Page 36 - Earth's Climate Past and Future

12 PART I • Framework of Climate Science

the one now covering the continent of Antarctica have On
the slowest response times in the climate system—many
thousands of years, as captured in the commonly used
word “glacial.” Climate forcing Slow change
in forcing
The concept also applies to vegetation, the organic
part of the climate system. Unseasonable frosts can kill Fast response
leaves and grass overnight, and abnormally hard freezes
can do the same to the woody tissue of trees, responses Off A
measured in hours. On the other hand, seasonal spring On
greening of the landscape and autumn loss of leafy
green material take weeks or months to complete. Pio- Fast change
neering vegetation that occupies newly exposed ground in forcing
(for example, bare ground left behind by melting glaci- Climate forcing
ers) may even take tens to hundreds of years or more to Slow response
come to full development because of the slow dispersal
of seeds and the time needed for them to germinate and Off
produce mature trees. On B Forcing

1-7 Time Scales of Forcing versus Response
The parts of this book differ considerably in their Climate forcing
emphasis on several factors: the forces that drive climate
change, the responses of the climate system, and the
interactions between forcing and response. Several Off Response
hypothetical examples shown in Figure 1-7 give a sense C
of some basic differences: Forcing
• The forcing is very slow in comparison with the response
of the climate system. This case is equivalent to Climate forcing On
increasing the flame of the Bunsen burner in
Figure 1-6 so slowly that the water temperature has
no problem keeping pace with the gradual Off Response
application of more heat. If the changes in climate
forcing are very slow in comparison with the Earlier Time Later
D
response time of the climate system, the system
simply passively tracks along with the forcing with FIGURE 1-7 Rates of forcing versus response Climate
no perceptible lag (Figure 1-7A). responses depend on the relative rate of changes in climate
This case is typical of many climate changes that forcing versus the response time of the climate system. (A) Fast
occur over the long tectonic time scales discussed in response times permit the climate system to fully track slow
Part II. For example, continents can be slowly forcing. (B) Slow response times allow little climate response
carried by plate tectonic processes toward higher or to fast changes in forcing. (C, D) Roughly equal time scales of
lower latitudes at rates averaging about 1 degree of forcing and response allow varying degrees of response of the
latitude (100 kilometers or 60 miles) per million climate system to the forcing.
years. As the landmasses move toward lower
latitudes, where incoming solar radiation is stronger,
or toward higher latitudes, where it is weaker, • The forcing is fast in comparison with the climate system’s
temperatures over the continents react to these slow response. At the other extreme, the response time of
changes in solar heating with an imperceptibly tiny the climate system may be slower than the time scale
year-by-year response. Because the response time of of the changes in forcing (Figure 1-7B). In this case,
air over land is short (hours to weeks; see Table 1-1), there is little or no response to the climate forcing.
the average temperature over the continent can This is equivalent to turning the Bunsen burner on
easily keep pace with the slow changes in average and off so quickly that the temperature of the water
overhead solar radiation over millions of years. in the beaker has no time to react.
Shorter-term changes also occur over tectonic time One example of this extreme case is a total solar
scales, but they are usually harder to resolve in older eclipse, which blocks Earth’s only source of external
records. heating for less than an hour. Air temperatures cool