Here are the summer days of one town, sorted by how hot they got. Most sit in the middle. A few are cool; a few are very hot. Only about 3 days in 100 go above 35°C. The numbers are made up, but the shape is how every place's weather looks.
Below 20°C5 days in 100
20 to 25°C22 days in 100
25 to 30°C35 days in 100
30 to 35°C28 days in 100
Above 35°C10 days in 100
Now warm every day by about one and a half degrees. Nothing else changes. The middle barely looks different. But look at the ends. Cool days halve. Days above 35 go from 3 to 10 in every 100, more than three times as many. The average moved a little. The edge moved a lot.
Above 35°C, before3 days in 100
Above 35°C, after10 days in 100
The average summer day got 1.5 degrees warmer. What happened to days above 35°C?
Each got 1.5 degrees hotter, and that is all.
Not yet. Each day did. But a line at 35 has two sides, and lots of days that sat just under it were pushed over. What changed most is how many days are past the line.
They became about three times as common.
Right. Many days that used to reach 33 or 34 now cross 35. The line did not move; the days did. So the count on the far side of the line jumped.
Nothing. 1.5 degrees is too small to notice.
Not yet. In the middle of the pile, 1.5 degrees is hard to notice. At the edge, few days sat there to begin with, so a small push changes the count a lot.
A once-in-ten-years hot day, how often it comes now2.8× as often
A once-in-fifty-years hot day, how often it comes now4.8× as often
The world's climate science panel, 2021, at about 1°C of warming.
Scientists checked this on the real record, not a made-up town. A hot day that used to come once in ten years now comes nearly three times as often. One that came once in fifty years now comes nearly five times as often. The rarer the day was, the more its count has grown.
Move the control to set how much warmer the world's average is. The numbers are the climate science panel's own, from 2021.
Once-in-ten-years hot day, times as often1×
Once-in-fifty-years hot day, times as often1×
0°CThe world of 1850 to 1900. Each kind of hot day comes as often as its name says: once in ten years, once in fifty.
Once-in-ten-years hot day, times as often2.8×
Once-in-fifty-years hot day, times as often4.8×
+1°CAt +1°C the once-in-ten-years day comes 2.8 times as often. The once-in-fifty-years day comes 4.8 times as often.
Once-in-ten-years hot day, times as often4.1×
Once-in-fifty-years hot day, times as often8.6×
+1.5°CAt +1.5°C the once-in-ten-years day comes 4.1 times as often. The once-in-fifty-years day comes 8.6 times as often.
Once-in-ten-years hot day, times as often5.6×
Once-in-fifty-years hot day, times as often13.9×
+2°CAt +2°C the once-in-ten-years day comes 5.6 times as often. The once-in-fifty-years day comes 13.9 times as often.
Once-in-ten-years hot day, times as often9.4×
Once-in-fifty-years hot day, times as often39.2×
+4°CAt +4°C the once-in-ten-years day comes 9.4 times as often. The once-in-fifty-years day comes 39.2 times as often.
Going from 2 to 4 degrees doubles the warming. What does it do to the once-in-fifty-years day?
Nearly triples how often it comes: from 14 times to 39.
Right. The edge grows faster than the middle. Each extra degree pushes more days across the line than the degree before, because more days are sitting just under it.
Adds two more days a year.
Not yet. These numbers are not counts of days. They say how many times more often a rare day comes than it did before. A once-in-fifty-years day coming 39 times as often is most years.
Doubles it, like the warming.
Not yet. The average doubled; the edge did more. At 2 degrees the rare day came 14 times as often, at 4 degrees 39 times. That is nearly triple.
Move the control.
Above 35°C, before3 days in 100
Above 35°C, after10 days in 100
In the made-up town, days above 35°C went from 3 in 100 to 10 in 100. A summer has 90 days. About how many days above 35 does the warmer summer have?
days
Ten in every hundred, and there are ninety.
Right. About nine, up from under three. The average is a number. The nine days are what a person, a crop or a railway line actually meets.
Frost nights a year, before30 nights
Frost nights a year, 1.5°C warmer18 nights
Made-up numbers; the direction is what matters.
The same shift works at the cold end, the other way. Frost nights get rarer. That sounds like good news, and in some places it is. But some things need the cold: fruit trees that must chill in winter, pests that a hard frost used to kill, snow that fills the rivers in spring.
In 2003 a heatwave sat over Europe for two weeks in August. About how many more people died that summer than in a normal summer?
Pick one. Nothing is scored.
About 70,000.Most guesses run far lower. Heat kills quietly: older people in top-floor flats, people already ill. It shows up only as extra deaths in the statistics, never as a scene on the news. That August ran several degrees above a normal one in the worst weeks.
Which of these is a claim about the average, and which is about the edge?
One card at a time. Tap the pile it belongs to.
Card 1 of 6
The planet is about 1.2°C warmer than in the late 1800s.
London reached 40°C for the first time on record in 2022.
Spain's 2023 was its hottest year on record.
Paris had five nights above 25°C this July.
The sea surface is about 0.9°C warmer than a century ago.
A wheat field lost its crop to three days over 38°C.
The planet is about 1.2°C warmer than in the late 1800s. → The average
Not yet. This is the average. It is every place and every day rolled into one number, which is how we measure the planet.
London reached 40°C for the first time on record in 2022. → The edge
Not yet. This is the edge. One city on one day crossed a line it had never crossed, which is where things break.
Spain's 2023 was its hottest year on record. → The average
Not yet. This is the average. A whole year for a whole country is many days added up, not one day at the edge.
Paris had five nights above 25°C this July. → The edge
Not yet. This is the edge. It counts the nights that crossed a line, and a hot night is where the harm happens.
The sea surface is about 0.9°C warmer than a century ago. → The average
Not yet. This is the average. The whole sea over a hundred years is the slow number, not a single hot day.
A wheat field lost its crop to three days over 38°C. → The edge
Not yet. This is the edge. Three days over a line killed the crop. The year's average would not have.
The average moves a little→Many days cross a line→The rare day comes often
Right. The average is how we measure the planet. The edge is where things break. A small move in the first makes the second several times more common, which is why the two kinds of headline both keep coming.
Lesson complete
Shift every day a little warmer, and the hottest days become several times more common.