Читать книгу Exploring the Solar System - Peter Bond - Страница 69

Earth is located in the habitable zone of the Solar System, where temperatures are neither too hot not too cold for life (as we know it) to exist. Although extremes do occur, notably in hot deserts and at the poles, the planet's mean temperature is a hospitable 7°C. (However, without the greenhouse gases in the atmosphere, notably water vapor, carbon dioxide, and methane, Earth's temperature would be well below the freezing point of water.)

The global radiation budget is determined by the amount of incoming solar radiation and the amount of radiation absorbed or lost by the planet (Figure 3.5). When solar radiation passes through the atmosphere, some wavelengths are absorbed by gases, dust, and cloud droplets. Some visible light is scattered by the gases, giving the sky its blue appearance. About half of the total insolation is absorbed by the surface, compared with one‐third which is reflected back into space from the clouds and the surface.

Of course, the surface does not retain all of its heat; some is re‐radiated at infrared wavelengths. A fraction of this is absorbed by the greenhouse gases and clouds, then re‐radiated in all directions.

The role of clouds and aerosols is crucial in this planetary balancing act. (Up to 70% of the world is covered with cloud at any one time.) They help to cool the planet by reflecting a substantial amount of solar radiation back into space. However, they can also raise its temperature by absorbing or re‐radiating downwards much of the infrared radiation that would otherwise escape. Scientists are keen to know how the climate would react to a long‐term increase or decrease in this cloud and aerosol blanket.

The pattern of surface temperatures is also influenced by a number of other factors. In winter, the amount of incoming solar radiation (insolation) is insufficient to compensate for the heat that escapes into space at night. The same effect, of course, on a much smaller scale, applies to day and night temperatures.

The daily and seasonal variation in overall insolation greatly influences surface temperatures, particularly over the continents. This is because land surfaces respond more rapidly than water to daytime heating and nighttime cooling. Oceans take a long time to warm, and an equally long time to cool. In effect, they act as storage heaters. (The lower “specific heat” of rock means that its temperature rises more quickly than that of water for the same input of energy.)

In Earth's southern hemisphere, which is largely covered in water, the slow response of the oceans to variations in insolation means that diurnal and seasonal temperature ranges are less extreme. (Places on the shores of large lakes experience similar moderating effects.) In the northern hemisphere, where land masses predominate, summer temperatures tend to be much higher, while thermometers plunge during the winter. As a result, places located in continental heartlands have a much greater variation in temperature than islands and coastal regions.

The nature of the surface also plays a part (Figure 3.6). Dark surfaces, such as those covered with vegetation and soil, have a low albedo (reflectivity). They heat up more quickly than lighter, more reflective surfaces, such as ice sheets. Ice and snow reflect some 80% of the solar energy they receive, compared with 20% for an area of grassland and 10% for a dry, black soil. This explains why scientists are concerned that the melting of polar ice sheets will contribute to global warming.

Figure 3.6 These images show the amount of long wavelength thermal radiation (heat) emitted to space from Earth's surface and atmosphere (left) and shorter wavelength solar radiation reflected by the ocean, land, aerosols, and clouds (right). The largest flux of long wave radiation is from cloud‐free tropical regions. Clouds over the equator and at high latitudes reflect a lot of incoming solar radiation back into space. Such images help to determine the factors that influence Earth's radiation balance.

(NASA)