Rebecca Emerton

One way of thinking about it is we often hear discussed kind of the level of humidity that you're experiencing during a heat wave and how that can make things worse.

That's an example of what the Universal Thermal Climate Index, or the UTCIE,

is trying to do.

It's trying to take into account as many different aspects of the environment as possible and how the human body reacts to the environment to really understand how the heat is affecting the human body.

So it accounts for the temperature, humidity, wind speed, solar radiation, lots of different factors.

So this can vary around the world and it can vary depending on the specific conditions you're looking at but just to give an example I took a look at the data for the recent heat wave that we had in May 2026 across much of Europe and quite often the UTCI or the fields like temperatures were four degrees and then sometimes five degrees higher than the air temperature and I've looked at previous heat waves for example one in August that impacted southeastern Europe where the UTCI reached even up to 10 degrees higher than the air temperature.

Yeah, I think it showcases why it's important to try and take into account these other factors as well as the temperature.

It's obviously important, but there's a lot of other factors at play.

Yes, exactly.

So we know there's trends in heat waves across the globe becoming more frequent, more intense, lasting longer.

And we really wanted to give this kind of global picture of how the heat stress in our climate has changed over recent decades.

So we kind of compare the most recent 10 years of the data, so 2015 to 2024, with how things looked in the 1970s.

Yeah, so there's a few different factors that we look at.

One is, for example, the maximum UTCI that you might experience on kind of the warmest days of the year.

And this has increased across almost all regions of the globe with some of the strongest warming in Europe, Northern Africa, the Arabian Peninsula.

some of those maximum utci values are reaching four or five degrees higher on the hottest days of the year than they did back in the 1970s we're also seeing more days per year with heat stress more of these so-called tropical nights where the temperature doesn't drop below 20 degrees c and we're also seeing that heat stress is becoming more widespread so it's not only affecting the areas that we might traditionally think of as being hot or experiencing heat waves

but it's expanding out into areas that have been previously unaffected by heat stress.

Yeah, that's exactly right.

So we know that across a lot of the tropics, there are places that experience kind of chronic heat stress, some level of heat stress almost all year round.

And we're seeing that in those places, that heat stress level is becoming more severe.