Target CO2 and non-carbon pollutants to slow climate change

ITHACA, N.Y.- Policies that focus solely on decarbonization will not be sufficient to keep the Earth’s temperature below the “tipping point” threshold scientists have long warned could result in a runaway greenhouse warming effect, according to research published May 23 in the Proceedings of the National Academy of Sciences.

While carbon dioxide is the chemical most responsible for climate change, four other pollutants – methane, black carbon soot, lower-atmosphere ozone smog and hydrofluorocarbon refrigerants – contribute almost half the heat trapped to-date that cause global warming.

Reducing atmospheric levels of all of these pollutants, known as super pollutants, will be necessary to keep global temperatures from rising beyond 1.5 degrees Celsius above pre-industrial levels, the internationally accepted guardrail beyond which the world’s climate is expected to pass irreversible tipping points.

“We have a near-term and long-term climate crisis,” said Veerabhadran Ramanathan, adjunct professor in the Department of Global Development in Cornell’s College of Agriculture and Life Sciences (CALS), and co-corresponding author of “Mitigating Climate Disruption in Time: a Self-Consistent Approach for Avoiding Both Near-Term and Long-Term Global Warming.”

“In the near-term, the planet is likely to cross the 1.5 degrees C warming threshold in the next 10 years,” Ramanthan said. “On the longer-term, with unchecked emissions, the warming can exceed the catastrophic 3 degrees C by end of century.”

The overwhelming majority of climate change mitigation efforts focus on the longer-term crises and decarbonization, such as phasing out fossil fuel use. This study considers both the near- and long-term threshold crossings and simulated the relative impacts of curbing carbon dioxide pollution vs. curbing both carbon dioxide and non-carbon climate pollutants.

It confirms that even if global leaders succeed in reducing carbon dioxide pollution, it will not be enough to stop global temperatures from exceeding 1.5 degrees C above pre-industrial levels by 2035. It will likely not even stop warming to the much more dangerous 2 degrees C guardrail by 2050.

“This study clarifies a long-standing misunderstanding on the role of non-CO2 climate warming pollutants,” Ramanathan said. “In so doing, it demonstrates why we should simultaneously reduce emissions of both CO2 and non-CO2 pollutants.”

Carbon dioxide can remain in the atmosphere for centuries. The four non-CO2 super pollutants remain in the atmosphere for a much shorter period of time, but can be far more damaging while there. For example, Ramanathan discovered in 1975 that certain cholorofluorocarbons (CFCs) have 10,000 times the greenhouse warming effect as carbon dioxide.

This dual approach would slow the rate of global warming 10 to 20 years earlier than decarbonization alone and make it possible for the world to stay below the 1.5C guardrail, the researchers found.

“We need to urgently broaden our approach to climate mitigation to target both carbon dioxide and other largely neglected pollutants to address the near-term and long-term impacts of climate disruption, slow self-reinforcing feedbacks reduce the risk of crossing irreversible tipping points, and maintain a livable planet,” the researchers wrote.

This research was supported by the Frieman Foundation at the University of California San Diego and by CALS.