Long-term effects of global warming
|This article's factual accuracy may be compromised due to out-of-date information. (April 2012)|
- The focus of this article is on the effects of global warming beyond 2100.
There are expected to be various long-term effects of global warming. Most discussion and research, including that by the Intergovernmental Panel on Climate Change (IPCC) reports, concentrates on the effects of global warming up to 2100, with only an outline of the effects beyond this.
Ice loss and sea level rise
Concern has been expressed about the stability of the West Antarctic ice sheet (WAIS). Vaughan & Spouge argued that the "WAIS most likely will not collapse in the next few centuries." In a recent inaugural article for members of the National Academy of Sciences elected in 2005, Timothy Lenton and others suggest that a collapse of the West Antarctic ice sheet could occur within a millennium. Specifically they state "Although the timescale is highly uncertain, a qualitative WAIS change could occur within this millennium, with collapse within 300 years being a worst-case scenario. Rapid sea-level rise (greater than 1 m per century) is more likely to come from the WAIS than from the Greenland ice sheet."
Greenland's ice sheet contains enough fresh water as ice to raise sea level worldwide by 7 metres (23 ft). Greenland may become warm enough by 2100 to begin an almost complete melt over more than 1,000 years. James E. Hansen suggests that inadequate attention is being given to this issue.
Some long-term effects happen over thousands, not hundreds, of years.
Disruption to thermohaline circulation
Early work with simplified models suggested that global warming could cause a shutdown of the thermohaline circulation. This effect is not replicated in more sophisticated coupled ocean-atmosphere global climate models, which do not shutdown but do display varying degrees of slowdown.
This may take a considerable time to occur, as Knutti and Stocker found, again from a simplified model, when :"…thermohaline shutdowns can occur thousands of years after the warming has stopped."
One study suggests that the amount of oxygen dissolved in the oceans may decline, with adverse consequences for ocean life. This effect was determined using a model run of 100,000 years. The researchers predicted:
…severe, long-term ocean oxygen depletion, as well as a great expansion of ocean oxygen-minimum zones for scenarios with high emissions or high climate sensitivity. We find that climate feedbacks within the Earth system amplify the strength and duration of global warming, ocean heating and oxygen depletion. Decreased oxygen solubility from surface-layer warming accounts for most of the enhanced oxygen depletion in the upper 500 m of the ocean. Possible weakening of ocean overturning and convection lead to further oxygen depletion, also in the deep ocean.
Other research by the Leibniz Institute for Marine Sciences predicts that minimum oxygen zones, particularly in the pacific are expanding, with an estimated total reduction of ocean oxygen levels from 1% to 7% by the end of the century. 
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 (estimated at 3000–11,000 Gton C).
MacDonald suggests clathrate volumes are "about 11,000 Gt of carbon for ocean sediments, and about 400 Gt for sediments under permafrost regions". Buffett and Archer predict "eventual releases of 2000–4000 Gton C in response to a ~2000 Gton C anthropogenic carbon release", for which they suggest no timescale.
Archer considered the timescale and determined that "…on longer time scales of 1–10 ky, there may be a positive feedback with ocean temperature, amplifying the long-term climate impact of anthropogenic CO2 release."
Long-term return to equilibrium
While the warm surface waters of the oceans have limited ability to absorb anthropogenic carbon dioxide, the coldest surface waters near the poles (2–3% of ocean surfaces) can transfer significant amounts of carbon dioxide to deep-ocean reserves. Over a period of many centuries, this process and the process of calcium carbonate absorption of carbon dioxide on land and in the oceans will remove 60–80% of the excess carbon dioxide.
Igneous rock when exposed to a near surface environment absorbs carbon dioxide through a very slow weathering rate, but weathering increases in a warmer, higher rainfall climate, speeding the process. This geological weathering will absorb the remaining 20–40% of anthropogenic carbon dioxide over the period of tens of thousands to hundreds of thousands of years.
- "Climate Change 2001: The Scientific Basis" (TABLE 11.3). Intergovernmental Panel on Climate Change. 2001-02-16. Retrieved 2007-12-24.
- Vaughan, D. G.; Spouge, J. R. (2002). "Risk Estimation of Collapse of the West Antarctic Ice Sheet". Climatic Change 52: 65–00. doi:10.1023/A:1013038920600.
- Lenton, T. M.; Held, H.; Kriegler, E.; Hall, J. W.; Lucht, W.; Rahmstorf, S.; Schellnhuber, H. J. (2008). "Inaugural Article: Tipping elements in the Earth's climate system". Proceedings of the National Academy of Sciences 105 (6): 1786. doi:10.1073/pnas.0705414105.
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The Greenland ice-sheet would melt faster in a warmer climate and is likely to be eliminated — except for residual glaciers in the mountains — if the annual average temperature in Greenland increases by more than about 3 °C. This would raise the global average sea-level by 7 metres over a period of 1000 years or more. We show here that concentrations of greenhouse gasses will probably have reached levels before the year 2100 that are sufficient to raise the temperature past this warming threshold.
- "Regional Sea Level Change" (FIGURE 11.16). Intergovernmental Panel on Climate Change.
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- Knutti, R.; Stocker, T. F. (2002). "Limited Predictability of the Future Thermohaline Circulation Close to an Instability Threshold". Journal of Climate 15 (2): 179–186. Bibcode:2002JCli...15..179K. doi:10.1175/1520-0442(2002)015<0179:LPOTFT>2.0.CO;2. ISSN 1520-0442.
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- Ocean Deoxygenation in a Warming World Ralph F. Keeling, Arne Körtzinger, and Nicolas Gruber
- Buffett, B.; Archer, D. (2004). "Global inventory of methane clathrate: sensitivity to changes in the deep ocean" (PDF). Earth and Planetary Science Letters 227 (3–4): 185. Bibcode:2004E&PSL.227..185B. doi:10.1016/j.epsl.2004.09.005.
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- Archer, D.; Buffett, B. (2005). "Time-dependent response of the global ocean clathrate reservoir to climatic and anthropogenic forcing" (PDF). Geochemistry Geophysics Geosystems 6 (3): Q03002. Bibcode:2005GGG.....603002A. doi:10.1029/2004GC000854.
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- David Archer (2009). The Long Thaw: How Humans Are Changing the Next 100,000 Years of Earth's Climate. Princeton University Press. p. 109. ISBN 978-0-691-13654-7.