The meaning of the term "record heat" depends on what the temperature is compared to. Thousands of meteorological stations around the world continuously monitor the temperature. A record may be set at a location if the measured temperature exceeds the previous maximum for a particular day, month or season.
Scientists determine both regional and global temperature records by combining data from meteorological stations, ships and ocean buoys, and then calculate average values for larger areas, writes Deutsche Welle.
How far back those comparisons go depends on the database being used. The oldest continuous series of temperature measurements in the world is the Central England Temperature Data Series, which has existed since 1659.
However, the three most complete databases - run by the US Goddard Institute for Space Studies, the National Centers for Environmental Information and the UK's Met Office Hadley Center - have systematically recorded temperatures since 1880.
Claims such as: "The last decade was the warmest on record" are based on that data.
Although the Earth has experienced warmer periods and extreme temperatures during its 4,5 billion year long history, today's warming is exceptional in the context of the last 12.000 years of human civilization.
How do we know what the climate was like before people and thermometers?
To reconstruct the climate of the distant past, scientists use natural records known as climate archives. They enable the reconstruction of long-term changes in temperature, precipitation and atmospheric composition.
Such archives include ice cores, tree trunks, ocean sediments and fossils.
"They store information about temperature, precipitation and the composition of the atmosphere that go back thousands and even millions of years," said climatologist Friederike Otto, founder of the research group World Weather Attribution (WWA), in an interview with DW.
The longest continuous record from ice cores comes from Antarctica and covers a period of more than a million years. In ancient bubbles of air trapped in the ice, researchers can measure former concentrations of carbon dioxide in the atmosphere.
"Since we know the physical laws that govern the climate system, we can also use climate models to see how changes in various factors affect climate and weather," says Otto.
This allowed scientists to determine that higher concentrations of greenhouse gases throughout Earth's history have been associated with higher global temperatures. About 56 million years ago, large amounts of carbon entered the atmosphere, probably due to volcanic activity and the release of methane from oceanic sediments, which caused strong global warming.
The difference with today's climate change is that the current warming is the result of human-caused burning of fossil fuels and is occurring much faster than at any time during the last million years, according to NASA.
How do scientists know if a heat wave is a trend or an exception?
Climatologists analyze data collected over decades to distinguish long-term climate change from normal weather oscillations. One particularly warm year is not in itself proof of a trend. However, the longer the data sets, the more reliably statistical analyzes and climate models detect persistent patterns.
"You're looking for consistency. You're looking for a match between all the available evidence," says climatologist Gerald Miles of University College Dublin.
Scientific institutions around the world—among them NASA, the US National Oceanic and Atmospheric Administration (NOAA) and the European Copernicus Climate Change Service—monitor global temperatures using a variety of methods and datasets. Despite the differences in approach, they all come to the same conclusion: The Earth is warming, and recent years have been among the warmest on record.
Scientists also use so-called attribution studies to assess the role of climate change in the occurrence of extreme weather events. Using climate models, they compare the probability and intensity of an event in today's climate with a hypothetical world in which there is no human-caused warming.
The research group World Weather Attribution (WWA) applied just such an analysis to the European heat wave of June 2026. They concluded that it was the most intense heat wave recorded so far in the analyzed area. Such temperatures, according to their estimates, would be almost impossible in the climatic conditions of 1976, which was long considered one of the warmest in Europe.
What indicators should be paid attention to in the weather forecast?
Air temperature alone does not fully show how heat affects the human body. That's why the meteorological services also publish data on the subjective feeling of temperature, that is, how hot the actual feeling is.
In the United States, the heat index is used, which takes into account the temperature and humidity of the air. British Meteorological Service (Met Office) in estimation human experience of temperature includes the influence of wind.
Why does high humidity make us feel even hotter?
When the air is very humid, it contains more water vapor, which is why sweat evaporates from the skin more slowly. This reduces the body's ability to cool down, so the same temperature seems significantly warmer than in dry conditions.
Scientists also use the so-called wet bulb temperature to assess the burden on the body. It is measured by covering the tank of the thermometer with a damp cloth. As the water evaporates, the thermometer cools – in the same way that the evaporation of sweat cools the human body.
The closer the wet bulb temperature is to the actual air temperature, the more saturated the air is with moisture and the harder it is for the body to cool down, increasing the risk of dangerous heat stress.
If the wet bulb temperature is around 35 degrees Celsius for a long time, the human body can overheat even when the person is resting in the shade and taking in enough fluids. However, dry heat is not harmless either, as it increases the risk of dehydration.
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