Terra Turns Five

Five years ago NASA’s Terra satellite began measuring Earth’s vital signs with a combination of accuracy, precision, and resolution the world had never before seen. Today, Terra completes the fifth year in what was scheduled to be at least a 6-year mission to advance understanding of Earth’s climate system, and to help improve our quality of life.

Launched on December 18, 1999, Terra’s five onboard instruments began science operations in February 2000. Terra’s goal is to assess the health of the planet by providing comprehensive information about Earth’s land, oceans and atmosphere. From its vantage point 438 miles above our world, Terra orbits the Earth more than fourteen times a day and observes nearly the entire globe.


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High-resolution artist’s rendering of Terra satellite (1.5 MB; courtesy Reto Stockli, NASA Earth Observatory)


NPP Globe

Sending home roughly 1 million megabytes of data per day, Terra is helping scientists all over the world tackle important questions about the causes and effects of environmental changes. While the mission is still in the process of fulfilling its main science objectives, Terra’s portfolio of achievements to date already marks the mission a resounding success.

A key focus of the Terra mission is to help scientists measure the movements of carbon through Earth’s climate system. To meet their needs for industry, agriculture, and transportation, humans annually release more than 7 billion tons of carbon into the atmosphere through the burning of fossil fuel. Yet, scientists cannot account for where all this carbon ends up. Between 1 and 2 billion metric tons of carbon per year are “missing” from the global carbon budget.

Today Terra is providing scientists with some important clues to help them solve the mystery of the missing carbon. For example, every eight days the mission’s Moderate Resolution Imaging Spectroradiometer (MODIS) instrument produces a global map of where and how much carbon dioxide is drawn out of the air and fixed by vegetation during photosynthesis.

“These maps give scientists the best global measure ever made of the amount of carbon taken up by plants,” said Jon Ranson, Terra Project Scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md. “Carbon fixation by plants is the basis for capturing and storing the energy that fuels our world’s living systems and forms the foundation of the food web.”

  This false-color map represents the Earth’s carbon “metabolism” — the rate at which plants absorbed carbon out of the atmosphere. The map shows the global, annual average of the net productivity of vegetation on land and in the ocean during 2002. The yellow and red areas show the highest rates, ranging from 2 to 3 kilograms of carbon taken in per square meter per year. The green, blue, and purple shades show progressively lower productivity. (NASA image courtesy Reto Stockli, Earth Observatory) More information.

Tracking plant growth is also an important way in which Terra gives back to society. Scientists and resource managers are using Terra’s Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and MODIS instruments to monitor large agricultural regions and assess the health of croplands. These instruments produce maps of the “greenness” of the landscape, which scientists use as a measure of how much plant growth is occurring.

  The Terra ASTER instrument acquired these images over the U.S. Department of Agriculture’s Grazing Lands Research Laboratory near El Reno, Oklahoma, on Sept. 4, 2000. In the top image, bright red colors indicate green vegetation, which at this time of year only includes irrigated lands and riparian zones. Gray-green colors represent harvested winter wheat fields. Dendritic drainage patterns are clearly depicted in the lower left and upper right portions of the scene. ASTER’s three visible and near-infrared bands were used to make this image. The second panel shows the abundance of green vegetation (referred to as Normalized Difference Vegetation Index, or NDVI). NDVI is a measure of vegetation density and health. In this image, blue represents dense vegetation cover and yellow and red shows progressively less dense vegetation. (NASA images courtesy Andrew French, ASTER Science Team) More information.

Moreover, by comparing today’s Terra vegetation greenness maps to long-term averages, scientists can gauge when plants are under stress due to extreme heat or drought. The U.S. Department of Agriculture’s Foreign Agricultural Service uses Terra data to produce maps of drought across major food production regions in developing countries. In terms of lives lost and economic impact worldwide, drought is the costliest of natural disasters. Terra data are helping resource managers improve foreign market access for U.S. agricultural products, particularly during times of need.

  Drought has been looming over Kenya for several years, leaving many regions of the country parched and hungry. Now, as the 2004/2005 harvest draws to a close, the cereal deficit has grown to 300,000 metric tons, which means that up to 2.7 million people will need food aid this season-an unusually high number for Kenya. This map shows Normalized Difference Vegetation Index (NDVI) anomaly, recorded by the Terra MODIS instrument on February 3, 2005. The brown clusters in the Coast and Eastern provinces show patterns of dryness where vegetation is less dense than it has been in the past. More pronounced drought areas surround Central Province. Grey pixels indicate regions where data were not available. An arch of green through the center of the country reveals where there has been plentiful rainfall and the vegetation is thriving. (NASA image courtesy Jesse Allen, Earth Observatory) More information.


Benefits to Society

Many of Terra’s achievements benefit society. For example, Terra data have helped improve scientists’ ability to predict the weather. Terra’s ability to track the speed, direction, and height of clouds allows scientists to accurately measure how hard and which way the wind is blowing over areas where they had precious little data before. Terra’s Multi-angle Imaging Spectroradiometer (MISR) measures of wind heights over the Pacific Ocean have proven more accurate than our best operational weather satellites. (Please read Tracking Clouds for more details.)


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Two years ago meteorologists at the European Centre for Medium-Range Weather Forecasts (ECMWF) began using Terra MODIS data to track clouds over the Arctic Circle. Before Terra, scientists had to make mostly educated guesses about wind strength and direction. But given Terra’s updates every 99 minutes over the Arctic, scientists are able to feed their computers with much more timely and accurate information. The result is a 3-hour advance in the accuracy of their forecasts. This advance may not seem like much but one expert equates that to a whole year’s worth of research and development by the ECMWF. (Please read Polar Wind Data Blow New Life into Forecasts for more details.)

Terra has helped weather forecasters in other ways too. Before Terra, scientists made assumptions about how much sunlight gets reflected by the surface in seldom-measured parts of the world, such as the Sahara Desert. Today Terra provides scientists with much more accurate information on Earth’s albedo (or reflected sunlight) over areas where before they could only make educated guesses. The end result is improved weather forecasts in Northern Africa, the Arabian Peninsula, and across great expanses of the mostly uninhabited boreal forests of North America and Asia. (Please read Measuring Earthshine: How New Terra Data are Improving Weather and Climate Forecast Models for more details.)

  A classical intense hurricane is shown in this view of Juliette, captured by the Terra MISR instrument on September 26, 2001. On the left is a true-color image produced by MISR’s nadir (vertical-viewing) camera. Impressive spiral arms wrapped around the eye are apparent. Several areas of convective clouds can be discerned along the arms. On the right is the cloud-top height field derived using automated computer processing of the data from multiple MISR cameras. Relative height variations are well represented, such as the clearing within the storm’s eye. (NASA image courtesy Image courtesy NASA/GSFC/LaRC/JPL, MISR Team.) More information.

MODIS Albedo of African & Iranian Peninsula


Another of Terra’s benefits to society has been the development of natural hazard warning systems. A mission goal is to help humans avoid emerging hazards where possible, to reduce the impact of hazards where avoidance is not possible, and to more accurately assess the damage where and when natural disasters happen.

The mission’s unique combination of sensors allows scientists worldwide to monitor fires, floods, severe storms, and volcanic activity in near real time. Today organizations all over the world use data from four different instruments aboard Terra as part of their ongoing efforts to monitor the causes and effects of natural hazards.

Moreover, more than 80 receiving stations positioned around the world routinely and freely receive Terra data as it is broadcast live and continuously. These receiving stations push Terra data to more than 800 operational users for a wide range of research and practical science applications.

“Timely data from the Terra satellite are being used to produce maps of where active fires are burning all over the world,” said Ranson. Used operationally by USDA Forest Service and the U.S. National Interagency Fire Center, Terra MODIS and ASTER data help firefighters develop strategies for fighting the fires as well as assessing the severity of the burns. (Please read From Space to the Outback and Satellites Aid Burned Area Rehabilitation for more details.)

“This same technology also allows scientists to map the locations of volcanic hot spots all over the world,” continued Ranson. “Such information has great potential to save lives, not just for people living near active volcanoes, but also for commercial airlines that routinely fly near them.”

  The new MODIS albedo data product reveals in striking detail how widely varied the terrain is across Northern Africa and the Arabian Peninsula. The variation across the land’s surface affects how much solar radiation is absorbed and how much is reflected back up into the overlying atmosphere. This new data product should significantly improve weather forecast models for that region. This image was produced using data composited over a 16-day period, from April 7-22, 2002. (Image courtesy Elena Tsvetsinskaya, Boston University) More information.


How Terra Tracks Pollution

Wildfires and volcanoes aren’t just hazards on the ground. They produce plumes of smoke and gas that can travel hundreds or even thousands of miles downwind, affecting the chemistry of the atmosphere and impacting the quality of the air we breathe. Terra carries three instruments designed to observe and measure these plumes.


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Terra’s Measurements Of Pollution In The Troposphere (MOPITT) instrument provides the first global maps of the concentration of carbon monoxide in Earth’s lower atmosphere. This gaseous pollutant results from fires and human burning of fossil fuel. It is important to monitor carbon monoxide because it is a precursor to the formation of poisonous ozone in the lower atmosphere, and because it shows one way in which humans affect the air we breathe.

Thus, Terra’s measurement capabilities allow scientists to distinguish between natural changes in Earth’s environment and changes caused by humans. As such, the mission can help warn of cases when we become hazardous to ourselves. (Please read NASA’s Terra Satellite Tracks Global Pollution for more details.)

The U.S. Environmental Protection Agency (EPA) picked up on this fact and began integrating Terra data into its ongoing efforts to monitor and regulate air quality. EPA scientists found that Terra’s combined precision and big picture perspective far exceed their ability to measure aerosol and carbon monoxide pollution from individual locations all across the United States. Now, whenever thick smoke plumes billow from wildfires in western states all the way to Maine, or whenever industrial smog drifts out of the Midwest and over the Eastern Seaboard, EPA scientists use Terra data to trace the pollution back to its source as well as predict its future pathways. (Please read A New IDEA in Air Quality Monitoring for more details.)

  Colorless, odorless, and poisonous, carbon monoxide (CO) is one of the six major air pollutants regulated in the United States (and by other nations around the world, as well). CO is created when carbon-based fuels, like fossil fuels or wood, don’t burn completely or efficiently. The image above shows the average amounts and geographic sources of CO from April, May and June for the years 2000 through 2004. Blue areas have little or no atmospheric CO, while progressively higher levels are show in green, yellow, orange, and red. (Images courtesy Cathy Clerbaux, NCAR Atmospheric Chemistry Division) More information.

What do all these changes in Earth’s environment and climate system say about the health of our home planet? Terra is designed to help scientists answer this question. But, scientists caution, this question is complex and multi-faceted so the answer will not come overnight.

  EPA and NASA scientists fuse ground-based measurements with satellite data, meteorological measurements, and computer models to get a big-picture view of air pollution. This data fusion produces a map with several layers of information. The background is satellite data from MODIS, with aerosol optical depth in color (aerosol concentrations increasing from blue to red) and cloud thickness in shades of gray (thickness increasing from gray to white). Wind speed and direction are shown with white arrows, and ground-based measurements of air quality appear as colored dots (air pollution increasing from green to red). Fire locations from the GOES-12 satellite are marked with pink or violet diamonds. The image shows conditions on July 19, 2004. The large area of dense aerosols across the Midwest was caused by widespread fires in Alaska and Canada (see animation). (Map courtesy NASA/EPA/NOAA/CIMSS IDEA program)


Balancing Earth’s Energy Balance

Unlike weather forecasting and natural hazard monitoring, which rely upon near-real-time regional observations made on time scales from minutes to days, assessing the health of Earth’s entire climate system relies upon collection of data over the entire globe on time scales from years to decades. Will Earth continue its present warming trend and, if so, how rapidly will it warm?

Because the Sun powers our world’s climate system, to answer that question scientists must determine whether Earth’s energy budget is in balance. That is, over the course of a year, does the amount of solar energy entering the climate system equal the total amount of energy that is escaping back into space?

“Generally, the answer is yes, Earth’s energy budget is in balance and the climate system is stable,” explained Bruce Wielicki, Senior Scientist for Earth Science at NASA’s Langley Research Center, Hampton, Va. “But over the last 150 years, as humans have introduced increasing amounts of greenhouse gases and aerosol particles into the atmosphere, we have changed how our world reflects and absorbs solar energy as well as how it emits thermal energy (or heat).”

According to Wielicki, some of these changes result in more sunlight reflected back into space, thereby cooling the planet. On the other hand, some of the changes result in more solar energy getting trapped within Earth’s climate system, thereby warming the planet. Which changes will predominate in the future and how rapidly will the changes occur?

“We don’t know,” said Wielicki. “To answer that question we have to track a long list of variables all over the Earth over a long period of time.”

Then we have to spend time and brainpower analyzing all those data. Among the “variables” Terra tracks are cloud and aerosol concentrations in the sky, snow and ice cover on the surface, areas of expanding deserts and cities, and areas of human deforestation. Each of these variables plays a part in determining how much total energy gets absorbed into the climate system and how much gets reflected and emitted back to space.

To make matters more complicated, as each of these climate variables change they cause other variables to change. In one case study, scientists set out to determine how smoke from biomass burning across the Amazon region in South America affect clouds. Their hypothesis was that the sooty aerosol would modify the clouds by making them brighter white and longer lasting, thereby enhancing their ability to reflect sunlight and to cool the surface. Instead, much to their surprise, satellite data revealed that the smoke effectively chokes or limits cloud formation. Thus, more sunlight gets absorbed into the system and the net result is a regional warming, rather than the enhanced cooling scientists had expected. (Please read Clouds Are Cooler Than Smoke for more details.)

All working in concert, and together with the fifteen other NASA Earth Observing System (EOS) satellites currently in orbit, Terra data are helping scientists untangle the myriad cause-and-effect relationships at work within Earth’s climate system. And when scientists get to the bottom line on Earth’s radiant energy budget, Terra’s Clouds and Earth’s Radiant Energy System (CERES) instrument will help ensure they get it right.


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At better than twice the accuracy of its predecessors, CERES makes daily global maps of how much sunlight and how much heat energy are escaping through the top of the atmosphere. Comparing CERES’ daily global maps of radiant energy flux with Terra’s other maps of changing variables allows scientists to more precisely measure the role each variable plays in how the whole system works.

“Terra is Earth science’s first great observatory,” said Wielicki. “It has provided the most comprehensive and the most accurate global view of the Earth’s climate yet on record. And it has pioneered the first comprehensive, multi-instrument approach to climate change research.”

It is important to note that Terra is not going it alone. The mission is part of an extended family that includes sixteen NASA Earth Observing System (EOS) satellites currently in operation.

  A portion of the sunlight that reaches Earth is absorbed into the system, while some of the light is reflected by our planet back into space. Some of the sunlight that gets absorbed is converted to heat and later emitted by the surface and atmosphere back up into space. This image shows Earth’s net radiation in June 2004. The term “net radiation” refers to the total amount of sunlight and heat energy that does not escape from the top of the Earth's atmosphere back into space. Regions of positive net radiation indicate areas of energy surplus in the Earth system (i.e., green regions over the tropics) and areas of negative net radiation signify regions of energy deficit (such as blue regions over high latitudes and the poles). (NASA image courtesy Reto Stockli, Earth Observatory)

Together, these EOS missions extend the legacy of global observations begun in the late 1970s by NASA’s Nimbus satellite and the series of increasingly advanced Earth-observing satellites launched through the 80s and 90s. And yet the data being collected today by Terra and its sister satellites show how NASA has made significant progress in measurement accuracy.

“The value of a climate record is directly proportional to the length of the record and to its accuracy,” said Wielicki. “There are no substitutes for these two data qualities.”

Ranson agreed, stating that Terra’s instruments brought new channels that provide data at better resolutions of time and space than previous satellites. Terra’s instruments are also better calibrated and collect data from multiple viewing angles, both of which result in data of sufficient quality for studies of the global climate system.

  Terra animation

Since February 2000, Terra’s five instruments have been comprehensively observing and measuring our planet’s climate system. A goal of the mission is to help scientists distinguish between natural changes and those changes that humans cause, and to help scientists construct better computer models of how the climate system works. (NASA animation courtesy Reto Stockli, Earth Observatory)