Changes in climate can lead to changes in (among other things) food production and availability, thus Earth's radiation budget has important impacts on human populations. Conversely, human modification of the environment can have significant repercussions for Earth's radiation budget and climate, resulting in a feedback cycle between human actions and environmental conditions that humans and other living things will experience in the future.
Environmental impact occurs as human populations change existing Earth-Atmosphere features or characteristics and upset balances in ongoing physical processes and sensitive ecological interconnections. Human activities affect Earth's surface and the atmosphere, setting in motion changes having potentially profound implications for global climate and habitat sustainability. Some brief discussion of human modifications is provided below; a more extensive treatment of the topic is given in Chapter 8.
Human populations have altered land surface albedos and heat budgets most obviously by deforestation and other changes to vegetation cover. Vegetation is a key player as solar energy interacts with Earth's surface. Generally, more thickly forested areas have lower albedos, so more shortwave energy falling on them is absorbed and less is reflected immediately back to the atmosphere. Urbanized areas and areas where trees have been clearcut tend to have higher albedos. These kinds of surface differences and changes in albedo affect localized radiation budgets. Albedo (reflectance) contrasts are evident in the image (Figure 2.10) of mangrove forests bordered by farmland.
Vegetation cover also influences local air temperatures by shading the ground and through the release of water vapor during the process of transpiration in which water is carried up through trees and discharged through leaf pores. Transpiration consumes energy (latent heat) and consequently has somewhat of a cooling effect. By steadily releasing water vapor into the atmosphere, transpiration also contributes to the hydrologic cycle.
When vegetation is removed from a surface the localized radiation budget changes. Though the surface albedo usually increases and hence relatively more insolation is reflected and less absorbed, the localized area may become hotter overall due to less shading and less evaporative cooling as a result of reduced transpiration. These dynamics lead to a hotter and drier climate in which crops are harder to grow. At times when vegetation is not transpiring (for example, during winter in cold climates), forested areas absorb more insolation and act as wind breaks; consequently, areas cleared of vegetation tend to be colder and windswept. The role of human activity in reducing vegetative cover is discussed further in Chapter 8.
Although most greenhouse (i.e., heat trapping) gases occur naturally, their concentrations in the atmosphere have rapidly increased through human activities, particularly the burning of fossil fuels. Addition of carbon dioxide (CO2) to the atmosphere has grown along with human population, primarily through greater fossil fuel consumption (Figure 2.11: data from ice cores and direct observations; Siple, South Pole, Mauna Loa). CO2 is also given off with the burning of woody biomass associated with deforestation. Another result of deforestation is to remove trees which normally take in atmospheric CO2 and produce oxygen (O2) as a byproduct of photosynthesis. The production of O2 by plant life has been a crucial process in the evolution of Earth's atmosphere and its ability to sustain life.
Manmade gases such as chlorofluorocarbons (CFCs) have also increased in the atmosphere. Used as refrigerants, solvents, and aerosol propellants, CFCs in the air are very strong absorbers of longwave energy. CFCs are also responsible for the depletion of stratospheric ozone (O3) which protects life on Earth from harmful ultraviolet (UV) radiation.
Anthropogenic forces have increased atmospheric concentrations of another important greenhouse gas, methane, in part because of growth in rice cultivation and the raising of ruminants (sheep and cattle), both significant methane sources. Methane levels have more than doubled since the advent of the industrial age, and the number of ruminants continues to rise as human population grows. Also adding methane to the atmosphere are processes associated with gas mining and production facilities, termites, wetlands, and trash landfills.
With increased concentrations of greenhouse gases in the atmosphere, there is an increase in atmospheric air temperature, particularly in the lower atmosphere close to the surface. Observations suggest that the global average land surface temperature has risen 0.45-0.60°C (0.8-1.0°F) in the last century. Greater increases in this trend (2-3°C) have been measured in the northern hemisphere due to a larger proportion of Earth's surface being land rather than water in this hemisphere (land temperatures increase much more quickly than water temperatures).
Rising temperatures and changes in soil moisture resulting from altered climate patterns are predicted to significantly affect the distribution and productivity of vegetation around the globe. The accurate prediction of exact changes in the distribution of major vegetation communities is very difficult due to the complexity of climate systems, lag times in vegetation's response to environmental dynamics, and physical barriers to the dispersal and migration of different plant types.
Despite possible beneficial effects (such as fertilization enhancement and more efficient water use) that greater atmospheric CO2 might have on some types of plants, the accompanying increase in temperature and aridity may couple with other factors such as increased fire frequency to create a net negative impact on vegetation properties and processes. A great deal of research is being done in this area for a numbers of reasons, not the least of which is that humans will be affected by changes in agricultural productivity.
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