Greenhouse Gases
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 (see Figure 1 in NASA Facts,Biosphere). 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 (see Figures 2a & 2b in NASA Facts,Biosphere). The effect is more pronounced in the northern hemisphere because most of the land on Earth is located there.
Modeling
To understand and predict climate change, the following types of models are needed:
Socio-economic models that predict future fossil fuel consumption and utilization of alternative fuels. These models depend upon technology, e.g., industrial production methods, energy efficiency, new materials, etc.; public policy and social attitudes, e.g., concern for the environment; and economic development, standard of living and reliance on energy and chemical-based products.
Chemical-physical-biophysical models of the Earth System that tell us what happens to gases released into the atmosphere, e.g., how much carbon dioxide is taken up by the oceans and the biosphere, and how industrial and agricultural uses of chemicals and natural processes on Earth's surface affect the release of methane, nitrogen oxides, and other greenhouse gases into the atmosphere.
Coupled ocean-atmosphere models to tell us how the climate system, e.g., temperatures, humidity, clouds, and rainfall, responds to changes in the chemical composition of the atmosphere.
Getting reliable predictions from models is difficult because many of the secondary processes are not understood. For example, when temperatures start to warm because of the direct radiative effect of increasing carbon dioxide? will clouds increase or decrease. Will they let in less radiation from the sun, or more? These secondary processes are important.
The direct radiative effect of doubling carbon dioxide is relatively small, and there is not much disagreement on this point among models. Where models conflict is in regard to the secondary, or feedback effects. Models that predict a very large warming from carbon dioxide show cloud cover changes that greatly amplify the warming effects, while models that predict more-modest warming show that clouds have a small or even damping effect on the warming.
Can we match the observation of temperature trends with the model predictions? The temperature record of the past hundred years does show a warming trend, by approximately 0.5 degrees C. However, the observed warming trend is not entirely consistent with the carbon dioxide change. Most of the temperature increase occurred before 1940, after which Earth started to cool until the early seventies, when warming resumed. Carbon dioxide, on the other hand, has been increasing steadily throughout the past century. Other factors that could have affected climate during this period include the possible change in the solar energy reaching Earth, the cooling effects of volcanic aerosols, and the possibility that sulfur dioxide and other pollutants might be affecting the amount of solar radiation that is reflected back to space. Some of these effects can cause a cooling that could counteract the warming due to carbon dioxide and other greenhouse gases. All of these effects would have to be taken into account and appropriately modeled in order to predict the changes that one might expect in the next century.
Original source : http://www.maui.net/~jstark/nasa.html
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 (see Figure 1 in NASA Facts,Biosphere). 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 (see Figures 2a & 2b in NASA Facts,Biosphere). The effect is more pronounced in the northern hemisphere because most of the land on Earth is located there.
Modeling
To understand and predict climate change, the following types of models are needed:
Socio-economic models that predict future fossil fuel consumption and utilization of alternative fuels. These models depend upon technology, e.g., industrial production methods, energy efficiency, new materials, etc.; public policy and social attitudes, e.g., concern for the environment; and economic development, standard of living and reliance on energy and chemical-based products.
Chemical-physical-biophysical models of the Earth System that tell us what happens to gases released into the atmosphere, e.g., how much carbon dioxide is taken up by the oceans and the biosphere, and how industrial and agricultural uses of chemicals and natural processes on Earth's surface affect the release of methane, nitrogen oxides, and other greenhouse gases into the atmosphere.
Coupled ocean-atmosphere models to tell us how the climate system, e.g., temperatures, humidity, clouds, and rainfall, responds to changes in the chemical composition of the atmosphere.
Getting reliable predictions from models is difficult because many of the secondary processes are not understood. For example, when temperatures start to warm because of the direct radiative effect of increasing carbon dioxide? will clouds increase or decrease. Will they let in less radiation from the sun, or more? These secondary processes are important.
The direct radiative effect of doubling carbon dioxide is relatively small, and there is not much disagreement on this point among models. Where models conflict is in regard to the secondary, or feedback effects. Models that predict a very large warming from carbon dioxide show cloud cover changes that greatly amplify the warming effects, while models that predict more-modest warming show that clouds have a small or even damping effect on the warming.
Can we match the observation of temperature trends with the model predictions? The temperature record of the past hundred years does show a warming trend, by approximately 0.5 degrees C. However, the observed warming trend is not entirely consistent with the carbon dioxide change. Most of the temperature increase occurred before 1940, after which Earth started to cool until the early seventies, when warming resumed. Carbon dioxide, on the other hand, has been increasing steadily throughout the past century. Other factors that could have affected climate during this period include the possible change in the solar energy reaching Earth, the cooling effects of volcanic aerosols, and the possibility that sulfur dioxide and other pollutants might be affecting the amount of solar radiation that is reflected back to space. Some of these effects can cause a cooling that could counteract the warming due to carbon dioxide and other greenhouse gases. All of these effects would have to be taken into account and appropriately modeled in order to predict the changes that one might expect in the next century.
Original source : http://www.maui.net/~jstark/nasa.html
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