How do you take the temperature of a whole planet?

You take Earth's temperature with thousands of thermometers, then average them with great care. Weather stations measure the air over land. Ships and buoys measure the sea surface. Since late 1978, satellites have added a view from space. Records with near-global coverage go back to about 1850.

Each method has a catch. Sailors once hauled up seawater in buckets, and a canvas bucket cooled faster than a wooden one. Later ships read the water at the engine intake instead, which runs about 0.6 °C too warm because of the engine room. Cities make their own heat, so a thermometer downtown reads warmer than the countryside around it. Satellites don't measure temperature at all: they measure radiation and convert it. And each satellite ages and drifts in its own way.

Scientists correct for all of this. City heat turns out to be a small effect. Left uncorrected, it is unlikely to account for more than a tenth of the warming measured on land.

The first person to add it all up was not a professional. Guy Stewart Callendar was an English steam engineer who studied climate as a hobby. In 1938, without a computer, he gathered decades of thermometer records and set them beside old carbon dioxide measurements. His conclusion: land temperatures had risen over the previous fifty years. A leading British meteorologist called it a coincidence. Callendar's estimates have since proved remarkably accurate.

Today several independent teams build their own records and reach the same answer. Earth warmed by about 1.1 °C (0.95 to 1.20 °C) between 1850–1900 and 2011–2020, and land warmed faster than the sea. The years 2015 to 2025 were the eleven warmest on record. 2023 alone was 1.48 °C warmer than 1850–1900.