Page 506 - Environment: The Science Behind the Stories

Among other greenhouse gases, ozone concentrations in 2
the troposphere have risen roughly 36% since 1750 because of
photochemical smog (p. 483). The contribution of halocarbon Tropospheric
gases to global warming has begun to slow because of the
Montreal Protocol and subsequent controls on their produc- 1 Soot
tion and use (p. 490). on snow
Water vapor is the most abundant greenhouse gas in our
atmosphere and contributes most to the natural greenhouse
effect. Its concentrations vary locally, but its global concen- Radiative forcing (watts/m 2 ) 0
tration has not changed over recent centuries. Because its con-
centration has not changed, it is not thought to have driven
industrial-age climate change. –1 Stratospheric
Land use
Most aerosols exert a cooling effect

Whereas greenhouse gases exert a warming effect on the atmos- –2
halocarbons
phere, aerosols (p. 475), microscopic droplets and particles, can Ozone Aerosols
have either a warming or a cooling effect. Soot particles, or “black Carbon dioxide CH 4 + N 2 O + Surface albedo Cloud albedo
carbon aerosols,” generally cause warming by absorbing solar
energy, but most other tropospheric aerosols cool the atmosphere
by reflecting the sun’s rays. Sulfate aerosols produced by fossil Figure 18.4 Radiative forcing measures the degree of influ-
fuel combustion may slow global warming, at least in the short ence that aerosols, greenhouse gases, and other factors
term. When sulfur dioxide enters the atmosphere, it undergoes exert over Earth’s energy balance. In this graph, radiative forc-
various reactions, some of which lead to acid precipitation (pp. ing is expressed as the warming or cooling effect that each factor
491–493). These reactions can form a sulfur-rich aerosol haze in has on temperature today relative to 1750, in watts/m . Red bars
2
the upper atmosphere that blocks sunlight. For this reason, aero- indicate positive forcing (warming), and blue bars indicate nega-
sols released by major volcanic eruptions can exert cooling effects tive forcing (cooling). Albedo (p. 516) refers to the reflectivity of a
on Earth’s climate for up to several years. This occurred in 1991 surface. A number of more minor influences are not shown. Data
with the eruption of Mount Pinatubo in the Philippines (p. 475). from IPCC, 2007. Fourth assessment report.

Radiative forcing expresses change which could lead to more evaporation and water vapor, in a
in energy input positive feedback loop (pp. 124–125) that would amplify the
greenhouse effect. On the other hand, more water vapor could
To measure the degree of impact that a given factor exerts on enhance cloudiness, which might, in a negative feedback loop
Earth’s temperature, scientists calculate its radiative forcing, (pp. 124–125), slow global warming by reflecting more solar
the amount of change in thermal energy that the factor causes. radiation back into space. In this second scenario, depending on
Positive forcing warms the surface, whereas negative forcing whether low- or high-elevation clouds result, they might either
cools it. Figure 18.4 shows researchers’ best calculations of the shade and cool Earth (negative feedback) or else contribute to
radiative forcing that our planet is experiencing today. warming and accelerate evaporation and further cloud forma-
When scientists sum up the effects of all factors, they find tion (positive feedback). We simply don’t yet know which effect
that Earth is now experiencing overall radiative forcing of about might outweigh the other. Because of feedback loops, minor
2
1.6 watts/m . This means that today’s planet is receiving and modifications of components of the atmosphere can potentially
2
retaining 1.6 watts/m more thermal energy than it is emitting lead to major effects on climate. This poses challenges for mak-
into space. (By contrast, the pre-industrial Earth of 1750 was in ing accurate predictions of future climate change.
balance, emitting as much radiation as it was receiving.) This
extra amount is equivalent to the power converted into heat
and light by 140 incandescent lightbulbs (or 650 CFLs) over a Climate varies naturally for several reasons
football field. For context, look back at Figure 18.1 and note that Besides atmospheric composition, our climate is influenced CHAPTER 18 • Glob al Cli M aT e Chan G e
2
Earth is estimated naturally to receive and give off 342 watts/m by cyclic changes in Earth’s rotation and orbit, variation in
of energy. Although 1.6 may seem like a small proportion of energy released by the sun, absorption of carbon dioxide by
342, over time it is actually enough to alter climate significantly.
the oceans, and ocean circulation patterns.

Feedback complicates our predictions Milankovitch cycles In the 1920s, Serbian mathemati-
cian Milutin Milankovitch described three types of periodic
As tropospheric temperatures increase, Earth’s water bodies changes in Earth’s rotation and orbit around the sun. Over
should transfer more water vapor into the atmosphere, but scien- thousands of years, our planet wobbles on its axis, varies in
tists aren’t yet sure how this will affect our climate. On one hand, the tilt of its axis, and experiences change in the shape of
more atmospheric water vapor could lead to more warming, its orbit, all in regular long-term cycles of different lengths. 505

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