12.3: Climate Change

Changes in the Earth’s radiation balance are a central issue in the phenomenon of climate change. Climate is the average weather over a relatively long period of time, often defined as 30 years. Thus, climate change refers to long-term change in the average weather patterns for a location, a region, or the globe. One of the most well-known examples of climate change is the ongoing increase in the average surface air temperature and ocean surface temperature around the world, which is referred to as global warming. Measurements from thousands of stations indicate that the Earth’s surface temperature has increased by approximately 0.75 °C since 1975 and approximately 1.25°C since 1900 (Fig. 12-5).That warming is not uniformly distributed around the globe, with greater warming over land than over the ocean sand greater warming in the Arctic than in lower latitudes, as highlighted in this animation of global surface temperatures from 1880-2019 [website]

Figure 12-5. Measured global land surface air temperatures and ocean surface temperatures from various sources from 1850 to 2020. Source: Wikipedia, RCraig09 [website]

These temperature increases are thought to be caused primarily by changes in the Earth’s radiation balance due to increased atmospheric concentrations of gases that absorb long wave radiation, which are commonly called greenhouse gases. The most prominent example of rising greenhouse gas concentrations is that of carbon dioxide (CO2). The atmospheric concentration of CO2 has increased by >30% since 1960, as evidenced by a famous set of measurements taken at Mauna Loa Observatory in Hawaii, USA (Fig. 12-6). This data set is often called the Keeling Curve in honor of the American scientist, Charles David Keeling, who began the observations.These rising CO2 concentrations are attributed primarily to burning fossil fuels and secondarily to human land management practices, such as deforestation, the associated burning of biomass,and the loss of soil organic carbon from land used for agriculture.

Figure 12-6. Atmospheric carbon dioxide (CO2) measurements at Mauna Loa Observatory in Hawaii, USA. Source: Scripps Institution of Oceanography, University of California-San Diego [website]

Carbon dioxide absorbs long wave radiation, thereby reducing the loss of long wave radiation from Earth to outer space. Atmospheric measurements at long-term monitoring locations in Oklahoma and Alaska, USA, indicate that the rising CO₂ concentration is resulting in an increase in surface radiative forcing of approximately 0.2 W m-2 per decade under clear sky conditions [5]. If this positive radiative forcing continues, the temperatures of Earth’s atmosphere, land, and oceans may continue to increase.As the atmosphere warms, it can store more water vapor, and increased water vapor in the atmosphere can make the surface radiative forcing 2-3 times larger [6]. This is called the water vapor feedback effect.Reduction of both fossil fuel burning and deforestation have been suggested to curtail CO₂ emissions to the atmosphere. Likewise, the adoption of alternative agricultural management practices, such as crop production without the use of tillage (i.e., no-till), has been suggested to increase soil organic carbon storage and remove CO₂ from the atmosphere. However, the carbon sequestration potential of no-till has likely been over-estimated in many studies because of inadequate soil sampling depths [7]