Causes Of Climate Change

The causes of natural climate variations, before human impacts, typically arose from one or more of the following: carbon- cycle changes, astronomical changes in the Earth- Sun configuration, large volcanic eruptions (especially plate- tectonics- related major volcanic episodes), asteroid impacts, or variations in the intensity of solar radiation output. Carbon-cycle changes may have acted on their own but were often also involved as a feedback in amplifying the climate responses to changes driven initially by the other mechanisms. In the following sections, we will look at each of these processes in turn. When we want to discuss the dominant changes in greenhouse gas concentrations, we focus mainly on CO2 and CH4 , of which CO2 is the dominant one on longer timescales because it exists in much higher concentrations and lasts much longer in the atmosphere than CH4. As mentioned before, we then commonly investigate things in terms of carbon (C) emissions and uptake because this allows us to relate variations directly to changes in the carbon cycle and how we humans are affecting it. The carbon cycle represents an intricate web of interactions that control carbon storage and exchange between the biosphere (life), hydrosphere (oceans, lakes, rivers), and lithosphere (rocks and sediments; Figure 4.1). We need to consider two critical terms when discussing it. The first is known as the reservoir volume. This stands for the volume of carbon held within each reservoir, such as land- plants and trees, the ocean, or carbonate rocks. The second term is known as flux, and it refers to the amount of carbon that is exchanged between two reservoirs in a year. Because the volumes of carbon that are involved are enormous, we commonly express them in gigatons (Gt). One gigaton is one billion (one thousand million) tons, where a ton is 1000 kg. Most frequently, this term will be used in the expression gigaton of Carbon, or GtC. There are several important reservoirs of carbon (Figure 4.1). The atmosphere holds an approximate volume of 750 GtC. The land-biosphere—living flora and fauna—comprises some 600 or 700 GtC of living material and more than 2000 GtC of dead material.