Experts disagree on the impact of marine clouds on global warming. Image Credit: Ship & Bunker
Two heavyweights of climate science have clashed on whether new data shows the introduction of the IMO 2020 rule has contributed to global warming.
The exchange began with the release last week of a new study, Global Warming in the Pipeline, lead by James Hansen that finds the introduction of the IMO 2020 rule has contributed to global warming.
The paper, that includes work by Hansen and seventeen other scientists, was published in the peer-reviewed open access journal Oxford Open Climate Change.
The study points to the impact of IMO 2020 being one of a number of factors causing a growing imbalance between the amount of warming solar energy being trapped on earth compared to that being released.
"That imbalance has now doubled. That's why global warming will accelerate," Hansen told reporters in a call alongside the study's release.
there were concerns that reducing emissions of SOx from ships would increase the amount of solar energy absorbed by the oceans, and thus increase global warming
Unlike the upcoming IMO 2030 and IMO 2050 GHG targets, IMO 2020 focused only on reducing sulfur emissions, and meant from January 1, 2020 the global sulfur cap for marine fuel dropped from 3.5% to 0.50%.
The idea that this sole focus on SOx could lead to a negative impact on global warming is not new.
Even back in 2016 when the decision was made to introduce the 0.50%S cap in 2020, rather than 2025, questions were raised over its impact on GHGs.
One concern was that the additional refining effort to produce the cleaner fuels would result in an increase in lifecycle GHG emissions.
Separately, there were concerns that reducing emissions of SOx from ships would increase the amount of solar energy being absorbed by the oceans, and thus increase global warming.
This is due to the idea that reducing SOx particles would also reduce the formation of so-called 'marine clouds' and the subsequent production of 'ship tracks'.
Marine clouds form when sulfate aerosol particles are emitted from ships' smokestacks. Water molecules collect on those particles and grow into clouds that then form long tracks along a ship's route in the ocean. Those clouds then reflect sunlight away from the earth.
With the reduced sulfur content of marine fuel from IMO 2020 and subsequent reduction in the emission of SOx, it was theorized that there would also be a meaningful reduction in the formation of marine clouds.
Leon Simons, Co-Author, Global Warming in the Pipeline
We now have about three and a half years of evidence of what happens ... to the oceans if you reduce sulfur emissions from shipping by 80%
Moreover, the clouds formed from sulfur dioxide seeds are thought to be substantially more reflective compared to clouds formed without sulfur dioxide.
Without those clouds reflecting sunlight, that solar energy is instead absorbed by the oceans. The theory was that this additional energy would be enough to result in a meaningful increase in global warming.
Hansen's study says that real-world data gathered since the introduction of IMO 2020 shows that the theory is true.
"We've never done the experiment of reducing emissions over the oceans by 80% before," climate scientist and co-author of the study, Leon Simons, told CBC News.
"So now we are starting to have the evidence. We now have about three and a half years of evidence of what happens ... to the oceans if you reduce sulfur emissions from shipping by 80%."
Hansen is a highly respected climate scientist and former director of NASA's Goddard Institute for Space Studies (GISS). His 1988 Congressional testimony on climate change is widely considered a key driver in bringing awareness of global warming into the mainstream.
Still, fellow climate science giant Dr. Michael E. Mann was among those to quickly raise issue with Hansen's work.
Out of the Mainstream
"It has always been risky to ignore his warnings and admonitions ... so it with no pleasure whatsoever that I find myself in a position to have to criticize his latest work," Mann wrote in a lengthy critique of Hansen et al's study on his personal website.
Dr. Michael E. Mann, director of the Center for Science, Sustainability & the Media, University of Pennsylvania
The existing literature on this ... indicates a net effect on global surface temperatures of 0.05-0.06C
"Jim and his co-authors are very much out of the mainstream with their newly published paper in the journal Oxford Open Climate Change. That's fine, healthy skepticism is a valuable thing in science. But the standard is high when you're challenging the prevailing scientific understanding, and I don't think they've met that standard, by a longshot."
In terms of the impact of IMO2020 specifically, Mann asserts that "there is no evidence that changes in ship-based aerosols have played any substantial role at all in recent warming trends.
"The existing literature on this ... indicates a net effect on global surface temperatures of 0.05-0.06C.
"For comparison, human greenhouse warming is responsible for more than 1C of warming. Indeed, the total impact of changes in sulphate aerosols (which are mostly from land-based coal fired power plants, not shipping) is very small over the past two decades."
Mann is the director of the Center for Science, Sustainability & the Media at the University of Pennsylvania. Among other things he is known for producing with colleagues Raymond S. Bradley and Malcolm K. Hughes the 'hockey stick' graph showing a steep rise in global temperatures corresponding to the Industrial Revolution.
Simons, however, was quick to brush Mann's criticism aside, saying he "hasn't studied this at all."
"He doesn't address the the most important scientific data, which is NASA satellite data."