Variability of the Earth’s climate - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 18

in the atmosphere will increase due to the higher vapor pressure of water with
increasing temperature. Depending on where the humidity increases, an additional
greenhouse effect due to water vapor can amplify the greenhouse effect of rising
CO 2 . Increases in humidity in dry areas will have a much greater effect than
humidity increases in humid areas. Climate models tend to make simplistic
assumptions in the absence of data on how humidity changes as the Earth warms.
Other important feedbacks include clouds, aerosols, wind patterns, ocean currents,
and dust. As the water vapor content of the atmosphere increases, more clouds are
likely to form. Some clouds may produce a net heating effect via absorption of IR
emitted by Earth, while others may produce a cooling effect by reflecting incident
solar irradiance out to space. The treatment of clouds remains one of the major
uncertainties in climate models.
The concentration of CO 2 in the atmosphere is reached as a balance between
opposing forces: CO 2 is supplied to the atmosphere by emissions from volcanoes
and other geological processes. (All of the above is prior to human production
of CO 2 ). CO 2 is removed from the atmosphere by what is called ''silicate rock
weathering'' which stores the CO 2 in CaCO 3 (limestone). Rainwater strips out
CO 2 from the atmosphere, and the rain thereby becomes slightly acidic. The
acidified rain reacts with rocks chemically, forming carbonates that store CO 2 in
the rocks (Walker, 2003). CO 2 is also removed by burial of organic matter in
sediments.
Over very long geological times, continental drift rearranges the positions of
the landmasses on Earth, and, depending on whether the continents are separate
or conjoined and also depending on their latitudes, the capability of the land to
take up CO 2 may increase or decrease. Silicate rock weathering is enhanced by
warm wet landmasses. Thus, landmasses located in the tropics enhance uptake of
CO 2 . An additional factor is the placement of the landmasses. If they are con-
joined, the humidity in the interior is likely to be low, thus reducing CO 2 uptake
by the land. Conversely, if the land is distributed as separate bodies with close
access to moisture from nearby oceans, CO 2 uptake by the land is enhanced.
Finally, large eruptions of basalt lava provide a rich source of calcium ions that
can readily remove CO 2 from the atmosphere to produce CaCO 3 . Hence, the CO 2
concentration in the atmosphere can go through wide variations over geologic
time as continental drift rearranges the continents, volcanoes pass through active
and passive periods, and occasional large emissions of basalt lava take place. It
is likely that variable CO 2 concentration has been an important

factor in

determining the Earth's climate over many millions of years.
Methane is supplied to the atmosphere by microbes (methanogens) that live in
poorly drained soils (e.g., tropical wetlands) and in organic-rich sediments below
the sea floor. It is removed via oxidation by the oxygen in the air. Methane has a
rather short lifetime in today's atmosphere and must be continually replenished or
its concentration will fall. In the early Earth prior to about 2.4 billion years ago,
the oxygen concentration was very low, so presumably the concentration of
methane was higher than today, producing a significant greenhouse effect that
counteracted the weaker Sun that prevailed at that time.