As the average global temperature increases due to human-caused global warming, the ratio between warm temperature and cold temperature records in the U.S. is increasingly skewed toward heat milestones, an Associated Press analysis shows.
The big picture: The analysis, shared with Axios, indicates that the biggest contributor to the trend is the rapidly shrinking number of cold records, although the extreme heat events are also becoming more common and severe.
Why it matters: The way we experience global warming is largely through extreme events, from heat waves to heavy precipitation events to wildfires. The fact that extreme cold records are getting more rare is one of the trends that's becoming clear to people.
Background: AP shared an analysis of warm temperature records versus cold records since 1920 within the Lower 48 states. The data consists of daily high maximum temperature records as well as daily low minimum temperature records set at a subset of 424 stations from the U.S. Historical Climate Network.
Context: Global warming, by raising average surface temperatures, is making high maximum temperatures easier to set than low temperature milestones. This shift is particularly visible when looking at decadal averages.
- AP reports: "Since 1999, the ratio has been two warm records set or broken for every cold one. In 16 of the last 20 years, there have been more daily high temperature records than low."
- In a stable climate (one that is not warming or cooling significantly overall), one would expect that ratio to be closer to 1:1.
A caveat: The analysis does not capture the increase in record warm overnight low temperatures, which is one hallmark of climate change and contributes to more dangerous heat waves, since warm overnight temperatures prevent the human body from cooling down.
What they're saying: "In a world without human-caused climate change, the rounds of record cold and record warm air would balance out over time," says Bernadette Woods Placky, chief meteorologist at Climate Central, a climate research and news organization.
- "That's not what's happening. Record warmth is far outpacing record cold. In fact, that's one of the reasons this winter's polar vortex was newsworthy, we just don't get record cold as often or intensely as we used to," she tells Axios.
- "People often have a hard time feeling and visualizing a rise in average temperature, but extremes and records smack you in the face. The extremes are what reshape lives — change camp and sports practice schedules, interrupt and stress outdoor work (from farms to construction), require cooling centers, increase heat-related illnesses, and sometimes even lead to death," she adds.
- Other scientists Axios contacted said the ratio of warm to cold temperature records is not an intuitive indicator of climate change over time, though it's tied to increasing average temperatures.