Efforts to curb sulphur dioxide revealed more planetary heating than we thought and faster temperature rise. While this will be short-lived, the skies above directly impact local and regional climate conditions, and our policies need to reflect it.
Numbers rarely lie. Nor do thermometers. But in this case, both have been misleading.
Between 1970 and 2008 our world experienced 0.18C of warming per decade. In December 2022, Dr James Hansen, a former-NASA scientist, presented a preprint study – peer reviewed and published the following year – projecting that temperatures could climb between 0.27C and 0.36C per decade until the mid-2050s.
Between 50 and 100% faster warming than we have been used to, recent years have consistently set new global temperature records so the idea rings alarmingly true as the bell tolls for hopes of limiting our planet to 1.5C above pre-industrial levels. In fact, Climate Path Tracker currently has us en route to 2.7 additional degrees by the end of this century.
Even if we avoid that, policymakers at local, regional, national and international levels must shift their position. This means developing more robust and comprehensive adaptation plans for warming now baked-in yet just beginning to make itself known, and the increase in extreme weather this will bring. But fit for purpose strategies need accurate data, and according to CICERO, one of Europe’s leading climate science bodies, this is a problem.
‘The main message we are trying to push is that air pollution has a much stronger local effect on climate than we probably realised,’ says Bjorn Hallvard Samset. ‘So air pollution, in general, has a cooling effect. That has been recognised for a while – it’s been cooling the planet by maybe 0.5C. Greenhouse gases are everywhere, and have an overall effect… Aerosols are much more regional. A lot of their effects happen where they are emitted.’
A Senior Researcher at Norway’s foremost climate science institution, Samset has spent most of the past 15 years focusing on air pollution’s role within the wider crisis. We approached him after a recent investigation triggered headlines claiming a dramatic fall in lethal sulphur dioxide fumes over Chinese cities like Beijing triggered an acceleration in the annual temperature rise globally. In reality, it’s more nuanced and complex than that, and he’s keen to point out that none of this means we should abandon cleaning up the atmosphere.
‘Colleagues around the world have run 10 different climate models, each phasing out aerosol emissions one region at a time. For East Asia, which is really just China and a couple of other countries that don’t really have big emissions, the model matched what has happened in the real world since around 2010,’ he tells us. ‘China’s war on air pollution and sulphur dioxide started around 2012, 2013. Between then and now, sulphur emissions have dropped by around 20 teragrams annually. Which is a really significant, sustained fall.
‘That means this change has now had time to start impacting our climate,’ he continues. ‘What we are finding is that the overall global warming effect – around 0.07C – is in itself not too big. That equates to around five years extra heating. But what’s interesting is when we look at the warming rate we’ve had since 2010, in several publications, including my own, it shows this is happening faster in the last decade or so.’
According to Samset, the fact that faster warming fits in with the timeline of China’s steps against sulphur is a positive thing. It means we understand why things have sped up and can more accurately predict how climate change, and specifically planetary heating, will play out in the coming years. ‘The alternative would be that there are some feedbacks in our climate system we don’t know about, and climate sensitivity is higher than we thought.’
Reassuringly, CICERO and a number of other teams, expect a slow down as the ‘China effect’ levels off. However, reasons to be cheerful come with caveats, emphasising our need to think carefully about, but act quickly on adaptation, in order to ready ourselves for incoming effects.
‘The more rapid climate change is, the less time we have to prepare,’ says Samset. ‘We are talking about reducing air pollution, which is easy to do in the context of mitigation – we’re reducing emissions in general. But how do we adapt to the change that’s already here now, and the changes we know are coming? First of all, this starts with accepting the need to adapt. Not everyone has done this yet.’
That doesn’t just mean bracing for warmer temperatures, and this is where things begin to get greyer. In areas where air pollution has been high enough to balance the impact of greenhouse gases, the two have likely been confused. Extreme rainfall, for example, is often dominated by air pollution rather than other potential contributing factors, like carbon, as has often incorrectly been assumed.
‘Policymakers should definitely work to remove air pollution, but then adaptation has to reflect this. Every country around the world now has climate projections, but these must take into account the aerosol gas patterns, and effects, alongside greenhouse gases,’ says Samset. ‘If we do this then we’re well on our way.’
The impact of China’s efforts to bring sulphur dioxide down is apparent. In contrast, another big drop in the gas is only beginning to play out. Since 2020, the International Maritime Organization [IMO] has mandated an 80% cut in the maximum acceptable sulphur emissions from shipping. A smaller scale change than China’s policy, southern England’s Plymouth Marine Laboratory still says the effect will be noticeable.
‘Atmospheric sulphur levels over the last decades have continuously declined – a good news story for air quality,’ says Dr. Mingxi Yang, Principal Scientist at the Plymouth Marine Laboratory who led on the organisation’s ACRUISE work around shipping, sulphur and global warming. ‘However there’s a temporary downside to this decline in sulphur, because such aerosols are very efficient at controlling clouds and radiation. The reduction in sulphur emissions post-2020 has led to increased solar radiation reaching the surface, so more surface warming.
‘The maximum fuel sulphur content was 3.5% by mass pre-2020, and the actual mean fuel sulphur content of the global shipping fleet was about 2%. Now the actual mean sulphur content is around 0.5%, as mandated – about a four-fold reduction,’ he continues. ‘Based on this, I don’t anticipate large, further reductions in sulphur emissions in the near future. However, as the Gettelman et al. 2024 paper indicates, the full extent of warming due to the IMO 2020 reduction has yet to be felt due to inertia in the climate system.’
Although Yang nods to a pronounced rise in Northern Hemisphere surface temperatures since 2022, this cannot be explained through shipping regulations alone as the ripple effect of this sea change will play out over the next five years. It is expected to peak around 2030.
He also tells us that, due to the overwhelmingly longer lifespan of carbon, decarbonisation must continue to be the priority for shipping and other sectors because this remains the key warming contributor. Of course, that’s easier said than done, as proposed alternatives to fuel stocks ‘such as hydrogen, ammonia and methanol all have their limitations’. Not to mention ‘potentially significant environmental impact’.
Just as worrying as the poorly understood ecosystem implications of these nascent energy sources are the relatively unknown effects on human health. But one thing is clear. Our route from here must be plotted with the minefield of hidden trigger points we are currently navigating kept in mind. The long-term must be prioritised over quick fixes, whether that’s plans to adapt, mitigate and prepare for what is happening – now at a faster pace than previously thought – or a just and feasible roadmap for transitioning away from carbon fuels and economics.
Image: Louis Reed / Unsplash
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