Coral reefs provide habitat for marine critters, as well as food and storm protection for local people. Consequently, they are very important components of the ecosystem.
Coral reefs are under threat by greenhouse gas (carbon dioxide) emissions, which increase ocean acidity and temperature. Both of these inhibit and destroy coral reefs.
Complicating predictions of future coral reef health is that greenhouse gases don't immediately change environmental conditions in the ocean. Circulation and other issues mean that there is a lag period between atmospheric dumping and effects on the ocean.
Simon Donner (University of British Columbia, Canada) has used computer models to ask whether or not coral reefs will withstand possible future shifts in world climate, taking into account this lag period, different predicted greenhouse gas emissions scenarios, as possible coral reef adaptation to increased ocean temperatures. He has concluded that atmospheric greenhouse gas levels may need to be reduced below what they are now to save coral reefs, unless they adapt naturally or through human intervention.
Changes in sea surface temperature.
Donner found that 0.5°C of the predicted sea surface temperature increases this upcoming century under a range of hypothetical greenhouse gas levels is due to greenhouse gases dumped into the atmposphere prior to and including the year 2000. Predicted sea surface temperatures do not diverge across scenarios (from the "baseline" scenario to the "business as usual" scenario) until approximately 2040.
What this says is that, no matter what direction humans intend to pursue regarding greenhouse gases, coral reefs face a similar fate for the next several decades. After that, however, sea surface temperaures may rise even more if greenhouse gases continue to be dumped into the atmosphere at their present rate.
Implications for coral reef bleaching.
Coral reef bleaching (loss their color) is commonly predicted in relation to the highest monthly average sea surface temperatures. Donner predicted the number of 2°C above normal heating months over ten-year periods up to 2100, under a range of hypothetical greenhouse gas levels, to predict the severity of coral reef bleaching.
Simply due to greenhouse gases that are already in the atmosphere, severe coral reef bleaching will happen every year in half of the world's coral reefs by the year 2080. Some regions of the world, such as the Caribbean, will be affected less than others, such as the equatorial Pacific.
Frequent coral reef mass bleaching by 2030 is predicted even if greenhouse gases are stabilized at 550 parts per million; only natural or artifical enhancement of the temperature-resilience of coral reefs will delay frequent bleaching until at least the year 2100 under these conditions. However, in the "business as usual" scenario (700 parts per million), over 80% of the world's coral reefs will still experience frequent bleaching by the year 2100.
Recommendations.
Without natural or artificial coral reef stabilization, atmospheric carbon dioxide levels need to be kept at or below 370 parts per million if we want to avoid frequent damaging bleaching events by the year 2100. With coral reef stabilization, levels need to be kept at or below 550 parts per million.
These results strongly indicate that greenhouse gases need to be reigned in if coral reefs are to be protected long into the future. The great unknown is whether or not people have the collective will to achieve this goal.
for more information:
Donner, S. D. (2009). Coping with Commitment: Projected Thermal Stress on Coral Reefs under Different Future Scenarios PLoS ONE, 4 (6) DOI: 10.1371/journal.pone.0005712