Is it really the hottest day ever?

Every summer, headlines about record-breaking heat dominate the news, causing thousands of deaths, wildfires, and straining resources like water, food, and energy. But what does it really mean when scientists declare a temperature record? How do they measure heat, and why do cities tend to be warmer than rural areas? DW spoke to climatologists to shed some light on these common questions about heat measurement and terminology.

Scientists track temperatures using thousands of weather stations worldwide. A location sets a record if its temperature exceeds the previous high recorded at that specific station. To assess regional and global records, data from weather stations, ships, and ocean buoys are combined and averaged across larger areas. The Central England Temperature Data Series, established in 1659, holds the title for the world's oldest continuous temperature record.

However, the most comprehensive datasets - maintained by NASA, NOAA, and the UK's Met Office - began recording comprehensively in 1880. These datasets form the basis of statements like "the past decade was the warmest on record."

While Earth has experienced hotter climates over its 4.5-billion-year history, the current warming is unprecedented in the context of human civilization, which spans 12,000 years. Scientists rely on climate archives - such as ice cores, tree rings, ocean sediments, and fossils - to understand past temperatures and atmospheric conditions extending back thousands to millions of years. These archives contain information about past temperatures, rainfall, and atmospheric conditions going back thousands to millions of years.

NASA, NOAA, and the EU's Copernicus Climate Change Service use different methods and data sets to track global temperatures, yet they all agree on a heating trend, with the last decade witnessing the hottest years ever recorded. Researchers also employ attribution science, comparing the likelihood and intensity of extreme weather events in today's climate with a world without human-induced warming.

Studies have shown that a heat wave in Europe during June 2026 would have been virtually impossible during a similarly hot year in 1976, highlighting the impact of climate change on extreme weather events.