Other Greenhouse Driven Changes Sea Level And Storm Intensity

The increase in the atmospheric concentration of CO2 and other greenhouse gases impacts other parts of the environment surrounding corals. The increase in temperature, for example, is greatest in the polar regions where it affects the melting of the ice caps in the spring and summer months. The increased melting of the ice caps increases the volume of the global ocean that, in addition to the thermal expansion of the ocean, increases sea levels across the globe. Mean sea level across the planet has risen by almost 17 cm over the past 100 years and is currently rising at 1-2 mm a year. This is an order of magnitude higher than the average rate over the previous several millennia. These changes are causing concern given the enormous human population and associated infrastructure (e.g. cities, ports) located in low lying coastal areas of the world. Some low lying countries in the Pacific such as Tuvalu, Kiribati and the north coast of Java (Indonesia) are already being affected by spring tides and storm surges that reach higher than ever before and now inundate towns and human dwellings on a regular basis.

How rising sea levels will impact coral reefs is unclear at this point. On one hand, projected changes in sea level are relatively slow compared to the rates of coral growth (up to 20-30 cm per year) and hence are unlikely to present major problems for healthy coral populations. Coral reefs have also generally kept up with even the most rapid changes in the past, as discussed in Chapter 3. The problem comes, of course, with a combination of sea level rise and reduced growth as a result of warmer and more acidic oceans, which raises the prospect of coral populations that literally get left behind (drown) as sea level rises.

Changing weather patterns also have the potential to affect coral reefs through impacts on river flows and drought (and hence the amount of sediment running off coastal areas), and through the physical impact of storms. Climate change is expected to increase the intensity of storms via its influence on sea-surface temperature, a fact emphasised by recent events such as hurricanes Katrina, Wilma, Dean and the other record storms in the Gulf of Mexico in 2005. On the GBR, cyclones, which are relatively rare in the north and more common in central and southern latitudes, are important drivers of the life expectancies of corals as well as reef and island building processes. Changing cyclone strengths, wave heights and return periods along the GBR will change coastal inundation and the wave climates on reefs, land runoff regimes and coastal salinities. These changes represent potential threats, although the precise details of how they will affect coral reefs, or their overall importance, is not entirely clear at this point.

Changes to weather patterns may have other important synergies. Recent trends on the Australian mainland, for example, show a drying trend along the eastern seaboard over the past 70 years as a result of removal of vegetation, causing destabilisation of sediments in river catchment areas. Coupled with more intense storms, destabilised sediments are likely to be increasingly washed down river catchments and into coastal areas where they may have impacts on coral reefs. Given that increased sediment runoff is a major factor affecting corals in coastal regions, these changes have the potential to have similar impacts to those seen when hard-hoofed cattle were introduced into river catchments over 100 years ago along the north-east coastline of Australia. Sediment flows in this case increased by 20 fold within a few decades of cattle farmers arriving, leading to the loss of inshore coral reefs in many parts of coastal Australia.

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