One of the big frustrations with trying to follow the debate on climate change is that most of the key questions are best answered with large, complicated models. Learning enough to assess these models in one subject area, even in general terms, is a huge task, and learning the details of any particular model is a full-time occupation. But, if we are going to make any real progress, we need numbers we can understand. It seems hopeless, but it isn’t entirely so. One thing I learned very early on about modelling is that, for almost any large complicated model, there’s a small simple model that gives much the same answers to the key questions of interest, if you use it correctly, and choose input parameters consistent with those in the big model. The big model (if it’s a good one) imposes consistency conditions you might miss in a simple model, and also gives detailed answers to lots of more specific questions, but a lot of the time, you can do without that. I’m writing a paper at the moment, trying to answer some of the important in a way anyone can check without spending years mastering a big model.

The biggest question of the moment is: what is the right price for carbon? I’m going to look at this question for the world as a whole, disregarding national differences and so on. If you’ve read the title of the post, you’ll know what answer I reach.

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The International Energy Agency reported today that global CO2 emissions hit a new record in 2010, and are well above where they should be for a path to stabilise CO2(+equivalent) concentrations at 450 ppm. The Global Financial Crisis has had a significant impact in the US and Europe but (not surprisingly) hardly any in China, where the impact of the crisis was short-lived, and rapidly offset by a strong fiscal stimulus. With the failure of policy in the US, things are not looking good. On the other hand, after playing the wrecker’s role at Copenhagen, China now seems to have embraced the idea of becoming the world leader in renewable energy.

The real good news is a new study undertaken by the IEA that refutes negative views about the variability of supply from PV and wind power (expressed by quite a few commenters here over the years, and the subject of numerous amateur analyses at blogs like Brave New Climate) and concludes that “the challenges of integrating large shares of variable renewables in power systems are far from insurmountable“. The analysis suggests that starting with existing grid characteristics, and employing balancing technologies now available, it would be possible to supply between 20 per cent (Japan) and 60 per cent (Denmark) of electricity generation using variable renewables, with an average of around 30 per cent. No specific date is given, but the discussion implies a time horizon around 2030.

Unfortunately, the PDF containing the detailed analysis is on sale at a price of 80 euros, which I don’t intend to pay, but the executive summary is online and gives a general idea of the argument.

An important point is that the most natural partners for variable renewables are sources that can be turned on and off easily. Hydro-electricity is the best example, but scope for expansion is limited. The next best case is a mixture of gas and variable renewables, and that seems like the sensible path to take over the next decade or two.

In the absence of any equally authoritative critique of the IEA analysis, I intend to treat this question as settled from now on, as with the prospects for nuclear power. Anyone seeking to make unsupported counter-claims based on their own intuition, BNC-style amateur analysis and so on should take them to the nuclear sandpit.

Summing up the news so far, if the world’s governments are willing to act to stabilise the global climate, they can do so at very low cost. It remains to be seen whether or not they will.