Scientists have proven that there has been a steady increase in the amount of carbon dioxide in the atmosphere since the beginning of the industrial revolution. There is mounting evidence that this and other atmospheric greenhouse gas increases are causing the average annual global temperature to rise. If so, climate warming may result in melting of continental ice and snow, as well as rising sea level and weather instabilities. Over the last 20 years, satellite observations have shown that the extent of snow cover in the Northern Hemisphere has decreased in conjunction with increases in surface air temperature.³ Satellites have also revealed recent deterioration of major ice sheets and an increase in atmospheric water vapor, both possible indicators of current global climate warming.⁴ Increases in air temperature are expected to cause changes in agricultural practices. Current models suggest that productivity is likely to increase where low temperatures are limiting plant growth and to decrease where the climate is currently warm and dry.⁵

What are the benefits of better understanding our planet?

A global monitoring system, coupled with other measurements and data analysis, will give us the tools to measure the environmental impacts of population expansion, overgrazing, and biomass burning in the tropics. To what degree do droughts, overgrazing, and deforestation affect generation of dust, change in the land radiative and hydrological properties, and forcing of climate? Do we know what the effects of global climate change have been so far and can we predict them in the future? What is the contribution of human use of resources: energy, land, and water, to climate change? Today's climate models are developed to a stage where, coupled with global observations—starting with Terra—they are expected to be useful for predicting the impact of natural and anthropogenic forcing of climate.

These NOAA AVHRR data products show normal sea surface temperatures (Oct. 1996) and those near the peak (May 1997) of the 1997 El Niño, one of the most severe this century. Under normal conditions the western tropical Pacific Ocean is a large "warm pool" of water, while relatively cool waters occur off the South American coast. During El Niño warm waters spread across the entire tropical Pacific (Jet Propulsion Laboratory).

Besides detecting longer term climate and land use change, there are more immediate economic and societal advantages to be gained by the implementation of an extensive Earth monitoring system. Better information on the state of the surface and the atmosphere will lead to more accurate extended weather forecasts affecting health, safety, commerce, recreation, agriculture, and natural hazard prediction. For instance, El Niño is known to disrupt the patterns of life of plants and animals in the Pacific Ocean, and is second only to the changing seasons in its impact on world climate. The term El Niño (Spanish for "the Christ Child") was coined by South American fishermen in reference to the warm ocean currents that periodically appear around Christmas-time and can last for months. Nine El Niños have occurred during the past 40 years—with each came warmer ocean temperatures, reduced oceanic productivity, and heavy rainfall in many areas of the Earth.

Scientists have shown that El Niño causes droughts in Africa, Australia, and Southeast Asia, while reducing the productivity in the Pacific Ocean and thus impacting the fisheries it supports.⁶ These dry conditions in Africa, coupled with extensive increases in local land use, generate a large amount of dust that is transported thousands of miles westward over the Atlantic Ocean by prevailing winds. In the atmosphere, dust alters the radiation budget of the Earth and, as it settles out, fertilizes the biota in the Atlantic Ocean, the Atlantic islands, and even the Amazon rain forest.

El Niño events cause fluctuations in temperature and rainfall patterns in the Pacific basin and beyond. These periodic variations that occur as a result of the interaction between the ocean and the atmosphere in the tropical Pacific region can affect ecosystems and human lives in far-flung regions of the globe. This ocean-atmosphere phenomenon, known as the El Niño/Southern Oscillation (ENSO), affects climate-sensitive human activities such as agriculture and fisheries. The symbols on this map indicate the known impacts of the 1982-83 El Niño (Figure courtesy of the NOAA Office of Global Programs). Click here to view enlarged image

El Niño has been linked to '93 and '95 flooding in the midwestern United States.^{7, 8} Crop yields have been found to be correlated with El Niño events. The economic gains due to improved El Niño forecasts in the southeastern United States alone are estimated to be more than $100 million a year. The ability to forecast El Niño events is another example of the benefits that are expected from EOS.

Alarm systems using satellite observations to monitor fire, famine, floods, and volcanoes are currently under development. For example, NASA scientists are processing data from meteorological satellites in near real time to provide the Famine Early Warning System (FEWS) with measurements of the condition of vegetation in Africa. Efforts like FEWS to mitigate the effects of disaster events will be more effective as more accurate and timely observations become available.

The Mt. Pinatubo eruption in 1991 gave scientists an opportunity to predict the expected global temperature impact and, subsequently, validate their predictions with real data. They predicted and confirmed that for the 2 years following the eruption the resulting stratospheric sulfate cloud intercepted and reflected back into space a significant part of the incoming solar radiation. Mt. Pinatubo caused, nearly as predicted, a 0.5°C reduction in the surface and tro- pospheric temperatures.⁹

The Terra mission will be complemented by Landsat-7 in 1998, Meteor-3M/SAGE III in 1999, and Jason-1 and Aqua (EOS PM-1) in 2000. EOS CHEM-1, ICESat-1, and missions that will continue the AM and PM main data sequence are currently under development, or in the planning stage for launch in the next millennium. These missions will provide repetitive and additive measurements to Terra.

Sea-viewing Wide Field-of-view Sensor (SeaWiFS) image showing the distribution of plant life on Earth, i.e., the Global Biosphere. On land, vegetated areas are shown as dark green and areas of little or no vegetation are brown. Ocean areas rich in phytoplankton are shown as green, yellow, and red, and areas of low concentration are blue and purple (SeaWiFS Project, Goddard Space Flight Center)

Schematic diagram of the Earth system and its interactions, encompassing the physical climate system, biogeochemical cycles, external forcing, and the effects of human activities [adapted from Earth System Science: Overview, NASA]. Projecting the future climate requires understanding and quantitatively predicting how the components and interactions will change a result of natural and human activities.

---