James Thinks

writing is a kind of thinking

2023 - The warmest year on record, especially the oceans

Anyone who has been paying attention to climate news will be aware that 2023 broke global temperature records, by a significant margin.

Copernicus graph showing global surface temperature anomalies with 2023 rising above previous years, from June onwards. The graph above shows Global surface temperature anomalies. It's easy to see that more recent decades have been warmer, but 2023 is something else. To those aware of the floods, droughts and other extreme weather that rising global temperatures are causing, this is worrying to say the least.

So far, I haven't heard a lot of discussion of the reasons for this besides the ongoing effect of climate change and the end of the change from a La Niña period to El Niño.

Curiously though, the temperatures were well above average by April 2023, before El Niño had really arrived. What's more, the temperature change was particularly striking in the oceans.

The effect of sulphate aerosol masking

I recently stumbled upon a podcast interviewing climate researcher Leon Simons who has worked recently with James Hansen. I encourage you to listen to the whole thing, and have a look at the slides, although it's nearly and hour and a half long. For those who prefer to read, I'll do my best to summarise the main points as I understand them here.

It seems that sulphur dioxide emissions from internationl shipping were having a masking effect reducing sea temperatures, which are a large proportion of global temperatures. Effectively cooling oceans and the planet while greenhouse gases warmed them. sulphur dioxide converts to sulphate aerosols in the atmosphere which act as many tiny mirrors, reflecting some sunlight back into space and cooling the planet. This is sometimes called global dimming. Sulphur particles have a secondary effect seeding clouds which also reflect sunlight back into space.

Graph showing International shipping SO2 emissions, rising from 1900 and a steep decline around 2020. Global sulphur dioxide emissions from international shipping. Sources: CEDS and Corbett et al. via The Great Simplification.

In the last few years, legislation has forced shipping fuel to greatly reduce its sulphur content. This is in many ways a good thing as sulphur can contribute to acid rain and is generally bad for human health. However, as compliance with this legislation increased and reflective sulphur particles in the atmosphere decreased, the cooling effect was reduced and temperatures rose. Meanwhile concentrations of warming greenhouse gases are still increasing, so the net effect is strongly heating.

Unlike greenhouse gases, which can stay in the atmosphere for decades or centuries, these aerosols are cleared in days or weeks. So stopping them has a faster effect. Some have tried to use this as an argument for continuing to burn fossil fuels (another source of sulphate aerosols), but research suggests this doesn't add up.

This was an accidental experiment in rapidly reducing reflective aerosols. A cooling effect has been removed, quite suddenly in climate terms.

What are the consequences?

Leon Simons has suggested that stopping this cooling effect might lead to an additional 0.2 to 0.9 degrees of warming in the first six months of 2024. A dramatic change that could take us over the dangerous threshold of 2 degrees of warming. This is likely to trigger further tipping points, making climate change much worse.

It is also like to disrupt or slow the Atlantic meridional overturning current (AMOC), of which the gulf stream is a part. That would have consequences for Atlantic weather patterns and probably on the ocean's ability to absorb further carbon dioxide and heat.

What have we learned?

The significant rise in temperatures suggests that the cooling effect of aerosols was underestimated. That in turn suggests that the climate is more sensitive to greenhouse gases than previously thought. In other words, climate sensitivity is towards the upper end of estimates.

This is very bad news as it means that existing predictions for future warming relying on climate sensitivity would also be underestimates, so we could expect more warming sooner, along with all the extreme weather that would bring.

This inadvertent experiment with atmospheric aerosols could be described as a "termination shock", when a cooling effect is suddenly stopped. That has long been a concern with the suggestion of intentional solar radition management by stratospheric aerosol injection. This is a method proposed to mitigate the warming effects of climate change by intentionally adding aerosols to the upper atmosphere. It's controversial and has huge risks of unintended consequences, especially if the world's ability or willingness to continue doing it suddenly faltered, unmasking a huge heating effect in a matter of weeks. Others fear it may be used as an excuse to continue with fossil fuel business as usual, neglecting the other dangerous effects of burning fossil fuels. Scientists are at least in agreement that geoengineering is not a substitute for the urgent decarbonisation of human activities.

So whether or not solar radiation management of some form might be safe or necessary, we are at least learning more about the likely effects and risks.

Category: Environment
Photo credit: [Leon Simons](https://static1.squarespace.com/static/61d5bc2bb737636144dc55d0/t/65a1a1dae6777975b57059aa/1705606248438/Leon+Simons+TGS+Slides)
Mugshot of James cycling on a road in the sunshine.

James Bradbury

I write about whatever is on my mind. I do so mostly to help me think more clearly. If other people find it interesting that's good too. :-)


I haven't heard a lot of discussion of the reasons for the heating

It seems that sulphur dioxide emissions from internationl shipping were having a masking effect reducing sea temperatures

This was an accidental experiment in rapidly reducing reflective aerosols.

A cooling effect has been removed, quite suddenly in climate terms