El Niño: reading 76 years of ocean data
Students use NOAA’s real El Niño record to describe trends, compare events, calculate change and judge a scientific forecast, then explain why a strong El Niño could make 2027 the warmest year on record.
Learning objectives
- Describe what El Niño is and how scientists measure it.
- Interpret a long-term bar chart and a comparison line graph, using figures from the data.
- Calculate and compare rates of change, and evaluate a probability-based forecast.
Key words
Starter 5 minutes
In October 2026, NOAA said there was an 83% chance that this El Niño would become the strongest since records began in 1950. In pairs: what does an 83% chance mean? Is the record certain? What would a 17% chance be the chance of?
Task 1: The long-term record 15 minutes
76 years of El Niño and La Niña
NOAA’s Relative Oceanic Niño Index, three-month seasons, 1950 to July–September 2026. Red bars are El Niño, blue bars are La Niña. The dashed line is NOAA’s +2.5°C record threshold.
- Which event reached the highest value on the chart, and what was that value? [2]The 1982–83 El Niño, which peaked at +2.40°C in December–February 1983 (1 mark for the event, 1 for the value).
- Has any event since 1950 reached NOAA’s +2.5°C record threshold? Use the chart to explain your answer. [2]No. Every bar stays below the dashed line; the closest is 1982–83 at +2.40°C, 0.1°C short.
- Describe the pattern of El Niño and La Niña over time. Use at least two pieces of data. [4]They alternate irregularly every few years rather than on a fixed cycle. The strongest El Niño events (1982–83, 1997–98, 2015–16) are roughly 15–18 years apart; La Niña events reach about −1.5 to −2°C; 2026 has reached +1.69°C by July–September. Credit any accurate use of dates and values.
Task 2: Comparing the giants 15 minutes
2026 compared with the strongest events
The index season by season, from May–July (MJJ) of the first year to April–June (AMJ) of the next.
- Rank the four events from strongest to weakest at the July–September (JAS) stage. [2]1997 (+1.84), 2026 (+1.69), 2015 (+1.54), 1982 (+1.14).
- Calculate how much each event strengthened between MJJ and JAS. Which strengthened fastest? [3]1982: +0.30°C; 1997: +0.64°C; 2015: +0.52°C; 2026: +0.72°C. 2026 strengthened fastest (2 marks for correct calculations, 1 for identifying 2026).
- 1982–83 started weakly but ended as the record. What does this suggest about using early readings to predict an event’s final strength? [2]Early readings are an imperfect guide: 1982 was the weakest of the four at JAS (+1.14°C) but went on to peak highest (+2.40°C), so forecasts use probabilities rather than certainties.
Task 3: Why it matters 15 minutes
New temperature records tend to follow El Niño winters
Global average surface temperature compared with 1991–2020, each year coloured by the El Niño state of its December–February winter.
- Describe the overall trend in global temperature from 1951 to 2025. [2]A clear rise: from about 0.6–0.8°C below the 1991–2020 average in the 1950s–60s to about 0.6–0.7°C above it in 2023–2025, with year-to-year ups and downs.
- Extended answer. The UK Met Office says 2027 is “very likely” to be the warmest year on record. Using the charts, explain why a strong El Niño in 2026 makes this likely, and why El Niño alone does not explain the long-term trend. [6]See the mark scheme in the teacher notes.
Extension
Open Greenly’s live El Niño tracker. Has the index changed since this pack was written (July–September 2026: +1.69°C)? Did the event break the +2.5°C record? Write a short news update using the new figure.
Teacher notes
Key facts. NOAA’s 8 October 2026 forecast gave a 54% chance of a record for September–November, 83% for October–December and 70% for November–January. NOAA switched to the relative index because the whole tropical ocean has warmed: in July–September 2026 the traditional index read +2.16°C but the relative index +1.69°C.
| Level | Mark scheme for question 8 (6 marks) |
|---|---|
| Level 3 (5–6) | Explains that El Niño releases heat from the ocean into the atmosphere, temporarily raising global temperature; uses chart evidence (e.g. records in 1998, 2016 and 2024 followed El Niño winters; 2026 is the second-strongest event at this stage). Also explains that the long-term rise continues through La Niña years and is caused by greenhouse gases, so each El Niño starts from a warmer baseline. |
| Level 2 (3–4) | Links El Niño to warmer years with some evidence from the charts, and mentions the long-term trend, but one side is less developed. |
| Level 1 (1–2) | Simple statements, e.g. “El Niño makes it hotter”, with little or no data. |
Common misconception. Students often think El Niño causes global warming. It moves heat between the ocean and the air; the long-term trend comes from greenhouse gases. Point out that 2023 and 2025, which followed La Niña winters, were still among the warmest years on record.
Differentiation. Support: give students the data table values pre-filled for question 5. Stretch: download the CSV and plot the 2026 line in a spreadsheet, adding a trend line.