There are a lot of ideas on how to limit emissions of CO 2 in order to avoid the worst impacts of climate change and ocean acidification. But most of those focus on future infrastructure and equipment; in the meantime, we have a large portfolio of power plants and vehicles that will continue to emit for as long as we use them, and we're unlikely to stop doing so. Just how significant are the carbon emissions that we've committed to? A study that will be released by Science today indicates that we're not in terrible shape yet, as we haven't built the hardware that could cause the most significant shifts in the climate.

The new analysis focuses on what it terms "committed emissions" by taking known values like a car's typical emissions per year of driving, and totaling those for the projected lifespan of the vehicle. The database the authors use for this has separate figures for passenger and industrial vehicles, and provides numbers for things like coal-fired power plants and the like. For land use changes, it relies on values in the IPCC report. It also has figures for fossil fuel use by industrial equipment and the like, but these are simply based on total energy consumption, as this hardware is too varied to project accurately.

There are a couple of additional limitations to the work. For example, they leave out the impact that building infrastructure, like a highway system or electric grid, has on emissions. In addition, they recognize that any effort to actually stop building further fossil fueled hardware is likely to extend the lifespan of the stuff that already exists. For these reasons, the authors caution that their "scenarios are not realistic."

Nevertheless, they suggest that the results could be valuable, as they will provide some guidance about when certain steps would have to be taken to meet targets for future emissions. Should the growth of fossil fuel use continue, they'd also provide a hint of when we might have to get serious about alternate approaches, like geoengineering and carbon capture and storage.

When the emissions figures are plugged into a climate model (the authors use one from the University of Victoria), some relatively good news comes out. The steady decline in emissions that would occur if no new hardware is built would cause a relatively quick stabilization in atmospheric CO 2 levels in even worst-case scenarios, with a peak of about 430 parts-per-million in the middle of the century. In more optimistic scenarios, the atmospheric CO 2 concentration would be declining by 2030.

This would only produce a warming of about 1.3°C over preindustrial times, or about a half-degree over the present atmospheric temperatures. "Because these conditions would likely avoid many key impacts of climate change, we conclude that sources of the most threatening emissions have yet to be built," the authors write.

Electricity production is one of the most significant influences on committed emissions, accounting for nearly half of the current output; since these plants have a long lifespan (typically in excess of 30 years), they'll also continue to emit for a while into the future. But there has been a significant shift to the dynamics of electricity production in recent decades. In the US and Europe, the average age of a coal plant is in the neighborhood of 30 years, and not a lot of new ones are being built (the US added over three times as much wind power as coal in the last decade).

In contrast, China has gone on a coal binge; even though it's increasingly focused on renewable power, its energy mix is very coal-heavy, and most of the plants that burn it are new. As a result, China alone accounts for over a third of the global committed emissions, and would quickly pass all other nations were we to stop building new fossil fuel burning hardware.

Although the report makes for some compelling reading, it's mostly based on a scenario that the authors themselves recognize is completely unrealistic. What's lacking here is any consideration of more realistic scenarios. The US, for example, is currently on a trajectory where wind and natural gas are displacing its old coal-fired plants. It would have been informative if the authors had projected this trend globally, and seen what resulted, and the data used for this analysis would seem to be sufficient to enable that.

Other, similar scenarios are easy to imagine. Unfortunately, we'll apparently have to wait until the follow-up paper.

Science, 2010. DOI: 10.1126/science.1188566 (About DOIs).