Samuel Lynch
Research Report
December 14, 2012
Many parameters within Earth’s climate directly impact other parameters, and
a surprising number of these second parameters directly affect the first ones in
return. For example, an increase in parameter 1 may trigger an increase in parameter
2, and an increase in parameter 2 may also trigger an increase in parameter 1. When
this is the case, an increase in either parameter will lead to a huge spike in both. This
is known as positive feedback. The two most prominent positive feedback loops that
affect the climate are that of water vapor and that of glacial ice. These loops act as
destabilizers, reinforcing either warming or cooling trends. There are also other less
prominent positive feedback loops, including several within the carbon cycle. There
are a couple of minor negative feedback loops, such as in net primary productivity
and the atmospheric lapse rate, as well as one powerful negative feedback loop
based on the Stefan-Boltzmann law. These stabilizing loops help to keep the climate
in check, ensuring that the positive feedbacks do not cause the temperature to spiral
out of control in either direction.
The most well-known
positive feedback loop in the
climate system relates to
water vapor. Water vapor is
a greenhouse gas, meaning
that while it is in the
atmosphere, it absorbs some
of Earth’s radiated energy
and radiates it back to Earth.
Thus, the more water vapor there is in the air, the warmer the air gets. Warmer air is
able to hold more water vapor, so the effect magnifies itself: as the air warms, it
holds more vapor, which causes the earth to warm more, allowing the air to hold
even more vapor (fig. 1). The reverse is also true: if the earth cools, the air becomes
less able to hold water vapor and thus the greenhouse effect is lessened, cooling the
earth further. This positive feedback can cause drastic changes in the climate with
only a small change in conditions.
There exists a similar feedback effect involving glacial ice. Ice reflects sunlight
back to space, so more ice cover means lower temperatures. This effect is known as
albedo. As the earth cools, its ice cover grows and the albedo is increased, which
cools the earth (fig.2). As
with water vapor, this
interaction can be reversed,
and this is what we see
today: the warming of Earth
is causing ice caps and
glaciers to melt, decreasing
the earth’s albedo. This is
causing the earth to warm
even more, further melting ice cover.
Other feedback loops may be found within the carbon cycle. Most of these
are positive feedbacks, such as permafrost melt, peat bog decomposition, forest
fires, and desertification. Most of these occur because large carbon reservoirs start to
be released into the atmosphere, and the warming this causes triggers faster carbon
release.
One such reservoir may be found in arctic permafrost. It is estimated that they
contain about 900 gigatons of carbon in frozen organic matter (Zimov, Schuur,
Chapin). As the earth warms, the permafrost melts, and the organic matter
decomposes and is released into the atmosphere as methane. Methane is a powerful
greenhouse gas, so its presence in the atmosphere exacerbates the warming and
causes further permafrost melting. Similarly, higher temperatures cause peat bogs to
dry, which then decompose faster. Just like organic matter from melting arctic
permafrost, this decomposition releases methane into the atmosphere.
Part of the climate change we have experienced due to higher temperatures
has been a reduction in rainfall. This dries out trees and other vegetation, making
them more susceptible to fire. As a result, forest fire frequency and destructiveness
has increased significantly in recent years (usgcrp.gov). Forest fires release carbon
from the vegetation into the atmosphere as carbon dioxide, which acts as a
greenhouse gas to cause more warming.
The aforementioned reduction of rainfall also results in drier soil, which is
more easily eroded (Philander). This drying and degradation of the soil can lead to
aridity, and eventually to desertification. Deserts do not support much vegetation, so
they are very poor carbon sinks. As more land turns into desert, less carbon is
absorbed by vegetation, and more carbon remains in the air where it acts as a
greenhouse gas. This warms the earth further, causing worse and worse
desertification.
There are also a number of stabilizing loops that appear to be affecting the
earth’s climate, although most are either minor or not understood very well. One
well-known negative feedback relates to net primary productivity: in theory, more
carbon dioxide in the air should cause plants to respire at an increased rate, thus
removing some of the extra carbon from the atmosphere. In reality, the effect is
minor, partially because the drier climate slows plant respiration.
A second stabilizing loop in the earth’s climate has to do with the reduction in
the lapse rate of the atmosphere. Our climate models predict that most of the
atmospheric warming due to greenhouse gases will occur within the upper layers of
the atmosphere (Water Vapour and…). This will lead to an atmosphere more uniform
in temperature. Currently, our atmosphere is much warmer near the surface than in
its upper reaches, and thus it emits much more longwave radiation down towards
the earth than out into space. If our models are correct and the upper atmosphere
warms more quickly than other layers, it will radiate more heat away from Earth than
it does now, and the net warming effect of the atmosphere’s radiation will be
reduced. This reduction will help to regulate the earth’s temperature.
Probably the most powerful stabilizing loop in the earth’s climate is that of
black-body radiation according to the Stefan-Boltzmann law. This law states that a
black body’s irradiance is directly proportional to the fourth power of its temperature
(Britannica.com). The earth is not a black body, as no such body exists, but it is a
grey body and a modified version of the law still applies. To account for the earth’s
imperfect absorption and radiation, a constant representing the earth’s emissivity
must be included in the equation:
Black-body Radiation:
Grey-body Radiation:
E = σT4
E = εσT4
E = irradiance
where ε (emissivity) < 1
σ = the Stefan-Boltzmann
constant
T = thermodynamic temperature
This change affects the amount
radiated, but not the proportional
change in radiation due to change in
thermodynamic temperature. This
means that Earth will still act like a
black body in that as its temperature
rises, its rate of radiation rises at a
much greater rate. When the level of
outgoing radiation meets the level in
incoming radiation, the earth will
reach radiative equilibrium and the temperature will cease to rise (Columbia
University).
However, the existence of these negative feedbacks does not mean that
people should not be concerned about climate change. We do not know how
powerful these stabilizing effects will be, and it is entirely possible that the
unprecedented amount of anthropogenic forcing will be enough to upset the
balance that these negative feedbacks have historically provided. It should instead
mean that we should not despair, because these stabilizing effects should buy us
time to find more permanent solutions to the problem of climate change.
Bibliography

Encyclopedia Britannica
www.britannica.com

Encyclopedia of Global Warming and Climate Change, edited by S. George
Philander
http://books.google.com/books/about/Encyclopedia_of_Global_Warming_and_Cli
ma.html?id=mNoW858izZcC

Permafrost and the Global Carbon Budget, by Sergey A. Zimov, Edward A. G.
Schuur, and F Stuart Chapin III
www.sciencemag.org/content/312/5780/1612.full

Solar Radiation and the Earth’s Energy Balance, by Yochanan Kushnir
http://eesc.columbia.edu/courses/ees/climate/lectures/radiation/index.html

Stefan-Boltzmann Law
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/stefan.html

US National Assessment of the Potential Consequences of Climate Variability and
Change
http://www.usgcrp.gov/usgcrp/nacc/

Water Vapour and Lapse Rate Feedbacks
http://stratus.astr.ucl.ac.be/textbook/chapter4_node7.html