Must-see: Bill McKibben on David Letterman

But Dave, though well-informed, gets one of his facts wrong

The founder of and the author most recently of the must-read book Eaarth “” has a great interview with David Letterman. Dave is more knowledgeable on climate and energy issues than the vast majority of ‘real’ journalists, though he makes one mistake:

Always amazing to see a person as well known as Letterman who sees the situation in more dire terms than a guy like McKibben (see Letterman rages on global warming: “We are so screwed!”)

Letterman does make an inaccurate statement toward the end (around 10:30) to effect that if we stopped all carbon emissions tomorrow, “the planet would continue to heat precipitously for another 60 years.” McKibben should not have said “that’s right.” He should have explained that this isn’t true. RealClimate explained this in March based on a Nature Geoscience letter by Mathews and Weaver (sub. reqd.):

The upper line is often what is referred to as the ‘climate change commitment’ (for instance Wigley, 2005). This is the warming you get if we keep CO2 (and other GHG and pollutant levels) constant at today’s values. (Technically, the figure shows the case staying at year 2000 values). In such a scenario, the planet still has a radiative imbalance, and the warming will continue until the oceans have warmed sufficiently to equalise the situation — giving an additional 0.3 to 0.8ºC warming over the 21st Century. Thus the conclusion has been that because of climate inertia, further warming is inevitable.


However, constant concentrations of CO2 imply a change in emissions — specifically an immediate cut of around 60 to 70% globally and continued further cuts over time. Matthews and Weaver make the point that this is a little arbitrary and that the true impact of climate inertia would be seen only with emissions cut to zero. That is, if we define the commitment as the consequence only of past emissions, then you should set future emissions to zero before you calculate it. This is a valid point, and the consequence of that is seen in the lower lines in the figure.

CO2 concentrations would start to fall immediately since the ocean and terrestrial biosphere would continue to absorb more carbon than they release as long as the CO2 level in the atmosphere is higher than pre-industrial levels (approximately). And subsequent temperatures (depending slightly on the model you are using) would either be flat or slightly decreasing. With this definition then, there is no climate change commitment because of climate inertia. Instead, the reason for the likely continuation of the warming is that we can’t get to zero emissions any time soon because of societal, economic or technological inertia.

That is an interesting reframing of an issue that comes up all the time in discussions of adaptation and mitigation. This is because it demonstrates that adaptation (over and above what is necessary to reduce vulnerabilities to current climate conditions) is unnecessary if mitigation is dramatic enough.

However, the practical implication of this reframing is small. We are clearly not going to get to zero emissions any time soon, and even the 60–70% cuts required to stabilise concentrations initially seem a long way off. Thus as a practical matter, it doesn’t really matter whether the inertia is climatic or societal or technological or economic because the globe will continue to warm under all realistic scenarios (what we do have a possible control over is the magnitude of that warming).

We don’t want to make an angels dancing on the head of a pin argument. We are, under any politically plausible scenario, committed to more warming. That said, humanity still controls its destiny at this point, and we almost certainly can avoid “precipitous” warming, but it would require us to take action that is much stronger and faster than the status quo media understands or the anti-science conservatives in this country will abide.

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