When a damaging heat wave occurs, how much responsibility do the major greenhouse gas-emitting countries bear? It's a question scientists say they're getting closer to answering at a country-by-country level.

As international climate negotiators meet in Germany this week, a team of scientists has published a method for estimating how individual countries' shares of global greenhouse gas emissions over time contributed to the risk of specific extreme climate events, like heat waves, occurring in other countries.

The ability to connect the risk of individual extreme events to climate change is still developing, but the technique the scientists describe plays into a thorny issue between rich and poor nations.

Responsibility for the greenhouse gas emissions that have been driving up global temperatures is among the most contentious issues on the international stage, particularly for less-developed countries that have contributed little to historic emissions but are seeing disproportionate damage from sea level rise and extreme weather fueled by global warming.

The new techniques "make it possible to assign extreme events to human-induced climate change and historical emissions," and "allow losses and damage associated with such events to be assigned country-level responsibility," the scientists, from World Weather Attribution and the Center for International Climate and Environmental Research, wrote in a study describing their methods. The peer-reviewed article was published in the journal Nature Climate Change.

Addressing the imbalance is implicit in the Paris climate agreement. While the agreement rules out assigning state-level liability, the new research can help guide discussions about how to help poor countries deal with climate impacts, the scientists say.

A Heat Wave Example

The scientists used the 2013-14 heat wave in Argentina as an example of how the process could work.

Previous research showed human-caused climate change made that heat wave 400 percent more likely. The new study calculated the change in the frequency of this event attributable to individual country or region's greenhouse gas emissions.

Overall, the Argentinian heat wave had about a one-in-12 chance of happening when all global greenhouse gas emissions are taken into account. When the European Union's emissions are subtracted from the global total, however, the risk falls to about a one-in-15 chance. The difference suggests that the EU's emissions alone increased the risk of the heat wave by 37 percent, the scientists say.

U.S. emissions, using the same type of calculation, were found to have increased the risk of the heat wave by 34 percent, while China and India—which have lower historic emissions—increased the risk by 21 and 18 percent, respectively.

The researchers said their method is particularly apt for heat waves, which have been strongly linked to overall human-caused warming.

Using Science as Guidance

The research shouldn't be construed as a finger-pointing exercise, said lead author Friederike Otto, with the Environmental Change Institute at the University of Oxford.

Rather, she said, such science can be part of the basis for a moral and ethical discussion about how to justly address the responsibility for climate change impacts, including paying for the costs of disaster mitigation after extreme events, or preparing for slow-onset changes like sea level rise.

In the UN's framework for climate talks, those questions are up for discussion under a negotiation track that has no deadline. Agreeing on methods is an important early step, Otto said.

"It enables us to get a realistic picture of what climate change means," she said. "Not every disaster that happens can be blamed on climate change, but at the same time, climate change has real impacts today," she said. "This allows us to disentangle where the hazard risk has really increased because of historic emissions."

Rather than providing fuel for outrage over climate impacts, she said the information can help provide evidence for valid claims of damage. "So it should be useful for both sides," she said.

Otto and her co-authors from the Center for International Climate and Environmental Research in Norway made clear that their paper only outlines a couple of ways to allocate shares of responsibility for climate change impacts, as a starting point for discussions. Much depends on what you count and when you start counting it — the contribution to particular events from specific countries varies, for example, if you add in the heat-trapping effects of industrial and commercial coolant gases.

What About Climate Liability?

There's little question that many less-developed countries are going to need help to pay for the damage caused by powerful storms or other extreme climate events; and the world has more or less agreed that the burden needs to be shared. That concept is built into the DNA of the climate talks, said attorney Lindene Patton, a risk management and climate resilience expert with the Earth and Water Group.

"Broadly, everyone agrees that we've created an environment with a climate of increased risk for less-developed countries," she said. "And the Paris agreement generally contemplates, on some basis, investment in development of economies in ways that are more resilient to climate change. But the agreement also specifies that "attributions will not be the basis for discussions on loss and damage."

Negotiators have approached the subject of how to account for loss and damage from climate change gingerly because rifts between countries involved in the negotiations could disrupt the overall process.

Outside the framework of United Nations climate negotiations, climate attribution studies could also play out in the legal arena eventually, Patton said.

In a study published in August, Patton and two co-authors concluded that rapidly advancing science is "improving our ability to detect human influence on extreme weather events. By implication, the legal duties of government, business and others to manage foreseeable harms are broadening, and may lead to more climate change litigation."

Otto said she doesn't think the science will unleash a flood of disruptive lawsuits. If anything, it could help inform courts already facing complex environmental cases, she said. "I think courts struggle with this kind of evidence mainly because there is no authoritative protocol that says how you do this kind of science right," she said. "But ... it might not be that far in the future that courts will be able to assess if the science is sound."

It may be a while before that day comes, said Michael Gerrard, director of the Sabin Center for Climate Change Law at Columbia University, pointing out that there have no judicial decisions yet made based on attribution of emissions. There may be an emerging framework of climate law, but it's still in an early stage.

There's also no recognized international court where one country could sue another over emissions, he said. Countries enjoy sovereign immunity and can only be liable if they assent to be sued—"I don't see a successful lawsuit," he said.

The new study fine-tunes long-available science on the attribution of emissions, said climate scientist James Hansen, who is also part of a citizen lawsuit challenging Norwegian government approval for drilling in the Barents Sea. His granddaughter, Sophie Kivlehan, is one of the plaintiffs taking the federal government to court in the Children's Climate Case.

The study "kind of serves as a warning to newly developing countries that there will be a disadvantage of burning a lot of fossil fuels to develop—probably better find some clean energy," he said.