Page 504 - Environment: The Science Behind the Stories

Incoming solar Reflected solar Outgoing longwave
radiation Units are in watts radiation radiation
342 per square meter 107 235

Reflected by Emitted by Emitted by
clouds, aerosols, atmosphere surface and passing
and atmosphere and clouds through atmosphere
77 195 40

Absorbed by
atmosphere Greenhouse gases in
67 Evapo- atmosphere
Thermals transpiration
24 78
Reflected by
surface
30 Radiation
Shorter- emitted by Back
wavelength surface radiation
UV and visible 390 324
light passes
through
atmosphere Longer-wavelength Absorbed
Absorbed by infrared radiation is by surface
surface absorbed and re-emitted 324
168 by atmosphere, creating
the greenhouse effect

Figure 18.1 Our planet receives 342 watts of energy per square meter from the sun, and it natu-
rally reflects and emits this same amount. Earth absorbs nearly 70% of the solar radiation it receives,
and reflects the rest back into space (yellow arrows). The radiation absorbed is then re-emitted (orange
arrows) as infrared radiation, which has longer wavelengths. Greenhouse gases in the atmosphere absorb
a portion of this long-wavelength radiation and then re-emit it, sending some back downward to warm the
atmosphere and the surface by the greenhouse effect. Data from Kiehl, J.T., and K.E. Trenberth, 1997. Earth’s annual
global mean energy budget. Bulletin of the American Meteorological Society 78: 197–208. © American Meteorological Society
(AMS). By permission.

molecule of carbon dioxide, and a molecule of nitrous oxide activity consist mostly of carbon dioxide. And CO has a much
2
is 298 times more potent than a CO molecule. longer residence time (pp. 475–476) in the atmosphere than
2
Although carbon dioxide is less potent on a per-molecule methane, so methane’s impact is greater in the short term than
basis than methane or nitrous oxide, it is far more abundant in the long term. According to the latest data, CO is causing
2
in the atmosphere, so it contributes more to the greenhouse nearly six times more warming than methane, nitrous oxide,
effect. Moreover, greenhouse gas emissions from human and halocarbons combined.

Table 18.1 global Warming Potentials of Four FAQ The greenhouse effect works just like CHAPTER 18 • Glob al Cli M aT e Chan G e
greenhouse gases a greenhouse, right?
RELATIVE HEAT-TRAPPING ABILITY Actually, not quite. A greenhouse helps plants grow because
GREENHOUSE GAS (IN CO EQUIVALENTS) its glass walls trap heat. In contrast, greenhouse gas
2
Carbon dioxide 1 molecules in our atmosphere absorb particular wavelengths
of light reflected up from the surface, then re-emit radiation
Methane 25
at different wavelengths. Some of this radiation travels back
Nitrous oxide 298
toward the surface, keeping the surface and lower atmosphere
Hydrochlorofluorocarbon 14,800 warmer than they would otherwise be. This phenomenon
HFC-23
differs from what happens in a greenhouse, but it was called
Data are for a 100-year time horizon, from IPCC, 2007. Fourth the “greenhouse effect” in the past, and the name has stuck.
assessment report. Climate change 2007: The physical science basis. 503

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