科学家们取消了最坏的气候情景——因为行动正在发挥作用

When major new climate change scenarios are released, there’s always strong interest. These scenarios lay out what our future climate will look like, depending on how fast we act to cut emissions.

But what was surprising about the seven new scenarios announced last week was that United States President Donald Trump took an interest.

Why? Because a high-emissions scenario – known as RCP8.5 and its successor SSP5-8.5 – had been removed. Under these worst-case scenarios, nations would make no effort to cut emissions and expand fossil fuel use. By 2100, carbon dioxide levels would almost triple, to 1,135 parts per million and the world would be around 4.5°C hotter than the pre-industrial period.

The climate scientists responsible for laying out the range of possible futures removed the RCP8.5 scenarios for a very good reason. Although often slow and incomplete, our efforts to tackle climate change have made a tangible difference. We have averted the worst climate future once thought possible.

The job is far from done. Emissions are at record highs and global warming is speeding up.

But the removal of this high-emissions scenario isn’t, as Trump and other climate sceptics have claimed, a sign of failed modelling, or that climate change was a hoax. It’s a sign the expansion of solar, wind, electric vehicles and batteries have slowed emissions growth.

How are these scenarios made?

Many climate impacts are becoming evident after about 1.4°C of warming – the level we’re roughly at now.

Because this period of extremely rapid climate change is due to human activities, it means we also have the opportunity to shape the future.

What will this look like? Will the world keep heating up, or will rapid action cut emissions and bring warming to a halt? The answer will make a big difference to the future humanity faces.

Predicting anything is difficult. But a group of scientists has created scenarios representing a range of possible climate futures.

Because the future is not set, scientists lay out a range of possible pathways for our future greenhouse gas emissions. They base them on what’s happened so far and what might happen in politics and technology over coming decades.

Then they select the emissions pathways deemed most plausible and then sample a range of different futures which are more or less optimistic about our fossil fuel use.

Scientific groups around the world then model these scenarios in depth using different climate models to ensure there’s a large amount of data available at global, regional and local levels.

These scenarios aren’t ranked by how likely they are. All are considered to be plausible futures. The huge range of temperature outcomes – approaching 2°C between the most and least optimistic scenarios by 2100 – points to how much of the future is in our hands.

Why the fuss about RCP8.5?

The two previous releases included two closely related scenarios – RCP8.5 and SSP5-8.5 respectively.

Here, “8.5” refers to radiative forcing – the level of extra heat (in watts) trapped per square metre by 2100.

In these worst-case scenarios, the world sharply boosts fossil fuel use. Unsurprisingly, this leads to very high amounts of global warming. Scientists have long argued over whether this was plausible in the first place.

None of the new scenarios are as pessimistic as RCP8.5/SSP5-8.5. The worst possible scenario now envisions high emissions leading to warming of around 3.5°C by 2100. That would still be very, very bad.

Sceptics acting in bad faith

Climate sceptics leapt on the removal of RCP8.5 as a sign the projections were wrong. These attacks were not made in good faith, but to cast doubt on climate science.

A clear eyed assessment is that RCP8.5 was removed because climate action is starting to work.

But while the worst outcome has been averted, we have also missed the window for the best future climate.

The new scenarios have no pathway as optimistic as the lowest emissions scenario from the last round of major climate projections. That scenario – SSP1-1.9 – envisaged strong climate action and rapid cuts to emissions, leading to global warming peaking at around 1.5°C.

Because global emissions haven’t yet begun to fall, the most optimistic new pathway would lead to warming peaking at about 1.9°C.

While we will definitely now pass 1.5°C, the hope is to only temporarily overshoot that level of warming while working to draw carbon dioxide back out of the atmosphere to get back to 1.5°C.

Our current emissions trajectory is somewhere in the middle – below the high emissions path but well above the most optimistic scenario. Based on current policies and countries’ actions, we’re looking at around 2.6°C warming by 2100.

You might wonder why we need to keep redoing these climate scenarios.

One reason: facts change on the ground. Solar keeps rolling out far faster than expected, but fracking has opened up large new fossil fuel deposits. Political shifts make climate action more or less likely.

Another is because our climate models are continually improving. The better the models get, the more accurate and detailed our projections of sea level rise and other climate impacts can be.

Yes, this is progress

Taking RCP8.5 off the table is a sign of progress – we’ve avoided the worst-case scenario. But we have also missed the best case future.

The next five years could play out in many different ways, leading to better or worse future climates. We must understand and prepare for what we’re facing – and double down on our efforts to create the best future possible.

Andrew King receives funding from the Australian Research Council and the National Environmental Science Program.