Jeff Tollefson

After last month's United Nations climate conference in Copenhagen failed to produce a binding agreement to cut greenhouse-gas emissions, the outlook still isn't getting any better.

A modelling study suggests that the massive mobilization necessary to halve greenhouse-gas emissions by 2050 is "barely feasible" unless global development patterns shift radically. Heroic efforts would still be required in the latter half of the century to cap the global temperature rise at 2° C by 2100, and even then we have a 50/50 chance of missing that target.

Some say emissions cuts need to look 10 to 20 years out, not 40.Punchstock

That's one scenario, perhaps the most likely, explored by new work looking at how emissions targets for the middle of the century feed into longer-term goals for capping global temperature and atmospheric carbon dioxide concentrations¹.

"There is a point at which postponing reductions until later will take some long-term options off the table," says co-author Brian O'Neill, of the National Center for Atmospheric Research in Boulder, Colorado, and the International Institute for Applied Systems Analysis in Laxenburg, Austria.

Malte Meinshausen, a modeller at the Potsdam Institute for Climate Impact Research in Germany, calls the study "another crucial cornerstone in our scientific understanding as to when we foreclose options by acting too little, too late when trying to avoid dangerous climate change".

The team merged a detailed assessment of energy economics with a simple climate model to analyse the costs and feasibility of various emissions pathways to mid-century, then work out what that might mean for climate in the period from 2050 to 2100. The analysis used two baseline emissions scenarios developed by the Intergovernmental Panel on Climate Change, one in which energy and consumption rises gradually and another that modelled fairly aggressive economic growth — more in line with recent emissions trends.

The study looks at the odds of meeting long-term temperature goals given different development trajectories and emissions cuts over the next 40 years. For instance, under the slow-development scenario, reducing global carbon emissions to 7 billion tonnes a year by 2050 — nearly 16% below 2000 levels — would confer a 45% chance of ultimately staying below 2° C. Under the more likely rapid-development scenario, that same reduction to 7 billion tonnes a year would provide just a 25% chance of staying below 2° C. Those odds rise to nearly 70% if the target is 3° C.

Too little, too late

Michel den Elzen, a climate modeller with the Netherlands Environmental Assessment Agency in Bilthoven, warns that even so, the study may be too optimistic about how fast new energy technologies could spread after 2050, particularly unproven ones that combine biofuels with carbon capture and sequestration to draw carbon out of the atmosphere. Such technologies might allow temperatures and carbon dioxide concentrations to overshoot the goals before coming down towards the end of the century.

"It's still in the range of what might be possible, but they are quite optimistic about implementation," den Elzen says. "We are not that optimistic."

The study underscores existing doubts about the 2° C target, which many politicians have used as a tool to delay more-credible short-term commitments, adds David Victor, who heads the Laboratory on International Law and Regulation at the University of California, San Diego. "It's interesting and helpful to do out the math, but this is not actually how political systems really get things done," he says. Victor argues instead that policy-makers should focus on commitments in the short term, over the next 10–20 years, instead of making grand promises for 2050.

The study does, however, provide new fodder for discussions about when the bulk of emissions reductions need to occur — and how much they might cost. If the goal is to limit warming to 2° C in a world that is developing rapidly and has no carbon limitations, as is currently the case, cumulative energy investments would need to rise from roughly US$47 trillion to $58 trillion. Other, cheaper scenarios would take advantage of declining technology costs.

Джефф Толлефсон | Nature