Heat waves and extremes
A small shift in the average makes extreme heat far more common. Learn what warming does to heat, rain and drought, and how scientists work out its role in a single event.
THE TAIL GROWS FASTEST
Shift the average, multiply the extremes
Picture all the days of summer as a bell-shaped curve: most near the middle, a few very hot ones at the edge. Warming slides the whole curve to the right. The middle barely looks different, but the hot edge grows a lot and the cold edge shrinks. It's lesson 1's loaded die: small change in the dice, big change in how often you roll a 6.
The IPCC estimates that a heat extreme that came about once a decade before industrialisation now comes about 3 times a decade. At 1.5 °C of warming it's about 4 times, and at 2 °C nearly 6. Cold extremes have become rarer and milder.
Check yourself
Hossein says: "The world has only warmed about 1 °C. One degree can't make heat waves much worse." What is he missing?
- Nothing: one degree is far too small a change for anyone to feel or notice
- A small shift in the average makes rare hot extremes several times as common
- Heat waves only look worse because thermometers and weather reporting have improved
- The 1 °C figure is for winter only; summers have warmed about 10 °C
Show the answer
A small shift in the average makes rare hot extremes several times as common
Right. Extremes live at the edge of the curve, and that's where a shift bites hardest. A once-a-decade heat extreme now comes about three times a decade.
HUMID HEAT
Why sticky heat is so dangerous
Your body cools itself mainly by sweat evaporating from your skin. In dry air that works well. In very humid air, sweat can't evaporate fast enough, so your body struggles to shed heat even at a lower temperature. That's why weather services track measures that combine heat and humidity, not temperature alone.
The Persian Gulf coast is among the places on Earth where humid heat already comes closest to what the human body can cope with. A humid 36 °C in Bandar Abbas can be harder on the body than a dry 44 °C in Yazd.
Check yourself
Ali's grandmother lives in Bandar Abbas. On a humid 36 °C day she is safer than his cousin in Yazd on a dry 44 °C day, because the temperature is lower.
Show the answer
False
False. In humid air sweat evaporates poorly, so the body can't shed heat well even at a lower temperature. For an older person in Bandar Abbas, a humid 36 °C can be just as risky or more. Judge heat by temperature and humidity, and by who's exposed.
WATER, BOTH WAYS
Heavier downpours and harsher droughts
Warmer air can hold about 7% more water vapour for each 1 °C. So when it rains hard, there's more water to fall, and heavy downpours have become more intense over most land regions with good records. The same warmth also pulls moisture out of soils and plants faster, so dry spells bite harder. More extreme wet and dry is not a contradiction: it's the same heat speeding up the water cycle.
A flash flood after a cloudburst and a farm drying out the same year can both be part of one warmer climate. Lesson 9 looks at water and drought in detail.
How sure are scientists?
Strong evidence of a change
Hot extremes more frequent and intense; cold extremes fewer. Heavy rainfall more intense over most well-measured land. Droughts that hurt farms more common in some regions.
Weak or unclear so far
Trends in tornadoes and hail. And rising disaster costs are partly about more people and property in harm's way, not only about the weather.
Check yourself
Is warming the main story here?
- Heat waves getting hotter and more frequent
- More homes built on a floodplain, so floods cost more
- Heavier bursts of rain in a warmer atmosphere
- A change in how often tornadoes strike
- Fewer frosty nights each winter
- A factory flooding because its drains were blocked
Show the answer
Clearly linked to warming: Heat waves getting hotter and more frequent, Heavier bursts of rain in a warmer atmosphere, Fewer frosty nights each winter
Mostly other causes, or unclear: More homes built on a floodplain, so floods cost more, A change in how often tornadoes strike, A factory flooding because its drains were blocked
ATTRIBUTION SCIENCE
Did climate change play a part in this event?
Scientists can't rerun history without our emissions, but they can compare two worlds. Using observations and many runs of climate models, they estimate how likely an event, say a heat wave, is in today's climate and in a simulated world without human warming. The answer comes out as odds or intensity: "made about 10 times more likely" or "about 2 °C hotter than it would have been".
In June 2021 a heat wave in western Canada and the northwest United States set Canada's all-time record, 49.6 °C. A rapid study by the World Weather Attribution group found it would have been virtually impossible without human-caused warming. An earlier landmark study found that human influence had at least doubled the risk of Europe's deadly 2003 heat wave.
Check yourself
A study finds human-caused warming made last summer's heat wave about 10 times more likely. Laleh has to write the headline. Which one is accurate?
- "Climate change caused last summer's heat wave"
- "Heat wave was ten times hotter because of climate change"
- "Climate change made the heat wave about 10 times likelier"
- "Scientists still unsure whether climate played any role at all in the heat wave"
Show the answer
"Climate change made the heat wave about 10 times likelier"
Yes. It keeps the study's real claim: a heat wave like this would have been about ten times rarer without human warming. Not a verdict, and not a shrug.
Reading a climate headline
- Ask what kind of event it is: heat claims rest on the strongest evidence; tornado claims on the weakest.
- Look for odds or intensity ("X times more likely", "Y °C hotter"), not a yes-or-no cause.
- Ask what else made it worse: building on floodplains, blocked drains, poor warnings.
- A single event neither proves nor disproves climate change; the pattern over many events does.
Check yourself
Match each attribution finding to what it means in plain words
Show the answer
- "Made about 10 times more likely" → Ten times rarer in a world without human warming
- "About 2 °C hotter than it would have been" → The same kind of event, but made hotter by warming
- "Virtually impossible without human-caused warming" → It practically couldn't have happened in the old climate
- "No clear climate signal found" → Other factors dominated, or the data can't tell yet
Lesson recap
- A small shift in the average makes hot extremes several times more common; cold extremes become rarer.
- Humid heat is especially dangerous because sweat evaporates poorly; check on the most vulnerable.
- Warmer air holds about 7% more water per °C: heavier downpours, and faster drying in dry spells.
- Attribution science compares today's world with one without human warming and reports changed odds or intensity.
- Confidence is highest for heat and lowest for tornadoes and hail; exposure and preparation shape the damage.