Thermometers, at least traditionally, don’t remember. Mercury expands in response to heat energy, and the red line rises. The temperature falls, the mercury contracts, the line descends. Yesterday’s measurement has no bearing on today’s.

But even if thermometers can’t measure it, rising temperatures leave a lasting mark. Forests die, ice sheets melt and species go extinct. Now that the world is within touching distance of 1.5°C of warming, we are getting hints of the climate chaos we can expect over the coming century. So how bad will it be, and is the damage reversible?

This isn’t just an abstract question. Last month, the United Nations finally acknowledged that holding warming to 1.5°C above pre-industrial levels would only be possible if Earth overshoots that threshold, then cools back down by 2100. “Overshoot, peak, decline” is the new mantra for decision-makers. Let’s put aside, for the moment, the rather large question of how global cooling on this scale could be delivered. What lasting damage do we risk by overshooting? It is a question scientists are racing to answer.

There is some good news. Important land carbon sinks like the Amazon rainforest seem to be able to bounce back from short-term climate shocks. We may even be able to resuscitate the AMOC, a vital ocean current, after a climate-induced collapse – although it won’t be easy.

When it comes to sea-level rise, we have locked ourselves in for a lot of future pain

But the bad news is that some climate impacts are locked in, even if we clean up our act immediately. As we report, Thwaites glacier in Antarctica looks to be beyond saving, even if ocean-driven melting were halted overnight. In short, when it comes to sea-level rise, we have locked ourselves in for a lot of future pain.

That isn’t a reason for despair. Once we pass 1.5°C of warming, we should double down on cutting emissions, not give up. But we should be aware that, even if we do successfully deliver global cooling, we won’t emerge from the overshoot years unscathed. What we do today matters for tomorrow.

After months of vacillation, the UK government has finally announced that drilling will go ahead at the Jackdaw gas field in the North Sea. Prime Minister Andy Burnham has described the move as “a pragmatic approach”, saying “we’ll still require oil and gas as part of our energy needs for decades to come”.

The thing is, the climate of our rapidly heating planet doesn’t recognise pragmatism. The only thing it acknowledges and responds to is how much carbon dioxide is added to the atmosphere, and it doesn’t care where it comes from.

reveals that the rate of global heating doubled over the period 2011 to 2025, compared with 1971 to 2025, so that the planet’s temperature is now rising at a rate of 0.4°C a decade. The corollary of this is that the 1.5°C limit is set to be permanently breached as soon as 2028 – a full decade sooner than previous projections. Furthermore, the speed of heating means that even the projected rise of 2.8°C by 2100 is now unrealistic, and points towards a much hotter world arriving far more quickly.

Nonetheless, a report published today by the UN Environmental Programme claims that, while the 1.5°C threshold is now dead in the water, it may still be possible to limit the global average temperature rise to just a few tenths of a degree above this.

, to have a “good” chance of staying this side of 1.7°C to 1.8°C, greenhouse gas emissions would need to fall 18 per cent by the end of this decade, 45 per cent by 2035 and a whisker under 70 per cent by 2040. That “good” is doing some seriously heavy lifting, but what’s more, the idea that emissions, in the real world, will drop by more than two-thirds in less than 15 years is pure fantasy. Especially at a time when fossil fuel corporations are planning for expansion, aided and abetted by the governments of the UK, the US and others.

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Instead, as pointed out by director of the Potsdam Institute for Climate Impact Research in Germany, the implementation of current national commitments would mean global emissions reaching close to 50 billion tonnes of carbon dioxide equivalent (a measure of the global warming potential of all greenhouse gases) by 2035. And that’s assuming that such commitments are adhered to, which is by no means certain as governments, including that of the UK, row back on green measures.

Today’s decision to go ahead with Jackdaw, and to allow drilling in adjacent areas, is entirely the wrong one. A separate decision about the far more carbon-intensive Rosebank oil field is expected by the end of the year. Going ahead with either project guarantees that the UK is now complicit in pushing up global greenhouse gas emissions, and reducing the chances of limiting the global average temperature rise to something more manageable.

At the same time, the decision marks a conscious commitment to retain fossil fuel infrastructure and lock in hydrocarbons to the country’s energy mix for decades to come. Not only will this endanger national carbon reduction goals, but it will hand other nations an excuse to green-light their own new fossil fuel projects.

Every tonne of CO2 we don’t emit makes our future that little bit less grim, and every tonne we pump out takes fully 400,000 years to remove from the atmosphere completely.

There is nothing pragmatic, then, about opening the North Sea to more drilling. The decision will do little to create jobs and make energy cheaper, but the resulting higher temperatures will ensure that the country is rewarded in the form of more drought, floods and wildfires.

Bill McGuire is professor emeritus of geophysical and climate hazards at University College London.

The world’s thickest glaciers may “calve” icebergs up to three times faster than previously predicted. However, such rapid retreat may not be sustained over long periods.

Instead, the large bodies of ice – which include Antarctica’s Thwaites glacier – may collapse in bursts, with intervening periods of stability.

The behaviour of tall ice cliffs is one of the biggest sources of uncertainty in predicting future sea level rise. Several of the world’s largest glaciers in Greenland and Antarctica extend out to sea, with their bases resting on the sea floor. Their coastal edges can form ice cliffs looming as high as 100 metres above the ocean’s surface.

For years, researchers have feared that such marine-based glaciers might be vulnerable to runaway cliff collapse. The idea here is that each time an ice cliff collapses, it exposes a new, taller – and so potentially even less stable – ice cliff further towards the glacier’s interior. The loss of ice due to runaway collapse of these ever-taller cliffs could , according to an Intergovernmental Panel on Climate Change report. But we don’t yet have enough empirical data to know how likely it is that runaway cliff collapse will occur, so previous studies have relied on ideal mathematical models to envision the process instead.

at the Massachusetts Institute of Technology-Woods Hole Oceanographic Institution Joint Program and her colleagues used satellite data to provide the first direct, large-scale observations of how the tallest ice cliffs actually behave, offering a firmer basis for estimating the danger of runaway collapse – and the rapid sea level rise that could result.

According to the study, the risk of runaway collapse depends on two main factors: how steeply the sea floor beneath the glacial ice deepens, and how quickly the ice flows seawards. Imagine an up-ended jar of thick honey spilling onto a plate: it won’t remain in a tall, stable shape, but will thin outwards at the edges. In a similar way, if the viscous flow of ice is fast enough, it will thin out the edge of a glacier, reducing cliff height. Even if the glacier is thicker further towards its interior, sufficiently rapid ice flow appears able to compensate, lowering those ice cliffs and preventing a runaway effect.

This idea has been suggested by idealised mathematical models, but had never been systematically measured in real glaciers. “None of the glaciers we observed underwent unstable retreat, even though they did retreat much faster than our current models predict,” says Needell.

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That retreat could speed up even further in the future. Needell notes that most glaciers in the study are “buttressed” by sea-floor features. For instance, a series of underwater ridges currently help stabilise Thwaites from beneath.

“Over the next decades, it’s going to jump from one pinning ridge to another,” almost like “tripping” downhill, says at the University of St Andrews in the UK. Eventually, Thwaites will reach a point where those ridges disappear. “The bed then begins to get real deep,” says Benn. At that point, which the glacier may reach within the current century, “the whole structural integrity of the ice becomes unviable”.

The ‘doomsday’ glacier’s giant ice shelf is about to break away

The floating ice shelf of world’s widest glacier – Thwaites glacier in Antarctica – is detaching, with worrying implications for global sea-level rise

That is when runaway collapse becomes a real possibility. However, those conditions may also speed up the flow of ice, thus thinning out any looming cliffs. The key question, says Needell, is whether thinning can outcompete rapid ice loss, restabilising the glacier. Her study suggests that it might – but the question remains open.

at the British Antarctic Survey points out that we haven’t yet seen this kind of runaway cliff collapse. One study suggests that it may have happened in the vicinity of Thwaites around 12,000 years ago. “The problem is, nobody was around to take observations at the time,” he says. “It’s a possibility we have to keep our eye on.”