Clarity about Key Climate Dynamic Creates More Pressure to Rein in Global Temperature Increase

New research suggests that the task of keeping global temperatures below manageable thresholds might be even more difficult to achieve than previously thought.

Scientists at Scripps Institution of Oceanography at UC San Diego and colleagues used a new method that enables more accurate depictions of the interplay between aerosols and clouds in the atmosphere. That led them to discover that the effect of aerosols such as sulfates in the atmosphere has hidden the full warming potential of greenhouse gases such as carbon dioxide (CO₂). Thus the planet’s susceptibility to future warming as CO₂ levels rise has been underestimated by half a degree Celsius, the researchers estimate.

Global leaders have agreed that the planet faces a more dangerous future if average annual temperatures rise more than 1.5℃ higher than those before fossil fuel use became widespread. The mantra has driven large-scale climate assessments and become the shorthand term for the goal of international efforts to limit greenhouse gas emissions.

Study lead author Casey Wall and co-authors Joel Norris and Nick Lutsko said that their analysis suggests that if CO₂ concentrations were to double from pre-industrial times, the prevailing estimate of how much the planet warms would need to be revised. Anticipated global warming would shift from a range of 2.5℃ to 4℃ to a range of 2.9℃ to 4.5℃.

CO₂ levels in the atmosphere never exceeded 280 parts per million (ppm) for thousands of years until the 19th century and the advent of fossil fuels as a power source. The doubling referenced by the researchers would therefore be 560 ppm. At present, annual CO₂ levels peak at roughly 420 ppm every May.

“We have these conversations about limiting warming to 1.5℃,” said Lutsko, a climate scientist at Scripps Oceanography. “Unfortunately, it’s going to be even harder for us to meet those targets.” 

Wall, Lutsko, Norris, and colleagues adopted a new methodology first described earlier in 2022 in an Oxford University-led paper to create a more accurate analysis. So-called “opportunistic experiments” let scientists understand how aerosols affect clouds and vice versa.  Since the beginnings of climate research, that interaction has been one of the largest sources of uncertainty impeding the reliability of climate forecasts.

Aerosols are fine particles ejected into the atmosphere from natural and human sources.  Some types of aerosols, like sulfates, can come from events like volcanic eruptions or the burning of fossil fuels. Sulfate aerosols are reflective and generally cool the atmosphere, reflecting sunlight back into space like tiny mirrors.

Aerosols influence the formation of clouds and the amount of solar energy clouds reflect back to space, a concept known as “radiative forcing”. Likewise, clouds influence the prevalence and nature of aerosols in the atmosphere.

“When we look at clouds and aerosols and how they’re correlated, it’s not always clear which direction the causality is going,” said Wall, who led the research at Scripps Oceanography as a postdoctoral researcher with Norris and Lutsko. “Aerosols can change clouds and clouds can change aerosols. It’s not always clear what’s causing what.”

But Wall, now a postdoctoral researcher at the University of Oslo in Norway, and his team took advantage of events in which the sequence of causation was clear (opportunistic experiments), in effect creating a control group.

One opportunity is afforded by Kilauea, the Hawaiian volcano that emits sulfur dioxide which turns into sulfate aerosol in the atmosphere. The team performed a statistical analysis of cloud behavior when the volcano was not erupting and then tested the models’ ability to project how clouds would form when the volcano was erupting and spewing quantifiable amounts of sulfur dioxide.

Similarly, the team looked at the history of cloud formation off the East Coast of the United States. That is a dynamic that has been altered in recent years by stricter regulations on emissions from coal-fired power plants.

In both cases, the researchers were impressed by the results of their analyses.

“We were using this analysis to test a method and it was so accurate,” said Lutsko. “It gets it bang on.”

The data from these natural control groups enabled Wall’s team to see what the real effect of CO₂ and other greenhouse gases is going to be with no more sulfate aerosols to counteract their effect. 

“Our results are not good news for limiting future global warming, but they help us understand how human activities influence climate,” Wall said.

The study appears in the journal Proceedings of the National Academy of Sciences. Besides Wall, Norris and Lutsko, co-authors include scientists from Goethe University Frankfurt, Germany; University of Wyoming, University of Miami, and the University Corporation for Atmospheric Research in Boulder, Colo. Funding came from NASA, NOAA, and Germany’s Federal Ministry of Education and Research.