Interactions between energy from the Sun, the Earth, and the atmosphere all have an effect on the Earth. This is called the global energy balance, an energy balance that also plays a role in climate. Because this energy balance has changed and, as a result, the atmosphere is retaining more heat, the process of global warming is already in full swing.
The global energy balance regulates the state of the Earth's climate. Modifications to it—forcings—can be by natural sources or human sources, and could cause global climate to change. Natural forcings might include variations in the Sun's intensity, a shift in the Earth's orbit around the Sun, a shift in the Earth's tilt, or an increase in volcanic activity. Human-influenced forcings could include burning fossil fuels, changing land use patterns, or deforestation.
Greenhouse gases in the atmosphere have an effect on the global energy balance. Without the natural greenhouse gases in the atmosphere, the Earth would be uninhabitable because it would be too cold. Several things have an effect on the energy balance, such as clouds and atmospheric aerosols. Clouds can interact in several ways with energy. They can block much of the incoming sunlight and reflect it back to space. In this way, they have a cooling effect. Clouds also act like greenhouse gases and block the emission of heat to space and keep the Earth from releasing its absorbed solar energy. The altitude of the cloud in the atmosphere also has an effect. High clouds are colder and can absorb more surface-emitted heat in the atmosphere; yet they do not emit much heat to space because they are so cold. Clouds can cool or warm the Earth, depending on how many clouds there are, how thick they are, and how high they are. It is not yet fully understood what effect clouds will have on surface temperatures if global warming continues into the next century and beyond.
Climatologists have proposed different opinions. Some think that clouds may help to decrease the effects of global warming by increasing cloud cover, increasing thickness, or by decreasing in altitude. Others think that clouds could act to increase the warming on Earth if the opposite conditions occurred. According to Anthony Del Genio of NASA's Goddard Institute for Space Studies, when air temperatures are higher, clouds are thinner and less capable of reflecting sunlight, which increases temperatures on Earth, exacerbating global warming. Del Genio, in an interview with CNN, said that "In the larger context of the global warming debate I'd say we should not look for clouds to get us out of this mess. This is just one aspect of clouds, but this is the part people assumed would make global warming less severe."
One way that scientists try to predict future climate change and the effects of global warming is through the analysis and interpretation of mathematical climate models. In these computer models, climatologists attempt to account for all items that affect climate. Cloud cover is one of those variables. Today, this is still one of the most difficult variables to control and interpret. The climate is so sensitive as to how clouds might change, that even the most complicated, precise models developed today often vary in their global warming prediction under all the different methods available for cloud modeling.
The main reason clouds are so difficult to model is because they are so unpredictable. They can form rapidly and complete their life cycle in a matter of hours. Other climate variables work on a much slower timescale. Clouds also occur in a relatively small geographic area. Other climate variables operate on a much larger scale. According to climate research scientists at NASA, the world's fastest supercomputers can only track a single column of the surface and the atmosphere every 50 to 200 miles (80-322 km). In comparison, a massive thunderstorm system might cover only 20 miles (32 km). Features that are small, fast, and short-lived are hard to predict. This is one of the reasons why predicting specific individual weather events is more difficult than predicting long-term climate changes over broad areas.
Clouds are just one thing that can change the global energy balance. Snow and ice can also do that. If the Earth becomes cold enough, allowing large amounts of snow and ice to form, then more of the Sun's energy will be directly reflected back to space because snow and ice have a high albedo. Over a period of time, this will change the global energy balance and the global temperature. Conversely, if the Earth warms, the snow and ice will melt. This lowers the surface albedo, allowing more sunlight to be absorbed, which will warm the Earth more.
Deforestation can also upset the global energy balance in many ways. If forested areas are removed and land is left bare, the ground can then reflect more sunlight back to space, causing a net cooling effect. On the other hand, if the forest material is burned, then the carbon stored in the trees is released into the atmosphere, contributing to global warming. Also, since forests are good reservoirs of existing carbon, storing and holding the carbon and keeping it out of the air, if the forest is burned, not only does the already stored carbon now enter the atmosphere, but any future storage potential of carbon in that forest is now destroyed—creating a double addition of carbon toward global warming.
Atmospheric aerosols can be added to the atmosphere by sources such as fossil fuels, biomass burning, and industrial pollution. Tiny smoke particles (aerosols) can either cool or warm the atmospheric temperature depending on how much solar radiation they absorb versus how much they scatter back to space. Fossil fuel aerosols can also pollute clouds. Scientists need to do a lot more research on aerosols before they fully understand the full impacts of aerosols on global warming. The composition of the aerosol, its absorptive properties, the size of the aerosol particles, the number of particles, and how high they are in the atmosphere all have an effect on whether they cool or warm the atmospheric temperature and by how much.
Another effect aerosols have on clouds is that as aerosols increase, the water in the clouds gets spread over more particles, and smaller particles fall more slowly. This could decrease the amount of rainfall. Scientists believe aerosols have the potential to change the frequency of cloud occurrences, cloud thickness, and amount of rainfall in a region. Like clouds, aerosols are also a challenge to accommodate in climate models because they occur over small areas, move rapidly, form and dissolve quickly, and interact with other variables (such as wind) in unexpected ways, making them a hard variable to control.
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