Saturday, April 05, 2008

Do the IPCC scenarios underestimate future emissions?

A commentary in this week's Nature by Roger Pielke Jr. and colleagues about whether the future scenarios used by the IPCC underestimate future emissions has caused all sorts of fits, defenses and sighs. The journal itself went so far as to include rebuttals from climate and energy experts in the same issue.

One of the causes of the furor is Pielke's reputation. The lead author's previous writings stressing adaptation to climate change give some the impression he is either climate skeptic/denier or a Lomborg-esque "delayer" (and drive many other people crazy). This is the problem with applying broad labels, especially ones with, intentional or not, serious historical overtones.

Let's be fair: Pielke (and his colleagues) don't question the basic science behind climate change, rather whether we should spend more effort on mitigation or adaptation. Like many, I'm troubled by some of the pro-adaptation arguments, both because of the particular way the science is interpreted in those arguments, and in the way that they can create soft bigotry of low expectations (we can't reduce emissions, so we won't reduce emissions). But, it is not like Pielke et al. ever argue that cosmic rays are to blame or that CO2 was higher in the mid-1800s.

The basic premise of the commentary is that all of the future scenarios used by the IPCC presume there will be substantial technological advances in the future, and that without that assumption, emissions would actually be much higher. The authors calculate how much future emissions are "reduced" in the scenarios by the technology assumption by comparing with a case in which technology is effectively frozen over time. The result is some big numbers.

The argument comes down to rates of technology change, measured here by the emissions "efficiency" of energy production, and whether we just naturally become more efficient. Graphs of declining energy intensity (energy/GDP) or emissions intensity (emissions/GDP) over the past century show that we're now producing far more "stuff" per unit of energy and per unit of emissions. That happened naturally, without any emissions or climate policy.

Pielke and colleagues argue the energy intensity cannot continue to drop as such rates, especially with the growth of China, and that's where the scenarios used by the IPCC go wrong. Either way, the real issue, I would think, is not the energy intensity but what's buried within it, the "GHG intensity".

If you look at the energy intensity plot, it is clear that the energy intensity improvements in the past have happened from producing more "stuff" (GDP) per unit of energy, not from emitting fewer GHGs per unit of energy. In other words, there's not been much advance in the GHG intensity. In fact, it is currently decreasing because so much GDP is being produced by coal burning in China.

Under a business-as-usual scenario, it is reasonable to assume the world will keep trying to produce more stuff per unit of energy, so energy intensity would continue to fall. That's the "technological advance" the authors of the scenarios supposedly assumed. One can -- and should -- debate the numbers. But the basic assumption the world would go in that general direction without prompting by policy follows from history. Under a mitigation scenario, I'd think the reduction in emissions should come more from producing far fewer emissions per unit of energy (or from dramatically cutting energy use), because we'd already naturally be trying to produce more per unit of energy.

In the end, whether one agrees with the logic of the commentary or not, or whether one likes the tone, it is hard to figure out what the take-home message of such an argument should be for policy. The authors are members of the Breakthrough Institute, which arguments in favour of funding new energy technologies, without much mention of climate change or emissions targets. The paper and the Institute present their arguments set up a false dichotomy between technology and climate or emissions policy, as if talking about climate change distracts us from developing new energy technologies, or other in the community think we can magically reduce emissions without any technological advances. I've argued about this before. There's no reason that a technology policy and emissions targets have to be mutually exclusive. In fact, the entire purpose of the binding emissions targets is to force things like the development of cleaner energy technologies.

At least the commentary has people talking about these issues. That we can all support.


EliRabett said...

Sorry Simon, the message as usual from the Nordhaus, Yohe school is not now, there will be time later. Permit a horselaugh.

Simon Donner said...

The Oreskes paper about the link between the early "skepticism" of William Nierenberg and the William Nordhaus / Gary Yohe argument that we can afford to delay emissions reductions is an impressive piece of scholarship. If scientists paid more attention to history, we might better understand why others don't trust our science.