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One possible explanation for the extremely hot temperatures of recent years: removing the sulfur dioxide from shipping fuels.
The world has been very hot lately. Like, really hot. Much hotter than you might expect from climate change alone.
In 2023, the global average temperature was nearly 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, above its pre-industrial level. It is nearly certain to exceed that milestone in 2024.
These are extreme leaps. For context, 2019 was the second-hottest year on record when it happened, and it was merely 0.95 degrees Celsius above the pre-industrial average. So Earth’s temperature has seemingly surged half a degree Celsius in five years.
These searingly hot temperatures aren’t completely outside the range climate models predict, but they are arriving sooner than most scientists thought, and climate researchers haven’t yet reached a consensus explanation for why they are happening.
This week, though, we got somewhat closer. A new study adds to a growing literature suggesting that a change to global shipping fuels has accidentally contributed to a surge in warming.
In 2020, the International Maritime Organization began enforcing rules that removed a toxic air pollutant, sulfur dioxide, from shipping fuels. Sulfur dioxide can inflame and irritate the heart and lungs, trigger asthma attacks, and can cause acid rain. But it can also reflect heat back into space, cooling the Earth.
These cooling effects of sulfur dioxide are very short-lived, and sulfur dioxide only sticks around in the atmosphere for about a week and a half. (Carbon dioxide, on the other hand, can persist in the atmosphere for a millennium.)
It now seems that all those sulfur aerosols were likely reflecting enough heat back into space to make a noticeable difference in the Earth’s temperature rise. The new study, written by the researchers Ilaria Quaglia and Daniele Visioni, finds that removing sulfur dioxide from shipping fuel likely increased the planet’s temperature by 0.08 degrees Celsius.
This change alone can’t explain the Earth’s recent surge in temperature rise. But the new rules likely made the record-breaking temperatures in 2023 roughly 12 times likelier than they would have been had the rules not changed, Visioni, an atmospheric chemistry professor at Cornell, told me.
“The likelihood of something like 2023 happening — was it made larger, was it made bigger, by this contribution? We found, yes,” he said.
The timing of the surge — and the fact that the most anomalously warm part of the planet has been the surface of the North Atlantic Ocean, a popular shipping route — also support the conclusion that the IMO rules are playing an effect.
Other factors — including natural fluctuations in Earth’s multi-year climate cycles, like El Niño — may have helped the surge along too, Visioni said. “If you take a probabilistic approach, you can say, even without the shipping rules, 2023 wouldn’t have been completely impossible,” he added. “But you cannot evaluate the truthfulness of probability from one outcome because you only have one world.” In other words, both climate change and our response to it are part of the same poorly designed experiment — and we can only run that experiment once.
Over the past 12 months, several other papers have reached a similar conclusion, although they disagree about the magnitude of the IMO’s accidental cooling effect. Quaglia and Visioni’s study finds one of the largest effects.
The literature suggests that sulfur dioxide’s effects are “in the range of three hundredths to eight hundredths of a degree Celsius, but I don’t know that we can say that we're on the high or low end of that,” Zeke Hausfather, a climate scientist who studies carbon removal technologies at the tech company Stripe, told me. Hausfather has his own estimate of how much shipping rules have affected the recent warming episode — about five hundredths of a degree Celsius — which he reached with Piers Forster, a climate physics professor at the University of Leeds.
The exact magnitude of the effect, though, might matter less than the fact that it happened at all. For Visioni, the results demonstrate that policymakers need to think more intentionally about the tradeoffs between cutting toxic air pollution emissions and losing the cooling effect those same toxic emissions produce.
Over the past few decades, humanity has gotten better at cutting toxic air pollution from power plants and industrial activities than previous climate models estimated. That means that, somewhat paradoxically, it might be more difficult to stay below the Paris Agreement’s 2 degrees C warming goal because the same levels of greenhouse gas emissions will now have a greater warming effect than they would have in 2015.
It’s time to discuss this trade-off frankly and head-on, he told me. That also means taking seriously — and beginning research — on the proposition that humanity may want to experiment with intentionally releasing some forms of aerosols to suppress the planet’s warming — something the international shipping community has historically been loath to do.
In 2013, a paper from Finnish researchers suggested that ships could retain the climate benefits of sulfur aerosol pollution — while mitigating most of their public health downsides — by burning clean fuels near the coasts, but dirtier fuel on the open ocean. Under that scenario, shipping emissions would actually have reflected even more heat than they did at the time. But the group downplayed that scenario in part because it was a potential form of geoengineering.
Is it? It’s not clear where the line of “intentionality” in geoengineering lies, Visioni said. If you stop doing something bad for the environment, but it has a warming effect on the climate, are you geoengineering? Or are you passing prudent environmental policy? The question of where geoengineering begins or ends gets harder and harder to adjudicate — especially while humanity conducts what is in essence the largest and most important geo-engineering experiment possible by burning fossil fuels and releasing billions of tons of greenhouse gas pollution into the atmosphere.
Visioni made a point to emphasize that he’s in favor of the IMO’s efforts to clean up shipping emissions. “Do we keep polluting? No. I think we should be forceful and say no,” he said.
“But on the other hand, my wish would be if we started discussions a bit more like, ‘Okay, so do we think that these [warming] thresholds are so important? And if so, are we willing to have a discussion about what we could do to prevent this warming from happening?”
Visioni’s paper is not the only new study that seeks to explain the warming blip. On Friday, a team of German researchers wrote in Science that a recent and mysterious decline in low-altitude clouds in the atmosphere has decreased the planet’s brightness. Clouds, like sulfur aerosol emissions, reflect heat back into space, and so their decline would also contribute to a warming surge.
They provide another piece of evidence that the surge in warming is caused by some fundamental change to the climate system and not by a multiu-seasonal hiccup like El Niño. “The big question that we have is: Is this a blip or not?” Hausfather said. “If we're in the world where El Nino is behaving weirdly, that’s kind of the comforting one, because it means we’ll go back to normal — normal here being a rapidly warming world. If the spike in warming over the past two years is due to natural variability, it means it will likely be shortlived.”
The more worrying possibility, he continued, is that something more fundamental has changed in the climate system. Climate scientists describe these shifts as a change in “radiative forcings,” meaning a change in the basic dynamics that force adjustments in the energy balance between the Earth, the Sun, and outer space.
“If this is a change in forcings — which clouds or aerosols would imply — then that change in forcing would likely persist. It would be a factor that continues affecting the climate in the future, rather than just a blip,” Hausfather said.
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A conversation with Mike Hall of Anza.
This week’s conversation is with Mike Hall, CEO of the solar and battery storage data company Anza. I rang him because, in my book, the more insights into the ways renewables companies are responding to the war on the Inflation Reduction Act, the better.
The following chat was lightly edited for clarity. Let’s jump in!
How much do we know about developers’ reactions to the anti-IRA bill that was passed out of the House last week?
So it’s only been a few days. What I can tell you is there’s a lot of surprise about what came out of the House. Industries mobilized in trying to improve the bill from here and I think a lot of the industry is hopeful because, for many reasons, the bill doesn’t seem to make sense for the country. Not just the renewable energy industry. There’s hope that the voices in Congress — House members and senators — who already understand the impact of this on the economy will in the coming weeks understand how bad this is.
I spoke to a tax attorney last week that her clients had been preparing for a worst case scenario like this and preparing contingency plans of some kind. Have you seen anything so far to indicate people have been preparing for a worst case scenario?
Yeah. There’s a subset of the market that has prepared and already executed plans.
In Q4 [of 2024] and Q1 [of this year] with a number of companies to procure material from projects in order to safe harbor those projects. What that means is, typically if you commence construction by a certain date, the date on which you commence construction is the date you lock in tax credit eligibility, and we worked with companies to help them meet that criteria. It hedged them on a number of fronts. I don’t think most of them thought we’d get what came out of the House but there were a lot of concerns about stepdowns for the credit.
After Trump was elected, there were also companies who wanted to hedge against tariffs so they bought equipment ahead of that, too. We were helping companies do deals the night before Liberation Day. There was a lot of activity.
We saw less after April 2nd because the trade landscape has been changing so quickly that it’s been hard for people to act but now we’re seeing people act again to try and hit that commencement milestone.
It’s not lost on me that there’s an irony here – the attempts to erode these credits might lead to a rush of projects moving faster, actually. Is that your sense?
There’s a slug of projects that would get accelerated and in fact just having this bill come out of the House is already going to accelerate a number of projects. But there’s limits to what you can do there. The bill also has a placed-in-service criteria and really problematic language with regard to the “foreign entity of concern” provisions.
Are you seeing any increase in opposition against solar projects? And is that the biggest hurdle you see to meeting that “placed-in-service” requirement?
What I have here is qualitative, not quantitative, but I was in the development business for 20 years, and what I have seen qualitatively is that it is increasingly harder to develop projects. Local opposition is one of the headwinds. Interconnection is another really big one and that’s the biggest concern I have with regards to the “placed-in-service” requirement. Most of these large projects, even if you overcome the NIMBY issues, and you get your permitting, and you do everything else you need to do, you get your permits and construction… In the end if you’re talking about projects at scale, there is a requirement that utilities do work. And there’s no requirement that utilities do that work on time [to meet that deadline]. This is a risk they need to manage.
And more of the week’s top news in renewable energy conflicts.
1. Columbia County, New York – A Hecate Energy solar project in upstate New York blessed by Governor Kathy Hochul is now getting local blowback.
2. Sussex County, Delaware – The battle between a Bethany Beach landowner and a major offshore wind project came to a head earlier this week after Delaware regulators decided to comply with a massive government records request.
3. Fayette County, Pennsylvania – A Bollinger Solar project in rural Pennsylvania that was approved last year now faces fresh local opposition.
4. Cleveland County, North Carolina – Brookcliff Solar has settled with a county that was legally challenging the developer over the validity of its permits, reaching what by all appearances is an amicable resolution.
5. Adams County, Illinois – The solar project in Quincy, Illinois, we told you about last week has been rejected by the city’s planning commission.
6. Pierce County, Wisconsin – AES’ Isabelle Creek solar project is facing new issues as the developer seeks to actually talk more to residents on the ground.
7. Austin County, Texas – We have a couple of fresh battery storage wars to report this week, including a danger alert in this rural Texas county west of Houston.
8. Esmeralda County, Nevada – The Trump administration this week approved the final proposed plan for NV Energy’s Greenlink North, a massive transmission line that will help the state expand its renewable energy capacity.
9. Merced County, California – The Moss Landing battery fire is having aftershocks in Merced County as residents seek to undo progress made on Longroad’s Zeta battery project south of Los Banos.
Anti-solar activists in agricultural areas get a powerful new ally.
The Trump administration is joining the war against solar projects on farmland, offering anti-solar activists on the ground a powerful ally against developers across the country.
In a report released last week, President Trump’s Agriculture Department took aim at solar and stated competition with “solar development on productive farmland” was creating a “considerable barrier” for farmers trying to acquire land. The USDA also stated it would disincentivize “the use of federal funding” for solar “through prioritization points and regulatory action,” which a spokesperson – Emily Cannon – later clarified in an email to me this week will include reconfiguring the agency’s Rural Energy for America loan and grant program. Cannon declined to give a time-table for the new regulation, stating that the agency “will have more information when the updates are ready to be published.”
“Farmland should be for agricultural production, not solar production,” Cannon wrote – a statement also made in the USDA report.
REAP is a program created in 2008 that exists to help fund renewable energy and sustainability projects at the level of individual farms and has been seen as a potential tool for not only building more solar but also more trust in agriculturally-focused communities. It’s without question that retooling REAP to actively disincentivize awardees from building solar on farmland could have a chilling effect, at least amongst those who receive money from the program or wish to in the future. This comes after Trump officials temporarily froze money promised to farmers, too.
As we’ve previously written in The Fight, agricultural interests can at times present as much a threat to the future of solar energy as any oil-funded dark money group, if not more so. Conflicts over solar production on farmland make up a large portion of the total projects I cover in The Fight every week, and it is one of the most frequently cited reasons for opposition against individual renewables projects. (Agricultural workforces are one of the most important signals for renewable energy opposition in Heatmap Pro’s modeling data as well.) I wrote shortly after Trump’s inauguration that I wondered when – not if – he would adopt this position.
It’s unclear what exactly led USDA to dive headlong into the “No Solar on Farmland” campaign, aside from its growing popularity in conservative political circles, but there is reason to believe farming interests may have played a role. USDA has stated the report was the product of discussions with farming groups and an industry roundtable. In addition, per lobbying disclosures, at least one agricultural group – the Pennsylvania Farm Bureau – advocated earlier this year for “congressional action and/or executive orders” to “balance renewable and conventional sources of energy” through “limit[ing] solar on productive farmland.” (The Pennsylvania Farm Bureau denied this in an email to me earlier this week.)
There’s also reason to believe some key stakeholders were caught off-guard or weren’t looped in on the matter.
American Farmland Trust has been trying to cultivate common ground between farmers, solar companies, and various agencies at all levels of government over the future of development. But when asked about this report, the nonprofit told me it couldn’t speak on the matter because it was still trying to suss out what was going on.
“AFT is meeting with the Trump administration to learn more about what they are planning in terms of policy and programs to implement this concept,” AFT media relations associate Michael Shulman told me.