Global warming is usually measured from a familiar starting point: 1850. That year anchors most modern climate datasets and underpins the idea that the world is now approaching 1.5°C of warming. But a newly released temperature dataset, published by Earth System Science Data, suggests that this reference point may already be skipping part of the story. Compiled by a group of UK-based scientists, the dataset - known as GloSAT - pushes global temperature estimates back to 1781. By extending the timeline nearly 70 years earlier than most existing records, it raises a fundamental question about how much the planet has already warmed - and how much of that warming happened before our official measurements even began.

The choice of 1850 has never meant that the Earth was untouched by humans before then. Industrialization was already underway, forests had been cleared for centuries, and greenhouse gases in the atmosphere rose by about 2.5% between 1750 and 1850. That increase alone would have caused some warming. GloSAT shows that the late 1700s and early 1800s were notably cooler than the 1850–1900 period that scientists currently define as “preindustrial.” This implies that part of the warming counted since 1850 may have already been in motion. At the same time, the early 19th century was shaped by massive volcanic eruptions that temporarily cooled the planet, complicating the picture and masking some early warming signals.

To untangle this history, researchers turned to some of the oldest temperature measurements humans ever recorded. While no one tracked global averages centuries ago, thousands of local observations exist. Europe hosts several of the world’s longest-running temperature records, including Central England, Uppsala in Sweden, and Hohenpeissenberg in the Bavarian Alps, where monks began recording temperatures in 1781. These records reveal substantial regional warming when modern temperatures are compared to those of the late 18th and early 19th centuries. Together, they form a patchwork that helps scientists estimate broader trends, even though the data becomes sparser the further back in time it goes.

Land measurements alone are not enough, since oceans cover roughly 70% of the planet. GloSAT also incorporates marine air temperature readings taken aboard ships during the 18th and 19th centuries. Trading vessels, including those operated by the British East India Company, routinely measured temperature and air pressure to improve navigation and commercial efficiency. Whaling ships in the Atlantic collected similar data, later compiled by early oceanographers. These records are uneven and require careful correction for biases, such as ships heating up during the day, but they remain invaluable. By combining land and ocean observations, GloSAT offers a more complete - though still uncertain - view of early global temperatures.