Chapter Two
6. Draw the key features of the radiation balance or budget on Earth and label all of these features.
NOTE: Incoming high energy, short wave radiation is partially absorbed at the surface of the Earth. Longer waver, lower energy IR-radiation is radiated back from the surface of the Earth where, greenhouse gases absorb some of that energy and so prevent its dissipation farther away from the surface of the Earth. The net effect is to warm the Earth's atmosphere (troposphere). Laws of physics mean that the amount of energy entering the Earth's atmosphere must be equal to the amount absorbed by the Earth's atmosphere + the amount reradiated back into space.
2. Why do scientists believe the role of carbon, cycling through natural systems, like the oceans and forests, is important when talking about CO2 as a greenhouse gas?
A key feature of all biogeochemical Earth cycles is that in order to maintain a constant level in any reservoir, sources and sinks need to be balanced. In the case of carbon in the atmosphere, the amount that accumulates there depends as much on how the sinks like forests and the ocean are responding to environmental conditions as it does on the sources of carbon that are adding CO2 to the atmosphere. The balance between sources and sinks is the key to understanding the complete cycling of carbon through the Earth.
1. Why is CO2 the key greenhouse gas that is monitored when there are other greenhouse gases in the atmosphere?
Even though CO2 is not the most effective of the GHG at warming the atmosphere it is the most important to monitor. There are two main reasons why CO2 is so important - relative abundance and long residence time. A) CO2 is the most abundant of all of the greenhouse gases in the atmosphere accounting for about 75% by volume of all greenhouse gases in the atmosphere. The six greenhouse gases that are emitted in the United States are: • Carbon dioxide (CO2) • Methane (CH4) • Nitrous oxide (N2O) • Industrial Gases: o Hydrofluorocarbons (HFCs) o Perfluorocarbons (PFCs) o Sulfur hexafluoride (SF6) B) CO2 has a long residence time in the atmosphere. The carbon dioxide molecule is very stable due, in part, to its two double bonds. As a result, CO2 that is emitted today could stay in the atmosphere for close to 100 years. Under these circumstances, without additional sinks, CO2 will accumulate in the atmosphere and result in additional warming.
5. Describe one positive and one negative feedback in the atmosphere.
Positive: Positive feedbacks amplify temperature change so they make the climate system more sensitive to the properties that trigger them. The more the temperature rises the more a positive feedback is accelerated and so the more it contributes to warming the climate etc.... • Ice-albedo feedback on solar radiation (positive). Rising temperatures cause polar glaciers and floating ice sheets to recede, decreasing Earth's albedo and raising temperatures. This feedback is very strong at times when polar ice has expanded widely, such as at the peak of ice ages. It can work in both directions, helping ice sheets to advance as Earth cools and accelerating the retreat of ice sheets during warming periods. There is relatively little polar ice on land today, so this feedback is not likely to play a major role in near-term climate change. However, temperature increases large enough to melt most or all of the floating ice in the Arctic could sharply accelerate global climate change, because ocean water absorbs almost all of the incident solar radiation whereas ice reflects most sunlight. • Water vapor feedback (positive). The atmosphere can hold increasing amounts of water vapor as the temperature rises, because the pressure of water vapor in equilibrium with liquid water increases exponentially with temperature. The presence of more water vapor as temperature rises increases the greenhouse effect, as well as the absorption of solar radiation, which further raises temperature. This is the strongest and best-understood feedback mechanism in the atmosphere, because it is based on the straightforward fact that warm air can hold more water vapor than cool air. • Cloud feedback on terrestrial radiation (positive). Because warmer temperatures increase water vapor amounts, they can increase cloudiness and further raise temperature. This is a very strong feedback that is not well understood. It is hard to know whether or how much cloudiness will increase as temperature does, because cloudiness depends more on upward air motion than on temperature or water vapor levels directly. (For details on how clouds form, see Section 5, "Vertical Motion in the Atmosphere.") Negative: Negative feedbacks have a dampening effect on temperature change, making the climate system less sensitive to the factors that trigger them. The more the climate warms the less a negative feedback contributes to temperature. • Hurricanes (negative): Early research seems to indicate that warmer sea surface temperatures create more and higher energy hurricanes. The hurricanes leave a trail of cooler water that they churn up to the surface from deeper within the ocean cooling sea surface temperatures which in turn help to cool the atmosphere. • • Vegetation feedback on solar radiation (negative). As temperatures rise, deserts may expand, increasing Earth's albedo and decreasing temperature. This is a very complex feedback. It is uncertain whether deserts will expand, or conversely, whether higher CO2 levels might stimulate higher plant growth levels and increase vegetation instead of reducing it. • Cloud feedback on solar radiation (negative). As temperature increases and atmospheric water vapor levels rise, cloudiness may increase. Greater cloudiness raises Earth's albedo, reflecting an increasing fraction of solar radiation back into space and decreasing temperature, although some cloud types are more reflective than others (Fig. 20). This is another very strong feedback that is not well understood because it is hard to know whether or how much cloudiness will increase with temperature. Also, as noted in the previous example, clouds can also absorb infrared radiation, raising temperatures.
3. Why do scientists focus on understanding sinks instead of concentrating solely on humans' role as a source of CO2?
If the mechanisms that control the rate of consumption by sinks can be well understood, it may be possible to increase the rate of carbon uptake by those sinks and so balance the excess amount of carbon released into the atmosphere from anthropogenic sources. For example, increased photosynthesis can absorb considerable CO2. Fuel sources from algae that are burned and turn around the carbon by its photosynthesis at almost the same rate as the carbon is released by burning it as fuel might serve as the perfect non-polluting fuel. In this case, the source of carbon in the atmosphere - burning the algae-based fuel to run cars and industrial operations - would, during the production phase, serve as a sink for the carbon through photosynthesis while the algae is growing. This is, of course, a technological dream at the moment but it illustrates the power of understanding all of the sinks and sources in the carbon cycle.
