Because climate change is such a complex, globe-spanning problem, it’s hard to really wrap your head around possible future scenarios. A future where no action is taken to slow greenhouse gas emissions is easy enough to grok, but what exactly does a “middle-of-the-road emissions world” entail?

These scenarios work well for outlining the range of futures available to us, but it can be hard to understand the steps necessary to get to that future. “What if?” scenarios are often easier to think about. What if we eliminated all greenhouse gas emissions tomorrow? Or, if those rainbow unicorns are too impractical for you, what if we didn't replace fossil fuel infrastructure when it reached the end of its life, replacing it with clean alternatives instead?

End of life

That’s the question that a new study led by the University of Leeds’ Chris Smith investigated. The basic idea is to find out how much warming the world’s existing fossil-fuel-burning machinery commits us to, given how long that machinery is likely to run before it naturally hits the scrap heap.

Power plants and industrial equipment like cement kilns generally have lifetimes of about 40 years. The researchers also included transportation, as cars tend to last around 15 years, while planes and ships fly/float (as applicable) for around 25 years. The assumption of a zero-emissions replacement requires a slight stretch of what is currently possible, but aircraft running on biofuels, for example, could potentially serve in this thought experiment without being too hard to imagine.

While these sources add up to about 85 percent of current greenhouse gas emissions, this scenario also assumes that the other 15 percent declines at the same rate, providing some simplicity. That would mean that things like agricultural emissions and deforestation are also reduced—however that is accomplished.

The researchers used a simple climate model to relate these emissions scenarios to global temperature. This also allowed them to get at the range of possibilities by trying out different numbers for things like the climate’s exact sensitivity to greenhouse gas emissions and the amount of greenhouse gas released by ecosystems due to warming. They even created multiple versions of their infrastructure-retirement scenario by adding or subtracting 10 years from the average lifetimes for each category.

Keeping cool

The results showed that, if we just started using green replacements for all dying infrastructure as of 2018, we could very likely limit global warming to 1.5°C above pre-industrial times—about 0.4°C higher than 2018’s global temperature. To be more specific, at least half of their model simulations stayed below 1.5°C in every scenario, with a greater portion when infrastructure lifetime was shorter. (The simulations that failed to stay below 1.5 °C were those where Earth’s climate sensitivity was at the high end of estimates.)

However, if we put off the start of this infrastructure retirement plan until 2030, the answer changes. This time, most of the simulations are on the wrong side of 1.5°C—but still under the 2°C mark that has long been the agreed-upon target for international negotiations.

So barring any major surprises thrown at us by a warming planet, it’s possible that simply replacing all old fossil fuel technology after it has had its time in the sun is a rapid enough transition to effectively limit global warming.

Of course, the word “simply” belies the fact that this would be an absolute technological revolution—several of them, really. But this is also a boiled-down scenario that doesn’t include other options like efficiency gains or efforts to actively remove CO 2 from the atmosphere, which could loosen the requirements. Reducing things to one type of action does nicely illustrate what it would take to truly slam the brakes on climate change, though: no new fossil fuel infrastructure.

Open Access at: Nature Communications, 2018. DOI: 10.1038/s41467-018-07999-w (About DOIs).