By Janet Raloff

Web edition: November 12, 2009

Weather-monitoring stations in the Lower 48 have been logging record daily highs in temperature at twice the pace of record lows. Yet more evidence of climate warming. 

Many people have pointed to colder than normal winters — or summers — as evidence that global warming is a myth. Climatologists have countered that weather, the meteorological features that we experience at any given hour or day, may show anomalies even as Earth’s overall climate warms. So weather can locally mask the planet’s overall slowly rising fever.

Except that any such mask appears to be disappearing throughout most of the United States, according to a new study. Where the climatic thermostat is stable, the ratio of record highs to lows should be about even. That 2:1 ratio of highs to lows means that “climate change is making itself felt in terms of day-to-day weather,” concludes the study’s lead author, Gerald Meehl of the National Center for Atmospheric Research in Boulder, Colo. And, he adds, “The ways these records are being broken show how our climate is already shifting.”

It's something that Guy Walton of the Atlanta-based Weather Channel suspected, a while back, based on recent record-temperature data. So when Meehl stopped by the television station to give a lecture on climate change to the station's meteorologists, Walton asked him why, since at least 2000, record highs had become twice as common as record lows.

Meehl had no idea. Before long he, Walton and others were analyzing several million temperature readings that had been gleaned since around 1950 — data from some 1,800 weather stations across the continental United States.

The 2:1 ratio that Walton turned up proved real, the researchers now confirm in a paper that’s due to appear in Geophysical Research Letters. And the trend stems from a paucity of record lows, not a stunning increase in record highs. In other words, a disproportionate amount of the excess warming is taking place at night — a finding that Meehl notes is consistent with modeling data on temperature trends expected in a warming world. (Moreover, Meehl points out, U.S. weather-monitoring stations have been recording warmer minimum temperatures over most of the last few decades, irrespective of whether they broke any records.)

During mid-century, the number of record U.S. temps were about evenly distributed between highs and lows. During the 1960s, record lows were slightly more common. But since the 1970s, daily temperatures have definitely been trending towards an excess of new highs.

Data from half a world away exhibit a similar trend. “We found out about a study being done in Australia, sort of in parallel with ours,” Meehl says. There too, scientists turned up a roughly 2:1 ratio of record highs to lows.

Much of the new U.S. paper focused on a statistical point: something known as 1/n, where “n” stands for the number of years that temperatures have been recorded at a given monitoring station.

So the first year amounts to 1/1 — or 100 percent. With no previous data to compare a day’s temperatures against, 100 percent of readings will constitute “record” values. But by year two, if climate is stable, you’ll have one-over-two; so you’d expect half as many records highs. The year after that, just one-third of days should constitute a record.

Now, after logging 60 years of data, each weather station should turn up only about six record temps per year. But “we’re seeing proportionately more record highs and record lows — by about a factor of two,” Meehl says. Indeed, this year isn’t over yet, but through September the 1,800 monitoring stations have already logged 11,711 new highs and 7,449 new lows. So they’re on track to average 14 new temperature records a piece, this year.

And down the road? Meehl’s team modeled what we might expect decades from now, based on a mid-range projection of likely greenhouse-gas emissions and climate warming. This exercise, meant to be illustrative, not truly predictive, indicated that as warming continues to increase, the ratio of record highs to lows will grow — significantly. Think 8:1 by mid-century; 20:1 by 2100.

One surprise: “When we started doing this modeling, I thought that by the end of the century everything was going to be so warm that record low temperatures would disappear,” Meehl recalls. Not so, if the models can be believed. Even then, he says, “on average, you’d still see the occasional record cold snap."