How high are we willing to let the temperature of the planet get? Many governments have signed on to international agreements that would limit greenhouse gas emissions to targets that will keep the Earth from exceeding a 2°C increase over preindustrial levels. However, two studies that came out in recent weeks suggest we're rapidly running out of time to do so. Both suggest we could be locked in to changes above 2°C before the decade is out, and perhaps in as little as five years.

Estimates of future warming are based on a term called the climate sensitivity, which is usually expressed in terms of the expected temperature rise caused by a doubling of atmospheric CO 2 concentrations. (The impact of each incremental increase goes down as the concentration rises, since there's a greater chance that some other CO 2 molecule will have already absorbed a given infrared photon.) The IPCC's best estimate is that the climate sensitivity is about 3°C per doubling, with uncertainties of about a degree in either direction.

How do we get locked in to future warming? On the simplest level, the impact of added greenhouse gasses is not immediate; emissions take time to equilibrate globally, CO 2 levels show seasonal variations, the oceans act as an enormous heat sink, etc. So there is a substantial delay between emissions and the actual temperature impact.

Furthermore, we can lock ourselves in to future emissions via infrastructure. Large power plants are expensive and have operating lifetimes on the order of decades, so we don't tend to retire them until they're obsolete. So a coal plant that's currently under construction can be expected to continue to produce emissions until mid-century, locking us in to its emissions for quite some time.

The concept of lock-ins feature highly in two recent analyses, one that appeared in Nature Climate Change, the other produced by the International Energy Agency. Both use the value of the IPCC climate sensitivity and the goal of 2°C, and figure out how likely we are to reach it.

The Nature Climate Change analysis is the simpler of the two. It uses a simplified climate model that can reflect the accepted climate sensitivity value, and link that with a series of models that produce various emissions scenarios. They then perform multiple modeling runs to provide probabilistic estimates of how likely we are to hit different climate targets. The good news is that we're not already locked in to emissions that would ultimately drive the climate to larger changes.

The bad news is that we're almost there. Eighty-five percent of the emissions pathways they sampled that keep us under 2°C of change all have peak emissions occurring before this decade is over. Many of the ones that don't rely on the extensive use of carbon capture and storage, which is currently a very immature technology, are just undergoing the first reasonably sized tests.

The IEA report includes the concept of infrastructure lock-in, and that makes matters even worse. The report's term for limiting climate change to 2°C is a "450 scenario," and its author looks at what it would take to get there: "Four-fifths of the total energy-related CO 2 emissions permitted to 2035 in the 450 Scenario are already locked-in by existing capital stock, including power stations, buildings and factories. Without further action by 2017, the energy-related infrastructure then in place would generate all the CO2 emissions allowed in the 450 Scenario up to 2035."

In other words, we've got five years to make a dramatic shift in the sorts of power plants we're building, or there's no way we're going to limit climate change to stated goals. It's also worth noting that the IEA ran a "New Policies" scenario that takes into account all current pledges by national governments. It resulted in much higher emissions and a likely temperature rise of 3.5°C. Still, if the New Policies targets aren't implemented, then the IEA estimates that we would be on track for a rise of 6°C.

Not only is the lack of progress risky, it'll probably be expensive. "For every $1 of investment in the power sector avoided before 2020," the IEA concludes, "an additional $4.3 would need to be spent after 2020 to compensate for the higher emissions."

Other tidbits from the IEA, including peak oil

Although the IEA's analysis is pretty grim, it comes as part of their annual report on the state of the energy economy, which projects trends out to 2035. And, as usual, it's filled with some fascinating figures. Some of these are largely trivia—for example, if Russia were to reach the energy efficiency levels of Western Europe, it would annually save as much energy as the entire UK uses. But a lot of the information is rather telling.

For example, subsidies to make renewable energy competitive with fossil fuel are often a point of contention, and the IEA calculates that they totaled $64 billion in 2010. In contrast, however, subsidies to fossil fuel use were over $400 billion. Renewable subsidies won't pass the current fossil fuels subsidies by the study's 2035 end point, at which point fossil fuel subsidies will have cleared $600 billion unless major policy changes are made.

In keeping with its earlier predictions, the IEA thinks peak oil has already passed, and production will stay at its current plateau, just below its 2008 peak. Total petroleum, however, will be supplemented with nontraditional sources, allowing total supplies of fuel to continue rising for a bit. Still, any shortage in investment in further extraction could easily produce future lags in the supply and a rapid increase in price. Most of that oil will go to the developing world, as industrialized countries will see continued gains in fuel economy and an increased electrification of their auto fleets.

Natural gas and coal will both continue to boom; gas will generate almost as much power as coal by 2035. Coal use is continued to rise in all but the most aggressive emissions scenarios, with China using about half of the global supply. China is now the world's largest investor in renewable energy, however, and is suffering severe pollution problems due to its reliance on coal, so there is a chance that policy decisions may change that dynamic. In any case, coal is poised to get a lot more expensive, and the supplies near the major points of consumption are dwindling, which will force more of it to be transported long distances.

The problems with coal will help drive a renewable expansion, with nearly half the new power installations put in place by 2035 being renewables. They'll grow faster than any other segment, but are starting from a low market share, so their impact will not be as large as the growth percentage would suggest. Nuclear power will also grow significantly, led by expansion in India, China, and Korea. However, the aftermath of Fukushima means that nuclear's future in markets where it's already established is uncertain, which could offset some of its growth elsewhere.

Nature Climate Change, 2011. DOI: 10.1038/NCLIMATE1258 (About DOIs).