Earth as a Planet: Atmosphere and Oceans 183
perturbations is highly nonlinear and is determined by feed-
backs in the climate system. Positive feedbacks amplify a
perturbation and, under some circumstances, can induce a
runaway process where the climate shifts abruptly to a com-
pletely different state. In contrast, negative feedbacks re-
duce the effect of a perturbation and thereby help maintain
the climate in its current state. Some of the more important
feedbacks are as follows.
Thermal feedback:Increases in the upper tropospheric
temperature lead to enhanced radiation to space, tend-
ing to cool the Earth. Decreases in the upper tropo-
spheric temperature cause decreased radiation to space,
causing warming. This is a negative feedback.
Ice-albedo feedback:Ice caps and glaciers reflect visible
light easily, so the Earth’s brightness (albedo) increases
with an increasing distribution of ice and snow. Thus,
a more ice-rich Earth absorbs less sunlight, promoting
colder conditions and growth of even more ice. Con-
versely, melting of glaciers causes Earth to absorb more
sunlight, promoting warmer conditions and even less ice.
This is a positive feedback.
Water-vapor feedback:Warmer surface temperatures al-
low increased evaporation of water vapor from the ocean
surface, increasing the atmosphere’s absolute humidity.
Because water vapor is a greenhouse gas, it promotes
an increase in the strength of the greenhouse effect and
hence even warmer conditions. Cooler conditions inhibit
evaporation, lessen the greenhouse effect, and cause ad-
ditional cooling. This is a positive feedback.
Cloud feedback:Changes in climate can cause changes in
the spatial distribution, heights, and properties of clouds.
Greater cloud coverage means a brighter Earth (higher
albedo), leading to less sunlight absorption. Higher alti-
tude clouds have colder tops that radiate heat to space
less well, promoting a warmer Earth. For a given mass
of condensed water in a cloud, clouds with smaller
particles reflect light better, promoting a cooler Earth.
Unfortunately, for a specified climate perturbation (e.g.,
increasing the CO 2 concentration), the extent to which
the coverage, heights, and properties of clouds will
change remains unclear. Thus, not only the magnitude
but even the sign (positive or negative) of this feedback
remains unknown.The sum of these and other feedbacks determine how
Earth’s climate evolved during past epochs and how Earth
will respond to current human activities such as emis-
sions of CO 2. Much of the uncertainty in current climate
projections results from uncertainty in these feedbacks. A
related concept is that of thresholds, where the climate un-
dergoes an abrupt shift in response to a gradual change. For
example, Europe enjoys temperate conditions despite its
high latitude in part because of heat transported poleward
by the Gulf Stream. Some climate models have suggested
that increases in CO 2 due to human activities could sud-
denly shift the ocean circulation in the North Atlantic into
a regime that transports heat less efficiently, which could
cause widespread cooling in Europe (although this might
be overwhelmed by the expected global warming that will
occur over the next century). The rapidity with which ice
ages ended also suggests that major reorganizations of the
ocean/atmosphere circulation occurred during those times.
Although thresholds play a crucial role in past and possi-
bly future climate change, they are notoriously difficult to
predict because they involve subtle nonlinear interactions.5.3 Recent Times
A wide range of evidence demonstrates that Earth’s global-
mean surface temperature rose by about 0.6◦C between
1900 and 2000. Since the mid-1970s, the global-mean rate
of temperature increase has been∼0.17◦C per decade (with
a greater rate of warming over land than ocean). As of 2006,
20 of the hottest years measured since good instrumen-
tal records started in∼1860 have occurred within the past
25 years, and the past 25 years has been the warmest 25 year
period of the past 1000 years. There is widespread consen-
sus among climate experts that the observed warming since
∼1950 has been caused primarily by the release of CO 2
due to human activities, primarily the burning of oil, coal,
natural gas, and forests: The greater CO 2 concentration has
increased the strength of the greenhouse effect, modified
by the feedbacks discussed in Section 5.2. Before the In-
dustrial Revolution, the CO 2 concentration was∼280 ppm
(i.e., a mole fraction of 2.8× 10 −^4 ), and in 2006 the CO 2
concentration was 380 ppm—a 36% increase. Interestingly,
only half of the CO 2 released by human activities each year
remains in the atmosphere; the remainder is currently ab-
sorbed by the biosphere and especially the oceans. The
increase in mean surface temperature has been accompa-
nied by numerous other climate changes, including retreat
of glaciers worldwide, thawing of polar permafrost, early ar-
rivals of spring, late arrivals of autumn, changes in the Arc-
tic sea-ice thickness, approximately 0.1–0.2 m of sea-level
increase since 1900, and various effects on natural ecosys-
tems. These changes are expected to accelerate in the 21st
century.5.4 Ice Ages
The repeated occurrence of ice ages, separated by warmer
interglacial periods, dominates Earth’s climatic record of
the past 2 million years. During an ice age, multi-kilometer-
thick ice sheets grow to cover much of the high-latitude
land area, particularly in North America and Europe; most
or all of these ice sheets melt during the interglacial periods
(however, ice sheets on Antarctica and Greenland have re-
sisted melting during most interglacials, and these two ice
sheets still exist today). The sea level varies by up to 120 m