Climate Science 101: Turning off the Tap

Climate disruption

By Preventable Surprises Board Member Rich Pancost

The challenges arising from growing carbon dioxide concentrations and climate disruption are clear to scientists and to the nearly 200 nations that have agreed to limit carbon emissions through the Paris Accord. What is less clear is the language used in this discourse.
Is the goal to limit warming to 2°C or to aspire to 1.5°C—or is neither feasible? Is the way to get there decarbonisation, carbon neutrality, or carbon zero? What do these different phrases mean and how do they guide decision-making? Which one is right? Which one is useful?
The answer is they are all right, and they are all useful. Both 2°C and 1.5°C are targets whereas decarbonisation, carbon neutrality and net-zero are pathways to achieving them.
We begin with targets and the assumptions that underpin them. Although science can predict many things and has taught us even more about natural systems, the Earth system is complex and non-linear. We do not know exactly what temperature thresholds will bring about profound changes in drought and food security, what combination of warming and ocean acidification will prove terminal for coral reefs, or how fast ice sheets will melt and sea level rise. But it is generally thought that exceeding 1.5°C of warming will eventually result in sea level rise that will inundate hundreds of millions of people, causing some nations and cultures to go extinct and forcing mass migration. Above 2°C, a range of more severe climatic and ecological impacts—including positive feedbacks that will exacerbate global warming, such as greenhouse gas emissions from thawing permafrost—will almost certainly come into play (and in some cases are already starting).
The broad recognition of impending danger led the nations of the Earth to agree to limit warming to “well below 2°C” with an aspiration to limit warming to 1.5°C. These are the targets. It should be noted that the Earth has already warmed 1°C, with more warming locked into the system due to the lag effect of accumulated carbon in the atmosphere. If we continue on our current course, warming is likely to exceed 3°C by the end of the century.
As for how we achieve the target, decarbonisation, carbon neutrality, and net zero are variations on a theme. And that theme is based on the fact that, although natural processes do remove carbon dioxide from the atmosphere, they do so slowly. On century timescales we can think of the atmosphere like a bath with a very slow drain: if you turn down the tap, the bath fills more slowly but it still fills. And if we continue emitting carbon dioxide, concentrations continue to rise and the Earth continues to warm.
So to limit warming below >4°C—let alone 2°C—we need to stop emitting carbon dioxide to the atmosphere from fossil fuel burning and deforestation. The cessation of fossil fuel use and deforestation for agriculture could mean the decarbonisation of our society. Alternatively, if we continue to use fossil fuels but we do not allow the generated carbon dioxide into the atmosphere (i.e. capturing it at the factory or power plant), we could create a society with zero emissions. Or we use some fossil fuels and allow some carbon dioxide into the atmosphere, but we balance that by capturing and storing the same amount, creating carbon neutrality.
These representations look at the challenge we face from different angles and make different assumptions about technological innovation, political interventions and behavioural changes. They aim for the same target but make different assumptions about how we get there. They all require the same thing: to limit warming, we must stabilise the concentration of carbon dioxide in the atmosphere; we have to turn off the tap.

Crucially, all of these assumptions and models require action now if we intend to limit warming to 2°C. The longer we wait, the more drastic and damaging will be the effort required to avert catastrophic change.

Pancost is director of the University of Bristol Cabot Institute, which conducts world-leading research on the challenges arising from how we live with, depend on and affect our planet.

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