Overshooting 1.5°C is risky – that’s why we need to hedge our bets

The global response to climate change has gained momentum since the 2015 Paris agreement, yet it remains inadequate to meet the scale of the challenge. That agreement established the goal of holding global warming to well below 2°C, and to pursue efforts to limit it to 1.5°C. To achieve this, greenhouse gas emissions should peak and decline as soon as possible.

The latest reports of the UN Environment Programme, the International Energy Agency and others have suggested that we are on the cusp of global emissions peaking. However, halting the increase in annual emissions is only the first step. Failure to act earlier and more decisively to bring emissions down to net zero has made limiting global warming to 1.5°C an uncomfortably close call.

The IPCC has looked at “pathways” to keeping 1.5°C in reach. In nearly all of these, temperature rises will exceed 1.5°C, after which warming is reversed by humanity removing more carbon from the atmosphere than it emits. This temporary breach of 1.5°C for at least a few decades is referred to as “overshoot”.

In a recent study in the journal Nature, we discuss the pitfalls of being overly optimistic about the feasibility and safety of such temperature overshoot scenarios. Excessive confidence could lead to underestimating the risks associated with going over 1.5°C – even temporarily.

There is a need to be clear about what climate science does and does not know about overshoot, and plan accordingly. This means that, while some risks can be directly reduced by global climate action, others may require additional measures. A responsible strategy to limit near- and long-term climate risks requires both stringent near-term emission reductions and to develop a large-scale carbon removal capacity.

What if the planet warms more than we expect?

Even if warming goes below 1.5°C after the overshoot, the impacts of climate change will not automatically and uniformly reverse. Overshoot comes with irreversible consequences for people and ecosystems, such as species extinction, and the world we return to will be different from the one we failed to safeguard.

We can’t be certain how much warming a given amount of greenhouse gas emissions will lead to, and overshoot projections are often based on a best estimate. The IPCC, for instance, talks about high overshoot pathways exceeding 1.5°C “by 0.1–0.3°C”.

But those numbers are just the middle of a wide range of possible outcomes. In reality, uncertainty about how some features of the Earth system will respond to warming, such as the carbon cycle, means that peak warming could be substantially higher – by up to 1°C or more. We cannot even rule out continuous warming after reaching net zero carbon emissions. Every fraction of a degree of warming counts – exceeding 1.5°C by as much as an additional 1°C would come with grave repercussions.

A capacity to remove several hundred billion tonnes of CO₂ in this century might be needed to hedge against the risks of high warming outcomes, and to ensure we can bring warming back to 1.5°C once this has been exceeded.

In fact, our results imply we might need close to 10 billion tonnes of CO₂ removal a year after 2050 (about 25% of current annual emissions). This would require a massive effort, but might just be possible with the rapid scaling up of a range of methods.

These include well-known strategies such as restoring forests and wetlands and managing the soil better. But it also includes novel methods such as direct air capture technology, in which carbon would be sucked directly from the sky, or bioenergy and carbon capture and storage, which involves extracting CO₂ from the atmosphere and storing it underground.

Some of these methods may not work out as envisioned due to technological, economic, social or sustainability limitations. But even if they do not work at the scale envisioned, or not at all, we still need to try.

Limiting near- and long-term climate risks

Because we can’t be certain exactly how much the climate will warm, we’ll need to limit the risks as much as possible.

First, we must reduce emissions as fast as possible to slow down Earth’s temperature increase, limit peak warming, and reduce how dependent we ultimately are on removing large amounts of CO₂ to achieve net zero emissions.

The Paris agreement accommodates such temperature reversal. Even if 1.5°C is exceeded, countries are obliged to hold peak temperatures to “well below 2°C” and to aim for long-term temperature decline.

However, every fraction of warming will disproportionately make poor and vulnerable people suffer greater hardship, so delaying stringent emissions cuts is not a resilient strategy. The urgency to reduce emissions now should guide the next round of countries’ targets for cutting emissions that are due early next year.

Second, we should consider hedging against high-risk, high-warming outcomes by building up our capacity to remove carbon and reverse warming. Just as governments hold strategic food and water reserves to weather unexpected disruptions, the world needs to develop the ability to remove large amounts of carbon from the atmosphere. But, given potential limits to how much carbon removal we can scale up in time, we also cannot afford to squander this capacity on any emissions that could be avoided in the first place.

Investing in this kind of removal capability, on top of pursuing the most ambitious emissions cuts possible, is a no-regrets strategy. Should we have certainty that a more fortunate climate outcome will materialise, being able to remove this scale of carbon would enable us to bring temperatures down faster. And if the warmer side of our projections are realised, we will have put ourselves in a position in which we are best equipped to make temperatures decline again.

Achieving temperature decline in the long run would limit longer-term climate impacts. For instance, in our study we showed that temperature decline could shave off about 40cm (and potentially up to 1.5 metres) of global sea level rise in 2300. This could be the difference between having a future or not for whole nations of people. It may also limit risks from triggering tipping points in the Earth system, such as the collapse of the Greenland ice sheet or currents in the Atlantic ocean.

The high-risk outcomes of overshooting 1.5°C means we need to do more, not less, right now – and to focus on bringing temperatures back below 1.5°C in the long run.

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Carl-Friedrich Schleussner received funding from European Union’s Horizon 2020 research and innovation programme under grant agreement No 101003687 (PROVIDE).

Gaurav Ganti received funding from European Union’s Horizon 2020 research and innovation programme.

Joeri Rogelj received funding from European Union’s Horizon 2020 research and innovation programme.