FEEDBACK LOOPS IN CLIMATE
CHANGE
Some effects of global warming themselves contribute
directly to further global warming. A potential danger of feedback
loops in climate change is that they may go beyond an amplifying
effect and and lead to an explosive effect. This would have devastating
consequences for humans.
Methane release from hydrates
Methane clathrate, also called methane hydrate,
is a form of water ice that contains a large amount of methane within
its crystal structure. Extremely large deposits of methane clathrate
have been found under sediments on the ocean floors of Earth. The
sudden release of large amounts of natural gas from methane clathrate
deposits, in a runaway greenhouse effect, has been hypothesized
as a cause of past and possibly future climate changes.
The release of this trapped methane is a potential
major outcome of a rise in temperature; it is thought that this
might increase the global temperature by an additional 5° in itself,
as methane is much more powerful as a greenhouse gas than carbon
dioxide. The theory also predicts this will greatly affect available
oxygen content of the atmosphere. This theory has been proposed
to explain the most severe mass extinction event on earth known
as the Permian-Triassic extinction event.
Carbon cycle feedbacks
There have been predictions, and some evidence,
that global warming might cause loss of carbon from terrestrial
ecosystems, leading to an increase of atmospheric CO2 levels. Several
climate models indicate that global warming through the 21st century
could be accelerated by the response of the terrestrial carbon cycle
to such warming.
All 11 models in the C4MIP study found that a
larger fraction of anthropogenic CO2 will stay airborne if climate
change is accounted for. By the end of the twenty-first century,
this additional CO2 varied between 20 and 200 ppm for the two extreme
models, the majority of the models lying between 50 and 100 ppm.
The higher CO2 levels led to an additional climate warming ranging
between 0.1° and 1.5 °C.
However, there was still a large uncertainty on
the magnitude of these sensitivities. Eight models attributed most
of the changes to the land, while three attributed it to the ocean.
The strongest feedbacks in these cases are due to increased respiration
of carbon from soils throughout the high latitude boreal forests
of the Northern Hemisphere. One model in particular (HadCM3) indicates
a secondary carbon cycle feedback due to the loss of much of the
Amazon rainforest in response to significantly reduced precipitation
over tropical South America]. While models disagree on the strength
of any terrestrial carbon cycle feedback, they each suggest any
such feedback would accelerate global warming.
Observations show that soils in England have been
losing carbon at the rate of four million tonnes a year for the
past 25 years according to a paper in Nature by Bellamy et al. in
September 2005, who note that these results are unlikely to be explained
by land use changes. Results such as this rely on a dense sampling
network and thus are not available on a global scale. Extrapolating
to all of the United Kingdom, they estimate annual losses of 13
million tons per year. This is as much as the annual reductions
in carbon dioxide emissions achieved by the UK under the Kyoto Treaty
(12.7 million tons of carbon per year).
Forest Fires
Rising Global temperature might cause forest fires
to occur on larger scale, and more regularly. This releases more
stored carbon into the atmosphere than the carbon cycle can naturally
re-absorb, as well as reducing the overall forest area on the planet,
creating a positive feedback loop. Part of that feedback loop is
more rapid growth of replacement forests and a northward migration
of forests as northern latitudes become more suitable climates for
sustaining forests. There is a question of whether the burning of
renewable fuels such as forests should be counted as contributing
to global warming.
Retreat of Sea Ice
The sea absorbs heat from the sun, while the ice
largely reflects the sun rays back to space. Thus, retreating sea
ice will allow the sun to warm the now exposed sea water, contributing
to further warming. The mechanism is the same as when a black car
heats up faster in sunlight than a white car. This albedo change
is also the main reason why polar temperatures are predicted to
rise up to twice as much as those of the rest of the world.
Negative feedback effects
A negative feedback loop is one that will counteract
the effects of climate change. The chemical equilibrium of the Earth’s
carbon cycle will shift in response to anthropogenic CO2 emissions.
The primary driver of this is the ocean, which absorbs anthropogenic
CO2 via the so-called solubility pump. At present this accounts
for only about one third of the current emissions, but ultimately
most (~75%) of the CO2 emitted by human activities will dissolve
in the ocean over a period of centuries (Archer, 2005; “A better
approximation of the lifetime of fossil fuel CO2 for public discussion
might be 300 years, plus 25% that lasts forever”). However, the
rate at which the ocean will take it up in the future is less certain,
and will be affected by stratification induced by warming and, potentially,
changes in the ocean’s thermohaline circulation.
Another negative feedback loop is that as temperature
rises, the Earth’s thermal radiation rises and will lose some heat
to space. The impact of these negative feedback effects in relation
to the positive feedback effects are usually taken into account
in global climate models.
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