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Sunlight is the source of energy for the Earth's oceans, atmosphere, land, and biosphere. This energy serves to heat the Earth to temperatures far above the minus 454° Fahrenheit (3° Kelvin) of deep space. Averaged over an entire year and the entire Earth, the sun deposits 342 Watts of energy into every square meter of the Earth. This is a very large amount of heat1.7 x 1017 watts of power that the sun sends to the Earth/atmosphere system. For comparison, a large electric power plant would produce 100 million watts of power, or 108 watts. It would take 1.7 billion such power plants to equal the energy coming to the Earth from the sunroughly one for every three people on the Earth! Where does all the Sun's heat go? Why doesn't the Earth just keep getting hotter?
Most of the sun's heat is deposited into the tropics of the Earth. This is because the Earth's rotational axis is almost perpendicular to the plane of Earth's orbit around the sun. The polar latitudes receive on average much less solar heating than the equator. If the tilt of the Earth's axis were exactly perpendicular to the orbit plane around the sun, then there would be no seasons! Climate in January would be the same as climate in April or July, all over the Earth. But the Earth's rotational axis tilts 23.5° away from perpendicular. Consequently, during one part of the orbit around the sun, the North Pole will be tilted 23.5° toward the sun and will be in sunlight 24 hours a day. Six months later at the opposite side of the orbit around the sun, the North Pole will be in total darkness 24 hours a day. Why is this important? The amount of solar heating of the polar latitudes varies greatly through the year. In the summer, polar latitudes receive almost as much solar energy as the tropics, while in the winter they receive no solar heat at all. Meanwhile, the tropics receive by comparison roughly constant solar heating throughout the year (hence the small seasonal cycles there). As a result, in the winter hemisphere, the difference in solar heating between the equator and the pole is very largea situation perfect for driving a strong "heat engine," or circulation of the atmosphere. This energy difference drives large mid-latitude storm systems as heat moves from the surplus in the equator to the deficit in the polar regions. In contrast, the summer has very similar heating at the equator and poles, such that the heat engine slows down, and mid-latitude storms lose their source of energy. Summer storms tend to be very small scale and local.
Solar heating of the Earth and its atmosphere drives the large-scale atmospheric circulation patterns, and even the seasons. The difference in solar heating between day and night also drives the strong diurnal (or daily) cycle of surface temperature over land. But with all this solar heating going on, we still haven't answered our earlier question: Why doesn't the Earth just keep getting hotter?
The answer might be loosely called the yang and yin, or the "duality of radiation fluxes." At the same time the solar energy that we can see with our eyes is heating the planet, there is radiation being emitted at much longer wavelengths that our eyes do not seecalled "thermal infrared radiation" (basically heat). The amount of heat emitted from a solid surface is proportional to the fourth power of the temperature of the surface. So as the temperature of the Earth rises, it rapidly emits and loses to space an increasing amount of heat. If the Earth were a ball of rock with no atmosphere, a simple calculation that equates the solar energy absorbed by the Earth to the heat emitted by the Earth would predict the global average Earth temperature to be 0° Fahrenheit, or 255 Kelvin-very cold, and not the Earth as we know it (this scenario assumes that an average rock reflects 30 percent of all light that hits it).
Earth's Invisible Blanket
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Figure 1. The atmospheric greenhouse effect. Shortwave solar radiation passes through the clear atmosphere relatively unimpeded, but longwave infreared radiation emitted by the warm surface of the Earth is absorbed partially and then re-emitted by a number of trace gases such as water vapor and carbon dioxide. |
Clouds: A hot topic or are we made in the shade?
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Figure 2. Stratus clouds, which are mostly composed of liquid water droplets, reflect most of the incoming shortwave radiation (thin lines), but re-emit large amounts of outgoing longwave radiation (thick lines). Their overall effect is to cool the planet. | |
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Figure 3. Cirrus clouds, which are mostly composed of ice crystals, transmit most of the incoming shortwave radiation (thin lines), but trap some of the outgoing longwave radiation (thick lines). Their overall effect is to warm the Earth. |
Given the large impact of clouds on the radiative energy balance, the critical question now becomes: What effect will clouds have on surface temperatures if global climate changes in the next century? No one knows. Clouds could act to dampen any greenhouse gas warming by increasing cloud cover, increasing thickness, or by decreasing in altitude. Conversely, clouds could act to increase warming of the planet if the opposite trends occur. In fact, the climate is so sensitive to how clouds might change, that our current best models of global climate can vary in their global warming predictions by more than a factor of three depending on how we try to model the clouds.
So why can't we model clouds? The biggest problem is that clouds are almost explosive in nature when compared to the rest of the climate system. Cumulus clouds can form in seconds to minutes, and the entire life cycle of a massive thunderstorm can be measured in hours. This thunderstorm cloud may only cover 20 to 50 miles of the Earth's surface, while our best global climate models on the world's fastest supercomputers can only track a single column of the surface and atmosphere every 50 to 200 miles.
Surface Absorption and Reflection
Clouds are just one example of things that can change Earth's radiant
energy balance. Snow and ice are other examples. If the surface of the
Earth becomes cold enough, snow and ice will cover the surface and
increase the amount of sunlight reflected back to space. So the amount of
snow and ice on the Earth will change the global energy balance and
therefore the global temperature. Conversely, as the planet warms, the
amount of snow and ice on the surface will decrease, and so the planet
will warm furtherthis is called a "positive feedback" because it
tends
to amplify climate change.
Less obvious, but still important, when vegetation is cleared from land surfaces (such as in deforestation or agricultural burning), the bare surface reflects more sunlight back to space and there is net cooling effect. But, there is also counter productive greenhouse gas effect that comes from deforestation and biomass burningthe release of carbon dioxide, as well as elimination of vegetation that would otherwise absorb carbon dioxide from the atmosphere during photosynthesis. Unfortunately, while deforestation/reforestation may take place on annual to decadal time scales, the lifetime of carbon dioxide in the atmosphere is 50 to more than 100 years. Consequently, the solar reflectance cooling and greenhouse gas warming due to biomass burning take place at very different time scales, leading to an initial cooling followed later by a warming trend.
Atmospheric Aerosols: Fossil Fuel and Biomass Burning
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Figure 4. Clouds polluted by aerosols from factories have more numerous and smaller drops, causing the cloud to become brighter and reflect more
of the Sun's radiative energy.
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As the number of aerosols increases, the water in the cloud gets spread over many more particles, each of which is correspondingly smaller. Smaller particles fall more slowly in the atmosphere, and decrease the amount of rainfall. In this way, changing aerosols in the atmosphere can change the frequency of cloud occurrence, cloud thickness, and rainfall amounts. Even the tiny aerosols (typically less than 50 millionths of an inch or 1 micron across) can affect the clouds, which in turn can change the radiation balance of the planet. Thus, aerosols can have both a direct effect on the energy balance, as well as an indirect effect (through clouds). It is thought that the indirect effect of aerosols can be even larger than their direct effect, but at present it is not known whether such an effect is a net cooling or warming of the planet.
From measurements to climate models
With clouds, aerosols, and surface properties varying greatly over the
Earth, and with our inability to model Earth's complex climate system, we
need a hierarchy of measurements varying from laboratory studies of cloud
particles and aerosol properties, through aircraft and surface-based
field experiment measurements, to observations of the entire Earth from
space. Only from space, however, can we observe the surface, aerosol, and
cloud changes in anything approaching a complete set of conditions
occurring in the climate system.
Ultimately, we are searching for a set of mathematical models that allow us to span the incredibly large range of space and time scales important to aerosols, water vapor, clouds, the land surface, and the oceans. These models must be capable of reproducing the variability shown in the data at both regional and global scales. They must be capable of reproducing El Niño, the Earth's diurnal and seasonal cycles, and the inter-annual variability in the climate system. The models must also be capable of reproducing the systematic changes in the radiative energy balance with changing aerosols, water vapor, clouds, and surface properties. Only then can we begin to trust the models to produce accurate global change predictions.
It should be noted that these are not the only tests such models must successfully pass, but they are a critical part of the story. Water and carbon cycles in the climate system are also critical. Moreover, the development, testing, and improvement of such models using global data sets will be an iterative process, with no assurance of success. The process will be a continual narrowing of the uncertainties.
The Earth's climate system, particularly its energy and water cycles, is complex and intricately interlinked. The discovery of these links, and the development of improved predictive computer models using these links, is at the heart of NASA's Earth Observing System (EOS) observations and science plan.
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