Climate change: the evidence and the numbers
How climate is defined, the greenhouse effect and feedbacks, the Keeling curve of carbon dioxide, global temperature and its baselines, sea level and ice loss, and what the IPCC concludes. With a data investigation of the CO₂ record.
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Climate is the statistics of weather over a long period, usually 30 years. Köppen climate zones are defined by thresholds: in a polar climate no month averages above 10 °C; in a temperate climate the coldest month averages between 0 °C and 18 °C and at least one month averages above 10 °C.[1][3]
The greenhouse effect is the warming that results when the atmosphere traps heat radiating from Earth toward space. The key greenhouse gases are carbon dioxide, methane, nitrous oxide, chlorofluorocarbons and water vapour. Water vapour acts as a feedback: a warmer climate holds more of it, which amplifies the warming.[5]
Carbon dioxide at Mauna Loa was 313 ppm when Charles David Keeling began measuring in March 1958. In May 2026 it peaked at 432.00 ppm, 1.8 ppm higher than in May 2025. It peaks every May and falls during the northern growing season as plants take up CO₂. The IPCC states that the 2019 value (410 ppm) was higher than at any time in at least 2 million years.[8][9]
Global surface temperature in 2011–2020 was 1.1 °C above 1850–1900 (IPCC). NASA reports 2025 as 1.19 °C above its 1951–1980 average and 2024 as the hottest year on record. Different baselines give different numbers, so always check what a temperature anomaly is compared with.[9][7]
Global mean sea level rose 0.20 m (range 0.15–0.25 m) between 1901 and 2018, and the rate in the last two decades is nearly double that of the last century. Greenland lost an average of 279 billion tons of ice per year in 1993–2019 and Antarctica about 148 billion tons. Melting all of Greenland's ice sheet would raise sea level by about 7.4 m, Antarctica's by about 60 m. Sea level rise comes mainly from land-ice meltwater and the thermal expansion of seawater.[9][6][11]
The ocean is changing too: the top 100 m has warmed by 0.33 °C since 1969, and the acidity of surface seawater has increased by about 30% since the Industrial Revolution.[6]
The IPCC concludes that human activities, principally through greenhouse gas emissions, have unequivocally caused global warming; that every increment of warming will intensify multiple hazards; and that limiting human-caused warming requires net zero CO₂ emissions.[9]
Try it: Data investigation: how fast is CO₂ rising?
You need
- Graph paper or a spreadsheet
- Calculator
- The CO₂ values in the lesson text (313 ppm in March 1958; 410 ppm in 2019; 430.2 ppm in May 2025; 432.00 ppm in May 2026)
- Optional: the monthly Mauna Loa data from NOAA or Scripps
Safety
No practical hazards: this is a data investigation.
What to do
- Plot CO₂ (ppm) against year for the four values given.
- Calculate the mean rate of rise from 1958 to 2026 in ppm per year.
- Calculate the rate from 2019 to 2026 and compare it with the 1958–2026 mean and with the 1.8 ppm rise between May 2025 and May 2026. Is the rise speeding up?
- The 1958 value is from March and the 2025 and 2026 values are May peaks. Explain how the seasonal cycle could affect your calculated rates.
- Optional: plot one full year of monthly data and measure the size of the seasonal cycle (highest minus lowest month).
- Write a conclusion with the evidence and the main uncertainties.
What happens, and why?
The rate from 1958 to 2026 is about (432 − 313) ÷ 68 ≈ 1.75 ppm per year, while recent years rise faster, about 3 ppm per year from 2019 to 2026, so the rise has accelerated. Because CO₂ peaks in May and falls in the growing season, comparing a March value with May peaks adds a small seasonal bias; a fair comparison uses the same month or annual means.[8][9]
Play: Where is the evidence?
Is each piece of evidence about the atmosphere, the ocean, or ice and snow?
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Quiz
Other ages: Ages 6–7 · Ages 10–11