Human-caused changes in the lower atmosphere have begun to affect the seasons, according to a study carried out at Lawrence Livermore National Laboratory.

The key atmospheric changes are related to increasing concentrations of greenhouse gases, especially carbon dioxide, the study concluded.

The study was published in Science magazine last week, at a time when news media around the world were reporting record high temperatures and devastating fires in places ranging from the far north of Scandinavia to southern Europe to Africa.

Unlike previous studies that have found the “fingerprints” of human activities in rising temperatures generally, the one announced last week focused on change as reflected in month-by-month averages.

While temperatures are rising globally, the study found, summer high temperatures have risen significantly faster than winter high temperatures, especially in the mid-latitudes in the northern hemisphere.

In the tropics, seasonal change is difficult to detect, while in the Arctic and Antarctic regions, winter temperatures have risen faster than the summer temperatures, the research found.

Five institutions collaborated with LLNL in the study: the University of Washington, the University of California at Santa Cruz, Remote Sensing Systems of Santa Rosa, MIT and the National Oceanic and Atmospheric Administration.

The work made use of 38 years of satellite-based temperature measurements, as well as the outputs of more than three dozen computer-based climate models around the world, according to Ben Santer, senior climate scientist at LLNL.

Comparing and publishing the disparate outputs of climate models running different software on a variety of computers has been one focus of LLNL climate research over the past three decades.

The effort has helped research groups around the world communicate and compare results as they converged on the view that humans are playing a role in climate change.

Santer compared the monthly temperature averages examined in the current study to ocean waves with a trough and a crest. The trough is the year’s low temperature, the crest the year’s high.

“The ‘wave’ isn’t really changing all that much in the tropics, which sort of makes sense because the tropics don’t have…big fluctuations in temperature between summer and winter,” he said.

“But as you go toward mid-latitudes, there is a very large seasonal cycle in the amount of sunlight that we receive…at the Earth’s surface.

“That is where you begin to see this increase in the size of the ‘waves’ over the 38 year satellite record, with all months warming but summer warming preferentially relative to winter.”

Santer said the reason for the more rapid mid-latitude warming appears to be that the soils in continental interiors are drying out.

“As you ramp up greenhouse gases and start warming the planet…you have less evaporative cooling. The soil is dryer and there is diminished flux of moisture from the soil into the atmosphere to cool things off.”

As he has done in the past, Santer expressed admiration for the ability of two Princeton scientists, Syukuro Manabe and Richard Wetherald, to forecast this phenomenon in the 1970s before climate satellites could begin to verify their predictions.

Different at the Poles

Polar regions, which are warming faster than any other, do not show the same effect; in fact, they show a declining gap because the lowest temperatures are warming faster than the highest, the study found.

Santer explained that a disproportionate amount of their rapid warming occurs in the fall and winter because the white, reflective ice cover is now disappearing in the summer, “exposing the dark underlying surface….

“The ocean begins to absorb (and) retain more heat and it takes longer for sea ice to form when it starts getting cold,” he said.

While the warming occurs throughout the year, “You get more warming at the colder times…than at the warmer times.”

Santer said the strength of the seasons should not be confused with their timing -- the question, for example, of whether spring is arriving earlier, as claimed by naturalists who record the arrivals of the first green leaves and then the first blooms in U.S. national parks.

Looking for human “fingerprints” connected to that reported change seems “of equal or perhaps even greater scientific interest” than the strength-of-seasons research just announced, he said. It is a topic that he expects to focus on in future research.

In California, he thinks this question will be particularly important in studies of water resources, a topic discussed at length last February at Livermore’s Bankhead theater by nationally known expert Roger Bales of UC-Merced.

Earlier spring can mean earlier mountain snowmelt and runoff, affecting drinking water supplies and agriculture in much of the state.

Shortly after his interview, Santer was leaving for Alaska to spend time on the Juneau Icefield lecturing summer students and interacting with other researchers and sometimes adventurous young business leaders.

This will be his fifth summer on the glaciers there, an experience he has found to be stimulating both physically and intellectually.