18.S: Summary

Summary

Included in Earth Science is the study of the system of processes that affect surface environments and atmosphere of the Earth. Recent changes in atmospheric temperature and climate over intervals of decades have been observed. For Earth’s climate to be stable, incoming radiation from the Sun and outgoing radiation from the Sun-warmed Earth must be in balance. Gases in the atmosphere called greenhouse gases absorb the infrared thermal radiation from the Earth’s surface, trapping that heat and warming the atmosphere, a process called the greenhouse effect. Thus the energy budget is not now in balance and the Earth is warming. Human activity produces many greenhouse gases that have accelerated climate change. CO₂ from fossil fuel burning is one of the major ones. While atmospheric composition is mostly nitrogen and oxygen, trace components including the greenhouse gases (CO₂ and methane are the major ones and there are others) have the greatest effect on global warming.

A number of positive feedback mechanisms, processes whose results reinforce the original process, take place in the Earth system. An example of a positive feedback mechanism of great concern is permafrost melting which causes decay of melting organic material that produces CO₂ and methane (both powerful greenhouse gases) that warm the atmosphere and promote more permafrost melting. Two carbon cycles affect Earth’s atmospheric CO₂ composition, the biologic carbon cycle and the geologic carbon cycle. In the biologic cycle, organisms (mostly plants and also animals that eat them) remove CO₂ from the atmosphere for energy and to build their body tissues and return it to the atmosphere when they die and decay. The biologic cycle is a rapid cycle. In the geologic cycle, some organic matter is preserved in the form of petroleum and coal while more is dissolved in seawater and captured in carbonate sediments, some of which is subducted into the mantle and returned by volcanic activity. The geologic carbon cycle is slow over geologic time.

Measurements of increasing atmospheric temperature have been made since the nineteenth century but the upward temperature trend itself increased in the mid-twentieth century showing the current trend is exponential. Because of the high specific heat of water, the oceans have absorbed most of the added heat. That this is temporary storage is revealed by the record-breaking warm years of the recent decade and the increase in intense storms and hurricanes. In 1957 the Mauna Loa CO₂ Observatory was established in Hawaii providing constant measurements of atmospheric CO₂ since 1958. The initial value was 315 ppm. The Keeling Curve, named for the observatory founder, shows that value has steadily increased, exponentially, to over 417 ppm now. Compared to proxy data from atmospheric gases trapped in ice cores that show a maximum value for CO₂ of about 300 ppm over the last 800,000 years, the Keeling increase of over 100 ppm in 50 years is dramatic evidence of human-caused CO₂ increase and climate change! As Earth’s temperature rises, glaciers and ice sheets are shrinking resulting in sea level rise. Atmospheric CO₂ is also absorbed in sea water producing increased concentrations of carbonic acid which is raising the pH of the oceans making it harder for marine life to extract carbonate for their skeletal materials.

Earth’s climate has changed over geologic time with periods of major glaciations. There was a high temperature period in the Mesozoic shown by fossils in high latitudes and the Western Interior Seaway covering what is now the Midwest. However, climate has been cooling during the Cenozoic culminating in the Ice Age. Since the Ice Age, several proxy indicators of ancient climate show that the rate and amount of current climate change is unique in geologic history and can only be attributed to human activity. Those who ignore the consequences of increasing global warming for our planet’s future do so at the peril of our posterity!