About 20,000 years ago, the spot where Toronto stands was buried under more than a kilometre of ice. Nobody was there with a thermometer, so how do scientists know? Nature keeps its own records. Ice, trees, lake mud and fossils lock in clues about the conditions when they formed. These natural records, called proxies, let us reconstruct climates long before weather stations, and compare today's changes with the past.
Reading past climates from proxies
| Proxy | What scientists measure | What it reveals | How far back |
|---|---|---|---|
| Ice cores | Air bubbles trapped in the ice; the ratio of heavy to light isotopes of oxygen and hydrogen in the ice; dust and volcanic ash layers | Past CO₂ and methane levels (directly, from the bubbles); past temperature (from isotopes); past eruptions and dustiness | About 800,000 years in published Antarctic records; a core finished in 2025 reaches more than 1.2 million years |
| Tree rings | Width and density of each annual growth ring | Wide rings mean good growing years (warm and moist enough); narrow rings mean drought or cold. Counting and matching rings dates each year | Hundreds of years from one tree; over 10,000 years by matching overlapping trees |
| Lake sediment cores | Annual layers (varves); pollen grains; charcoal; remains of tiny organisms | Which plants grew nearby (and so the climate); past fires; how fast sediment built up | Thousands to tens of thousands of years |
| Ocean sediment cores | Shells of tiny plankton and their chemistry | Past ocean temperatures and global ice volume | Millions of years |
| Fossils and preserved organisms | The kinds of plants and animals found in rock, peat or permafrost | Tropical corals or warm-climate plants in rock that is now cold suggest a warmer past climate there (allowing for continents moving) | Hundreds of millions of years |
Natural factors that change climate
| Natural factor | How it changes climate | Time scale |
|---|---|---|
| Plate tectonics | Moving continents change where land sits relative to the poles and rearrange ocean currents | Tens to hundreds of millions of years |
| Mountain uplift and weathering | Newly exposed rock reacts with CO₂ in rain as it weathers, slowly drawing CO₂ out of the air | Millions of years |
| Orbital (Milankovitch) cycles | Changes in the shape of Earth's orbit, the tilt of its axis and the wobble of the axis shift how sunlight is spread across seasons and latitudes; these pace the ice ages | Tens of thousands to about 100,000 years |
| Solar output | The Sun's brightness changes slightly, for example over the 11-year sunspot cycle | Years to centuries |
| Volcanic eruptions | Large eruptions put sulfur particles high in the atmosphere that reflect sunlight and cool Earth briefly; volcanoes also release some CO₂ | Cooling for 1–3 years after a big eruption |
Indicators of present-day change
- Global temperature: long-term warming is now estimated at about 1.3–1.4 °C above the 1850–1900 average. The World Meteorological Organization reported that 2024 was the warmest year on record, about 1.55 °C above that baseline and the first calendar year above 1.5 °C. One year above 1.5 °C is not the same as crossing the Paris Agreement's 1.5 °C goal, which refers to warming averaged over about 20 years.
- Canada: Canada warmed about 2.0 °C from 1948 to 2023, about 65% faster than the global average over that period and almost twice as fast since 1970; northern Canada warmed about 2.6 °C, more than twice the global rate (Canada's Changing Climate Report, 2026).
- Sea level: global sea level has risen about 10 cm since satellite measurements began in 1993, and the yearly rate has more than doubled over that period, from melting glaciers and ice sheets and the expansion of warming seawater.
- Glacial and polar ice: Arctic sea ice at its late-summer minimum has shrunk by roughly 12% per decade since satellite records began in 1979; mountain glaciers such as the Athabasca Glacier in the Rockies are retreating; the Greenland and Antarctic ice sheets are losing ice.
- Wind and weather patterns: storm tracks and the jet stream respond to warming, and heavy rainfall and extreme heat are becoming more frequent in many regions.
- The global carbon budget: careful accounting of how much carbon is emitted and where it goes shows atmospheric CO₂ rising by about 2 to almost 4 ppm per year in recent years (a record jump of about 3.75 ppm at Mauna Loa in 2024), and chemical fingerprints in that CO₂ show the extra carbon comes from fossil fuels.
Testing a cause-and-effect hypothesis with time-trend data
Hypothesis: "Rising CO₂ since the Industrial Revolution is causing global temperatures to rise." You cannot run a controlled experiment on a whole planet, so scientists use time-trend data: records of each variable through time. The table shows rounded decade averages.
| Decade | Average CO₂ (ppm) | Temperature above 1850–1900 average (°C) |
|---|---|---|
| 1960s | about 320 | about 0.2 |
| 1980s | about 345 | about 0.5 |
| 2000s | about 380 | about 0.9 |
| 2010s | about 400 | about 1.1 |
Both variables rise together: they are positively correlated. But a correlation alone does not prove that one causes the other; ice cream sales and drownings both rise in summer because of a third factor, hot weather. To support causation, scientists need a mechanism (CO₂ absorbs infrared, which has been measured in labs since the 1850s), and they must rule out other explanations. The Sun's output has not increased since the 1950s and has dipped slightly since the 1980s, while temperatures climbed. Volcanoes cause brief cooling, not decades of warming. And the pattern of change fits greenhouse warming, not a brighter Sun: the lower atmosphere is warming while the stratosphere above it is cooling, and nights have generally warmed faster than days. Many independent lines of evidence point to the same cause, which is why the IPCC concludes that human influence is the dominant cause of the warming since the mid-20th century.