NOAA/GFDL

Some 2,000 climate scientists are flocking to Paris this week to chew over their research ahead of December’s crucial round of negotiations in the French capital. After the sobering failure of previous policy talks, scientists are keen to reiterate what is at stake. Although the urgency of cutting greenhouse-gas emissions is the main topic of discussion, researchers have also organized the meeting — titled ‘Our common future under climate change’ — to discuss the future of their field, which still has many unanswered questions. Nature asked those at the conference about some of the most pressing research issues for climate scientists:

Understanding climate sensitivity

As global concentrations of atmospheric carbon dioxide continue to shoot upwards, researchers need a better handle on how global temperatures could respond. That means narrowing the value of ‘climate sensitivity' — the increase in temperature caused by a given rise in atmospheric CO 2 concentration. This number is disturbingly uncertain because it depends on poorly-understood feedback effects from clouds, water vapour and sea ice. In its most recent report1, the Intergovernmental Panel on Climate Change estimated that if CO 2 concentrations were to double —for example from the pre-industrial value of 270 parts per million (p.p.m.) to around 540 p.p.m. — global temperatures would probably rise somewhere between 1.5 °C and 4.5 °C. It added that feedbacks might lead to an even higher value.

Monthly global CO 2 concentrations exceeded 400 p.p.m. in March, probably for the first time in several million years, and researchers think that the chances of limiting global warming to 2 °C above pre-industrial temperatures — the broadly accepted threshold for dangerous climate change — are diminishingly small. “We’re on track with emission scenarios that easily exceed 2 °C,” says Richard Betts, a climate scientist at the University of Exeter, UK. “The question is, how much and how soon?”

“We are entering uncharted territory,” says Sandrine Bony, a climate researcher at the Laboratory of Dynamic Meteorology in Paris. “It’s uncertain at which CO 2 level we may exit the safety zone, and it’s unclear what might happen if we do so.”

Improving models of atmospheric circulation

To understand precisely what rising CO 2 levels mean — not just at a global scale, but also at regional levels — researchers need climate models that can better resolve clouds in the atmosphere and eddies in the ocean.

“Atmospheric circulation controls where it is dry or excessively warm, where rain comes down and where it doesn’t, and where powerful storms form and hit,” says Bony, who will give a keynote talk on 7 July about the future of climate science. “If we can run models that are able to resolve clouds and convection — and if we can be sure that they are doing it right — we will be able to answer many questions.”

Although computationally expensive, these models are creeping into use. But scientific understanding is also limited by a lack of global observations of wind. Bony says that the European Space Agency’s wind-profile mission, the Atmospheric Dynamic Mission Aeolus, scheduled for launch next year, promises to advance climate research. It will use a laser system to generate about 100 3D profiles an hour of the strength and direction of wind at different altitudes. Those data will shed light on global atmospheric circulation, precipitation systems and ozone and aerosols transport.

Early warning of disastrous ‘tipping points’

Scientists say that if the world warms by more than 4°C, it is exceedingly likely that some large-scale component of the Earth’s climate system will shift — or pass a ‘tipping point’ — with potentially catastrophic consequences for climate and ecosystems. That could include the melting of the Antarctic and Greenland Ice Sheets, dieback of the Amazon rainforest or the collapse of the heat-balancing Atlantic Ocean circulation1.

Some researchers hope to get early warning of coming changes, for example, by scrutinizing the strength of the ocean’s circulation at various latitudes. The UK-funded Rapid Climate Change monitoring array programme has followed ocean circulation at a specific northern latitude since 2001, and has detected surprisingly large fluctuations in its strength at various depths. But it is not yet possible to determine whether average circulation strength is in decline.

Tim Lenton, a climate scientist also at the University of Exeter, is not optimistic about early detection. “The rate at which a system recovers from small perturbations is an indicator of its stability, but gaining reliable early warning of approaching tipping points is a long way off,” he says.

Lenton says that a safer way to avoid disastrous climate disruptions would be to shut down carbon emissions by mid-century. But results from a cost–benefit assessment, which he will present at the Paris meeting on 8 July, suggests that the feat would require a carbon tax in excess of US$500 per tonne of carbon emitted — at least an order of magnitude above what most countries are willing to consider to ward off climate change.

Adapting to climate change

Because fossil fuels will probably drive emissions for decades, another topic at the meeting will be how to adapt to the effects of climate change.

Ideas for adaptation range from large-scale engineering projects to a plethora of less-costly solutions. In agriculture, for example, information systems are now used in some countries in Africa and Asia to allow poor farmers to download real-time soil data and fertilization recommendations.

But Betts warns that researchers must strike a delicate balance between reasonable precaution and misguided activism. “We can’t afford to wait until there is absolute certainty, say, on the extent of sea-level rise,” he says. “But you also don’t want to spend billions now on mammoth adaptation measures that might later prove unnecessary.”

Rather than build huge, expensive things now, he says, we should take reasonably scaled measures that can be modified and upgraded later. Good examples include London’s flood-defence system (the Thames Barrier) and the Delta works in the Netherlands — a large flood-protection system that shelters the Dutch coastline. And any adaptation efforts, from flood defence to water-management and irrigation schemes, need to be guided by scrupulous Earth and climate observations, Betts adds.