plants called

phytoplankton, which serve as the foundation for the marine food chain. Like theirland-based relatives, phytoplankton use the sun’s energy and photosynthesis toproduce carbohydrates. Small fish and whales eat the phytoplankton, bigger fish eatthe smaller fish, and humans eat many of the bigger fish. 

Periodic regional ocean temperature shifts, such as occur during an El Niño event,

can have dramatic effects on the health and distribution of phytoplankton, which in turncan have adverse effects on the animals that depend upon phytoplankton for nutrition. Becausephytoplankton selectively absorb red and blue light, and reflect green light, satellites can preciselymeasure the abundance of microscopic marine life in the ocean based on its color. Scientistscan also measure the difference between incoming sunlight and that which is reflectedback up into the atmosphere (see Figure 6). This difference in sunlight represents theamount being used for photosynthesis, from which the abundance of phytoplanktonis derived.

Covering more than 70 percent of the Earth’s surface and containing 97 percent of its surface water, the ocean has been labeled “the heat engine of global climate” due to its influence on the timing and patterns of climate change. The ocean

possesses a vast capacity to absorb and store sunlight as heat. This heat is transported by currents and eventually released into the atmosphere where it directlyinfluences temperature and precipitation patterns, and indirectly influences land vegetation through precipitation or drought events. These ocean temperature trends alsoaffect the marine life in the ocean. When the sea surface is cold, deeper waters flow to thesurface, bringing critical life-sustaining nutrients with them. Yet when sea surface temperatures increase during an El Niño event, there is less vertical circulation of water, which means nutrients at the surface become more scarce andphytoplankton productivity drops. Larger fish and mammals that require phytoplankton to survive migrate to where the food source is more abundant.South American fishermen who fish the Pacific know well that El Niño is atime when their livelihood is interrupted. 

How will the frequency and severity of El Niño affect ocean productivity?

How will concentrations of phytoplankton shift in response to changes inocean circulation? How do changes in ocean productivity affect ocean transparency, and therefore impact the exchange of heat between the ocean andatmosphere? The Terra satellite will measure these parameters with unprecedented accuracy to help scientists answer these questions.

Figure 6. (top) The ocean has been dubbed the “global

heat engine.” Energy escapes the ocean in the form of

heat and water vapor. As the atmosphere warms, temperature gradients are created, resulting in surface winds

that, in turn, drive ocean currents. These winds and water

vapor also dramatically affect meteorological conditions,

resulting in the formation of clouds or even rainstorms

that are vital for life on land. (bottom) There is ongoing

chemical “dialogue” between the ocean and atmosphere

that influences regional and global climate in many ways.

Sea spray and water vapor form low clouds that ultimately cool temperatures at the surface. Meanwhile, desert

dust and carbon dioxide settle into the ocean and act as

“fertilizer” to stimulate the growth of phytoplankton, thereby enhancing the ocean's ability to absorb carbon dioxide

from the atmosphere—a process known as the “biological

pump.” Over geologic time, more than 90 percent of the

world's carbon has settled into the deep ocean (Courtesy

of David Herring, SSAI).