Temperature-induced mass coral bleaching causing mortality on a wide geographic scale started when atmospheric CO2 levels exceeded 320 ppm. When CO2 levels reached 340 ppm, sporadic but highly destructive mass bleaching occurred in most reefs world-wide, often associated with El Niño events. Recovery was dependent on the vulnerability of individual reef areas and on the reef’s previous history and resilience. At today’s level of 387 ppm, allowing a lag-time of 10 years for sea temperatures to respond, most reefs world-wide are committed to an irreversible decline. Mass bleaching will in future become annual, departing from the 4 to 7 years return-time of El Niño events. Bleaching will be exacerbated by the effects of degraded water-quality and increased severe weather events. In addition, the progressive onset of ocean acidification will cause reduction of coral growth and retardation of the growth of high magnesium calcite-secreting coralline algae. If CO2 levels are allowed to reach 450 ppm (due to occur by 2030–2040 at the current rates), reefs will be in rapid and terminal decline world-wide from multiple synergies arising from mass bleaching, ocean acidification, and other environmental impacts. Damage to shallow reef communities will become extensive with consequent reduction of biodiversity followed by extinctions. Reefs will cease to be large-scale nursery grounds for fish and will cease to have most of their current value to humanity. There will be knock-on effects to ecosystems associated with reefs, and to other pelagic and benthic ecosystems. Should CO2 levels reach 600 ppm reefs will be eroding geological structures with populations of surviving biota restricted to refuges. Domino effects will follow, affecting many other marine ecosystems. This is likely to have been the path of great mass extinctions of the past, adding to the case that anthropogenic CO2 emissions could trigger the Earth’s sixth mass extinction.
J.E.N. Veron, O. Hoegh-Guldberg, T.M. Lenton, J.M. Lough, D.O. Obura, P. Pearce-Kelly, C.R.C. Sheppard, M. Spalding, M.G. Stafford-Smith, A.D. Rogers (2009) The coral reef crisis: The critical importance of <350 ppm CO2. Marine Pollution Bulletin 58:1428-1436
Charlie, at pages 1429-1430 of the article your team allows for a lag time of 10 years between carbon dioxide levels and coral bleaching.
Obviously you were trying to make some realistic assumptions about a complex reality and to make a conservative conclusion on the true levels of CO2 that led to the impacts. However, I was surprised not to find a reference to the warming effect of other greenhouse gases and the uncertainty over the cooling effect of aerosols confounding the analysis you were undertaking.
Did you simply ignore other greenhouse gases and aerosols during the analysis or did you assume that they cancel each other? The latter is a fairly big assumption.
Given the widespread use in international climate change policy of climate stabilization targets based on carbon dioxide equivalent (CO2-eq) levels, for any future articles it would be useful to include a brief explanation of the difference between CO2 and CO2-eq targets. I doubt that many policy-makers will understand the important differences between the CO2 levels you referred to in the article and the CO2-eq targets being used in international negotiations.
The confusion between CO2 and CO2-eq targets is something that has been addressed here before: http://www.climateshifts.org/?p=683
[...] marine protected areas and reducing CO2 emissions would go a long way towards helping to restore the reefs. Nature remains full of resilience and [...]
Hey, that post leaves me felenig foolish. Kudos to you!