Half the global population saw all-time record temperatures over past decade

Carbon Brief reveals visually just how much the world’s most populous regions have been affected by extreme heat since 2013.

More frequent and intense extreme heat is one of the major impacts of climate change.

As the Intergovernmental Panel on Climate Change’s (IPCC) sixth assessment confirmed, it is “virtually certain” that “there has been increases in the intensity and duration of heatwaves and in the number of heatwave days at the global scale from 1950”.

Extreme heat events can have serious health impacts – Europe’s 2003 summer heatwave, for example, caused more than 70,000 deaths. Extreme heat can also worsen air quality problems and ground-level ozone, exacerbate drought and wildfire risk, reduce labour productivity, damage infrastructure and reduce crop yields.

There are many ways to measure extreme heat. Absolute temperature is one, but a key factor for the impacts of heat extremes is also how much higher temperatures are compared to normal for a specific location.

To assess how the incidence of extreme heat has changed over time, Carbon Brief has examined which regions experienced all-time daily heat records in each year since 1950.

The analysis shows that the number of people experiencing all-time high heat events has increased dramatically over the past three decades. Overall, about half the world’s population is living in regions that saw their hottest daily temperatures since 1950 during the past 10 years.

In 2022 alone – which was the fifth or sixth warmest year on record for the Earth’s surface as a whole across different datasets – approximately 380 million people saw their hottest single hourly temperature ever recorded.

Record heat over the past decade

In the map below, the red shading indicates areas that saw their warmest daily temperatures since 1950 in 2022.

As you scroll, you will see the areas experiencing their warmest temperatures on record over the past two years (that is, 2021 and 2022) and all the way back to the past 10 years (2013-2022). Over the past decade, approximately 40% of the Earth’s surface experienced its warmest temperatures on record.

When scientists talk about record temperatures it can mean a number of different things. Often scientists focus on the hottest month or year on record for the Earth as a whole. This generally refers to the largest increase in temperatures relative to the historical average – known as the temperature “anomaly”.

An alternative metric to look at the changing climate is how the hottest temperature recorded over the course of the year has changed over time. This only provides part of the story – there are real impacts of warming in other parts of the year – but is important in assessing extreme heat stress.

In this analysis, Carbon Brief uses ERA5 – a state-of-the-art reanalysis product from the European Centre for Medium-Range Weather Forecasts (ECMWF) – to look at how the hottest maximum daily temperature of the year has changed over time for every part of the planet.

In essence, a reanalysis product is a weather model run backwards in time. To produce daily weather forecasts, groups such as ECMWF take into account massive amounts of data from satellites, aircraft, radar, weather balloons, weather stations and automated buoys. A reanalysis record creates these “weather forecasts” for each day backwards in time and can track changes in different data input sources to detect and correct for changes in the way that measurements are made over time.

The figure below shows the global average surface temperature from ERA5 for each day between 1950 and the end of 2022. While there is a lot of day-to-day variation in global temperatures – and even more in any individual part of the planet – the long-term warming trend is quite clear in the daily temperature data, with current temperatures around 1C warmer on average than those of the 1950s.

Increasing portion of the global population experiencing record heat

While the world as a whole has warmed rapidly and experienced an increasing amount of record heat, people do not live in that global average.

In order to assess the number of people experiencing record heat, Carbon Brief combined the ERA5 all-time hottest temperature data with global population estimates from NASA and Columbia University.

As global population has increased over time, more people would be experiencing record heat events even if the climate were not changing – simply due to the fact that there are more people. To isolate the effect of climate change on the number of all-time record heat events the population is experiencing, the analysis holds the global population constant at 2020 levels and looks back in time at how many all-time record heat events would have been experienced if population remained unchanged.

The figure below shows the number of people who would have experienced all-time record heat events in each year if the global population remained fixed at 2020 levels.

If the climate were not warming, there would still be some year-to-year natural variability in the number of record heat events experienced, but the line would remain relatively flat. However, the world is warming and, thus, the portion of the population that has experienced all-time record heat has increased dramatically.

At the 2020 level of global population, approximately 6.3 billion people live in areas that have seen all-time record heat over the past 30 years – and 3.5 billion people have experienced all-time record heat in the past decade alone.

During the past decade, a number of regions stand out as experiencing particularly widespread extreme heat events. The maps below overlay global population density data on top of areas experiencing record heat over the past decade, and zoom in on regions including Europe, China and the Caribbean to show large cities that set new all-time records in recent years.

These extreme heat events have large negative impacts for both human and natural systems. They can cause heat stress that contributes to premature death, particularly in vulnerable populations. They can worsen local air quality, hurt crop yields, reduce soil moisture and exacerbate droughts, and increase the risk of the catastrophic wildfires that have been seen in the western US, Europe and Australia in recent years. Extreme heat can also reduce labour productivity and damage infrastructure by buckling roads, bridges and rail lines.

Extreme heat events and all-time hottest daily temperature records will both become more common as the world continues to warm. The only way to slow this down is for the world to cut global emissions of CO2 and other greenhouse gases down to net-zero.

Methodology

To produce the analysis in this article, Carbon Brief used KNMI Climate Explorer to analyse ERA5 daily reanalysis data which is available via the website on a grid with a resolution of 0.5 degrees latitude by 0.5 degrees longitude (e.g. ~50 km by ~50 km near the equator). The analysis examined the daily maximum temperature (Tmax), which is based on the hottest hour of the day, and used absolute temperatures rather than anomalies.

Next, Carbon Brief took the hottest Tmax value for the year for each grid cell and determined which year in the dataset the hottest Tmax value occurred. The ERA5 reanalysis dataset extends back to 1950 in the version of the dataset used. Unlike monthly anomalies, it is not currently possible to get accurate high-resolution daily absolute temperature values between the mid-1800s and early-1900s given data limitations, though some reanalysis products are working to extend their records further back in time. The percent of the globe setting all-time maximum temperature records in each year is based on a weighted average of grid cells setting records, with the grid cell weights based on their area.

Finally, the analysis uses gridded global population estimates from NASA and Columbia University, provided at a resolution of 30 arc-second (e.g. ~1km by ~1km near the equator) and combined it with the temperature data to determine what portion of the global population experienced their hottest temperature on record in each year.