Лекция: Short-term forcing and feedback

Internal radiative forcing agents are mainly those that involve changes in atmospheric composition, cloud cover, aerosols and surface albedo. Although subject to long-term changes, it is their susceptibility to short-term anthropogenic changes that makes them of particular interest to contemporary climatologists. The interactive relations between short- term external solar radiative forcing and these internal radiative forcing agents lie at the heart of the understanding and prediction of short-term global climatic changes, through a complex set of feedback mechanisms, which can be either positive (i.e. self-enhancing) or negative (i.e. self-regulating or damping).

Positive feedback mechanisms affecting global climate appear to be widespread and to be partic­ularly effective in response to temperature changes, which is a matter of especial current concern. Increases in global temperature produce increases in atmospheric water vapour, increases in plant respi­ration, decreases in CO2 dissolved in the oceans, and an increase in methane emissions from wetlands. All of these, in turn, tend to increase the global concentration of greenhouse gases and, hence, to increase global temperature further. Ice and snow cover is involved in especially important positive feedback effects in that a more extensive cover creates higher albedo and lower temperatures, which, in turn, will further extend the ice and snow cover, producing additional cooling. Conversely, a warming effect, which melts ice and snow cover, decreases the surface albedo, allows the absorption of more incoming solar radiation and leads to increased surface heating and higher temperatures.

Unfortunately, negative feedback mechanisms appear to be much less important in the face of short-term radiative forcing and it is important to understand that, for example, they can only reduce the rate of warming but cannot, of themselves, cause global cooling. Cloud cover is a particularly com­plex global feedback mechanism, producing both positive and negative effects. For example, negative feedback may operate when increased global heating leads to greater evaporation and greater amounts of high-altitude cloud cover, which will reflect more incoming solar radiation and thus damp down the global temperature rise.


Упражнение 9.

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Упражнение 10.

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Changes in climate involve factors both external to and within the climate system. External ones include solar variability, astronomical effects on the earth's orbit, and volcanic activity. Internal factors include natural variability within the climate system, and feedbacks between the atmos­phere, ocean and land surface. During the last century, human-induced climatic change on local and global scales has become a reality, primarily through changes in atmospheric composition and surface properties. Climatic changes on geological time scales involve continental drift, volcanic activity and possible changes in solar output. Over the last few million years, glacial-interglacial cycles appear to have been strongly controlled by astro­nomical variations in the earth's orbit, although atmosphere-ocean-cryosphere feedbacks must also be involved in amplifying the initial changes in solar radiation.

Possible causes of climatic change are exam­ined from the point of view of the global atmos­phere-earth-ocean-ice system and with respect to forcing and feedback mechanisms. Whereas longer-term changes are probably due to astro­nomical forcing mechanisms, short-term changes (i.e. the last 100 years) appear to be more obvi­ously linked to anthropogenic factors. These are mainly changes in atmospheric composition, including aerosol loading, depletion of ozone and destruction of world vegetation. Natural and anthropogenic aerosol effects appear to be particularly important, but their net effect remains uncertain.

Упражнение 11.

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Упражнение 12.

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