The bad news just got worse: A new study finds that reining in greenhouse gas emissions in time to avert serious changes to Earth's climate will be at best extremely difficult. Current goals for reducing emissions fall far short of what would be needed to keep warming below dangerous levels, the study suggests. To succeed, we would most likely have to reverse the rise in emissions immediately and follow through with steep reductions through the century. Starting later would be far more expensive and require unproven technology.
Published online today in Nature Climate Change, the new study merges model estimates of how much greenhouse gas society might put into the atmosphere by the end of the century with calculations of how climate might respond to those human emissions. Climate scientist Joeri Rogelj of ETH Zurich and his colleagues combed the published literature for model simulations that keep global warming below 2°C at the lowest cost. They found 193 examples. Modelers running such optimal-cost simulations tried to include every factor that might influence the amount of greenhouse gases society will produce —including the rate of technological progress in burning fuels efficiently, the amount of fossil fuels available, and the development of renewable fuels. The researchers then fed the full range of emissions from the scenarios into a simple climate model to estimate the odds of avoiding a dangerous warming.
The results suggest challenging times ahead for decision makers hoping to curb the greenhouse. Strategies that are both plausible and likely to succeed call for emissions to peak this decade and start dropping right away. They should be well into decline by 2020 and far less than half of current emissions by 2050. Only three of the 193 scenarios examined would be very likely to keep the warming below the danger level, and all of those require heavy use of energy systems that actually remove greenhouse gases from the atmosphere. That would require, for example, both creating biofuels and storing the carbon dioxide from their combustion in the ground.
"The alarming thing is very few scenarios give the kind of future we want," says climate scientist Neil Edwards of The Open University in Milton Keynes, U.K. Both he and Rogelj emphasize the uncertainties inherent in the modeling, especially on the social and technological side, but the message seems clear to Edwards: "What we need is at the cutting edge. We need to be as innovative as we can be in every way." And even then, success is far from guaranteed.