The basic message of the last few posts: climate change projections require assumptions about human behavior and these assumptions may be questionable. For instance, the Intergovernmental Panel on Climate Change (IPCC) has presented a “business-as-usual” trajectory of green house gas concentrations that would result in a mean global temperature rise of 3.7°C (2.6 to 4.8°C range) by 2100, meaning that such concentrations are plausible if present trends continue.  Problem is, the “trends” assumed by this trajectory are the exact opposite of what’s actually occurring. Please see previous posts for details. One could argue that it really doesn’t matter whether climate change projections are reasonable based on the current evidence – what matters is that people and governments are mobilized to take serious action now, the idea being that the specter of sure disaster mobilizes better than the specter of possible harm. This works well enough when most everyone agrees the awfulness will happen soon unless averted by decisive action.

But what if the really, really bad stuff isn’t supposed to happen for decades and there is no consensus about how bad it will be. (The IPCC actually offers a range of possible climate change scenarios, from the manageable and not too scary to the calamitous and super-scary. Guess which one gets the most press?) What if the messenger isn’t considered trustworthy, having raised false alarms in the past?  Will the little boy who cries ‘Wolf!’ get a serious hearing the louder he screams, even though he’s been screaming loudly for a very long time and no wolf is in sight?

Assumptions about the magnitude, rate, and effects of climate change make all the difference in the world if you're trying to formulate  policies to mitigate and adapt to climate change. All policies involve trade-offs and all have potential downsides. Whatever we do about climate change is bound to help in some ways and hurt in others, whether we take an incremental or aggressive approach.  Sometimes baby steps work best; sometimes great leaps forward. The devil, as always, is in the details.

Next: what details?