Geoscience Reference
In-Depth Information
Another factor affecting the climate of the Earth is ocean circulation. Ocean
currents from the tropical zones toward polar areas transfer heat to higher lati-
tudes, thus helping to prevent the equatorward spread of ice. As this ocean water
moves poleward, its density increases due to evaporation and cooling, and the
dense brine eventually sinks and returns toward tropical zones at deeper levels.
One theory is that this ''great ocean conveyor'' can on occasion be shut down,
leading to increased cooling of high latitudes and expansion of glacial conditions
there. This, in turn, produces an increase in global albedo, and ice ages may result.
Thornalley et al. (2011) found evidence of such shutdowns as the Earth came out
of the last ice age. The ocean conveyor is affected by the placement of the con-
tinents and the connectivity of the oceans. There is some evidence, for example,
that the conveyor changed about 3.2 million years ago when the Isthmus of
Panama became closed off, leading to a long series of repeated ice ages and a
gradual cooling of the Earth. Spencer Weart asserted that the Earth's climate is
largely governed by the oceans, because the main ingredients of climate are not in
the Earth's tenuous atmosphere but in the oceans, where the top few meters alone
store as much heat energy as the entire atmosphere, and the average depth of the
oceans is 3.7 km. Most of the world's water is there too, of course, and even most
of the gases, dissolved in the water. However, as Figure 8.4 shows, the atmosphere
actually delivers a higher heat flux to high latitudes than the oceans.
If the irradiance of the Sun were to change, that would also affect the Earth's
climate. We know that solar irradiance has gradually increased over billions of
years but we don't know the degree to which the ''solar constant'' has been con-
stant over the past few hundred thousand years. Variations in sunspot count and
solar cycle period over the past four centuries suggest that solar irradiance may
have wavered but there is no definitive analysis.
As the Earth moves in its orbit about the Sun, subtle changes in the orbital
characteristics occur over many tens of thousands of years. These changes can
affect the yearly input of solar energy to higher latitudes on a secular basis. It is
widely believed that the ebb and flow of ice ages is tied to this phenomenon
whereby variations in the solar energy input to high latitudes act as ''triggers'' to
initiate feedback mechanisms that produce climate extremes. However, this theory
remains dicult to validate exactly.
The biosphere also affects climate by generating greenhouse gases (principally
CO 2 , but also CH 4 ) and by uptaking these gases as part of the natural life/death
cycle of the plant kingdom. Land clearing reduces the ability of the biosphere to
absorb CO 2 and acts as a virtual source of CO 2 .
Hence, variations in the Earth's climate are due to many (sometimes
conflicting and opposing) factors, some of which provide positive feedback to
enhance trends, once started. Understanding climate variations is very challenging
because it is dicult to separate out and quantify the various contributing factors.
The Earth has gone through incredibly wide climate changes over hundreds of
millions of years. A number of factors contributed including a gradually strength-
ening Sun, drift of continents, incidence of volcanism, etc. But it is widely believed
that the Earth's thermostat was controlled mainly by prevailing CO 2 concen-
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