What “record-breaking heat” actually means

Like an overturned truck that had been carrying vinyl LPs, 2023 is overflowing with broken records.

The Atlantic Ocean’s surface temperature is at a record high for this time of year while Antarctic sea ice is at a record low for the season. Earth just broke records for its hottest day and hottest June, and is poised to experience its hottest July. Countries like China, India, and Spain set new record highs, while Greece and Italy are likely to set their own records this month. The bottom of the scale is rising too: Phoenix, Arizona, set its highest low temperature on record last week.

It’s not just heat, either. Rainfall records fell in California, North Carolina, and Vermont, while Kentucky is on track to set one too. Warm air can hold onto more moisture, leading to more torrential downpours. The National Oceanic and Atmospheric Administration (NOAA) reports that thousands of temperature and precipitation records have already been matched or exceeded so far this year, and more are likely to fall.

The point of a record is to put events into context, and the numbers this year show the planet is assuming a state unlike any humanity has experienced before.

El Niño, the warm phase of the Pacific Ocean’s temperature cycle, is especially strong this year, driving up temperatures around the world. It’s aligning with the warm phases of regional cycles in places like Antarctica and the Atlantic Ocean.

Beneath it all, the global climate is changing as humans burn fossil fuels and spew heat-trapping gases into the atmosphere. The world’s average temperature has increased just over 2 degrees Fahrenheit (1.2 degrees Celsius) since the dawn of the industrial revolution in the 1800s. That’s fueling more frequent and intense heat waves. The World Weather Attribution research team, a group of scientists analyzing humanity’s role in extreme weather, reported on Tuesday that the July heat waves in Southern Europe, as well as the heat across the US and Mexico, would be “virtually impossible” without human-caused climate change and will become more common.

“A heatwave like the recent ones would occur every 2-5 years in a world that is 2°C [3.6° F] warmer than the preindustrial climate,” the scientists wrote.

However, there’s a lot baked into the idea of a “record” that often goes unspoken. Not every record is equally long or robust. Past measurements may not be as reliable as modern ones. People colloquially use “record” in different ways to mean different things. And too often, these nuances are lost, and “hottest day on record” becomes “hottest day ever” or “all-time high.”

“I cringe when I read that,” said Chip Konrad, director of NOAA’s Southeast Regional Climate Center and a professor at the University of North Carolina at Chapel Hill. There were likely higher temperatures before humans walked the Earth, when the continents were in different places and the atmosphere had a different mix of gases. Reporters (including myself) don’t always clarify that “ever” and “all-time” mean “since record-keeping began.” And those records only go back to when weather monitoring stations were installed in a given part of the world.

“The longer a weather station has been there, the more meaningful the record is,” Konrad said. “We have some that go back to the 1800s, but the vast majority were put in around World War II, the middle of the 20th century.” In addition, individual weather monitoring stations can be biased by other human activity like urban sprawl. So not every broken record carries the same weight.

“That being said, it’s still meaningful how impressively warm it’s been this summer,” Konrad said. Just because records only go back so far doesn’t mean scientists can’t figure out what conditions were like before then, and despite the normal chaos of weather, far more heat records are falling than cold records year after year. Their trajectory points toward a world that will continue to heat up.

Strengthening the context of current climate records, then, could clarify what’s in store for the future — and one of the most important ways to do this is to extend our view of the past.

There are several things to keep in mind when thinking about temperature and rainfall records after a new one is set. As mentioned, it’s worth noting how the record was obtained and how far back observations extend. But it’s also important to understand the conditions leading up to a record and the limitations of measurements.

For example, the hottest temperature ever recorded on Earth is 134°F on July 10, 1913, in Death Valley, California. However, weather historian Christopher Burt has argued that this measurement was likely an error. Temperatures from surrounding areas were nowhere near as hot at the time, and the record was several standard deviations higher than average high temperatures recorded in the area before or since. If that’s the case, the world’s actual hottest temperature on record might have occurred this year.

To increase the integrity of new heat records, scientists can take multiple measurements over a wider area, though truly nailing down a new record often requires an investigation that can take weeks or months. NOAA, for instance, will conduct a site analysis around weather instruments to make sure the equipment was working correctly and that nothing nearby, like a highway or a factory, was altering the record-breaking measurement. “We have to make sure there’s no bias there,” Konrad said. “There are very detailed investigations to make sure that that record isn’t spurious.” Such records, validated with hindsight and multiple measurements, are more significant than individual preliminary records.

Scientists can also fill in gaps by using computer models and by augmenting measurements on the ground with readings from space. From there, they can paint a picture of temperatures around the world. That’s how scientists were able to find that global daily average temperatures are at record highs, though these measurements only date back to around 1979, when weather satellites capable of measuring atmospheric air temperature began taking orbit.

But based on other historical records and trends, scientists can then place these temperatures on longer time scales. “Because warmth must be truly global to push these numbers very high, many scientists have concluded it may have been thousands if not more than 100,000 years (since before the last ice age) when global daily temperature was last this high, but this is more of an expert judgment than a calculated finding,” Bob Henson, a meteorologist and journalist with Yale Climate Connections, said in an email.

Events like heat waves and severe rainfall events can also be described in terms of their probabilities. World Weather Attribution researchers said that the July heat wave in China was a one-in-five-year event, but absent human-caused climate change, it would’ve been a one-in-250-year occurrence. The rain that drenched Vermont and New York earlier this month was described as a one-in-1,000-year storm. This is not a description of history, saying that the last such storm was 1,000 years ago. Rather, it’s a statement of probability, that a storm this severe occurs on average once in every 1,000 years, or that there’s a one-in-1,000 chance of this level of rain in any given year.

“Note that these calculations are based on the assumption that climate is stationary, when in fact we know that the most extreme rains are getting heavier in many/most places,” Henson said. “There’s no simple way to adjust for this, other than to point out that what is a ‘100-year rainfall’ may be occurring more often in a warming climate.”

Another way to contextualize a record is to see how far it is from what’s typical. The record-low sea ice in Antarctica right now has been described as a five-sigma event, meaning it’s five standard deviations below the average for this time of year. “Which means that if nothing had changed, we’d expect to see a winter like this about once every 7.5 million years,” Edward Doddridge, an oceanographer at the Australian Antarctic Program Partnership, told the Australian Broadcasting Corporation.

As the world braces for a warmer future, it’s more critical than ever to understand the past. A number of sites around the world lay claim to being the oldest weather monitoring station. One of the earliest claims, at more than 2,000 years old, comes from the Tower of Winds in Athens, Greece. But continuity is the other key factor, and some of the oldest continuous records date back to 1724 in Beijing, China.

The problem is that most of the continuous temperature and rainfall records were taken in an era when humanity was already altering the planet, if not with fossil fuels, then with deforestation and agriculture. It’s tricky to get a sense of humanity’s influence on heat, rain, and extreme weather and sift out natural variability without a way to compare it to the climate before such impacts. “We are not looking at a clean world when we’re looking at these early records,” explained Sturt Manning, an archaeologist who runs the Cornell Tree-Ring Laboratory at Cornell University.

To get a picture of the climate from even further back, before thermometers existed, scientists are piecing together records from proxies like ice cores and rocks, which can reveal the state of the atmosphere and oceans from long ago. One of the most robust sources of historical climate data is tree rings. Trees are abundant in most of the world, and as they grow, they capture annual snapshots of air, water, soil, and sunlight conditions. These tree rings can even reveal trends in extreme weather like hurricanes.

“Hundreds of different tree species produce visible annual tree rings that we can work with and observe, and those relate to calendar years,” Manning said. The record is built from the present using living trees and then extended using fossilized trees or trees preserved in bogs and rivers.

This way, researchers can build a local climate record going back millennia. Already, scientists have reconstructed climate records in Sweden dating back nearly 8,000 years, and with some long-lived Kauri trees found in New Zealand, they could push the timeline back further.

“We have the potential probably, in time, to build maybe 50-, 60,000 years of tree ring records,” Manning said.

These reconstructions likely won’t ever reach a high enough resolution where we can compare the weather on a specific day to a day thousands of years ago, but they provide a sharper picture of the natural variability in the global climate system. That, then, can help explain how we’re changing the planet now, and how future changes will affect us all.