Are emissions growing faster than the models say?

There have been a number of recent studies warning that greenhouse gas emissions will skyrocket in the next three decades, thanks to rising energy use in China, India, and the developing world. The new International Energy Outlook from the US Energy Information Administration reports that global CO2 emissions from fossil fuels will increase 51% by 2030.

Projecting future emissions is by no means simply. The underlying conceit in much of the discussion (e.g. see Pielke et al.) of late is that the projected emissions in the scenarios used by all the climate models are too low. In other words, climate change will be even worse than the models say, and the runaway emissions train will be even harder to stop than we think.

I had to plot some emissions data for a presentation, so I did a quick comparison of the SRES scenarios, the group of future emissions scenarios used by all the climate models in the last IPCC assessments. The global carbon emissions from fossil fuels only in the four main scenarios are plotted in the figure to the right. [A note: These are the means for each scenario. Despite what most people think, A2, A1b, etc. do not each refer to one set of emissions data, rather a series of datasets generated by different economics / emissions models using the same inputs].

Now here's the same graph with the high growth, medium (reference scenario) and low growth scenarios from the IEO (coarse grey/black lines). And the surprise: the path of the highest IEO scenario lies in between SRES A1b and SRES A2, basically what people have been calling "business as usual" for quite some time. The low growth scenario parallels SRES B1, a scenario in which thew world seeks "global solutions to economic, social, and environmental sustainability, including improved equity". The emission path in all the IEO scenarios fall below that of SRES A1F1, the fossil fuel intensive scenario. In other words, the scary new report is no worse than what we were already projecting. That is hardly a reason to celebrate, yes, but it may be a reason to argue with those claiming emissions are rising so fast that climate change mitigation is prohibitively expensive.

There is a caveat. The red line on the graph comes from a recent paper by Sheehan in Climatic Change (see Pielke's commentary). This scenario attempts to take into account the "abrupt shift to rapid growth based on fossil fuels" that has occurred in Asian countries. The paper argues that the SRES scenarios assumed too much decarbonization of the energy system and the economy. The SRES approach is reasonable -- as I've argued before, the emissions intensity of the world economy, the amount of carbon pumped out per unit of GDP, has been declining for decades as we became more efficient at making producing stuff. The problem is that we're at a moment where Asian are using coal and oil to expand, so the global decarbonization is slowing down or even stopping (because the carbon emissions per unit energy production is rising). How and whether recent Asian fossil fuel growth should be extrapolated 25 years into the future, I can't say. I'd be interested to see a comparison of Sheehan-style scenarios with those of the IEO.

(By the way, the IPCC is planning a new set of scenarios for the fifth assessment)