Page 39 - Earth's Climate Past and Future

CHAPTER 1 • Overview of Climate Science 15

reaches a particular region over intervals of thousands of times of the major components in the climate system.
years. Also assume that the fast-response curve represents Most parts of the system will begin to respond relatively
the rapid heating of landmasses at lower and middle lati- quickly to the greenhouse-gas forcing, but others (those
tudes, while the slow-response curve represents changes most closely tied to the ice sheets) will respond more
in the size of ice sheets lagging thousands of years later. sluggishly. A large part of the challenge facing climate
In this scenario, the asterisk in Figure 1-10 marks a scientists is to sort out these different responses and all
time when large ice sheets have built up in Canada and their interactions.
Scandinavia (as has actually happened many times in the
past, most recently 20,000 years ago). At this point in 1-9 Feedbacks in the Climate System
the sequence, the slow-responding ice has not yet Another important kind of interaction in the climate
begun to retreat, even though the heating from the Sun system is the operation of feedbacks, processes that
has begun to increase and the land far south of the ice alter climate changes that are already underway, either
sheets has begun to warm. by amplifying them (positive feedbacks) or by sup-
Given this situation, how do you think the air tem-
peratures just south of the ice limits would respond? pressing them (negative feedbacks). Figure 1-11
shows the basic way these feedbacks operate.
Would the air warm with the initial strengthening of Assume that some external factor (again, perhaps a
the overhead Sun and heating of the land? If so, its change in the strength of radiation from the Sun) causes
response would track right behind the initial forcing Earth’s climate to change. That change will consist of
curve in Figure 1-10. many different responses among the various internal
Or would air temperatures still be under the chilling
influence of the large mass of ice lying just to the north
and not begin to rise until the ice starts its retreat? In
this case, the ice would in a sense be acting as a semiin- Initial Initial
dependent player in the climate system by exerting an climate climate
influence of its own on local climate. Although the ice forcing response
initially acts as a slow climate response driven by slow
changes in the Sun, it then exerts its own effect on cli-
mate separate from the immediate effects of the Sun.
Both these explanations probably sound plausible, Response amplified
by
and they are. The air temperatures just south of the ice climate system
sheets will be influenced by both the overhead Sun and
the nearby ice. The actual timing of the air-temperature
response in such regions will fall somewhere in the mid-
dle, faster than the response of the ice but lagging
behind the forcing from the Sun. As this example sug-
gests, Earth’s climate system is very dynamic, with A Positive feedback
numerous interactions.
The response-time concept is directly relevant to Initial
Initial
projections of climate change in the near future. Part V climate climate
of this book addresses the effects of humans on climate forcing response
through the buildup of greenhouse gases, primarily
CO produced from burning fossil fuels such as coal,
2
oil, and natural gas. The changes in the next few cen-
turies will be unusual in the sense that both the large Response reduced
climate forcing produced by humans and the warming it by
will cause will arrive with unusual speed. Within a few climate system
centuries, the fossil fuels that generate excess CO in
2
the atmosphere will be largely used up, CO emissions
2
will fall, and Earth’s climate will begin to return toward
its previous cooler state. But before that happens, Earth
will face a century or more of very substantial warmth, B Negative feedback
along with many other changes.
Scientists and the public in general want to know FIGURE 1-11 Climate feedbacks (A) Positive feedbacks
how large the disruption caused by the several centuries within the climate system amplify climate changes initially
of high CO concentrations will be, and the answer caused by external factors. (B) Negative feedbacks mute or
2
requires an understanding of the different response suppress the initial changes.