Page 28 - Earth's Climate Past and Future

4 PART I • Framework of Climate Science

Climate and Climate Change 30%, the land, is partly blanketed in green, darker in
forested regions and lighter in regions where grass or
Even from distant space, it is obvious that Earth is the shrubs predominate. Even the pale brown deserts and
only habitable planet in our solar system (Figure 1-1). some of the white ice contain life.
More than 70% of its surface is a welcoming blue, the Earth’s favorable climate enabled life to evolve on
area covered by life-sustaining oceans. The remaining our planet. Climate is a broad composite of the average
condition of a region, measured by its temperature,
amount of rainfall or snowfall, snow and ice cover, wind
direction and strength, as well as other factors. Climate
specifically applies to longer-term changes (years and
longer), in contrast to the shorter fluctuations that last
hours, days, or weeks and are referred to as weather.
Earth’s climate is highly favorable to life both in an
overall, planet-wide sense and at more regional scales.
The surface temperature of the Earth averages a com-
fortable 15°C (59°F), and much of the surface ranges
between 0° and 30°C (32° and 86°F) and can support
life (Box 1-1).
Although we take Earth’s habitability for granted,
climate can change over time, and with it can change
the degree to which life is possible, especially in vulner-
able regions. During the several hundred years in which
humans have been making scientific observations of
climate, actual changes have been relatively small. Even
so, climatic changes significant to human life have
occurred. One striking example is the advances of valley
glaciers that overran mountain farms and even some
small villages in the European Alps and the mountains
of Norway a few centuries ago because of a small cool-
ing of climate. Those glaciers have since retreated
to higher positions, as shown in the introduction to
Part I.
Scientific studies reveal that these historical changes
in climate are tiny in comparison with the much larger
changes that happened earlier in Earth’s history. For
example, at times in the distant past ice covered much of
the region that is now the Sahara Desert, and trees
flourished in what are now Antarctica and Greenland.

1-1 Geologic Time

Understanding climatic changes of the past begins with
a difficult challenge: coming to terms with the enor-
mous span of time over which Earth’s climatic history
has developed. Human life spans are generally mea-
sured in decades. The phases of our lives, such as child-
hood and adolescence, come and go in a few years, and
our daily lives tend to focus on needs and goals that we
hope to satisfy within days or weeks.
Almost all of Earth’s long history lies immensely far
beyond this human perspective. Earth formed 4.55 billion
FIGURE 1-1 The habitable planet Even seen from distant
space, most of Earth’s surface looks inviting to life, especially years (Byr) ago (4,550,000,000 years!). Most of the earli-
its blue oceans and green forests but also its brown deserts est part of Earth’s history is either a blank or is known
and white ice. All these areas are prominent parts of Earth’s only in a sketchy way. One reason for this gap in our
climate system. (NASA.) knowledge is the climate system itself: the relentless