What’s the big deal about a 2˚ C increase in temperature?

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 would be the coolest roller-coaster ever…

This shape has been documented for a huge number of organisms including my little friends, the sea urchin Lytechinus variegatus.

He was hopped up enough to make some bad decisions…

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.

From Lemoine and Burkepile (in press) Temperature-induced mismatches between consumption and metabolism reduce consumer fitness. Ecology. http://dx.doi.org/10.1890/12-0375.1

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.


2 thoughts on “What’s the big deal about a 2˚ C increase in temperature?

  1. Temperature is an energy density. I am an engineer as well as statistician, so my interpretation comes from that perspective.

    If someone claims “Warming is X degrees Celsius”, I am going to ask, “Plus or minus What?” As the blog post cites, warming may be much bigger in places like the Arctic, and it could be less.

    Also, what does warming mean? If it is an energy density, that means the planet has more energy available or stored in its systems. What does that do? What does that enable? Does it get stuffed some place and never come out with no consequences? Is that reasonable?

    I think not.

    If energy is being absorbed by oceans, that is going to have biologic and other consequences. Ocean systems don’t change quickly, but they do change. If CO2 is being absorbed by oceans, that means it gets dissolved as carbonic acid. Do that enough and you’ll not see a lobster dinner again after some time.

    Energy increases don’t get averaged away across the planet. Even if sea level rises, few know its effects are not like a bathtub …. Some places see the effects more than others, because of the dynamics of fluids sloshing around a spinning planet with irregular coastlines. So some places may, unfortunately, have a lot of that energy concentrated over a relatively short time. Unfortunately for them ….

    Question is, do people really want to play this game of trying to dodge these effects, and do they really want to say they’ll not help those who are devastated by them, even if they themselves got off easily this time?

    Can’t answer that.

    Earth is a wonderfully complicated, awesome place, with lots of intermingled ecosystems and geophysical systems and the like. There is no doubt we will be affected by whatever happens. The Earth has a future. It just doesn’t pay to be overly sure of our own future in it. As Raup in his book on EXTINCTION argued, “All species go exist”, offering it as a fundamental characteristic of being a species. When we will?

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