The Warmest Year, Until the Next One: What a Decade of Climate Records Is Telling Us

This week a heatwave settled over Europe, and somewhere in the reporting — between the maps glowing red and the advice to check on elderly neighbours — was a number that should stop you. France's public-health agency, Santé publique France, counted around a thousand excess deaths during the worst of it, most of them people aged sixty-five and over. Not a thousand who died of heat the way one might die of a falling tree — dramatically, singly — but a thousand who died sooner than they otherwise would have, because the air around them was hotter than a human body is built to bear.

Let me be clear at the outset about what this piece is and isn't. It is not about French weather. A single hot fortnight, however brutal, is weather, and weather has always had its extremes. This is about the thing the heatwave is a symptom of: the slow, measured, almost boring accumulation of heat in the planet's systems, recorded not in headlines but in datasets that are remarkably hard to argue with. The heat you feel is the part that makes the news. The trend underneath it is the part that matters — and it is the part I want to lay out, honestly, with the numbers and with the caveats, because an argument that hides its weak points isn't worth making.

A word on the heatwave itself first, because it is the doorway in. When scientists at World Weather Attribution run their now-standard analysis on these European heat events, they keep finding the same thing: this is not natural variability dressed up for the cameras. Their work on recent European heatwaves concludes that fossil-fuel warming has made them dramatically more likely, and several degrees hotter, than they would have been a few decades ago — with the hottest daytime temperatures now rising at roughly three times the rate of global warming, and the warm nights, when bodies fail to recover, at about twice. The heat is not an accident of a bad summer. It is the trend, arriving on schedule.

The thermometer doesn't argue

Start with the simplest instrument we have. 2024 was the warmest year in the entire instrumental record — not by the judgement of one agency but by the agreement of all of them; the World Meteorological Organization put the global average at about 1.55 °C above pre-industrial levels, give or take 0.13. It was also, by the Copernicus reckoning, the first full calendar year to come in more than 1.5 °C above the nineteenth-century baseline — the very line the Paris Agreement was written to stay under. NASA reached the same verdict from its own independent analysis.

A degree and a half sounds trivial — the gap between two thermostat settings you'd never notice in a room. At the scale of a planet it is not trivial; it is the difference between the climate that built our agriculture and coastlines and a visibly more hostile one.

But here is where the honesty has to start, because this is the figure most easily abused. A single year above 1.5 °C is not, by itself, a breach of the Paris goal — that target is measured as a multi-decade average, and the long-term, human-caused warming signal today sits nearer 1.3 °C. 2024 ran especially hot because an El Niño stacked itself on top of the underlying trend; 2025 came in a little cooler under a La Niña, ranking second or third rather than first. So if you want the honest, durable version of the fact, don't reach for any single year's record. Reach for this: the ten most recent years are the ten warmest ever measured. That is not a spike. That is a staircase.

What we've poured into the air

Temperature is the symptom; the cause is a change in the composition of the atmosphere that we can now measure to a fraction of a part per million. Carbon dioxide reached a record annual average of about 423 parts per million in 2024 — roughly 52% above where it had sat for the ten thousand years before the Industrial Revolution, and higher than at any point in at least 800,000 years of ice-core record. It is not only high; it is rising faster: about 2.5 ppm a year over the past decade, roughly three times the rate of the 1960s when continuous measurements began.

CO₂ is the headline, but it keeps company. Methane now stands more than two and a half times its pre-industrial level; nitrous oxide about a quarter above. NOAA's index of the combined heat-trapping power of these gases has risen 54% since 1990 alone. We are not easing off the accelerator. We are pressing it harder.

The ocean keeps the receipts

If the atmosphere is where we made the change, the ocean is where most of the consequence is being quietly filed away. The sea has absorbed something like ninety per cent of all the excess heat the system has trapped — which is the only reason the air has warmed as "little" as it has, and a debt the ocean does not forgive. Ocean heat content has set a new record eight years running. Warm water expands, and melted ice runs into it, so the sea is rising: about 21 to 24 centimetres since 1880, with the rate of that rise roughly doubling over the satellite era — from around 2 millimetres a year in the early 1990s to over 4 today.

And the warmth is killing the most delicate thing the ocean holds. The fourth global coral bleaching event, running since 2023, is the most extensive ever recorded: heat stress severe enough to bleach has struck something like 84% of the world's reef area, across more than eighty countries and territories. Bleaching is not always death. But it is the reef gasping.

The ice keeps its own ledger

Go to the cold ends of the Earth and the same story is written in mass. The Greenland and Antarctic ice sheets are now losing ice roughly four times as fast as in the early 1990s, Greenland alone shedding on the order of 169 billion tonnes a year. Arctic sea ice at its September low is shrinking about 12% per decade; every one of the lowest minimums ever measured has come in the last twenty years. The world's mountain glaciers have now lost mass every single year for over three decades, and the last few years have been the worst on record. Ice is the planet's slowest, most patient accountant, and its ledger does not flatter us.

Loaded dice

None of this stays politely in the data. It comes back as weather, which is where we started. The IPCC — the most cautious, most consensus-bound body in the field, which states nothing it cannot calibrate — finds it virtually certain that hot extremes have become more frequent and intense across most land regions since the 1950s, with high confidence that human-caused warming is the main driver. In their language "virtually certain" means a 99-to-100% probability. They do not write that lightly.

Warming doesn't cause any one heatwave the way a match causes a fire. It loads the dice. Every additional half-degree, the same report finds, produces a clearly discernible increase in the intensity and frequency of extreme heat. So when France counts its dead this June, the honest way to read it is not "climate killed these thousand people," but something more unsettling: the warming has reweighted the odds so that summers like this one — once rare — now come around again and again, and each time the most vulnerable pay first.

The toll shows up in the aggregates. Over the half-century to 2021 the WMO counts a roughly five-fold rise in weather and climate disasters — though, to its great credit, far fewer deaths than before, because early-warning systems have improved dramatically. Heat is the quiet exception to that good news. The Lancet Countdown estimates the rate of heat-related mortality has risen about 23% since the 1990s, with heat now linked to an estimated half a million deaths a year. One European summer alone — 2022 — was later estimated to have caused around 61,000 heat deaths across the continent. This month's thousand in France is not an aberration. It is a single line item.

The bill, and who pays it

Numbers at this altitude go numb quickly, so it is worth saying plainly who the bill lands on. The IPCC reckons that between 3.3 and 3.6 billion people — roughly half of humanity — live in contexts highly vulnerable to climate change. In 2023, weather-related hazards drove around 26 million new internal displacements — people forced from their homes inside their own countries, the overwhelming majority by floods and storms. The World Health Organization's deliberately conservative projection is about 250,000 additional deaths a year between 2030 and 2050, from heat, undernutrition and disease — and it calls that figure conservative because it counts only a handful of the ways a hotter world kills.

These are estimates, modelled, with wide error bars, and I'll come back to that honesty in a moment. But the direction of the harm is not in doubt, and neither is its distribution. It falls heaviest on the old, the poor, and the countries that did least to cause it.

The lines we can't uncross

There is one more category of risk that deserves real care, because it is the most frightening and the most easily overstated: tipping points. These are thresholds beyond which a part of the climate system flips into a new state and won't flip back on any timescale that matters to us. A major 2022 assessment in Science concluded that around five of them may already be possible at today's level of warming — among them the die-off of warm-water coral reefs, the long-term loss of the Greenland and West Antarctic ice sheets, and a disruption of currents in the North Atlantic. A large 2023 synthesis of more than two hundred researchers drew broadly the same map.

Here the caveats matter more than anywhere else. "Possible" is not "happening," and crossing a threshold is not the collapse completing — committed ice-sheet loss would play out over many centuries to millennia, not years. The Atlantic overturning circulation — the system that, weakened far enough, could upend European weather — is genuinely contested: the IPCC judges a full collapse this century unlikely, but no longer something it can rule out with high confidence, and that downgrade from its previous assessment is itself the news. I am not telling you the current will collapse. I am telling you the scientists who used to be fairly sure it wouldn't are now only medium-sure. That is the kind of sentence that ought to make a person pay attention.

The trend we're still bending the wrong way

So where does the line actually point? Despite everything — the pledges, the summits, the genuinely astonishing collapse in the price of solar and wind — global emissions are still going up, to another record in 2025. On the policies countries are actually implementing, the world is now on track for roughly 2.6 to 2.8 °C of warming by 2100 — and yes, a range is the honest way to state it, because the sources differ.

There is real news inside that number, and not all of it is bad. A decade ago the same trackers pointed closer to 3.5 or 4 °C; even last year the central estimate was a few tenths higher than it is now. The trajectory is bending. It is simply not bending fast enough, or soon enough. To hold the 1.5 °C line, emissions would need to fall something like 43% by 2030; instead they are still rising, and the remaining carbon budget for that target is down to a handful of years at current rates.

That is the uncomfortable shape of the thing. We are doing more than we were, and it is still not enough — which is the most dangerous kind of problem, because it is the kind you can feel virtuous about while losing.

What honesty requires

I promised caveats, and a piece like this is worthless without them, so here they are gathered in one place.

The single-year temperature records differ between agencies — 1.55, 1.60, 1.47 °C for 2024 — because they use different baselines and methods; the right move is always to name your source, keep the uncertainty band, and never mistake one hot year for a permanent step. The mortality figures — half a million heat deaths a year, sixty-one thousand in Europe in 2022 — are modelled estimates with wide confidence intervals, not bodies counted one by one; they are the best we have, but they remain estimates. The rise in disaster losses partly reflects that there is simply more built stuff standing in harm's way now, not warming alone. The tipping points are risks, not schedules. The warming projections are ranges, revised every year.

None of these caveats reverses the picture — which is exactly why it is worth stating them. Strip out every contested number and keep only what is not seriously disputed — that greenhouse gases are at their highest in hundreds of millennia and still climbing; that the last ten years are the ten warmest; that the ocean and the ice are changing in step; that human-caused warming has measurably loaded the dice toward extreme heat — and you are left with a picture that does not need the scary numbers to be alarming. The cautious version is enough.

What the data asks

The honest position on climate is neither of the two we're usually handed. It is not the denial that waves the whole thing away — the data simply does not permit that. But it is not doom either. Doom is only denial with the sign flipped: an excuse to do nothing, dressed as despair. The trajectory has bent before, and it can bend again; it is built, a little at a time, out of what gets decided.

What the data asks of us is not panic. It is seriousness — the recognition that the thousand who died in France this month are not a French story, or a freak story, but the leading edge of a trend we can read clearly in instruments that have no opinion. They are the part of the bill that comes due first. The rest of it is being written now, a little each year, in numbers quiet enough to ignore and clear enough not to. The least we can do, having measured the thing this precisely, is to stop pretending we don't know what it says.