Over the past 30 years, a number of satellite missions have been launched to obtain the data about Earth's radiation budget that are critical to understanding the greenhouse effect. Some of these missions are listed in the accompanying table.
Another very important aspect of greenhouse investigations has been the development of models. A number of climate models have been developed by NASA, and one of the most detailed is a General Circulation Model (GCM) developed by the Goddard Institute for Space Studies (GISS) in New York City. A GCM uses extremely high speed computers to solve the basic equations governing atmospheric motions and processes by numerical techniques. The GISS group, using its model, predicted that the annual global temperature would reach a new record high sometime during the first three years of the 1990's. Indeed, that record was reached in 1990. However, in June 1991, the Mount Pinatubo volcano erupted and sent 25 to 30 million tons of sulfur dioxide into the stratosphere. There, the sulfur dioxide reacted with water vapor to produce a long lasting haze of sulfuric acid droplets.
The GISS group then inserted the new information into the model, estimated how much sunlight the Pinatubo aerosol cloud would block, and predicted that the global temperature would drop about 0.3 degree C. Again, the predicted change actually occurred. Although these successful climate predictions are encouraging, most scientists agree that much remains to be done to improve climate models before we will be able to predict future climate in a credible manner.
An important need in the further development and verification of climate models is the acquisition, assembly, and analysis of reliable climate data. The highly accurate, self consistent, and long term data sets that will be acquired by the Earth Observing System (EOS), as part of NASA's Mission to Planet Earth with a series of satellite launches beginning in 1998, are designed to fulfill that need.
NASA Investigations of the Greenhouse Effect
Greenhouse Gases - NASA
To predict climate change, one must model the climate. One test of the validity of predictions is the ability of the climate models to reproduce the climate as we see it today. Elements of the models such as the physics and chemistry of the processes that we know or think we know are essential to represent in the models. Therefore, the models have to embody the characteristics of the land and the oceans that serve as boundaries of the atmosphere represented in the models. Models also have to take into account the radiative characteristics of the gases that make up the atmosphere, including the key radiative gas, water vapor, that is so variable throughout the atmosphere.
Global records of surface temperature over the last 100 years show a rise in global temperatures (about 0.5 degrees C overall), but the rise is marked by periods when the temperature has dropped as well. If the models cannot explain these marked variations from the trend, then we cannot be completely certain that we can believe in their predictions of changes to come. For example, in the early 1970's, because temperatures had been decreasing for about 25 to 30 years, people began predicting the approach of an ice age! For the last 15 to 20 years, we have been seeing a fairly steady rise in temperatures, giving some assurance that we are now in a global warming phase.
The major gases in the atmosphere, nitrogen and oxygen, are transparent to both the radiation incoming from the sun and the radiation outgoing from the Earth, so they have little or no effect on the greenhouse warming. The gases that are not transparent are water vapor, ozone, carbon dioxide, methane, nitrous oxide, and the chlorofluorocarbons (CFCs). These are the greenhouse gases.
There has been about a 25% increase in carbon dioxide in the atmosphere from 270 or 280 parts per million 250 years ago, to approximately 350 parts per million today. The record of carbon dioxide in the atmosphere shows a variation as seasons change. This variation is more pronounced in the northern hemisphere, with its greater land area, than in the southern hemisphere because of interactions in the atmosphere caused by vegetation. In the growing season, during daylight vegetation takes in carbon dioxide; at night and in the senescent season, vegetation releases carbon dioxide. The effect is more pronounced in the northern hemisphere because most of the land on Earth is located there.
Greenhouse Gas
- CO2 Carbon Dioxide
- CH4 Methane
- HFCs Hydrofluorocarbons (a class of several gasses)
- N2O Dinitrogen Oxide
- PFCs Perfluorocarbons (a class of several gasses)
- SF6 Sulfur Hexafluoride
What Causes the Greenhouse Effect?
Life on earth depends on energy from the sun. About 30 percent of the sunlight that beams toward Earth is deflected by the outer atmosphere and scattered back into space. The rest reaches the planet’s surface and is reflected upward again as a type of slow-moving energy called infrared radiation.
As infrared radiation is carried aloft by air currents, it is absorbed by “greenhouse gases” such as water vapor, carbon dioxide, ozone and methane, which slows its escape from the atmosphere.
Although greenhouse gases make up only about 1 percent of the Earth’s atmosphere, they regulate our climate by trapping heat and holding it in a kind of warm-air blanket that surrounds the planet.
This phenomenon is what scientists call the "greenhouse effect." Without it, scientists estimate that the average temperature on Earth would be colder by approximately 30 degrees Celsius (54 degrees Fahrenheit), far too cold to sustain our current ecosystem.
The Greenhouse Effect
Over the last 400,000 years the Earth's climate has been unstable, with very significant temperature changes, going from a warm climate to an ice age in as rapidly as a few decades. These rapid changes suggest that climate may be quite sensitive to internal or external climate forcings and feedbacks. As can be seen from the blue curve, temperatures have been less variable during the last 10 000 years. Based on the incomplete evidence available, it is unlikely that global mean temperatures have varied by more than 1°C in a century during this period. The information presented on this graph indicates a strong correlation between carbon dioxide content in the atmosphere and temperature. A possible scenario: anthropogenic emissions of GHGs could bring the climate to a state where it reverts to the highly unstable climate of the pre-ice age period. Rather than a linear evolution, the climate follows a non-linear path with sudden and dramatic surprises when GHG levels reach an as-yet unknown trigger point.
September 25, 2007 NASA-Melting Arctic sea ice has shrunk to a 29-year low, significantly below the minimum set in 2005, according to preliminary figures from the National Snow and Ice Data Center, part of the University of Colorado at Boulder. NASA scientists, who have been observing the declining Arctic sea ice cover since the earliest measurements in 1979, are working to understand this sudden speed-up of sea ice decline and what it means for the future of Earth's northern polar region.
At the end of each summer, the sea ice cover reaches its minimum extent and the ice that remains is called the perennial ice cover, which consists mainly of thick multi-year ice floes. The area of the perennial ice has been steadily decreasing since the satellite record began in 1979, at a rate of about 10% per decade. But the 2007 minimum, reached around Sept. 14, is far below the previous record made in 2005 and is about 38% lower than the climatological average. This data visualization shows the annual sea ice minimum from 1979 through 2007.
The Advanced Microwave Scanning Radiometer (AMSR-E) is a high-resolution passive microwave Instrument on NASA's Aqua satellite. AMSR-E provides a remarkably clear view of sea ice dynamics in greater detail than has ever been seen before. Researchers use this information to study polar bear habitats, plan expeditions to the ice, and to study the interactions between the ocean and sea ice from season to season. This data visualization shows Arctic sea ice from Jan. 1, 2007 to Sept. 16, 2007.
Because Arctic ice cover varies so much year to year, it can be dangerous to look at any one year and draw too much of a conclusion from it," said Waleed Abdalati, head of Goddard's Cryospheric Sciences Branch. "But this year, the amount of ice is so far below that of previous years that it really is cause for concern. The trend in decreasing ice cover seems to be getting stronger and stronger as time goes on."NASA developed the original capability to observe the extent and concentration of sea ice from space using passive microwave sensors. More recently, NASA launched an advanced microwave instrument in 2002 -- the Advanced Microwave Scanning Radiometer (AMSR-E) on the Aqua satellite -- that provides a view of sea ice dynamics in greater detail than has ever been seen before. Researchers use this information to study polar bear habitats and the unique movements of sea ice from season to season. AMSR-E is a joint project of NASA and the National Space Development Agency of Japan.
In September 2007, the Northwest Passage was ice-free for the first time since satellite records began. The passage is a direct route from Europe to Asia for ships traveling through the Arctic. The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA's Terra satellite captured this image of the ice-free Northwest Passage on Sept. 15, 2007.
Current satellites, however, can map sea ice in two dimensions, but it is much more difficult to find out how the thickness of the ice contributes to the change in the total volume of the ice. NASA's ICESat spacecraft (Ice, Cloud, and land Elevation Satellite), launched in 2003, with the primary goal of determining how much ice sheets are contributing to sea-level rise. ICESat is also collecting data that enables scientists to make estimates of sea ice thickness with unprecedented detail."What we need to truly understand the interaction of the ice, ocean and atmosphere in the Arctic is sea ice thickness information," said Abdalati. "The new capability we have with ICESat is expected to be extended into the next decade based on recent recommendations by the National Research Council for a follow-on mission. Ultimately, like the 29-year record we have now of sea ice cover, a long-term ice thickness record will help scientists understand these complex interactions and what the changes in the ice cover will mean to the ecology of the Arctic and to life on Earth."NASA has been observing sea ice from space since the 1970s, beginning with the Electricallly Scanning Microwave Radiometer (ESMR), Scanning Multichannel Microwave Radiometer (SSMR) and Special Sensor Microwave/Imager (SSM/I) sensors on the US Defense Meteorological Space Program (DMSP) satellites, and now with the AMSR-E instrument on NASA's Earth Observing System/Aqua satellite. Data collected by these instruments have been instrumental in shaping public policy and international perspectives on the Arctic.
