CRITICALLY EVALUATE THE FACTORS INFLUENCING THE GLOBAL HEAT BUDGET - 40 MARKS
The global heat budget is the balance between incoming insolation, and outgoing radiation from Earth. There are many factors that influence this heat budget, some of which are more important than others. In this essay I am going to describe and explain each factor involved.
Insolation is the incoming energy from the sun in shortwave ultraviolet (UV) radiation. The distribution of insolation over the Earth’s surface is complicated; however there are patterns to be seen. Insolation has numerous aspects to it that each affects it in turn, which are divided into three categories of long term, short term and local. Quite possibly the most important aspect is latitude, a long term factor. The effect of latitude on the heat budget can be seen all over the globe, but the most striking examples are that the annual amount of insolation in the Arctic region, located 66° north of the equator, is 75 W/m2 however it is 275 W/m2 in Egypt, located 26° north of the equator. Latitude holds such a large effect on insolation because a variation in latitude means variations in the altitude of the sun. Altitude of the sun affects insolation because as the angle of the sun in the sky decreases and becomes more oblique, the land area to be heated up by the rays and the depth of atmosphere they must travel through increase. Therefore the amount of insolation lost through absorption, scattering and reflection increases. Areas in lower latitudes have higher temperatures than those in higher latitudes, because the sun is at a higher angle of incidence. One might expect the highest amount of insolation to be found on land directly below the equator, due to the sun’s highest angle of incidence being located here, meaning the equator receives more energy as solar radiation strikes the Earth head-on. However, as a result of this large amount of sunlight, lots of plant life has grown on land beneath the equator, forming rainforests such as the Amazon Rainforest, Congo River Basin Rainforest and Rainforests of South East Asia. This in turn leads to increased cloud cover above, from higher levels of evapotranspiration, affecting the albedo. Albedo is a measure of the reflectivity of a surface, expressed as a percentage. It is 30-40% from thin cloud cover and 25% from deciduous forest. Due to this increased level of albedo, more of the sun’s radiation is instantly reflected back into the atmosphere, therefore the areas with the highest amounts of insolation are those above and below the equator around the tropics, where there is a similar angle of incidence. These areas include Australia, Mexico and southern Africa, each having 225 W/m2, 250 W/m2, and 250 W/m2 annual amount of insolation respectively. This evidence clearly shows how big an influence latitude has upon the global heat budget, as it affects levels of insolation in every area of the world. Another long term factor affecting insolation is height above sea level. This can be seen in the 2013 average temperatures of Aspen and Denver, two different areas located in the same state of Colorado, USA. Aspen measured 0°C and Denver measured 6°C. Aspen is 900m higher in altitude at 2400m than Denver at 1500m. The reason for this decrease in temperature from Denver to Aspen is that the atmosphere is not warmed directly by the sun but by heat radiated from Earth’s surface, distributed by conduction, convection and radiation. As altitude increases, regions become mountainous and there is a decreasing area of land surface to heat the surrounding air. Density and the pressure of the air also decrease, and so does its ability to retain heat giving a low heat capacity. The molecules retaining heat become fewer and more widely dispersed with an increase in height, meaning less heat is passed on through conduction and radiation, and as a result temperature decreases as altitude increases – this is known as the environmental lapse...
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