I’ve spent the last few posts trying to determine the causes of skepticism about anthropogenic climate change, be it the science version of fire-and-brimstone pulpit speeches or hypocrisy from those who encourage lifestyle changes to deal with climate change. This post takes on a different issue: the “So what?” response.
A number of climate models predict average global temperatures to rise anywhere from 2˚ – 7˚ C over the next half- to full century, although this varies among regions. For example, the arctic is warming way faster than the rest of the earth. Actually, it looks like we’re going to overshoot 2˚ C considerably, suggesting that this might not be a realistic baseline for experimental studies (but that’s another issue).
Some people may hear you say ‘Temperatures are going up by 2 degrees!’ and they say, ‘So what? It’s only two degrees’. How do you respond?
First, I’m not sure how big of an issue this next point is, but Americans are used to thinking about degrees Fahrenheit, not Celsius. A 2˚ C increase is a 3.6˚ F increase. However, 2˚ is conservative. The more likely outcome, 7˚ C increase, is a 9˚ F increase. So tell people imagine a summer where instead of 90, it’s 100. A spring that’s 85 instead of 75. Essentially, imagine this summer’s heatwave on a more permanent basis. Again, I’m not sure how big a deal this really is, but I think part of the message might be getting lost in the conversion.
Second, let me hit you with some knowledge. All ectothermic species (i.e. almost everything beyond birds and mammals) have what are called ‘thermal response’ curves describing how well they perform at various temperatures (side note: endotherms have these to, but homeothermy makes them less interesting, at least to me 🙂 ). More often than not, these curves are asymmetrical, where species have a thermal optimum, beyond which things come crashing down rapidly. Super rapidly. Ridiculously rapidly. Etc. In the graph below, ‘Fitness’ refers to an organism’s capacity to reproduce, commonly measured in eggs, size, growth rate, or any number of proxy variables that indicate how successful an individual is at reproducing.
This shape has been documented for a huge number of organisms including my little friends, the sea urchin Lytechinus variegatus.
In fact, I have a paper in press showing that the cause of such fitness reductions is a severe mismatch between metabolic demand and consumption at high temperatures. Metabolism keeps going up while consumption rates crash just beyond the urchin’s thermal optimum.
With less energy available (because they’re eating less), and more energy needed to fuel metabolism, fitness related traits (growth, reproduction, etc.) crash as well. This happens for a huge number of species, and it all happens within the space of 2 – 3˚ C.
Consider that most tropical species are already living near their thermal optimum (say, 29˚ C as in Fig. B). A 2˚ increase pushes the species past its thermal optimum into the crash zone (31˚ in Fig. B). The ramifications for population growth rates under such thermal stress are enormous. Organisms reproduce less, population growth rates decline, and extinction risk increases (because slow-growing populations are more likely to go extinct in variable environments).
So when someone asks, ‘What’s the big deal about 2˚?’, just draw that graph for them. It probably won’t change their minds, but at least you’d have an answer.
P.S. I realize this is the same kind of fear-inducing tactic I’ve recently suggested avoiding, but sometimes it can’t be helped. I like urchins too much.