Anyone who has been paying attention to climate news will be aware that 2023 broke global temperature records, by a significant margin.
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.
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.
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 suplur 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. Suplur dioxide converts to suplate 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.
Global suplur 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 suplur content. This is in many ways a good thing as suplur can contribute to acid rain and is generally bad for human health. However, as compliance with this legislation increased and reflective suplur 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 suplate 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.
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.
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.
I haven't heard a lot of discussion of the reasons for the heating
It seems that suplur 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.