Was is it really the hottest day ever?

Published 7 August 2026 · india

Every time a new heat record is broken, skepticism follows. How do scientists know a day or a summer was really the hottest on record

Every time a new heat record is broken, skepticism follows. How do scientists know a day or a summer was really the hottest on record or that a heat wave was historic? DW has the answers. Another summer, another wave of headlines about record-breaking heat. From Northeast Asia to Europe to the United States, extreme temperatures are contributing to thousands of deaths, fueling wildfires and straining water, food and energy supplies. But what do the words behind the headlines mean? How do scientists know if temperatures are record-breaking? Why do cities warm up more than rural areas? And does humidity really make everything feel hotter? DW spoke to climatologists to answer some common questions about how heat is measured, and why understanding heat terminology matters for everyone. What does 'record-breaking heat' mean anyway? It depends on what's being compared. Thousands of weather stations around the world track temperatures. A location can set a record if its temperature exceeds the previous high recorded at the station for a specific day, month or season. Scientists also calculate regional and global temperature records by combining data from weather stations, ships and ocean buoys and determining the average across larger areas. How far back that comparison goes depends on the dataset. The oldest continuous record of temperatures in the world is the Central England Temperature Data Series, which started in 1659. The 10 warmest years in modern global record-keeping have all occurred within the past decade Image: Thorsten Wagner/W2Art/picture alliance But the three most complete datasets — maintained by the United States' Goddard Institute for Space Studies and Centers for Environmental Information, plus the United Kingdom's Met Office Hadley Centre — began recording comprehensively in 1880. These are the datasets behind statements such as "the past decade was the warmest on record." When media outlets use the shorthand "hottest day ever," they are talking about recording temperatures.

While Earth has experienced hotter climates and extreme temperatures over its 4.5-billion-year history, today's warming is exceptional in the context of the 12,000 years of human civilization. But how do scientists know about Earth's climate before humans and thermometers were around? To understand Earth's earlier climate, scientists rely on natural records known as "climate archives" that help them recreate long-term patterns in temperature. Those include ice cores, tree rings, ocean sediments and fossils. "These preserve information about past temperatures, rainfall and atmospheric conditions going back thousands to millions of years," Friedrike Otto, a climatologist and founder of research group World Weather Attribution (WWA), told DW in an email. The most complete continuous ice core record, drilled in Antarctica, stretches back more than a million years, for example. It contains ancient air bubbles that allow researchers to measure past atmospheric carbon dioxide levels. But other "climate archives" stretch back much further. "As we know the equations that govern the physics of the climate, we can also use climate models to see how changes in different drivers produce different climate and weather conditions," said Otto. That's helped scientists discern that higher greenhouse gases are closely linked with warmer global temperatures throughout Earth's history. For instance, about 56 million years ago, large amounts of carbon entered the atmosphere, likely from volcanic activity and methane released from ocean sediments, triggering a major period of global heating. The difference today is climate change is primarily caused by humans burning fossil fuels and is happening at a much faster rate than over the last million years or so, says NASA. But how do scientists know if heat waves are a trend or an anomaly? Climate scientists analyze decades of data to distinguish long-term climatic changes from normal weather fluctuations.