The physics says adding greenhouse gases should warm the planet. Has it? Answering that takes measurements from millions of thermometers, thousands of ocean robots, decades of satellites and many independent teams who check one another's work. The verdict from all of them is the same, and it comes with fingerprints that point to the cause.

The temperature record

Reliable thermometer records go back to about 1850. Several groups build global averages from them independently, including NASA, the US National Oceanic and Atmospheric Administration (NOAA), the UK Met Office with the University of East Anglia, Japan's weather agency and the European Copernicus service. They use different methods and make different choices about gaps and corrections, yet their results agree closely.

One check came from a skeptic. In 2010 the Berkeley physicist Richard Muller, doubting the existing records, set up the Berkeley Earth project, partly funded by a foundation linked to the fossil-fuel billionaire Charles Koch. His team reanalyzed the data from scratch, including the worry that cities make thermometers read warm, the urban heat island effect. They found the warming was real and matched the earlier records. In 2012 Muller wrote that he now considered himself a converted skeptic.

The planet has warmed by about 1.1°C from the late 1800s to the 2011 to 2020 decade, and the pace has picked up since. The ten warmest years in the record have all come within roughly the last decade. In 2024 the yearly average reached roughly 1.5°C above pre-industrial levels for the first time. A single year at that level does not mean the Paris Agreement target has been missed, because that target refers to an average over about 20 years, but it shows how close we are.

The ocean holds most of the heat

The air is only part of the story. More than 90 percent of the extra energy trapped by greenhouse gases has gone into the ocean. Since the early 2000s, a fleet of roughly 4,000 robotic Argo floats has drifted through every ocean, diving to 2,000 meters and surfacing to radio home temperature profiles. Ocean heat content has risen almost every year, and because the ocean is vast and slow, this is one of the clearest measures of an energy imbalance.

Rising seas and shrinking ice

Global average sea level rose about 20 centimeters between 1901 and 2018, and the rate has more than doubled since the 1990s to around 4 millimeters a year. Two causes dominate: water expands as it warms, and melting glaciers and ice sheets add water to the ocean. Satellites that measure Earth's gravity show Greenland and Antarctica together losing hundreds of billions of tonnes of ice a year. Mountain glaciers are retreating on every continent. The area of Arctic sea ice left at the end of summer has fallen by roughly 40 percent since satellite records began in 1979.

Living things are responding too. Spring arrives earlier in many places, and many plants, birds and fish are shifting toward the poles or up mountains as their familiar climates move.

What about the satellites and the "pause"?

Satellites can also estimate the temperature of the lower atmosphere from the microwaves that oxygen gives off. Early satellite analyses in the 1990s seemed to show little warming, and skeptics leaned on them heavily. Researchers then found errors, including a failure to account for the satellites' orbits slowly sinking and drifting. Once corrected, the satellite records also showed clear warming. Similarly, a slower rise in surface temperatures from about 1998 to 2012, sometimes called the "pause," turned out to reflect natural ocean variability and gaps in Arctic coverage. The ocean kept gaining heat throughout, and record warm years soon followed. Short stretches of any noisy record can mislead; long records over the whole system do not.

Fingerprints of the cause

Warming alone does not say what caused it. Different causes leave different patterns, and scientists use them like a detective uses fingerprints.

• A cooling upper atmosphere: if the Sun were brightening, the whole atmosphere would warm. Instead, the lower atmosphere is warming while the stratosphere above it is cooling, just as greenhouse physics predicts.
• Nights warming faster than days: a stronger heat-trapping blanket matters most when the Sun is down.
• Less heat escaping at CO₂'s wavelengths: satellites measuring the infrared leaving Earth see dips in exactly the bands that CO₂ and methane absorb.
• A steady Sun: satellite measurements show the Sun's output has been flat or slightly down since the 1980s, while temperatures have climbed.

Worked example: "it's just the Sun"

A relative argues that the Sun, not CO₂, is behind recent warming. Walk through the evidence.

• Check the trend: the Sun's energy output has been measured from space since 1978 and shows its regular 11-year cycle with no upward trend. Temperatures rose strongly over the same years.
• Check the pattern: a brighter Sun would warm the stratosphere as well. It is cooling.
• Check day and night: a brighter Sun would warm days most. Nights are warming faster.
• Offer the better explanation: rising greenhouse gases predict all three observations at once.

Good reasoning does not stop at "the climate has changed before" or "the Sun is powerful." It asks which explanation fits all of the evidence, and here only one does.