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7.5: The Science of Climate Change

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    What is causing global climate change?

    Scientists have identified the source of our current global climate change as being the increased human-caused emissions of greenhouse gases such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), since the industrial revolution. Greenhouse gases are defined as large (at least three atoms) gas molecules that participate in the greenhouse effect. While you already know about the “big three” greenhouse gases (CO2, CH4, and N2O), it’s important to realize that water vapor (H2O) is also a greenhouse gas. While humans have little direct impact on water vapor concentrations in the atmosphere, is it still an essential component of the natural greenhouse effect that occurs in our atmosphere.

    The Earth receives energy from the sun and in turn radiates energy back into space. When these two energies are equal, a stable temperature of the Earth is achieved. This temperature can be calculated from basic physics and is equal to about -18°C (0°F). This thermal equilibrium temperature is obviously much colder than that of the surface of the Earth. The actual average value of the Earth’s surface temperature is about 15°C (59°F). The difference between these temperatures is due primarily to the natural greenhouse gas concentrations in the atmosphere, causing the greenhouse effect. If the Earth had no naturally occurring atmospheric greenhouse gases, the temperature at the surface of the Earth would equal the thermal equilibrium temperature. The influence of these greenhouse gases, mainly water and some CO2, moderates the Earth’s climate and makes life possible (Figure \(\PageIndex{1}\)).

    As solar radiation reaches the Earth’s atmosphere, there are a variety of possibilities for its fate. Some solar radiation is reflected by the Earth and its atmosphere, and does not contribute to warming. Some passes through the atmosphere and reaches the surface of the Earth. When this solar radiation is absorbed by objects on Earth’s surface, it is re-emitted as infrared radiation (heat) that escapes to space. However, some of this heat is intercepted in the atmosphere by greenhouse gases. These gases absorb and re-emit the radiation in all directions. This creates a warming impact on the Earth’s surface Radiation can be bounced around from one greenhouse gas molecule to another, becoming trapped, and increasing its warming potential. For this reason, an increased greenhouse gas concentration causes an increase in the overall warming potential of the Earth’s atmosphere.

    Screenshot (91).pngFigure \(\PageIndex{1}\): This diagram shows the Earth's greenhouse effect. The Earth absorbs some of the energy it receives from the sun and radiates the rest back toward space. However, certain gases in the atmosphere, called greenhouse gases, absorb some of the energy radiated from the Earth and trap it in the atmosphere. These gases essentially act as a blanket, making the Earth’s surface warmer than it otherwise would be. While this greenhouse effect occurs naturally, making life as we know it possible, human activities in the past century have substantially increased the amount of greenhouse gases in the atmosphere, causing the atmosphere to trap more heat and leading to changes in the Earth’s temperature. Credit: US EPA

    On a geological time scale, the climate has changed many times in the past, even before the presence of humans. These changes occurred naturally because man had not yet evolved. A well-known example of past climate change is the occurrence of ice ages. Ice ages have occurred repeatedly throughout Earth’s history, the most severe ice age of which scientists have reliable data occurred around 650,000 years ago. During this time, glacial ice covered much of Canada, the northern United States, and northern Europe; the level of the ocean decreased 120 m, and the temperature decreased by 5°C.

    A geologic history of ice events is preserved in the ice sheets covering Antarctica and Greenland. This history has been uncovered over the past decades by scientists who have cored deeply into the ice and deciphered the temperature and atmospheric composition records stored in the ice. This process of obtaining ice cores is shown in Figure \(\PageIndex{2}\). The temperature at which the ice originally formed can be obtained from an interpretation of the measured ratio of the stable isotopes (see Chapter 1 supplement for a description of isotopes) of oxygen in the molecules of water forming the ice. The atmospheric gas composition is taken from air bubbles trapped in the ice at the time of formation. From these data, scientists have gathered a set of reliable data that track atmospheric temperature and gas concentrations that dates back 800,000 years. These data helped scientists come to the conclusion that the Earth’s temperature and greenhouse gas concentrations are directly correlated to one another (Figure \(\PageIndex{3}\)). During the ice age 650,000 years ago, the Earth was experiencing depressed temperature and atmospheric CO2 concentrations below 200 parts per million (ppm). We can also see from these data, that CO2 concentrations can be naturally elevated to as high as 300 ppm, correlating with increased temperatures.

    Screenshot (92).pngFigure \(\PageIndex{2}\): On Dec. 8, 2010, Michelle Koutnik, of the University of Copenhagen's Center for Ice and Climate, prepared ore of Antarctic ice to be wrapped and put into core tubes for transport back to labs at Brigham Young University in Utah. But first, Koutnik measured the core's length, diameter and weight. The traverse was the first of two field campaigns to study snow accumulation on the West Antarctic Ice Sheet and tie the information back to larger-scale data collected from satellites. 120 m, and the global average Credit: NASA/Lora Koenig. Screenshot (94).pngFigure \(\PageIndex{3}\): Estimates of the Earth’s changing CO2 concentration (top) and Antarctic temperature (bottom), based on analysis of ice core data extending back 800,000 years. Until the past century, natural factors caused atmospheric CO2 concentrations to vary within a range of about 180 to 300 ppm. Warmer periods coincide with periods of relatively high CO2 concentrations. NOTE: The past century’s temperature changes and rapid CO2 rise (to 400 ppm in 2015) are not shown here. Source: Based on data appearing in NRC (2010).

    The 100,000 year major cycle of the ice ages and some variations within the cycles agree very well with predicted periodic relationships between the Earth’s orbit around the sun, generally referred to as the Milankovitch cycles. Milankovitch cycles describe the very slight “wobbles” that occur in the Earth’s tilt and path as it moves around the sun. The Earth is always slightly tilted on its axis with respect to the sun. The angle of this tilt, however, changes periodically, varying from about 22° to about 25°. A less severe tilt will cause milder summers and winters close to the poles, preventing full summer ice melt in the northern- and southernmost regions, and allowing for a buildup of ice from year to year.

    The path through which the Earth travels on its journey around the sun also changes from a more circular to a more elongated shape. Again, a round orbit will cause milder summers and winters close to the poles. These are very long term changes, and the results of the Milankovitch cycles can be observed in the changes in temperature and atmospheric CO2 concentration shown in Figure \(\PageIndex{3}\). The climate change event that scientists are currently documenting is occurring much more rapidly than could be explained by Milankovitch cycles. Therefore, scientists agree that the cause of our currently changing climate is due to human impacts and not natural forces.