There is general international agreement that we must limit temperature increase to 2°C if we are to minimise the impact of climate change. This is known as the 2°C temperature limit. But in some respects, global temperature is a rather arbitrary figure. Why not, for instance, set a target for the maximum ocean acidification or the maximum sea level rise we are prepared to tolerate? After all, rising carbon emissions do not just translate to an increase in surface temperatures but also to a rise in sea level, an increase in ocean acidification and a possible loss of net primary production (net carbon influx into an ecosystem through photosynthesis) on land.
Ocean acidification is a good target to set because the consequences of rising acidity are so tragic. As the sea water increases in acidity, it becomes corrosive to the shells of marine organisms and is a death sentence for coral reefs. I have been snorkeling on the Great Barrier Reef and it was an extraordinary experience, one of the best I have ever had. Losing the Great Barrier Reef would be a catastrophe by every measure of the word. There will also be enormous economic losses from tourism and fishing if this were to happen. The Caribbean earns over 50% of GDP from activities associated with the reefs. Coral reef ecologist, Peter Sale, suspects that loss of reefs will also impact coastal fish populations negatively. We are talking now about the collapse of an entire ecosystem.
So the question is, how much CO2 would we be allowed to emit if we set targets not just for global surface temperatures, but for other factors as well? A new paper in Nature – Allowable carbon emissions lowered by multiple climate targets – seeks to answer exactly this question.
The authors, from the University of Bern in Switzerland, argue that the climate system refers not just to global mean temperature but also to the hydrosphere, biosphere, geosphere and all of their interactions. If we are to ensure sustainable ecosystems and food production, then we need to consider other factors in addition to global temperature.
They set six targets: global mean surface temperatures, sea level rise, two targets for ocean acidification and two targets addressing the impact on the terrestrial biosphere. What they find is these additional targets can “considerably reduce the allowable CO2 emissions.” The ocean acidification target, in particular, is much more restrictive and puts a tighter limit on how much carbon we can afford to add to the atmosphere. This target specifically measures the loss of habitat associated with coral reefs. If CO2 rises above 550ppm, it is likely (>33%) that more than 90% of these habitats will be lost. We are likely to reach 500ppm by mid-century. That is very little breathing space for our coral reefs.
Something I have found somewhat irritating about certain media reports and political blogs is an obsession with the apparent temperature “hiatus”. The implication is that surface temperatures have not risen for a decade or so and therefore there is no global warming. There are lots of problems with this view which I’m not going to address here. I just want to say that climate change is more than just a measure of global surface temperatures. This paper is a good example of why we need to look at other factors as well.