How sensitive are you to climate change?
In discussing predictions of future climate change, the subject of climate sensitivity often comes up. This is the projected amount of temperature rise from a doubling of carbon dioxide (CO2). Our best guess at this number is about 5.4 degrees Fahrenheit (or 3 degrees Celsius), but it could reasonably range anywhere from as low as 3.6 to 8.1 degrees Fahrenheit.
That range necessarily introduces a good deal of uncertainty into future climate predictions. If we should be lucky and the climate sensitivity falls at the low end of that range, we have time to get our greenhouse gas emissions in order before the climate gets too out of hand. If we're unlucky and it's at the high end, we're in store for some very hot times as the climate system catches up to the greenhouse warming we've already committed to. (Read more in my post on America's Climate Choices.)
But the climate sensitivity only considers the internal (i.e., non-societal) aspects of the climate system; that is, it relates the response of the climate system to a CO2 doubling without addressing any response or change in people's actions. But that human factor is also important. What we as a global society do in the coming decades in terms of economic development and energy needs and uses and how we generate that energy will greatly affect the trajectory of greenhouse gas emissions and that in turn will have a huge impact on the future climate trajectory. For example, in the fourth assessment report by the Intergovernmental Panel on Climate Change, it is projected that average global temperatures by 2100 could be 7.2 degrees Fahrenheit higher for a scenario with rapid economic expansion and no policies or 3.2 degrees Fahrenheit for a scenario with slower economic expansion to as little as 1 degree Fahrenheit for a scenario where policies are implemented to keep greenhouse gas warming constant at year 2000 levels.
In most climate models, the human response to climate change is treated as an external and independent parameter. We adopt a given scenario of emissions and then calculate the climate response. We then adopt another scenario and calculate its climate response.
The problem with this approach is that it assumes that human activities that affect climate are carried out in a way that is decoupled from the climate itself. But is that the case? Not really, argue Andrew Jarvis of Lancaster University and co-authors in a recent paper published in the journal Nature Climate Change. If we as a species are to alter our actions to avert large, potentially catastrophic climate change, we will do so because there is a feedback between climate change (e.g., temperature change), the concomitant impacts of that change on us as a society, and our actions vis-à-vis greenhouse gas emissions. Using an analogy to the climate sensitivity discussed above, you can think of this as the human climate sensitivity -- how much we alter our CO2 emissions in response to a given increase in temperature. For large climate disruptions to be averted, that response must work as a negative feedback where rising temperatures cause humans to respond by lowering emissions.
Jarvis et al. use this concept of a human climate sensitivity to ask an interesting question: How sensitive or responsive do we need to be to keep future temperaturesfrom exceeding the threshold identified by the global community to avoid "dangerous anthropogenic interference with the climate" (i.e., 3.6 degrees Fahrenheit [2 degrees Celsius] above pre-industrial levels).
Their conclusions are daunting. Using the value of 3.6 degrees Fahrenheit (2 degrees Celsius) to avoid dangerous anthropogenic interference, they estimate that "society will have to become ~ 50 times more responsive to global mean temperature change than it has been since 1990." (The responsiveness from 1990 to the present was estimated on the basis of the global proliferation of renewable energy sources over that period.)
A 50-fold increase -- that's a whole lot of sensitivity training. Can we do it? The problem is, it may be moot. It could be that it if we don't do it, it will be sort of "done for us." How so? Well, consider that, because we humans are part of the system, we are also subject to self-correcting aspects of that system.
The human/social response to climate change can occur via three pathways:
We live in the natural world and in some respects are at the mercy of the weather and therefore the climate. And so the climate-human system can have some pretty serious "self-corrections" built into it. Could be that we either lower our global warming emissions or the climate will force us to do it anyway -- and that forcing may be unpleasant, to say the least. When it comes to global warming and greenhouse gas emissions, it may turn out to be a pay-me-now-or-pay-me-later kind of thing.
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