How climate science works: what we measure and how models are tested

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Climate science can seem like a single headline number about warming. Behind it sits a large body of measurement, physics and model testing carried out by thousands of researchers. This guide explains where the evidence comes from and how scientists check that their models are trustworthy.

Weather is not climate

Weather is what happens over hours and days. Climate is the long-term average and range of weather, usually judged over 30 years or more. A cold week does not disprove warming, and one hot summer does not prove it. What matters is the trend across decades.

What is measured, and how

Surface temperature: tens of thousands of weather stations on land, plus measurements from ships and buoys at sea. Several independent teams, including NASA, the US National Oceanic and Atmospheric Administration, and the UK Met Office working with the University of East Anglia, compile global records using different methods. Their results agree closely.
Ocean heat: a global network of robotic floats drifts through the oceans, diving and surfacing to record temperature and salinity. Most of the extra heat trapped by greenhouse gases ends up in the ocean.
Atmospheric carbon dioxide: continuous measurements on Mauna Loa in Hawaii began in the late 1950s, and many other stations now take part. They show a steady rise with a yearly wiggle as northern forests grow and decay.
Satellites: they track sea level, ice sheet mass, sea ice extent, temperature through the atmosphere, and the balance of energy arriving from the Sun and leaving the Earth.
Ice and glaciers: repeated surveys and gravity-measuring satellites show how much ice Greenland, Antarctica and mountain glaciers are gaining or losing.

Looking further back

Instruments only go back a century or two. For earlier periods scientists use proxies, natural records that respond to climate.

Ice cores trap tiny bubbles of ancient air, giving direct samples of past carbon dioxide over hundreds of thousands of years.
Tree rings, corals, lake sediments and cave formations record past temperature and rainfall.

These records show that climate has changed naturally before, and they help scientists understand how sensitive the climate is to changes in greenhouse gases.

How we know the warming is caused by people

The physics of how carbon dioxide absorbs heat has been understood since the nineteenth century. Several lines of evidence point to human activity rather than natural causes:

The extra carbon in the air carries a chemical fingerprint showing it comes largely from fossil fuels.
The lower atmosphere is warming while the upper atmosphere is cooling, which matches greenhouse warming but not a brighter Sun.
Natural factors such as solar cycles and volcanoes have been measured and cannot account for the trend.

What climate models are

A climate model divides the atmosphere, oceans, land and ice into a three-dimensional grid and uses the laws of physics to calculate how energy, air and water move between the boxes over time. Processes too small for the grid, such as individual clouds, are represented by simplified rules. Different centres around the world build their own models independently.

How models are tested

Hindcasting: models are run over the past, given only the known natural and human influences, to see if they reproduce the observed record. They do this well when human influences are included and poorly when they are left out.
Checking against events: after large volcanic eruptions, models have been used to predict the temporary cooling that followed.
Comparing past projections with what happened: studies have looked back at models published decades ago and found that most projected the subsequent warming reasonably well once actual emissions were taken into account.
Model intercomparison: groups worldwide run the same experiments so differences between models can be studied. The spread between them is a useful measure of uncertainty.

What the big assessments say

The Intergovernmental Panel on Climate Change (IPCC) does not run its own research. It assesses published studies, with thousands of reviewer comments, in reports produced in recurring cycles. Its most recent full assessment found that global surface temperature in 2011 to 2020 was around 1.1°C above the 1850 to 1900 average. The World Meteorological Organization later confirmed 2024 as the warmest year on record, at about 1.55°C above that baseline, while noting that a single year above 1.5°C is not the same as the long-term Paris Agreement threshold being passed.

Where to read more

The IPCC's summaries for policymakers, NASA's climate pages and the Met Office's climate explainers are written for general readers and updated regularly. What part of climate science would you like explained in more depth? Ask in the comments.