Global efforts to tackle climate change rest on a common goal: to keep the planet’s temperatures from rising beyond dangerous thresholds. But what if the gases that come from cars and power plants are harming the planet themselves, even without the warming they cause?

Some scientists think that’s a risk.

Rising carbon dioxide levels in the atmosphere may have direct effects on the climate system, they say. In other words, even if global temperatures stay locked in at a certain point, higher CO2 concentrations could continue to affect the planet.

As a result, some experts are calling for an explicit cap on carbon dioxide concentrations, in addition to the global temperature targets of 2 degrees Celsius outlined in the Paris climate accord.

A study in last week’s Nature Climate Change is the latest to illustrate the point. The research suggests that rising CO2 levels may cause an increase in extreme weather and climate events, regardless of what happens with average global temperatures.

Broadly speaking, the Earth warms proportionally with increasing carbon dioxide concentrations. But there’s still some uncertainty about exactly how much warming is associated with a given amount of greenhouse gas emissions—a metric known as the planet’s “climate sensitivity.” Scientists are still working to understand some aspects of how the Earth responds to rising greenhouse gases in the atmosphere.

In the study, the researchers investigated what would happen to the climate system under a range of possible carbon dioxide levels, assuming the average global temperature remained steady. They chose the 1.5-degree target—the world’s most ambitious goal—as a reference point.

The scientists ran a series of model simulations using a range of CO2 concentrations consistent with 1.5 degrees of warming. The effect on extreme climate events was striking. Even though average global temperatures stayed the same, higher CO2 concentrations caused a significant increase in extreme heat and precipitation events in certain regions around the world.

Scientists are still figuring out exactly why these effects occur, said lead author Hugh Baker, a Ph.D. candidate at the University of Oxford. In part, it may be that extra carbon dioxide affects atmospheric temperatures differently across regions of the world—even if the average global temperature remains more or less constant.

“What this does is it can cause changes in circulation, changes in wind patterns,” Baker said. “This is what is driving the extremes.”

The new findings may be some of the most striking yet, but they’re not the first of their kind.

Previous studies also suggested that carbon emissions may directly affect the climate beyond just raising average global temperatures. One 2013 study found that carbon dioxide levels alter precipitation and atmospheric circulation patterns independently of average warming.

And other research notes that it’s not just weather affected by carbon dioxide levels. One of the biggest concerns about rising CO2 is its contribution to ocean acidification, which is harmful to marine ecosystems.

Reevaluating global goals

The 1.5- and 2-degree temperature targets are currently the centerpiece of international climate mitigation efforts—the ultimate goal that world leaders are striving to meet under the Paris climate agreement. But some experts are concerned that these targets may not encompass all the risks associated with climate change. And some say they may not be promoting the level of global climate action originally hoped for.

As a result, some experts are now advocating for additional climate goals that tackle the issue in different ways.

Baker and his co-authors suggest adopting a concrete cap on atmospheric carbon dioxide concentrations—a goal to keep carbon dioxide levels from passing a certain threshold.

A CO2 target would be a new step for world leaders, but it’s not necessarily a new idea. Some climate activists have argued for similar strategies. The climate advocacy organization 350.org, co-founded by prominent activist Bill McKibben, was established around the idea that world leaders should work to get atmospheric carbon dioxide concentrations below 350 parts per million.

For Baker and colleagues, the idea is to use a CO2 target to avoid bad outcomes if the planet’s climate sensitivity ends up being on the lower side after all.

“If you have a temperature target, and you have a low climate sensitivity, that sounds great because it means you can emit more carbon dioxide into the atmosphere,” Baker said.

But even if it turned out that a 1.5- or 2-degree goal could be achieved with greater emissions than previously thought—a larger “carbon budget,” in other words—the new research shows that the extra emissions could still be damaging on their own. Adding a carbon dioxide limit on top of the temperature target would address this issue.

It could also help address some recent criticisms of the “carbon budget,” which some experts say may not be as useful for global climate policy as once thought.

A temperature target is only useful if policymakers know how to achieve it. As a result, numerous studies in the last decade have attempted to determine the amount of carbon the world can emit without exceeding certain temperatures. This value would inform policymakers about how quickly they need to reduce greenhouse gas emissions in the coming years.

The problem is that the budgets don’t always agree with one another. A wide variety of assumptions can influence a carbon budget estimate—everything from how much warming the Earth has already experienced to the role of non-CO2 greenhouse gases, like methane—and scientists are still debating many of them.

As a result, different research groups have produced different estimates over the years. In the last six months, for instance, several studies have suggested that the carbon budget is significantly larger than previously thought (Climatewire, June 19).

Recently, some experts have suggested that these uncertainties may confuse policymakers, or allow them to argue there’s still time to reduce emissions later—especially if recent estimates seem to suggest that there’s more leeway than before (Climatewire, May 22).

A CO2 goal, in addition to a temperature goal, could address some of those issues, says Glen Peters, from the Centre for International Climate Research, who has previously questioned the carbon budget’s usefulness in international climate policy.

An unmovable target just for carbon dioxide would take “a big part of the uncertainty out” regarding the planet’s climate sensitivity, he said in an email. It would also eliminate the need to deal with other uncertainties that affect carbon budget calculations, such as how much the planet has already warmed or when the baseline “preindustrial era” actually started.

But it could also create new problems, he cautioned. There’s some uncertainty about exactly how emissions translate into carbon dioxide concentrations in the atmosphere. Scientists are still trying figure out how much carbon dioxide is soaked up by the land and the ocean in the planet’s natural carbon cycle. And setting a carbon dioxide cap also doesn’t address the role of other greenhouse gases, like methane.

“So, a carbon budget for CO2 concentrations would solve some problems but create others,” Peters noted.

Oliver Geden of the German Institute for International and Security Affairs, another scientist critical of the carbon budget’s policy use, also noted that setting a CO2 cap isn’t the same thing as developing a concrete timeline for reducing greenhouse gas emissions. Ideally, the target would lead to the timeline, but as he pointed out, merely setting a goal “does not tell a single government what to do.”

So both temperature targets and CO2 targets have their limitations. Baker and his co-authors recommend adopting both—keeping the temperature target and adding a CO2 limit on top of it. Peters says that would “make sense” as a way to at least narrow down the uncertainties, even if it doesn’t solve all the problems.

Thomas Stocker, a climate scientist at the University of Bern in Switzerland, also suggests that multiple targets may be the way to go. He previously co-authored research suggesting that a temperature threshold doesn’t capture all the risks associated with climate change. Adopting a set of specific, individual climate goals—for instance, limits on the amount of global and regional warming, ocean acidification, agricultural losses or sea-level rise—would require greater reductions in carbon emissions than those needed to meet a global average temperature goal alone, the research suggests.

Setting a carbon dioxide limit would be “another complication in the political process,” he acknowledged. But if it were paired with existing temperature targets, it could be another step in the right direction.

“We all realize that whilst the 1.5- and 2-degree Celsius limits in the Paris agreement are politically easily communicable and understandable goals, for the protection of the climate system as a whole they are not sufficient,” he told E&E News. “In other words, one has to really look at what generates impacts, and the various components of the climate system—and to what extent are we willing to accept the level of impacts that a given climate change is affecting.”

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.