Geoscience Reference
In-Depth Information
The ImpacT of Global chanGe
2
Warming
has risen by 1.7 degrees C (3.1 degrees F), a
number that is typical of the rapidly warm-
ing high latitudes. The temperate and tropi-
cal zones are warming more slowly.
Although the greenhouse effect is global,
each of the continents has a slightly dif-
ferent average temperature curve for the
twentieth century. This is because a myr-
iad of processes other than greenhouse
gases and solar radiation have an impact
on temperatures. These include ocean cur-
rents, wind patterns, the presence of ice,
the distribution of land areas, vegetation
patterns, and volcanic eruptions. The most
marked regional changes have occurred in
the Earth's polar regions. Overall the Arctic
has warmed in recent decades, even though
temperatures dropped dramatically in the
early 1960s after having risen during the
1930s and 1940s. The Arctic has warmed
more than the Antarctic because there is
more land area in the northern polar re-
gions to absorb solar radiation. Land areas
warm twice as fast as the upper layers of the
sea, in part because the ocean loses much
more heat by evaporation.
Local effects are quite variable, however,
and local temperature curves may vary
The evidence that the Earth is warming
consists of its rising sea levels, warming
atmosphere and oceans, and widespread
melting of sea ice, ice sheets, glaciers, and
permafrost. Short-term temperature num-
bers over the last century were measured
by thermometers, of course, but longer-
term records are mostly derived by inter-
preting ice cores from ice sheets and gla-
ciers, deep-sea sediment cores, tree rings,
and corals.
Atmospheric temperature measurements
since the beginning of the twentieth cen-
tury indicate that the Earth's average tem-
perature has increased from 13.5 to 14.5
degrees Celsius (56 to 58 degrees Fahren-
heit). Temperature warming varies with
the season and with latitude, as illustrated
by the warming trends in Alaska. Between
1949 and 2008, summer temperatures in
Alaska have risen by an average of 1.2 de-
grees C (2.1 degrees F), fall temperatures
by 0.5 degrees C (0.9 degree F), winter tem-
peratures by 3.3 degrees C (6 degrees F), and
spring temperatures by 1.9 degrees C (3.5
degrees F). Overall the average temperature
