Coaching Centre
Reading the climate record
How scientists reconstruct past and present climate from thermometers, tide gauges and satellites and from proxies such as ice cores, tree rings, corals and lake sediments; what the records show; the fingerprints that identify human causes; and how climate models are tested.
At Law Dome, near Australia's Casey station in Antarctica, so much snow falls each year that the ice preserves every single year as a separate layer, back more than 2000 years. Each layer holds a little sea salt blown in from the Southern Ocean, and scientists noticed that the amount of salt rises and falls with patterns in Pacific Ocean temperatures, the same patterns that bring rain to eastern Australia. Reading the salt like a rain gauge, they found that eastern Australia has had "megadroughts" much longer than anything in our written records, one lasting around 39 years in the 1100s and 1200s. An ice core from Antarctica turned out to be a weather diary for New South Wales.
Measuring the climate directly
Climate is the average weather of a place over decades, usually 30 years or more. To see whether it is changing, you need long, reliable records:
- Thermometers: enough stations around the world to calculate a global average from about 1850. Australia's official dataset, ACORN-SAT, starts in 1910, when instruments were standardised in Stevenson screens. That's why Australian warming is usually quoted "since 1910".
- Tide gauges: Sydney's Fort Denison gauge has measured sea level since 1886, one of the longest records in the Southern Hemisphere.
- Air samples: carbon dioxide measured at Mauna Loa in Hawaii since 1958 and at Cape Grim in Tasmania since 1976 (Year 9, 3A).
- Satellites and Argo floats: global measurements of sea level, ice, sea-surface temperature and Earth's energy budget since the 1970s–2000s.
The records agree. Global average surface temperature was about 1.1 °C higher in 2011–2020 than in 1850–1900, and the ten warmest years on record are 2015 to 2024. The World Meteorological Organization found 2024 was the warmest year yet, about 1.55 °C above pre-industrial; one year above 1.5 °C doesn't break the Paris goal, which refers to a 20-year average, but it shows how close we are. Australia has warmed by about 1.5 °C since 1910 (State of the Climate 2024), slightly faster than the global average, because land warms faster than ocean.
Proxies: reading the past
To look back before thermometers, scientists use proxies: natural records that respond to climate in a measurable way.
- Ice cores: bubbles in the ice are samples of ancient air, so CO₂ and methane can be measured directly. The ratio of heavy to light water molecules in the ice records the temperature when the snow fell, dust shows how dry and windy the world was, and sulfate spikes mark big volcanic eruptions. The EPICA core from Dome C in Antarctica reaches back about 800 000 years.
- Tree rings: trees grow one ring per year, wider in good growing seasons. Huon pines on Mount Read in western Tasmania have given a temperature record more than 3000 years long.
- Corals: massive corals lay down yearly bands. On the Great Barrier Reef, rivers in flood carry humic acids from the land that make bands glow under UV light, recording floods hundreds of years back, and the coral's chemistry records sea temperature.
- Lake and swamp sediments: pollen shows which plants grew, and charcoal shows how often fires burned, over tens of thousands of years.
No single proxy is perfect. Each is calibrated against thermometer records where they overlap, and scientists trust a result more when different proxies from different places agree.
The curves in this model are simplified, but their shape follows the real EPICA Dome C ice-core record.
- Drill slowly to the bottom. How many ice ages (cold glacial periods) can you count in 800 000 years?
- What were the lowest and highest CO₂ levels in the record? How do they compare with each other?
- Describe how CO₂ and temperature are related in the record.
- Add the air of 2025. How does today's CO₂ compare with the whole record, and how fast has it changed?
Ice ages are paced by slow, regular changes in Earth's orbit and tilt (Milankovitch cycles) that change how sunlight is spread over the seasons. The orbital changes start the warming or cooling; CO₂ released from or absorbed by the oceans, and ice–albedo feedback, then amplify it into a full ice age or interglacial. Today's change is different: CO₂ is leading, not following, and it has risen by about 145 ppm in under 200 years, a change that took thousands of years at the end of the last ice age.
How do we know it's us?
Correlation alone doesn't prove cause, so scientists look for fingerprints: patterns that one cause would produce and others wouldn't.
- The upper atmosphere is cooling while the lower atmosphere warms. More energy from the Sun would warm both.
- Nights are warming faster than days, and winters faster than summers, as expected when less heat escapes.
- The Sun's output has been flat or slightly falling since the 1980s, while temperatures have risen fastest.
- The extra carbon is fossil carbon. Carbon from coal, oil and gas contains no carbon-14 and less carbon-13 than carbon in the air. As we burn it, the air's mix of carbon isotopes is changing in exactly that direction, and oxygen in the air is falling in step.
The IPCC's 2021 report concluded that it is unequivocal that human influence has warmed the atmosphere, ocean and land. Climate models, including Australia's ACCESS model developed by CSIRO, the Bureau and universities, are tested by checking that they reproduce the past: the 20th-century warming, the cooling after Pinatubo (7D) and the ice ages. Models that leave out human greenhouse gases cannot reproduce the warming since 1950.
Test yourself
Air bubbles trapped in Antarctic ice are useful because they
Scientists measure the CO₂ and methane in them directly.
Which observation is evidence that greenhouse gases, not the Sun, are causing the current warming?
A brighter Sun would warm both layers.
Australian warming is usually measured "since 1910" because
ACORN-SAT uses reliable, comparable records from 1910.
The start and end of past ice ages were triggered mainly by
Milankovitch cycles, amplified by CO₂ and ice feedbacks.
Name what each proxy records.
Sort the sources of climate data.
Natural records that respond to climate, such as tree rings, are called . The EPICA ice core reaches back about 800 000 . Carbon from fossil fuels contains no carbon-. A pattern that identifies a particular cause is called a . The IPCC says human influence on warming is .
A friend says, "The climate has always changed naturally, so there's no way to know whether today's warming is caused by people." Use at least three pieces of evidence from this page to evaluate this claim.
It's true that the climate has changed naturally: ice cores show ice ages and warm periods every 100 000 years or so, triggered by changes in Earth's orbit and amplified by CO₂. But natural changes have causes that can be identified, and the evidence points to people this time. First, the upper atmosphere (stratosphere) is cooling while the lower atmosphere warms. A brighter Sun would warm both; trapping heat with greenhouse gases warms the bottom and cools the top. Second, satellites show the Sun's output has been flat or slightly falling since the 1980s, exactly when warming has been fastest. Third, the extra CO₂ in the air has the isotope signature of fossil fuels: it lacks carbon-14 and is low in carbon-13, and oxygen is falling as it would if carbon were being burned. Finally, CO₂ is now about 425 ppm, far above the 180–300 ppm range of the last 800 000 years, and it rose by about 145 ppm in less than 200 years, much faster than any natural change in the record. Together these show the claim is wrong: we can tell that today's warming is caused by human greenhouse gas emissions.