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Environment > Biosphere >

Dangerous Climate Change and Tipping Points

Paper ID: 606 Last updated: 31/01/2012 09:08:31
Criteria: bullet Impact:  Likelihood:  Controversy:  Where: Global When: 50yrs+ How Fast: Unknown
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Summary bullet

There is no single definition of dangerous climate change but one way of describing it is climate change severe enough to have a major effect on societies, economies and the wider environment. This would include the occurrence of ‘tipping points’ in the climate system. Evidence from models, theory and records of past climate [1], [2] indicates that the climate system can change abruptly if it passes so-called ‘tipping points’ - critical thresholds at which a small change induces a large response in the climate system.

Discussion bullet

Potential climate tipping points include:

• rapid melting of the Greenland Ice Sheet
• collapse of the West Antarctic Ice Sheet
• abrupt retreat of Arctic summer sea-ice
• shut-down of the over-turning circulation in the Atlantic Ocean – also known as the ‘Atlantic Meridional Overturning Circulation’ (AMOC) or the Atlantic Thermohaline Circulation
• dieback of the Amazon rainforest
• rapid release of methane from permafrost or ‘methane hydrates’. Methane hydrates are structures consisting of methane locked into a water-ice lattice. They are sometimes referred to as ‘methane clathrates’. They occur in deep ocean sediments and under permafrost (frozen ground)
• sudden changes to marine ecosystems resulting from ocean acidification

The risk that ‘tipping points’ will be exceeded because of global warming, cannot be quantified yet, as the global temperatures at which they could occur are still highly uncertain. The IPCC’s Fourth Assessment Report concluded that abrupt climate changes are unlikely to occur in the 21st century, but that the likelihood of such changes increases as temperatures rise. More recent scientific evidence has reinforced this view. [3]

If ‘tipping points’ are passed, the effects would be virtually irreversible over timescales of hundreds or even thousands of years.

The UN Framework Convention on Climate Change’s (UNFCCC)’s target of limiting global warming to 2°C above pre-industrial levels gives the world a good chance of avoiding most known potential ‘tipping points’.

Implications bullet

Polar Ice sheets

The Antarctic and Greenland ice sheets are vast and contain enough freshwater to cause global sea-level rise of 66 and 7 metres respectively, if they were to completely melt. Large-scale decline of these ice sheets, as temperatures rise, is not expected for centuries, although the total masses of both the Greenland and the West Antarctic ice sheets have started to reduce and in the last decade have caused an average global sea level ocean rise of about 0.5 mm per year.

* Antarctic ice sheets
Antarctica has two ice sheets, the West Antarctic (WAIS) and the far larger East Antarctic (EAIS). The Antarctic Peninsula has warmed significantly over recent decades, which has already caused some melting of the WAIS, while temperatures over the rest of the continent have shown little change. [4]

Although the risk of significant decline in the coming centuries is greater for the WAIS, it is uncertain how either ice sheet may melt in the future because of global warming, and whether any ‘tipping points’ could occur, leading to irreversible ice loss.

* Greenland ice sheet
Temperatures have risen almost twice as quickly in the Arctic as in the rest of the world over the past 50 years or so, [5] causing some loss of mass in the Greenland Ice Sheet over the past couple of decades.

If global warming continues unchecked, the ice sheet would probably take several thousand years to melt completely. [6]

Recent research [7] suggests there could be multiple ‘tipping points’ in the ice sheet’s melting process. If these ‘tipping points’ were passed, the ice sheet would only partially recover even if global carbon dioxide concentrations subsequently return to pre-industrial levels. It would then take an ice age – which is unlikely to happen for tens of thousands of years – for the ice sheet to fully recover.

Arctic Sea Ice

Arctic sea ice extent follows a seasonal cycle throughout the year, reducing in summer as the ice warms and melts, and increasing in winter months as it freezes.

There has been a long-term decline in the extent of summer Arctic sea ice since the satellite record began in 1979. This decline has accelerated over the last decade to 10% per decade (72,000 km2 per year), with the four lowest minima being recorded over the last four years. [8] The ice has also become thinner, making it more susceptible to summer melting.

Climate models indicate that this decline will continue over coming years and that the first ice-free summers could occur between 2040 and 2080. [1] The loss of Arctic sea ice has important implications, including positive feedback effects which can accelerate regional warming, and significant impacts on ecosystems, the livelihoods of native populations and on Arctic geo-politics. [9]

North Atlantic Thermohaline Circulation (THC)

The THC is a global ocean circulation, driven primarily by density differences (relating to heat and salt content) in the ocean between high and low latitudes. Regionally, the THC (of which the Gulf Stream is a component) transfers warmer surface water from the tropical Atlantic into the North Atlantic, giving North West Europe a milder climate than it would otherwise have.

There are concerns that the THC will weaken as a result of climate change, which would have important implications for Europe’s maritime climate .

Scientific consensus (as represented in the IPCC’s Fourth Assessment Report) suggests it is very likely that the THC will slow down gradually during this century, offsetting a fraction of the greenhouse gas warming around the North Atlantic. However, a total collapse of the system, by 2100, is considered very unlikely.

Ocean current measurements have not so far identified any long term change in the THC.

Amazon Forest Dieback

The Amazon rainforest plays a vital role in the Earth’s climate system, taking in and storing vast amounts of carbon dioxide (CO2) from the atmosphere. But forest loss causes release of CO2 from the breakdown of vegetation and reduces capacity for absorbing CO2 from the atmosphere, leading to further climate change. [10]

Climate modelling has shown that global warming could reduce rainfall and dry out soil in the Amazon basin, with an associated long-term risk of extensive forest dieback. The research suggests that the rainforest is likely to react slowly, with many impacts not being seen for several decades. However, the forest would then decline for many years after that, even if temperatures were stabilised. It could take 500 to 2,000 years for the rainforest to recover, even if the climate returned to its pre-industrial state.

Although there are large uncertainties in these projections, and some other studies have suggested a lower vulnerability, there is strong evidence that drought poses a serious risk to the Amazon rainforest and may lead to an ‘irreversible switch’ from tropical ecosystem to savannah.

Methane release from hydrates and permafrost

Although shorter lived and less abundant in the atmosphere than CO2, methane is a potent greenhouse gas that is emitted from both human and natural sources. Very large amounts of methane are currently stored, directly or indirectly, in methane hydrates deep under the oceans and in permafrost. There is a risk that global warming could at some stage cause a release of these stored forms, creating a positive feedback effect which would enhance climate change.

* Methane hydrates
Immense quantities of methane are stored as methane hydrates - structures consisting of frozen methane gas locked into a water-ice lattice - on the floor of deep oceans. The low temperature and high pressure conditions currently keep them stable. If ocean temperatures warm sufficiently, the long term risk of methane hydrates being released over centuries to millennia is real but difficult to quantify. There is considerable uncertainty about this risk but even if only a fraction of these methane deposits were eventually destabilised, it could lead to rapid warming.

* Permafrost methane
Permafrost, the frozen organic-rich soils found at high latitudes around the Polar regions, contain large stocks of carbon. If the permafrost thaws, the carbon decomposes to form either methane or CO2, which can then be rapidly and irreversibly released as the permafrost dries out. This process has already been observed to a small extent in parts of Siberia.

It is still very uncertain how much methane (and CO2) could be released this way in the future and so this process is not currently included in most climate prediction models. There is, however, ongoing research to incorporate this feedback into the next generation of Earth System Models, so that quantitative projections can be made.

Scientists have also observed methane escaping from the seafloor permafrost in shallow Arctic seas but it is unclear whether the releases are caused by global warming. There is no current evidence to suggest that the Arctic system has reached or is close to, a methane-related climate ‘tipping point’.

Ocean acidification

The oceans are absorbing more CO2 and becoming more acidic, as a result of rising CO2 concentrations in the atmosphere. Ocean acidification could pose a major threat to many marine ecosystems and their associated food chains, because of the effects of changes in ocean chemistry on some marine species. [11] More research is needed into the mechanisms and impacts of ocean acidification.

Early indicators bullet

Continuing decline in the extent of Arctic summer sea ice
Observed decrease in ocean pH
Faster loss of ice from polar glaciers and increased calving of icebergs
Ice shelf loss in Western Antarctica
Ice melt from polar ice caps contributing to sea level rise
Methane release from permafrost thawing in a few parts of Siberia

Drivers & Inhibitors bullet

Increasing concentrations of greenhouse gases, due to emissions from human activity
Rising global temperatures, as a result of greenhouse gas emissions
Positive feedback effects in the Earth’s climate system
Reducing efficiency, or loss, of carbon sinks such as forests
International efforts towards mitigating greenhouse gas emissions
Development of renewable energy and carbon capture and storage technologies
Improvements in the efficiency of energy use derived from fossil fuels
Development of low carbon economies
Public awareness and support for limiting climate change
Aerosol emissions from major volcanic activity

Parallels & Precedents bullet

Global warming during the Mesozoic era, over 65 million years ago, when sea levels may have been considerably higher than today
A number of abrupt events in the geological past where ocean acidification increased rapidly in response to natural emissions, the largest being the Paleocene-Eocene Thermal Maximum (PETM) event, about 55 million years ago.
Theory that release of methane from clathrates under the deep oceans caused major global warming during the PETM
A slow-down in the North Atlantic Thermohaline Circulation at the end of the last Ice Age, after melting of the North American ice sheet resulted in a sudden reduction in the salinity of the North Atlantic surface waters.

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Sources bullet

1IPCC2007Fourth Assessment Report, Working Group I: Chapter 10, Global Climate Projections, Meehl G.A, Stocker T.F. (lead coordinators)Visit siteEnv
2Earth System Modelling Group06/03/2009Tipping Points in the Earth System, Timothy M. LentonVisit siteEcon
3European Union10/2010Scientific Perspectives after Copenhagen Information Reference Document, Eric Fee (Ed.)Visit siteEnv
4IPCC2007Fourth Assessment Report, Working Group I: Chapter 11, Regional Climate Projections, Christensen J.H., Hewitson B. (lead coordinating authors)Visit siteEnv
5Geophysical Research Letters16/07/2009Chylek P., Arctic air temperature change amplification and the Atlantic Multidecadal Oscillation Visit siteEnv
6Journal of Climate01/12/2005Ridley et al., Elimination of the Greenland Ice Sheet in a High CO2 ClimateVisit siteEnv
7Climate Dynamics21/08/2009Thresholds for irreversible decline of the Greenland ice sheet, vol. 35, No.6Visit siteEnv
8National Snow and Ice Data CenterArctic Sea Ice News and AnalysisVisit siteEnv
9National Oceanic and Atmospheric AdministrationFuture of Arctic Climate and Global ImpactsVisit siteEnv
10Science2008Malhi Y. et al., Climate Change, Deforestation, and the Fate of the AmazonVisit siteEnv
11Royal Society30/06/2005Ocean acidification due to increasing atmospheric carbon dioxideVisit siteEnv
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