Garnaut, 2008:

"Sea-level rise will come from two main sources—thermal expansion and the melting of land-based ice sheets and glaciers. The greater the temperature increase, the greater the sea-level rise from thermal expansion and the faster the loss of glacial mass (IPCC 2007a: 830). Current IPCC estimates predict that thermal expansion of the oceans will contribute 70 per cent to 75 per cent of the projected rise to 2100. The level of understanding about the magnitude and timing of contributions to sea-level rise from ice melt is low. Melting of some ice sheets on Greenland and the west Antarctic has accelerated in recent decades (IPCC 2007a: 44), and observed data suggest that sea-level rise has been at the high end of previous IPCC projections (Rahmstorf et al., 2007). The accelerated response in Greenland may be the result of meltwater from the surface lubricating the movement at the base of the ice sheet and increasing the dynamic flow of solid ice into the sea. The west Antarctic ice sheet is  grounded below sea level, which allows warming ocean water to melt the base of the ice sheet, making it more unstable (Oppenheimer & Alley 2005). The IPCC estimated sea-level rise in 2100 for a scenario similar to the no-mitigation case (SRES scenario A1FI) at 26–59 cm. This figure does not include the potential for rapid dynamic changes in ice flow, which could add 10 and 20 cm to the upper bound of sea-level rise predicted for the 21st century. A key conclusion of the IPCC sea-level rise projections was that larger values above the upper estimate of 79 cm by 2100 could not be excluded (IPCC 2007a:14). A significant change between the IPCC’s Third and Fourth Assessment Reports was the revision of the lower estimate of sea-level rise upwards. However, the lower end of the range is still only slightly higher than the sea-level ‘committed’ rise that would occur if greenhouse gas emissions ceased, and leaves little room for contributions for additional ocean warming and land-ice melt, making such a low outcome unlikely (Rahmstorf., 2007; Pew Center 2007). If a sufficiently warm climate were sustained which, is causing ongoing melting of the Greenland and west Antarctic ice sheets, than these  ice sheets would be largely eliminated over a long period. If the Greenland and west Antarctic ice sheets were to melt completely, they would add an estimated 7 m and 6 m to global sea level respectively (IPCC 2007a: 752; Oppenheimer & Alley., 2005). Current models suggest that once a certain temperature is exceeded, major reduction of the Greenland ice sheet would be irreversible. Even if temperatures were to fall later, the climate of an ice-free Greenland might be too warm for the accumulation of ice (IPCC 2007a: 752, 776). Sufficient global temperature rise to initiate ongoing melt of the Greenland ice sheet lies in the range of 1.2–3.9ºC relative to 1990 (IPCC 2007a: 752). A simple reading of the scientific literature suggests a high probability that, under business as usual, the point of irreversible commitment to the melting of the Greenland ice sheet will be reached during this century."

AND

"Atmospheric carbon dioxide concentrations as low as 500 ppm will result in coral communities that no longer produce sufficient calcium carbonate to be able to maintain coral reef structures."

AND

"Tropical cyclones are likely to increase in intensity and to generate greater precipitation in their vicinity. Most models suggest a decrease in the total number of storms. There will be a poleward shift of storm tracks, particularly in the southern hemisphere."

AND

AND

"The Great Barrier Reef is the world’s most spectacular coral reef ecosystem. Lining almost 2100 kilometres of the Australian coastline, the Reef is the largest continuous coral reef ecosystem in the world. It is home to a wide variety of marine organisms including six species of marine turtles, 24 species of seabirds, more than 30 species of marine mammals, 350 coral species, 4000 species of molluscs and 1500 fish species. The total number of species is in the hundreds of thousands. New species are described each year, and some estimates suggest that we may be familiar with less than 50 per cent of the total number of species that live within this ecosystem. In addition to housing a significant part of the ocean’s biodiversity, coral reefs provide a barrier that protects mangrove and sea grass ecosystems, which in turn provide habitat for a large number of fish species. This protection is also important to the human infrastructure that lines the coast. The Great Barrier Reef is threatened by increased nutrients and sediments from land-based agriculture, coastal degradation, pollution and fishing pressure. Climate change is an additional and significant stressor.

The IPCC recognises coral reefs globally as highly threatened by rapid human-induced climate change (IPCC 2007). The Great Barrier Reef waters are 0.4°C warmer than they were 30 years ago (Lough., 2007). Increasing atmospheric carbon dioxide has also resulted in 0.1 pH decrease (that is, the ocean has become more acidic). These changes have already had major impacts. Short periods of warm sea temperature have pushed corals and the organisms that support their development above their thermal tolerance. This has resulted in episodes of mass coral bleaching that have increased in frequency and intensity since they were first reported in the scientific literature in 1979 (see Brown., 1997; Hoegh-Guldberg., 1999; Hoegh-Guldberg et al., 2007). The Great Barrier Reef has been affected by coral bleaching as a result of heat stress six times over the past 25 years. Recent episodes have been the most intense and widespread. In the most severe episode to date, in 2002, more than 60 per cent of the reefs within the Great Barrier Reef Marine Park were affected by coral bleaching, with 5–10 per cent of the affected corals dying. Consideration has recently been given to how reef systems will change in response to changes in atmospheric greenhouse gas composition.

If atmospheric carbon dioxide levels stabilise at 420 ppm and the sea temperatures of the Great Barrier Reef increase by 0.55°C, mass bleaching events will be twice as common as they are at present. If atmospheric carbon dioxide concentrations increase beyond 450 ppm, together with a global temperature rise of 1°C, a major decline in reef-building corals is expected. Under these conditions, reef-building corals would be unable to keep pace with the rate of physical and biological erosion, and coral reefs would slowly shift towards non-carbonate reef ecosystems. Reef ecosystems at this point would resemble a mixed assemblage of fleshy seaweed, soft corals and other non-calcifying organisms, with reef-building corals being much less abundant, even rare. As a result, the three dimensional structure of coral reefs would slowly crumble and disappear. Depending on the influence of other factors such as the intensity of storms, this process may happen either slowly or rapidly. Significantly, this has happened relatively quickly (over an estimated 30 to 50 years) on some inshore Great Barrier Reef sites. A carbon dioxide concentration of 500 ppm or beyond, and likely associated temperature change, would be catastrophic for the majority of coral reefs across the planet. Under these conditions the three-dimensional structure of the Great Barrier Reef would be expected to deteriorate and would no longer be dominated by corals or many of the organisms that we recognise today. This would have serious ramifications for marine biodiversity and ecological function, coastal protection and the tourism and associated service industries reliant on the reefs (Hoegh-Guldberg & Hoegh-Guldberg., 2008)."

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Citation and/or URL

Garnaut, R. 2008, The Garnaut Climate Change Review, First edn, Cambridge University Press, Victoria, Australia.

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Spatial Coverage

Global

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Temporal Coverage

This volume is a compilation of information collected from  many sources and spanning many time frames

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Update Frequency

Not applicable 

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Other Information

None