Page 507 - Environment: The Science Behind the Stories

25˚ Ocean circulation Ocean water exchanges heat with the
22˚ atmosphere, and ocean currents move energy from place to
place. In equatorial regions, such as the area around the Mal-
dives, the oceans receive more heat from the sun and atmosphere
Orbital Equator than they emit. Near the poles, the oceans emit more heat than
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they receive. Because cooler water is denser than warmer water,
the cooler water at the poles tends to sink, and the warmer sur-
face water from the equator moves to take its place. This is one
principle underlying global ocean circulation patterns (p. 442).
The oceans’ thermohaline circulation system has influen-
(a) Axial wobble (b) Variation of tilt tial regional effects (p. 443). For example, it moves warm tropi-
cal water northward toward Europe, providing the European
continent a far milder climate than it would otherwise have.
Scientists are studying whether freshwater input from Green-
Earth land’s melting ice sheet might shut down this warm-water flow
(p. 443). Such an occurrence would plunge Europe into much
Sun colder conditions.
Earth Multiyear climate variability results from the El Niño–
Southern Oscillation (pp. 443–444), which involves system-
atic shifts in atmospheric pressure, sea surface temperature,
and ocean circulation in the tropical Pacific Ocean. These
(c) Variation of orbit shifts overlie longer-term variability from a phenomenon
known as the Pacific Decadal Oscillation. El Niño and La Niña
Figure 18.5 There are three types of Milankovitch cycles:
(a) an axial wobble that occurs on a 19,000- to 23,000-year events alter weather patterns from region to region in diverse
cycle; (b) a 3-degree shift in the tilt of Earth’s axis that occurs on a ways, often leading to rainstorms and floods in dry areas and
41,000-year cycle; and (c) a variation in Earth’s orbit from almost drought and fire in moist areas. This leads to impacts on wild-
circular to more elliptical, which repeats every 100,000 years. life, agriculture, and fisheries.

These variations, known as Milankovitch cycles, alter the way FAQ The climate changes naturally, so why
solar radiation is distributed over Earth’s surface (Figure 18.5). worry about climate change?
By modifying patterns of atmospheric heating, these cycles Earth’s climate does indeed change naturally across very long
trigger long-term climate variation. This includes periodic periods of time. However, no known natural factors can account
episodes of glaciation during which global surface tempera- for the rapid speed of the change we are experiencing today.
tures drop and ice sheets advance from the poles toward the Moreover, our civilization has never before experienced the sheer
midlatitudes, as well as intervening warm interglacial periods. amount of change predicted during this century. The quantity by
which the world’s temperature is forecast to rise is greater than
Solar output The sun varies in the amount of radiation it the amount of cooling needed to bring on an ice age. Green-
emits, over both short and long timescales. However, scientists house gas concentrations are already higher than they’ve been
are concluding that the variation in solar energy reaching our in over 800,000 years, and are still rising. Our entire civilization
planet in recent centuries has simply not been great enough arose only in the last few thousand years during an exceptionally
to drive significant temperature change on Earth’s surface. stable period in Earth’s climate history. Unless we reduce our
Estimates place the radiative forcing of natural changes in emissions, we will soon be challenged by climatic conditions the
solar output at only about 0.12 watts/m —less than any of the human species has never lived through before.
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anthropogenic causes shown in Figure 18.4. Moreover, solar
radiation has been decreasing since the 1970s, not increasing,
so it clearly cannot explain Earth’s recent warming trend.
Studying Climate Change
Ocean absorption The oceans hold 50 times more carbon
than the atmosphere holds. They absorb carbon dioxide from the To comprehend any phenomenon that is changing, we must
atmosphere when this gas dissolves directly in water and when study its past, present, and future. Scientists monitor present-
marine phytoplankton use it for photosynthesis. However, the day climate, but they also have devised clever means of infer-
oceans are absorbing less CO than we are adding to the atmos- ring past change as well as sophisticated methods to predict
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phere (see Figure 5.17, p. 140). Thus, carbon absorption by future conditions.
the oceans is slowing global warming but is not preventing it.
Moreover, recent evidence indicates that the rate of absorption is Proxy indicators tell us about the past
decreasing. As ocean water warms, it absorbs less CO because
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gases are less soluble in warmer water—a positive feedback Evidence about paleoclimate, climate in the ancient past, is
506 effect (pp. 124–125) that accelerates warming of the atmosphere. vital for giving us a baseline against which we can measure

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