Page 367 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 11 Climate Change 331
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▲Figure 11.22 Concentrations of methane, nitrous oxide, and fluorinated gases since 1978. gas concentrations are in parts per billion (ppb) or parts per trillion (ppt), indicating the number of mole- cules of each gas per billion or trillion molecules of air. [From Greenhouse gases continue climbing; 2012 a record year, nOaa, august 2013, available at research.noaa.gov/news/newsarchive/latestnews/Tabid/684/artMiD/1768/ articleiD/10216/greenhouse-gases-continue-climbing-2012-a-recordyear.aspx.]
substances, have been increasing since the early 1990s. In general, fluorinated gases are potent greenhouse gases with the longest atmospheric residence times.
Sources of Radiative Forcing
We learned in Chapter 4 that Earth’s energy balance is theoretically zero, meaning that the amount of energy ar- riving at Earth’s surface is equal to the amount of energy
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eventually radiated back to space. However, Earth’s cli- mate has cycled through periods where this balance is not achieved and Earth systems are either gaining or losing heat. The term radiative forcing, also called climate forc- ing, describes the amount by which some perturbation causes Earth’s energy balance to deviate from zero; a posi- tive forcing indicates a warming condition, a negative forc- ing indicates cooling.
Anthropogenic Greenhouse Gases Scientists have mea- sured the radiative forcing, quantified in watts of energy per square metre of Earth’s surface (W·m−2), of greenhouse gases on Earth’s energy budget since 1979. Figure 11.23, which compares the radiative forcing (RF) exerted by 20 greenhouse gases, shows that CO2 is the dominant gas af- fecting Earth’s energy budget. On the right side of the fig- ure is the Annual Greenhouse Gas Index, as measured by NOAA, which reached 1.32 in 2012. This indicator con- verts the total radiative forcing for each gas into an index by using the ratio of the RF for a particular year compared with the RF in 1990 (the baseline year). The graph shows that RF has increased steadily for all gases, with the pro- portion attributed to CO2 increasing the most.
Comparison of RF Factors In their 2007 Fourth Assess- ment Report on Climate Change, the International Panel on Climate Change (IPCC) estimated the amount of radi- ative forcing for a number of natural and anthropogenic factors (Figure 11.24). The factors included in the analysis were long-lived greenhouse gases, stratospheric and tropo- spheric ozone, stratospheric water vapour, changes in sur- face albedo related to land use and pollution, atmospheric aerosols from both human and natural sources, linear con- trails, and solar irradiance (the output of energy from the Sun, discussed earlier).
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Parts per trillion (ppt) Parts per billion (ppb)
Parts per billion (ppb)