In a mark of just how warm 2016 has been in the U.S., record high temperatures are outpacing record lows by more than 5-to-1, according to data from the National Oceanic and Atmospheric Administration.

This trend is one of the hallmarks of climate change, and is expected to become even more lopsided in favor of record heat if greenhouse gas emissions are not curtailed.

In a climate without a background warming trend, record highs and lows would be expected to balance out over time, even if there are months or years where one side dominates. And the longer a temperature dataset has been kept, the less likely it should be to set any type of record (it’s easier to set a record high or low with only 20 years of data than with 120 years).

highs beating lows

But because of decades of human emissions of carbon dioxide and other heat-trapping gases into the atmosphere, Earth’s baseline temperature has risen. As it does so, it makes it easier to see extreme, even record heat and harder to see record cold. This is the case from the perspective of daily highs and lows all the way up to annual average global temperatures.

Of the five hottest years globally, four have occurred in this decade. Of the 15 hottest, 14 have been during the 21st century. The last record cold year was 1911. Through June, 2016 is beating the current hottest year on record, 2015, by a long shot.

In the U.S., the trend of record highs outpacing record lows can clearly be seen in daily temperature data, and has become particularly noticeable since the 1980s.

Through July, the number of daily maximum temperatures that have set a record high this year have outpaced record low daily minimum temperatures by a rate of 5.5-to 1.

Extreme heat
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The imbalance becomes even more pronounced when looking only at daily overnight minimum temperatures: Record high daily minimums are beating record low ones by 7-to-1.

The steeper trend in daily minimums is an example of how the coldest times of day (usually overnight) are warming more rapidly than the hottest (usually in the afternoon). The same thing is true of the coldest times of the year and the coldest places on the planet.

While sky-high afternoon temperatures tend to grab headlines, warm overnight temperatures can be a bigger threat to public health. The young and old and those with certain medical conditions don’t get much recovery time if temperatures stay warm overnight.

Finally, when looking at all-time record highs and lows, or those that were record high for any date and not just the one they occurred on, highs are beating lows by 9-to-1. This ratio has gotten a major boost from some of the heat waves that have swept across large swaths of the country this summer.

“All-time highs speak more to the extent and coverage of single events that happen at the warmest time of the year,” Deke Arndt, the head of the climate monitoring division at NOAA’s National Centers for Environmental Information, said in an email. With background warming, “over time, we'd expect more all-time highs than lows, but in the course of a given year, it's more a matter of the timing and coverage of the ‘Heat Dome.’”

That dome covered a large chunk of the eastern portion of the country during mid-July, helping to keep the Lower 48 on track for its third warmest year in more than 130 years of record keeping.

If greenhouse gas emissions continue on the pace they have in recent years, the trend in record highs far outpacing record lows will only intensify, a 2009 study found. By mid-century, highs may beat out lows by 20-to-1, and by century’s end, that ratio could balloon to 50-to-1.