Scientists study shipping and cloud shifts for 2023's record heat

• If so, then a faster warmup — with potentially devastating consequences — could happen sooner than climate projections have shown.

The big picture: Two new studies provide insights about factors that may have helped boost the planet's temperatures during the last two years, above what would normally be expected during a strong El Niño episode in the equatorial tropical Pacific Ocean.

• A study published Nov. 28 in the journal Earth Systems Dynamics examined the effects of a switch to reduced sulfur fuel sources for container ships that went into effect in January 2020.
• The fuel shift was made to reduce these ships' air pollution. But in doing so, it cut down on emissions of tiny particles known as sulfate aerosols.
• These particles can reflect incoming solar radiation and exert a cooling influence on the planet.

Climate scientists are asking whether this fuel change, which the International Maritime Organization mandated, has influenced the climate to the extent that it largely, or even completely, explains the past two years' anomalous warming.

Zoom in: The study, by two Cornell University researchers, found that the marine shipping emissions shifts made 2023's record global heat significantly more likely than what would have occurred without it.

• Specifically, the odds were about 1-in-250 from natural variability alone, compared with about 1-in-10 from the boost provided by the difference in aerosol emissions, according to lead author Daniele Visioni.
• "Our conclusion is that maybe 2023 would have been very warm anyway (due to an El Niño year coming after 3 La Niña years)," Visioni told Axios via email. "But the push from less sulfate brought it very likely into 'unprecedented' territory."

Yes, but: The researchers' estimates of the shipping contribution to warming came out on the higher end of what the scientific literature shows on this topic.

The intrigue: Visioni notes the tradeoffs between cutting pollution and boosting warming.

• "Overall, we should still clean up our air from pollution!" he told Axios. "But we should keep in mind these trade-offs. Aerosols cool the planet, and us cleaning up our air is making warming just a little bit worse."
• "Perhaps it's time to stop burying our head in the sand and discuss these trade-offs a bit more openly — something that even the scientific community hasn't done always very well around this specific issue," Visioni said.

Friction point: Scientists disagree on the extent to which changes in human-contributed maritime aerosols are responsible for the stepped-up warming rate during the past few years.

• Some argue that warming is accelerating mainly due to upward trends in greenhouse gas concentrations in the atmosphere, for example.
• Others say the jury is still out.

Another new study, published in Science on Dec. 5, investigates declining low-cloud cover in the northern mid-latitudes and tropics that could be causing increased warming rates.

• Reducing low clouds can trap more heat in Earth's atmosphere. Clouds' net warming effects depend on their altitude and composition, and changes in clouds have vexed climate modelers for years.
• Depending on the driving forces behind the cloud trends, the study's findings are a potential warning sign since they could imply faster warming rates would continue, if not accelerate, in the coming years.
• However, scientists who didn't participate in the study told Axios the mechanism behind the reduction in low cloud cover needs to be more fully studied.

Zoom out: Climate scientist Zeke Hausfather, who wasn't involved in either of the studies, has been closely watching the steady drumbeat of studies looking into the maritime fuel switch.

• He said they all come out to a warming influence of less than 0.1°C (0.18°F).
• "The takeaway here is that the phaseout of sulfur in marine fuels is likely a part of the puzzle to explain recent spiking global temperatures, but is far from a complete explanation."

Texas A&M climate scientist Andrew Dessler told Axios via email that the "troubling thing" is how warm global temperatures still are despite the La Niña-like state in the tropical Pacific Ocean.

• Such events tend to suppress global average temperatures for a time, but that hasn't happened this year, and 2024 will beat 2023 for the title of the warmest year to date.

The bottom line: "I think it's still an open question whether we've broken the climate or not (by which I mean the climate has shifted into a new regime).  We probably won't know the answer to that for a few years," Dessler said.