The Earth's Radiation Budget

When it comes to climate and climate change, the Earth's radiation budget is what makes it all happen. Swathed in its protective blanket of atmospheric gases against the boiling Sun and frigid space, the Earth maintains its life-friendly temperature by reflecting, absorbing, and re-emitting just the right amount of solar radiation. To maintain a certain average global temperature, the Earth must emit as much radiation as it absorbs. If, for example, increasing concentrations of greenhouse gases like carbon dioxide cause Earth to absorb more than it re-radiates, the planet will warm up.

One of the most important scientific contributions of the Nimbus missions was their measurements of the Earth’s radiation budget. For the first time, scientists had global, direct observations of the amount of solar radiation entering and exiting the Earth system. The observations helped to verify and refine the earliest climate models, and are still making important contributions to the study of climate change. As scientists consider the causes and effects of global warming, Nimbus radiation budget data provide a base for long-term analyses and make change-detection studies possible. The Nimbus technology gave rise to current radiation-budget sensors, such as the CERES instruments on NASA's Terra and Aqua satellites.

This summary was adapted from the presentation of Tom Vonder Harr, of the Department of Atmospheric Science at Colorado State University, on the contribution of Nimbus satellites to the study of Earth's radiation budget.

A Weather Forecasting Revolution

When it comes to weather satellites, it's not a stretch to say that nearly everything that sensors are capable of today has its roots in the pioneering technology tested during the first Nimbus missions. Today, anyone with an internet connection and even the slightest interest can pull up the latest satellite image showing the weather over his or her hometown. But 40 years ago, the idea that we could observe something as intangible as air pressure using a satellite orbiting hundreds of miles above the Earth was revolutionary. With each Nimbus mission, scientists broadened their ability to collect atmospheric characteristics that improved weather forecasting, including ocean and air temperatures, air pressure, and cloudiness. The global coverage provided by Nimbus satellites made accurate 3-5 day forecasts possible for the first time.

The balance between short-wavelength energy coming in from the Sun and long-wavelength energy radiating from the Earth’s surface and atmosphere is at the root of global climate. Among the natural events that influence this incoming-outgoing balance are volcanic eruptions and El Niño-La Niña cycles. The graph shows how various events lead the Earth and atmosphere to radiate more or less longwave energy than average. The black line at zero represents the average. Nimbus missions have provided much of the long-term record of global energy-balance measurements. (Figures reproduced from Wielicki, B.A., Wong, T., Allan, R.P. Slingo, A., Kiehl, J.T., Soden, B.J., Gordon, C.T., Miller, A.J., Yang, S.-K., Randall, D.A., Robertson, F., Susskind, J., and Jacobowitz, H. (2002) Evidence for Large Decadal Variability in the Tropical Mean Radiative Energy Budget, Science, 295:5556, 841-844.)

The ability of the Nimbus satellites to detect electromagnetic energy in multiple wavelengths (multi-spectral data), in particular the microwave region of the electromagnetic spectrum, made it possible for scientists to look into the atmosphere and tell the difference between water vapor and liquid water in clouds. In addition, they were able to measure atmospheric temperature even in the presence of clouds, a capability that allowed scientists to take the temperature in the "warm core" of hurricanes.

This summary was adapted from the presentations of Bill Smith, of Hampton University and Dave Staelin, of the Department of Electrical Engineering and Computer Science, at Massachusetts Institute of Technology, on how Nimbus satellites revolutionized the study and prediction of Earth's weather and climate.

The Ozone Layer

Even before the Nimbus satellites began collecting their observations of Earth's ozone layer, scientists had some understanding of the processes that maintained or destroyed it. They were pretty sure they understood how the layer formed: solar radiation breaks apart the stable, double-atom form of oxygen (O₂) into two unstable singles that quickly latch on to whatever is around, sometimes re-forming as O₂, but also occasionally glomming on to an existing O₂ molecule to make ozone (O₃). They knew from laboratory experiments that halogens (chlorine, bromine, etc) could destroy ozone. Finally, weather balloons had revealed that the concentration of ozone in the atmosphere changed over time, and scientists suspected weather phenomena or seasonal change were responsible. But how did all of these pieces of information work together on a global scale?

Scientists conducted experiments from NASA experimental aircraft and proved that atmospheric chemicals such as the chlorofluorocarbons (CFCs) released from refrigerants and aerosol sprays did destroy ozone. As Nimbus 7 satellite observations accumulated between 1978 and 1994, it became increasingly clear that CFCs were creating a hole in the ozone layer each winter season over Antarctica. Not only that, but despite some year-to-year variations, it appeared the hole was becoming larger.

When forecasters began incorporating Nimbus observations of atmospheric temperature at different altitudes, predictions improved. These graphs show error in the 24-hour forecast of temperature over New Zealand and Australia in degrees Celsius when Nimbus-6 observations were included in the forecast (dashed line) and when they were not (solid line). Height is shown in terms of the air pressure at given altitudes (pressure decreases as altitude increases). Forecast errors decreased when scientists used Nimbus data. (Figures reproduced from Kelly, G. A. M., Mills, G. A., and Smith, W. L. (1978). Impact of Nimbus-6 temperature soundings on Australian region forecasts. Bulletin of the American Meteorological Society, 59, 393-405.)

Public concern gave rise to the Montreal Protocol, which bound the countries that signed the treaty to phase out the use of ozone-depleting chemicals. Without the Nimbus measurements, we would probably not have been aware of how severe the ozone hole problem was until many years later — perhaps until we began to see alarming increases in the rate of skin cancer and other negative effects of ozone loss.

This summary was adapted from the presentation of Arlin Krueger, of the Joint Center for Earth Systems Technology at University of Maryland, Baltimore County, on how long-term collection of observations of stratospheric ozone led to our awareness of the ozone hole and the dangers it posed to life on Earth.

Nimbus 7 measurements collected over the South Pole beginning in 1978 identified a previously unknown hazard: the large-scale destruction of ultraviolet-blocking ozone by chemicals released into the atmosphere through refrigeration devices and aerosol sprays. These globes show ozone concentrations over Antarctica in selected Octobers from 1979-85 and 2000-2003. Nimbus observations began to point to a drop in ozone (blue areas) as early as 1980, with more extreme decreases developing in 1985. Credit: Paul Newman, Richard Stolarski, Mark Shoeberl, Arlin Krueger

The Color of the Ocean

Anyone who has lived or stayed near the ocean for a long enough time can tell you how the sea seems to change color from day to day, from deep sparkling blue on a bright sunny day to slate gray beneath a thick layer of clouds. What fewer people know is that the color of the ocean changes as concentrations of sediment, organic matter, and ocean plant life change. These changes in ocean color signal biological processes that affect marine life as well as public health, particularly in coastal areas.

When the Nimbus 7 satellite launched in 1978, it carried on board the first sensor engineered to observe the ocean in visible wavelengths of light. Originally intended to be only a one-year technology demonstration, the Coastal Zone Color Scanner (nicknamed "CZCS") ended up delivering science data over selected test sites for the next 8 years!

With CZCS, NASA gave ocean biologists their first global-scale pictures of ocean plant growth, changing scientists' views of the marine biosphere. Scientists discovered that ocean plant life matched land-based plant life in terms of its rates of photosynthesis and seasonal changes. CZCS data also began to reveal the effects of land-based pollution on coastal ecosystems. The success of the mission paved the way for Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the ocean science sensors of NASA's Earth Observing System series of satellites in orbit today.

This summary was adapted from a presentation by Wayne Esaias, of NASA's Goddard Space Flight Center, on the legacy of Nimbus 7's Coastal Zone Color Scanner.

A Sea of Change

When the Nimbus 5 spacecraft launched in 1972, scientists planned for its Electrically Scanning Microwave Radiometer to collect global observations of where and how much it rained across the world. However, a new priority for the sensor evolved in the months following its launch: mapping global sea ice concentrations. When Nimbus 7 launched in 1978, technology had improved enough for scientists to distinguish newly formed (i.e., "first year") sea ice from older ice. The data it collected during its 9-year lifespan provide a significant chunk of the long-term record of Earth's sea ice concentration that today's scientists use for studies of climate change.

These graphics show global maps of changes in ocean chlorophyll in the last two decades. The top map combines data from the Nimbus 7 Coastal Zone Color Scanner (CZCS) with observations from buoys and research ships. It shows the average chlorophyll concentrations (milligrams per cubic meter of seawater) in summers from 1979 to 1986. The middle graphic shows average chlorophyll concentrations observed by the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) during the summers from 1997 to 2000. The bottom graphic shows the differences in ocean chlorophyll between the SeaWiFS era and the CZCS era. Credit: Watson W. Gregg (NASA) and Margarita E. Conkright (NOAA)

Among the most serendipitous discoveries that the Nimbus missions made possible was that of a gaping hole in the sea ice around Antarctica in the Southern Hemisphere winters of 1974-76. In a phenomenon that has not been observed since, an enormous, ice-free patch of water, called a polynya, developed three years in a row in the seasonal ice that encases Antarctica each winter. Located in the Weddell Sea, each year the polynya vanished with the summer melt, but returned the following year. The open patch of water may have influenced ocean temperatures as far down as 2,500 meters and influenced ocean circulation over a wide area. The Weddell Sea Polynya has not been observed since the event witnessed by the Nimbus satellites in the mid-70s. Without those images, we might never have known that an event like that did—or even could—occur.

This summary was adapted from a presentation by Per Gloersen of NASA's Goddard Space Flight Center on the contribution the Nimbus missions have made to the collection of long-term records of Earth's "vital signs" that we must have to study the causes and effects of climate change.

Satellite Search and Rescue and Data Collection Systems

Today, it isn't too big a challenge for people to figure out exactly where they are on the Earth. Between cell phones and pocket-sized Global Positioning System (GPS) devices, getting your bearings is only as complicated as deciphering your users manual. Thirty years ago, however, locating and tracking the position of something or someone on the Earth's surface was a tougher task.

NASA's Nimbus satellites (beginning with Nimbus 3 in 1969) blazed the trail into the modern GPS era with operational search and rescue and data collection systems. The satellites tested the first technology that allowed satellites to locate weather-observation stations set up in remote locations and to command the stations to transmit their data back to the satellite. The most famous demonstration of the new technology was through the record-breaking flight of British aviator Sheila Scott, who tested the Nimbus navigation and locator communication system when she made the first-ever solo flight over the North Pole in 1971.

In the mid-1970s, Nimbus sensors were recording sea ice concentrations in the Southern Ocean surrounding Antarctica when they detected a large ice-free opening in the ice pack. This patch of open water, called a polynya, recurred each winter between 1974-76, and has not retuned since. In the image at left, Antarctica is colored black and sea ice concentrations appear in shades of red and orange. The Weddell Sea polynya is the light blue (open water) area in the upper left quadrant of the image. Credit: Claire Parkinson (NASA GSFC)

In a series of "first-ever" experiments, Nimbus satellites tracked the movements of free-floating buoys in the Arctic Ocean, the erratic travels of weather balloons, and the movements of animals from sea turtles to puffins. Scientists set up ground-based observation stations in remote or even dangerous environments -- such as right outside a black bear's den in Yellowstone National Park -- and used Nimbus sensors to retrieve the data on environmental conditions that the ground station was recording.

The Nimbus ground-to-satellite-to-ground communication system demonstrated the first satellite-based search and rescue system. Among the earliest successes were the rescue of two hot air balloonists who went down in the North Atlantic in 1977 and, later that year, tracking a Japanese adventurer on his first attempt to be the first person to dogsled solo to the North Pole through Greenland. Tens of thousands of people over the past three decades have been rescued through the Search and Rescue Satellite-aided Tracking (SARSAT) operational system on NOAA satellites.

This summary was adapted from a presentation by Charles Cote of NASA's Goddard Space Flight Center on how Nimbus missions became the proving ground for satellite-based search and rescue technology.

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British aviator Sheila Scott became the first pilot to make a solo flight across the North Pole in 1971. On her mission, Scott tested the Nimbus navigation and locator communication systems that paved the way for the Global Positioning System technology available today. Credit: NASA