The upcoming edition of the Proceedings of the National Academy of Science (PNAS) features two articles that address global climate change, both of which have already generated press headlines. Rahmsdorf and Coumou have a paper on the expected increase in extreme weather events in a warming world, while Antoniades and colleagues have a paper that attempts to reconstruct the Holocene history of the (currently shrinking) Arctic ice shelves. These papers fall closely on the heels of an announcement by physicist and noted climate change skeptic, Richard Muller, that his group at Berkeley (BEST – Berkeley Earth Surface Temperature project) is finally convinced that the data for global warming is conclusive…the planet is getting warmer at an accelerating rate.
Climate change is a controversial topic and one that I occasionally address in teaching about human evolution. The reason I venture into this area is that climate is a hugely important factor in human evolution. Climate shapes environments on a global and local scale, thereby changing evolutionary dynamics and influencing how and in what ways selection acts on populations. Additionally, even a cursory glance at data on atmospheric temperature data covering the time period of human evolution shows dramatic fluctuations in the temperature of the planet. The following image comes from Anna Behrensmeyer’s review of the topic published in Science a few years ago.
The wiggle in the line is broadly consistent with changing global surface temperatures from cold to warm extremes. Life on the planet has certainly existed during periods of climate much warmer than the present. Which raises one of the points I often here not against climate change, per se, but against its significance – the planet has been hot before, it is not a big deal if it gets hot again. This, quite simply, is wrong. Yes, it has been hot before, but the current cycle of change matters a lot.
One of the things about human evolution, even recent human evolution, is that it involves change…lots of it. Humans persisted in the midst of significant and fairly rapid changes in the climate during the past two million years. Indeed, there are even some paleoanthropologists who think this climatic volatility was essential in shaping the evolutionary trajectory of humans. However, for most of our evolutionary existence humans were likely organized into small, thinly dispersed groups of mobile hunter-gatherers. Now, most of the planets 7 billion people (more on that later) live in large and intricately organized cities. Indeed, it is not hard to make the argument that the transition to more immobile and densely populated living conditions, a process we have been in the midst of for the last 15,000 years, is the biggest ecological change in the last two million years of human evolution. We don’t just live in cities, we depend on them. Things like working sewage systems do wonderful things to keep us healthy, live longer, and have more babies. But they are also fixed structures.
You can take a step back further and consider the development of the modern city, much of which goes back to the enlightenment period, a time period in which the predominant world view was one of a permanent and unchanging world. This world view is, we know now, utterly inconsistent with how the world works. Things change…pretty much everything, all the time. At the moment, the climate appears to be changing pretty quickly relative to what we consider normal (suggesting we have more than a little to do with it), and certainly faster than what we have experienced for the past several thousand years. If city planners 100 years ago had understood more about the history of the planet, the design of urban centers and their location would likely be quite different. Perhaps images like those coming out of Bangkok today or my neck of the woods recently, would be less frequent. There is an additional anthropological irony in this, in that many of the hypotheses for the origins of sedentary lifestyles and permanent structures in the archaeological record actually invoke a period of fairly rapid climate change, referred to as the Younger Dryas, as a significant causative factor.
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1. D. Antoniades, P. Francus, R. Pienitz, G. St-Onge, and W.F. Vincent, Holocene dynamics of the Arctic’s largest ice shelf. PNAS 2011; October 24, 2011, doi:10.1073/pnas.1106378108
2. S. Rahmstorf and D. Coumou, Increase of extreme events in a warming world. PNAS 2011; October 24, 2011, doi:10.1073/pnas.1101766108
3. A.K. Behrensmeyer, Climate Change and Human Evolution. Science 27 January 2006: Vol. 311 no. 5760 pp. 476-478. DOI: 10.1126/science.1116051
