Put a pan of cold water on the stove, turn the heat up all the way and what happens? Nothing at first. But wait for five minutes and the water will boil.
The time difference between turning up the heat on the stove and the water coming to boil is explained by the heat capacity of water, the number of calories needed to raise the water temperature from room temperature to boiling point. The same effect occurs in the ocean. Global climate change is causing the atmosphere to warm up and the extra heat is transferred to the ocean. But the heat capacity of the oceans is enormous.
The top three meters of the ocean has the same heat capacity as the entire atmosphere. And the oceans are four kilometers deep on average. This means that the oceans can store a lot of heat. And because of that there is a time lag between warming of the atmosphere (which heats up practically instantaneously) and the warming of the ocean.
We know that rising levels of carbon dioxide in the atmosphere are the main cause of global warming. As the carbon dioxide concentration increases the greenhouse effect causes more longwave radiation to be trapped in the atmosphere, which becomes warmer. This heat is transferred first to the surface of the ocean and eventually into its deeper reaches, but it takes a long time. Decades, in fact.
Ocean surface warming lags behind global atmospheric CO2 levels by around 25 to 50 years, over which time about 60% of global warming will take place (Hansen et al. 2004).
To put this in context, it has been predicted that a coral bleaching tipping point occurred when atmospheric CO2 levels exceeded around 350 parts per million (ppm) (Veron et al. 2009). But we passed 350 ppm almost 30 years ago back in 1988, and we are only now in the midst of a coral bleaching crisis that has affected 70% of the world’s reefs in the last three years. That's because it has taken the oceans 30 years to warm up in response to those long-past atmospheric CO2 levels.
Meanwhile, CO2 levels have now passed 400 ppm and the warming continues. The CO2 dissolved into the ocean makes it more acidic, which adds yet another burden to corals struggling to survive. We know now that the biological effects of acidification are far-reaching and are of growing concern to marine scientists.
Even if we could halt CO2 levels at their current levels it would take 25 to 50 years before just 60% of the full warming effect will take place. There is nothing we can do to stop this ocean warming from occurring. The future ocean will be much warmer than today’s temperatures and these have already passed the tipping point for tropical coral reefs.
That's the lag effect. It is one of the most insidious aspects of climate change. Inaction on climate change in the late 1980s effectively condemned coral reefs to death by bleaching 30 years later. We will not have to pay the full price for our inaction on climate change today – that cost will borne by our children and our grandchildren, once the ocean begins to equilibrate with today’s more than 400 ppm CO2 level.
The reefs we have now will not exist in the future – many will not survive, but some coral species will be able to adaptat to warmer, more acidic oceans. Their transition will need to be carefully managed if reef ecosystems are to survive to the end of this century (Hughes et al. 2017).
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