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GLOBAL WARMING
RELATED ISSUES
Ocean acidification
Increased atmospheric carbon dioxide increases the amount of CO2
dissolved in the oceans.[5] Carbon dioxide gas dissolved in the ocean
reacts with water to form carbonic acid resulting in ocean
acidification. Since biosystems are adapted to a narrow range of pH,
this is a serious concern directly driven by increased atmospheric CO2
and not global warming.
Relationship to ozone depletion
Although they are often interlinked in the mass media, the
connection between global warming and ozone depletion is not strong.
There are five areas of linkage:
The same carbon dioxide radiative forcing that produces near-surface
global warming is expected (perhaps somewhat surprisingly) to cool the
stratosphere. This, in turn, would lead to a relative increase in ozone
depletion and the frequency of ozone holes.
Radiative forcing from various greenhouse gases and other
sourcesConversely, ozone depletion represents a radiative forcing of the
climate system. There are two opposed effects: Reduced ozone allows more
solar radiation to penetrate, thus warming the troposphere instead of
the stratosphere; the resulting colder stratosphere emits less long-wave
radiation down to the troposphere, thus having a cooling effect.
Overall, the cooling dominates; the IPCC concludes that "observed
stratospheric O3 losses over the past two decades have caused a negative
forcing of the surface-troposphere system"[6] of about −0.15 ± 0.10
W/m2.
One of the strongest predictions of the greenhouse effect theory is that
the stratosphere will cool. Although this cooling has been observed, it
is not trivial to separate the effects of changes in the concentration
of greenhouse gases and ozone depletion since both will lead to cooling.
However, this can be done by numerical stratospheric modeling. Results
from the NOAA Geophysical Fluid Dynamics Laboratory show that above 20
km (12.4 miles), the greenhouse gases dominate the cooling.
Ozone depleting chemicals are also greenhouse gases, representing 0.34
±0.03 W/m2, or about 14% of the total radiative forcing from well-mixed
greenhouse gases.
Relationship to global dimming
Scientists have stated with 66-90% confidence that the effects of
volcanic and human-caused aerosols have offset some of global warming,
and that greenhouse gases would have resulted in more warming than
observed if not for this effect.
For comparison of the relative significance of these factors:
The best estimate for the magnitude of radiative forcing from the
long-lived greenhouse gases CO2, CH4, and N2O alone is +2.3 watts/m2.
Radiative forcing from the halocarbon class of long-lived greenhouse
gases is about +0.34 watts/m2.
The cooling effects of aerosols are estimated to be:
Direct cooling effects of -0.5 watts/m2
Cloud albedo cooling effects of -0.7 watts/m2
Total warming effects from post-industrial human activity including the
above and other cooling and warming factors are estimated at +1.6
watts/m2.
Pre-human global warming
Further information: Paleoclimatology
The Earth has experienced natural global warming and cooling many times
in the past and these processes can offer useful insights into the
present. It is thought by some geologists[citation needed] that a rapid
buildup of greenhouse gases caused the Earth to experience global
warming in the early Jurassic period, with average temperatures rising
by 5 °C (9.0 °F). Research by the Open University published in Geology
(32: 157–160, 2004 ) indicates that this caused the rate of rock
weathering to increase by 400%. As such weathering locks away carbon in
calcite and dolomite, carbon dioxide levels dropped back to normal over
roughly the next 150,000 years.
Sudden releases of methane from clathrate compounds (the Clathrate Gun
Hypothesis) have been hypothesized as a cause for other past global
warming events, including the Permian-Triassic extinction event and the
Paleocene-Eocene Thermal Maximum. However, warming at the end of the
last glacial period is thought not to be due to methane release.
Instead, natural variations in the Earth's orbit (Milankovitch cycles)
are believed to have triggered the retreat of ice sheets by changing the
amount of solar radiation received at high latitude and led to
deglaciation.
Using paleoclimate data for the last 500 million years Veizer et al.
(2000, Nature 408, pp. 698–701) concluded that long-term temperature
variations are only weakly related to carbon dioxide variations. Most
paleoclimatologists believe this is because other factors, such as
continental drift and mountain building have larger effects in
determining very long-term climate. However, Shaviv and Veizer (2003)
proposed that the biggest long-term influence on temperature is actually
the solar system's motion around the galaxy, and the ways in which this
influences the atmosphere by altering the flux of cosmic rays received
by the Earth.[12] Afterwards, they argued that over geologic times a
change in carbon dioxide concentrations comparable to doubling
pre-industrial levels, only results in about 0.75 °C (1.3 °F) warming
rather than the usual 1.5–4.5 °C (2.7–8.1 °F) reported by climate
models. They acknowledge (Shaviv and Veizer 2004) however that this
conclusion may only be valid on multi-million year time scales when
glacial and geological feedback have had a chance to establish
themselves. Rahmstorf et al. 2004 argue that Shaviv and Veizer
arbitrarily tuned their data, and that their conclusions are unreliable.
Pre-industrial global warming
Paleoclimatologist William Ruddiman has argued that human
influence on the global climate began around 8,000 years ago with the
start of forest clearing to provide land for agriculture and 5,000 years
ago with the start of Asian rice irrigation. He contends that forest
clearing explains the rise in carbon dioxide levels in the current
interglacial that started 8,000 years ago, contrasting with the decline
in carbon dioxide levels seen in the previous three interglacials. He
further contends that the spread of rice irrigation explains the
breakdown in the last 5,000 years of the correlation between the
Northern Hemisphere solar radiation and global methane levels, which had
been maintained over at least the last eleven 22,000-year cycles.
Ruddiman argues that without these effects, the Earth would be nearly 2
°C (3.6 °F) cooler and "well on the way" to a new ice age. Ruddiman's
interpretation of the historical record, with respect to the methane
data, has been disputed.[
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Terminology
History of warming
Causes
Greenhouse gases in the atmosphere
Solar variation
Attributed and
expected effects
Mitigation
Kyoto Protocol
Climate models
Other related issues
Ocean acidification
Relationship to ozone depletion
Relationship to global dimming
Pre-human global warming
Pre-industrial global warming
References |
