KS3 · Chemistry

Evidence for global warming

For 2,000 years Earth's average temperature barely moved. Since about 1850 it has climbed fast. How do scientists know — and how do they know it's us? You're the judge.

You're the judge

Is Earth warming — and is it us?

The claim

Earth has warmed rapidly since about 1850, and human activity is the cause.

Place each piece of evidence to load the balance. Mark the strong ones — they count double.

  1. Tree rings and ice cores show that Earth's average temperature stayed at about 13 °C for 2,000 years, changing only slightly. Since about 1850 it has risen rapidly, to over 14 °C.

    Evidence 1: does it support or challenge the claim?

  2. Temperature measurements are unevenly spread: lots where many people live and on land, far fewer on the ocean — even though about 70% of Earth's surface is ocean.

    Evidence 2: does it support or challenge the claim?

  3. Tiny amounts of air are trapped in ice-core layers each year. They show that for 800,000 years carbon dioxide went up and down but stayed below 300 parts per million (ppm) — until 1950. Since then it has passed 300 ppm and kept rising fast.

    Evidence 3: does it support or challenge the claim?

  4. Weather satellites can measure the temperature of the whole planet's surface from space — but they are less accurate than measurements taken at the surface.

    Evidence 4: does it support or challenge the claim?

  5. Direct measurements of carbon dioxide in the air began in Hawaii in 1958. The record has risen throughout, with small ups and downs each year around a rising average.

    Evidence 5: does it support or challenge the claim?

  6. Before about 1880 there weren't enough thermometer measurements to work out a global temperature, so earlier temperatures are only estimates.

    Evidence 6: does it support or challenge the claim?

  7. Since about 1850 the carbon dioxide that humans add to the atmosphere has increased significantly — and the amount of carbon dioxide in the atmosphere has risen in step with it. Methane has done the same.

    Evidence 7: does it support or challenge the claim?

  8. Earth's temperature has changed before: between about 1500 and 1850 it was slightly cooler than the long-term average.

    Evidence 8: does it support or challenge the claim?

Predict, then check

A tree adds one new ring every year as it grows. Here's the question that turns a tree into a thermometer.

A tree grows through a warm, wet year. How will that year's ring compare with the ring from a cool, dry year?

Inside an ice core

A frozen diary

An ice core is a long cylinder of ice drilled out of a glacier, an ice sheet or an iceberg. Step down through it layer by layer.

↑ Top of the core — the most recent years

↓ Deep in the core — the oldest ice

Zone 1 of 5

The top layers

Every year a new layer of ice is laid down on top of the last one. So the layers at the top of the core are the most recent — the newest page of the diary.

The deeper you go, the older the ice — so one core holds a year-by-year record of both temperature and the air itself.

The human link

?

Reason it through

How does human activity lead to global warming?

Link 1 of 4

First link · your turn

Think about burning fossil fuels for electricity, heating and transport — plus industry, farming and rubbish rotting in landfill sites. What do these release?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

What if we cut emissions?

Would Earth cool down?

Suppose the world cuts its greenhouse gas emissions — releasing less each year than it does now.

Which is closest to what you think would happen?
How sure are you?

Test the model

The bath with no plug

Sort each feature of the bath: does it match the real atmosphere, or not?

Still to sort

Matches the atmosphere (0)

This part of the model behaves like greenhouse gases really do.

Breaks down (0)

This part of the model is not like the real atmosphere.

6 of 6 still to sort.

Picture greenhouse gases in the atmosphere as a bath with no plug being filled from a tap. The tap is gas being added; water escaping down the plughole is gas leaving.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Is Earth really warming — and are we the cause? Put the evidence on trial and decide for yourself.

What you need to know

  • How Earth's surface temperature is measured — and why satellites trade accuracy for coverage
  • Why global temperature is hard to calculate, and why the result can still be trusted
  • How tree rings and ice cores reveal temperatures (and air) from before the thermometer record
  • What the temperature and carbon dioxide records show, and how they link global warming to human activity
  • What cutting greenhouse gas emissions would — and would not — do

The big picture

Independent records agree. Thermometers since about 1880 — checked by four research centres that get the same answer — and tree rings and ice cores before that all show Earth's temperature steady for about 2,000 years, then rising rapidly since about 1850. Carbon dioxide has risen in step with the carbon dioxide humans add, which is the evidence that human activity is causing global warming.

Key points

1Earth's surface temperature is measured automatically in thousands of places — by weather stations on land and weather buoys at sea, usually every hour. Satellites measure the whole planet from space but are less accurate.
2Global surface temperature is the average of all the places on Earth's surface. The measurements are unevenly spread, but four research centres using different models on the same data get very similar results — so the data can be trusted.
3The thermometer record goes back to about 1880. Before that, temperatures are estimated from tree rings (wider in warm, wet years) and ice cores (a layer each year; deeper is older) — and the two agree.
4For about 2,000 years Earth's average temperature stayed near 13 °C. Since about 1850 it has risen rapidly to over 14 °C. This rapid, significant rise is global warming.
5Carbon dioxide stayed below 300 ppm for 800,000 years until 1950, and has been rising ever since. It has risen in step with the carbon dioxide released by human activities such as burning fossil fuels.
6Cutting greenhouse gas emissions makes levels rise more slowly and slows warming. Only cutting enough makes levels fall — it does not cool Earth straight away.

Worked example

Problem

Your friend says: "Earth's temperature has always gone up and down. The warming since 1850 is just another natural wobble." How would you answer them, using the evidence?

⚠ Watch out

Thinking that cutting emissions will make Earth cool down straight away. Cutting emissions only makes greenhouse gas levels rise more slowly. Levels fall only once more gas is leaving the atmosphere than going in — and even then, that could reverse warming over time, not overnight.

🧠

Memory hook

An ice core is a frozen diary. Every year writes a new page on top: the atoms record that year's temperature, and a little trapped air keeps a sample of that year's atmosphere. Read it from the top down and you travel back in time.

✓

Check yourself

Without looking back: which two records reveal Earth's temperature before about 1880 — and why can the thermometer record since then be trusted?

Flashcards

(14)
How is the temperature of Earth's surface measured every day?
Automatically, in thousands of places — weather stations on land and weather buoys in the seas and oceans — usually once an hour, with the readings collected at a central data point.
Weather satellites vs weather-station thermometers: what is the trade-off?
Satellites measure across the whole planet but are less accurate. A thermometer at a weather station measures most accurately, but only where it is.
What is global surface temperature?
The mean (average) temperature of all the places on Earth's surface, calculated from tens of thousands of measurements worldwide and compared with each region's long-term average.
Why is global surface temperature difficult to calculate?
There is an enormous number of measurements, and they are unevenly spread — more on land and where more people live, fewer on the ocean, although about 70% of the surface is ocean. Mathematics and data models are used to overcome this.
Why can the global temperature data be trusted?
Four large research centres (NOAA, NASA, the UK Met Office and the Japanese Meteorological Agency) each calculate it from the same data using different models — and get very similar results.
How far back does the measured global temperature record go?
To about 1880. Before that there weren't enough measurements to calculate a global surface temperature.
How do tree rings help estimate past temperatures?
A new ring forms each year: wider in warm, wet years, thinner in cooler, drier years. Comparing old rings with rings from measured years estimates temperatures for years with no measurements.
What is an ice core, and how does it record the past?
A long cylinder of ice drilled from a glacier, ice sheet or iceberg. It has a layer for each year (darker in winter, lighter in summer), deeper layers are older, and the atoms in each layer indicate that year's average temperature.
What is global warming?
The rapid, significant increase in Earth's average surface temperature since about 1850 — from about 13 °C, where it had stayed for 2,000 years, to over 14 °C.
What does the carbon dioxide record measured since 1958 show?
The first measurements were taken in Hawaii in 1958. Carbon dioxide has increased throughout, with small annual fluctuations around the rising average.
What does the air trapped in ice cores show about carbon dioxide?
Over the last 800,000 years it went up and down but stayed below 300 parts per million until 1950. Since then it has passed 300 ppm and kept rising rapidly — faster and higher than normal.
What links the rise in atmospheric carbon dioxide to humans?
Since about 1850 the carbon dioxide added by human activity has increased significantly, and the amount in the atmosphere has risen in step with it. Methane levels have risen in the same way.
Which human activities release carbon dioxide and methane?
Burning fossil fuels for electricity, heating and transport; industry; farming; and waste decomposing in landfill sites.
Where does the bath-with-no-plug model of greenhouse gases break down?
Greenhouse gases are mixed throughout the air, they are only a tiny part of an atmosphere made mostly of nitrogen and oxygen, and different greenhouse gases leave at different rates.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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