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The planet does not have one clear "dangerous" temperature threshold. Two things needs to be explained clearly. First, the definition of dangerous climate change or unacceptable climate change is a normative choice. That choice could be based on analyses of scientific results but it is not a direct outcome of the science. Second, even if society could agree on one definition of "dangerous", climate science could not possibly produce one exact temperature threshold (e.g. 2 deg C), rather a frequency distribution of possible values. Besides, if climate science could produce an exact number, it would be an amazing coincidence that the number turned out to be an integer.
The precise evolution of 2 degree C towards its exalted status in the media is a bit of a mystery to me – I’m sure some of the readers have followed these developments in more detail, and their comments on the subject are welcome. It more or less began with efforts by scientists to define climate change impacts that could be construed as dangerous under the UN Framework Convention on Climate Change. The common list of "dangerous" is the loss of major ice sheets, slowing of the thermohaline circulation, drops in food production, triggering a positive feedback like the release of methane from clathrates in ocean sediments, or loss of a major ecosystem like coral reefs.
Researchers tend to focus on physical and seemingly non-linear impacts like ice sheet melt rather than biological and gradual impacts like drops in food production or drops in biodiversity. True, there’s no ethical or scientific reason we should value sea level more than biodiversity or food production. In terms of policy, however, it is a more useful metric because we’re more likely to agree on whether losing the Greenland ice sheet is dangerous than what percent change in crop yields or biodiversity is dangerous. As well, future food production is fraught with technological, demographic and ethical uncertainties – although one could certainly argue the same is true for sea level.
Science can provide guidance for decision makers by projecting the impacts of different levels of warming, albeit with error bars. Beyond 2 deg C, paleo-climate results suggest possible long-term melting of the Greenland and /or West Antarctic Ice Sheets; studies have also pointed to the potential methane feedbacks and the degradation of the world’s coral reefs occurring at around 2 C.
But science alone cannot declare the "right" policy decision. Even such seemingly black-and-white definitions of dangerous like the loss of coral reefs or major ice sheets involve a normative judgment. Much of the world, for example, may decide that the productivity of coral reefs is acceptable collateral damage – it’s certainly possible that we’ve already inadvertently chosen that path. Although my colleagues and I who study the value of coral reefs to tropical cultures are likely to seriously disagree with such a decision to allow harmful impacts to reefs, we recognize (at least I do) that there is no immutable right or wrong policy decision that can emerge purely from our scientific results. Policy decisions need to consider real-world trade-offs that are often ignored by scientific analysis.
In sum, the oft-cited line that 2 degrees C is “what scientists conclude the world needs to avoid” is inaccurate. For one, the scientists disagree (Part II, coming soon). Moreover, though many of us may wish otherwise, the threshold can and should not emerge solely from science.
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