Our Bodies Are Cooling Down

7:49 minutes

a woman looking at personal thermometer
Credit: Shutterstock

150 years ago, it was safe to assume that a healthy person’s average body temperature was 37 degrees Celsius, or 98.6 degrees Fahrenheit. 

But since about 1860, the average healthy American body temperature has been declining. Physicians have noticed this for decades, but thought it was a measurement error. But new research published in the journal eLife last week now confirms the change is real: We’ve been cooling off by fractions of a degree per birth decade. The average American man is about 1 degree Fahrenheit cooler, while the average American woman is about half a degree cooler.

What could be the reason? Science writer Eleanor Cummins joins Ira to explain why a lower disease burden could shift the norm for healthy body temperatures. Plus, she shares other stories from the week, including new insights into the birth of an underwater volcano and an update on the warming of the oceans.

Further Reading

Segment Guests

Eleanor Cummins

Eleanor Cummins is a freelance science journalist based in New York, New York.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. Later in the hour, political polling and a look at the fish of the Great Lakes. But first, take the back of your hand and touch it to your forehead, just like your parents might have when they were trying to figure out if you were sick. Remember, just like that? OK.

Do you feel warm? Maybe a little feverish? Hopefully not. Hopefully, you feel right about average.

But what is average for the human body temperature? 150 years ago, the answer was 98.6 degrees Fahrenheit, 37 degrees Celsius. I’m sure we’re all thinking about that now, too, right? Well, not anymore.

At least in the US, body temperatures, healthy ones, have been dropping for decades. And chances are good that you too are cooler and you’ve been told you should be. What’s going on here? Here to help explain the story, plus other short subjects in science, Eleanor Cummins, freelance science journalist living in New York. Welcome back.

ELEANOR CUMMINS: Thanks for having me.

IRA FLATOW: OK, can’t still quite wrap my head around this. We’re getting cooler?

ELEANOR CUMMINS: Yeah. It’s crazy to me too. So a new study came out this week in a journal called eLife. And some scientists at Stanford evaluated 157 years of American temperatures. So they went all the way back to the Civil War. And then they have some more recent data sets.

And they were comparing them. And it looks like with each generation, we’ve gotten a little bit colder. So what this means is that the average guy walking around today, like yourself, would probably be about 1.06 degrees Fahrenheit colder than your 19th century counterpart.

IRA FLATOW: And then the reason for this is?

ELEANOR CUMMINS: So that’s what they were trying to figure out. At first, they were like, this must be a problem with the methodology, right? Like, maybe there was some mistake back in the olden days.

But what they’ve actually landed on is that it’s nothing technological at all. It’s inflammation. So they think that as our lifestyles have gotten better– because of economic development, dental hygiene, tuberculosis is not so common anymore in the United States– our inflammation has gone down. And as a result, our body temperatures have gone down too.

IRA FLATOW: Very interesting idea. We don’t know, but that’s an interesting idea, right?

ELEANOR CUMMINS: Yeah, it’s definitely interesting. And yeah, challenges–

IRA FLATOW: Some for men and women, same thing?

ELEANOR CUMMINS: So women are also their temperatures have been going down, but by about 0.58 degrees Fahrenheit, so a little bit less of a difference. But there are sex variations in body temperatures anyway that we already knew about. So while women actually have a slightly higher core body temperature than men, they tend to have a cooler skin temperature, which is often why women are the ones asking their office places to turn the temperatures up in the winter.

IRA FLATOW: Explains a lot it.


IRA FLATOW: And OK, so let’s move on. Because we know as the human body is cooling off, as you’ve told us, the oceans are warming. And in fact, 2019 was another record year, right?

ELEANOR CUMMINS: It was. So yeah, the oceans have been taking on a lot of the excess heat we’re seeing from climate change. They’ve been a repository for about 90% of that heat. And this new study that came out showed that 2019 was another record-breaking year, the warmest our oceans have been since we started recordkeeping.

And they calculated this in joules, which is a common unit of energy. So they found that in the last 25 years, the oceans have warmed by about 228 sextillion joules. And realizing that that was a lot of zeros, a very hard number to wrap your head around, the lead study author put it another way. He compared it to adding 3.6 billion Hiroshima bombs worth of heat to the oceans in the last 25 years.

IRA FLATOW: Wow. That really does bring it home.


IRA FLATOW: And we’ve talked about things that happen when the oceans are subject to climate change– coral bleaching. Other animals feeling the heat too?

ELEANOR CUMMINS: Absolutely. So another study came out today looking at the common murre, which is a seabird in the Pacific. And what happened was is, from about 2014 to 2016, they estimate a million of these birds died, which is a massive die-off, something we haven’t really seen before. So this study tried to evaluate what was going on. And at that time in the Pacific, we were seeing that giant blob, right? This incredible heatwave.

And so what they’ve come to conclude is that the birds just couldn’t get enough to eat. They say it was sort of system-wide. Like, the zooplankton at the very bottom of the food cycle, it wasn’t as nutritious. There was more competition with fish. And so these birds became emaciated and died off en masse.

IRA FLATOW: And you know, there’s got to be a limit to what the ocean can take in, right?

ELEANOR CUMMINS: Absolutely. We’re not sure what that limit is. But because the ocean has been storing more than 90% of this excess heat, it means that very little has actually warmed our atmosphere, which is crazy to think about, because it seems like everything is getting so hot.

IRA FLATOW: Oh, the ocean is key.



ELEANOR CUMMINS: So it’s been an incredible repository. And as it gets warmer, we think that it may be able to store less carbon, which would mean more would warm the atmosphere.

IRA FLATOW: Meanwhile, speaking of the ocean, there was a strange seismic hum in the ocean–


IRA FLATOW: Last year, right?

ELEANOR CUMMINS: Yeah. So starting in May 2018, there were these rumblings. And nobody really knew what was going on. It was in a very quiet part of the Indian Ocean between Madagascar and Mozambique. And that part doesn’t see a lot of activity. But there were earthquakes up to 5.9 magnitude.

So scientists were like, OK, we have to look into this. They were seeing sensor data from all over the world showing these rumblings. Like, Hawaii, Chile, it was showing up. And it ended up being really the first time that scientists have observed a seafloor a volcano being born.

IRA FLATOW: Wow. You mean lava and everything coming out?


IRA FLATOW: That was a volcano.

ELEANOR CUMMINS: So that data was– we were able to– in this recent study, they were able to show how the data actually maps onto the volcano’s development.

IRA FLATOW: Well, that’s cool.

ELEANOR CUMMINS: So they can show where the sounds were emanating, as this magma– which came from a really, really deep reservoir 19 miles, they think, beneath the seafloor– they can show in the data as it rises up, as it reaches the seafloor, the whole process.

IRA FLATOW: Wow. [INAUDIBLE] to find one of those giant mats they found down there, floating lava.


IRA FLATOW: Found a plane.


IRA FLATOW: Maybe this will happen.

ELEANOR CUMMINS: Totally. And that’s such a great example of how little we’ve known. So to be able to track it this way is incredible.

IRA FLATOW: Now, a really crazy story, the last one, and this is the oldest matter found on Earth and a meteorite that landed in Australia in 1969?

ELEANOR CUMMINS: Yes. So the Murchison meteorite has been quite a gift. Scientists have been studying it for a long time. And in this most recent paper, they have identified some grains inside the meteorite that are 7 billion years old, which is vastly older than Earth and the Sun. It was so exciting.

IRA FLATOW: Wow! Well, wait a minute. So it came from outside. If it’s older than the Sun–


IRA FLATOW: –it’s a real alien.

ELEANOR CUMMINS: Yes. Yeah, absolutely. They call them presolar grains. So it’s literally stardust. In this case, it was silicon carbide grains. And so these were things that the meteorite was collecting and as it formed and as stars were forming in our galaxy.

IRA FLATOW: So this came from a meteorite we already had–


IRA FLATOW: –we knew about. Has– What was the oldest one before this one? Was that the same meteorite?

ELEANOR CUMMINS: It was also from the same meteorite, because this meteorite is 220 pounds. So they keep finding new things inside. In this case, when they broke of the sample, they pulverized it so they can find these presolar grains. And they said it smelled like rotten peanut butter. So they really– the scientists here really did us a service by–

IRA FLATOW: Rotten peanut butter?


IRA FLATOW: Have you ever smelled rotten peanut butter?

ELEANOR CUMMINS: They haven’t. Another one of the study authors was asked, did it smell like rotten peanut butter? And he was like, I don’t know what that smells like. But it was bad.

IRA FLATOW: It would be rancid, right?


IRA FLATOW: Like, peanut butter goes rancid, and– I wonder if they have any more of these meteorites around that they could find all this stuff. Well, yeah, maybe they will.

ELEANOR CUMMINS: Yeah, definitely.

IRA FLATOW: Well, this is exciting. Thank you, Eleanor. Always good stuff you bring for us.


IRA FLATOW: Eleanor Cummins, freelance science journalist here in New York.

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Christie Taylor was a producer for Science Friday. Her days involved diligent research, too many phone calls for an introvert, and asking scientists if they have any audio of that narwhal heartbeat.

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Ira Flatow is the host and executive producer of Science FridayHis green thumb has revived many an office plant at death’s door.

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