The news has been full of headlines about findings from a United Nations report: The world is set to exceed the 1.5 C global warming target set out in the 2015 Paris Agreement. The report has raised concerns and questions about what a 1.5 C increase could mean for our world.

As an Earth system scientist who has researched many aspects of the carbon cycle and long-term climate change, I was not surprised at the report’s findings. The scientific community saw this grim milestone coming from a long way off.

The vagaries of weather might make it seem that climate is fickle and prone to abrupt turns. Climate extremes often strike without warning, suddenly erupting across our news feeds — or, if we are unlucky, into our own lives — like bolts from the blue.

But global warming is like a freight train. It is massive, slow and has tremendous inertia. That inertia comes from the slow thermal adjustment of the planet’s oceans and atmosphere to the carbon dioxide we emit.

Once released, carbon dioxide is slowly absorbed by the ocean or taken up by plants, and as long as it’s in the atmosphere, the planet inexorably accumulates heat, day after day, year after year. This process does not make sudden movements, but just rolls along the track at a speed that’s set by the global rate of emissions.

So 1.5 degrees was visible from a long way off, and it was clear just how quickly we needed to reduce emissions to avoid crossing it. Instead, the freight train continued to accelerate, the engine stoked with more coal, oil and natural gas than ever. Rolling across the 1.5 C threshold is a sign that we are now entering profoundly dangerous territory.

The worst is still avoidable

Crossing 1.5 C is a call for greater action, not despair.

It’s not a magic number. At Paris, the global community chose 1.5 C as a safe target — because the science showed that most of us would probably be fine if we stopped by this point.

However, even in 2015, it seemed to those I knew in the field that it would have taken a miracle of global co-operation to bring climate change to a halt before the 1.5 C increase was passed. It was a wonderful aspiration for the world, but it was a long shot.

Some recent headlines tell us that climate change is past the point of no return and its worst impacts are now unavoidable. This is wrong.

Climate change has been past the point of no return for many decades, in the sense that we have never had a way to return to the pre-industrial climate of the 19th century.

More importantly, the worst impacts are still very much avoidable. The risk of crossing tipping points increases with every degree of warming, so warming to 2 C would still be much safer than warming to 3 C. The damage we do to life on Earth, including our own life-support system, will continue to intensify the further we go.

If we were to burn all the accessible fossil fuels we know of, the temperature would keep rising — beyond 8 C, 10 C or more. This would be truly catastrophic, and the world would be unrecognizable. We are still far from the worst.

Stop burning fossil fuels

The further the freight train goes, the more important it becomes to put on the brakes as quickly and as strongly as possible. There is no cliff that we know of at 1.5 C; the train track simply continues into a progressively harsher and more dangerous world.

The UN report refers to what we are in now as an “overshoot,” and discusses how we can best limit its duration and peak warming. And as the report says, “every fraction of a degree matters.”

The only sure way to stop climate change is to stop burning fossil fuels. To do this, we need to build a new energy system that supports human well-being while ensuring that we shut down the old fossil-fuel system as quickly as we can.

With the fossil-fuel system retired, deliberately removing fossil carbon from the atmosphere may help to bring the temperature back down by a few tenths of a degree. However, our ability to remove carbon from the atmosphere remains uncertain and unproven.

Natural solutions, like planting trees, could be undone by future changes like increased forest fires, and novel technological solutions remain untested and prohibitively expensive.

Tackling climate change has never been easy, but it is entirely possible, and the essential component remains — as always — that we end the burning of oil, coal and natural gas. Exceeding 1.5 C of global warming is a giant warning flag telling us not to give up, but to redouble our efforts to slam on the brakes.

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