How we know it's warming
About 1.1 °C of warming by the 2010s, measured by thermometers, ships, floats and satellites that all agree. Learn to read the evidence and spot a misleading chart.
THE HEADLINE NUMBER
About 1.1 °C warmer
The IPCC, the UN body that assesses climate research, concluded that the planet's surface was about 1.1 °C warmer in 2011–2020 than in 1850–1900, the period used as a stand-in for pre-industrial times. Land has warmed more than the ocean: roughly 1.6 °C against 0.9 °C. Each of the last four decades has been warmer than any decade before it since 1850.
1.1 °C sounds tiny next to a daily swing of 15 °C. But remember lesson 1: this is a global, year-round average, and the whole difference between today and the last ice age was only about 6 °C of that same average.
Temperature anomaly
How much warmer or colder a place or period is than a chosen baseline average, rather than its actual temperature. Climate charts use anomalies because a mountain station and a coastal one have very different temperatures, but their ups and downs from normal move together, so they can be combined fairly.
"This July was +1.2 °C against the 1991–2020 average" is an anomaly. Always check which baseline a chart uses: the same year looks warmer against 1850–1900 than against 1991–2020.
Check yourself
2024 was the warmest year on record, about 1.5 °C above pre-industrial, helped by an El Niño. Arash says: "So we've already broken the Paris Agreement's 1.5 °C limit." What's the most accurate reply?
- Yes, one year over 1.5 °C means the limit is permanently broken
- No, 2024 was hot only because of El Niño, so it tells us nothing at all about long-term warming
- Not yet: Paris means the ~20-year average, so one hot year is a warning, not a breach
- The Paris limit is measured against the 1991–2020 baseline, so 2024 was nowhere near it
Show the answer
Not yet: Paris means the ~20-year average, so one hot year is a warning, not a breach
Right. Scientists judge the Paris goals on long-term averages, precisely so that one El Niño year doesn't count as a breach. But single years above 1.5 °C show how close the average is getting.
Many instruments, one answer
- Thousands of weather stations on land, some with records going back over 150 years.
- Ships and buoys at sea, and since the 2000s about 4,000 robotic Argo floats that dive and measure the upper ocean.
- Satellites measuring the atmosphere, sea ice and sea level; weather balloons measuring the air above us.
- Separate teams, including NASA, NOAA, the UK Met Office, the Japan Meteorological Agency and Berkeley Earth, process the data independently. Their curves agree closely.
Check yourself
Match each tool to what it tells us
Show the answer
- Argo floats → How much heat the upper ocean holds
- Antarctic ice cores → CO₂ in the air long before thermometers
- Satellites over the Arctic → How much sea ice covers the ocean each summer
- Long-running weather stations → Local air temperature trends since the 1800s
FINGERPRINTS
Could it be the Sun?
Scientists checked. Satellites have measured the Sun's output since the late 1970s, and it hasn't risen; if anything it has dipped slightly, while Earth warmed fastest. And the pattern of warming points at greenhouse gases: the lower atmosphere has warmed while the upper atmosphere, the stratosphere, has cooled. A brighter Sun would warm both. Greenhouse gases trap heat low down, so less reaches the layers above.
It's like telling a heater from a better-insulated room: both make the room warm, but only insulation leaves the attic colder.
Check yourself
Behnaz's uncle says: "Weather stations sit in cities that have grown hotter, so the global warming trend is really just city heat." This argument holds up.
Show the answer
False
False. The urban heat effect is real, but it's known and corrected for. Records from rural stations alone show the same warming, and the ocean, which covers about 70% of the planet and has no cities, has warmed too, as have satellite measurements of the air.
Reading a temperature chart in four checks
- 1 · What's measured?
Global or local? Air or ocean? One city in one month is weather, not the global climate.
- 2 · Against what baseline?
Anomalies depend on the reference period. Compare charts only if they use the same one.
- 3 · How long is the window?
Anything under about 20–30 years can be dominated by noise and by the choice of start year.
- 4 · Who made it?
Is the data from a major dataset, and does the chart match what the others show?
Check yourself
Farzaneh is shown a chart of global temperature from 1998 to 2012 that looks nearly flat. The poster says warming has stopped. 1998 was an exceptionally hot El Niño year. What's the best way to judge the claim?
- Accept it: 14 years is plenty of data
- Check the full record by decade; the flat look comes from starting on a hot spike
- Reject it only if a newer version of the chart shows those same 14 years sloping upward
- Check whether 2012 was colder than 1998 in her own city
Show the answer
Check the full record by decade; the flat look comes from starting on a hot spike
Exactly. Starting on a spike hides the trend. Over the full record each decade is warmer than the last, and the years after 2012 went on to set new records.
What this means for you
- You can state the core fact calmly: about 1.1 °C of global warming from the late 1800s to the 2010s, and more since, measured many independent ways.
- The strongest evidence is the agreement: air, ocean, ice and living things all tell the same story.
- Before trusting any chart, run the four checks: what, which baseline, how long, who.
Check yourself
Is it good evidence of global warming on its own?
- Mountain glaciers shrinking on almost every continent
- One record-hot afternoon in your city
- Ocean heat content rising for decades
- A friend saying summers feel worse than they used to
- Plants flowering earlier in spring across many regions
- One country's unusually hot summer
Show the answer
Good evidence: Mountain glaciers shrinking on almost every continent, Ocean heat content rising for decades, Plants flowering earlier in spring across many regions
Not enough on its own: One record-hot afternoon in your city, A friend saying summers feel worse than they used to, One country's unusually hot summer
Lesson recap
- The planet was about 1.1 °C warmer in 2011–2020 than in 1850–1900; land has warmed more than the ocean.
- Stations, ships, Argo floats, balloons and satellites, processed by independent teams, all agree.
- About 90% of the extra heat has gone into the ocean.
- The Sun hasn't brightened, and the cooling stratosphere is a fingerprint of greenhouse gases.
- Watch for cherry-picked start years and short windows; judge trends over decades.