It many ways this summer's weather has been a yawner. No record droughts or floods, and just one little tornado.

Nonetheless, it has carried some unexpected and troubling signals of a rapidly changing climate.

Lake Superior around the Apostle Islands had its first massive algae bloom. The Twin Cities area experienced its highest-ever nighttime temperatures. And Minnesota reported a record number of bad air days, due largely to the catastrophic forest fires in California and Canada, with perhaps more on the way.

"This summer is indicative of what we expect to see ... as we move into the future," said Kenny Blumenfeld, a climatologist at the state climatology office.

Few Twin Cities residents could have missed the string of stinky, eye-itching bad air days that dragged on through August, thanks to the forest fires.

Around the world, such fires have increased in duration and intensity, in large part due to longer summers and more severe droughts. Minnesota has its share of fires as well, including one that's burning now in the northern part of the state.

But this month, the winds uncharacteristically came from the west and northwest, bringing acrid smoke from the massive blazes in California and Canada.

Most summers, the wind is out of the south — from Iowa, Missouri and Illinois, say meteorologists. That's what brought warm, damp air to Minnesota in the first part of the summer. But in the second half, the direction switched, and generally stayed that way, another new pattern that's cropped up in recent years.

"We seemed to be spending more time in that northwest flow," said Blumenfeld. It may be just another sign that weather patterns of all kinds stick around longer than they used to, he said, the result of the changing behavior of the jet stream around the northern part of the globe.

That causes weather systems to stall, "and we get stuck," he said.

In a year without forest fires, that might have meant more of the brilliant, cool days that are the delight of a Minnesota summer. Instead, there were seven air alert days in August, bringing the summer's total to nine — by far the highest number recorded since the state started using the current warning system in 2010.

Though the state's overall air quality has improved markedly — thanks to fewer coal-burning power plants and stricter pollution and fuel efficiency requirements — airborne pollution still contributes to the deaths of about 2,000 Minnesotans annually, according to the Minnesota Department of Health.

Forest fires aggravate a common air pollutant: The smoke carries high levels of fine particulates, 2.5 microns or less, that penetrate the lungs and enter the bloodstream. That can trigger attacks of asthma, heart disease and stroke.

In some places around Minnesota this month, where concentrations reached 100 or more nanograms per cubic meter, breathing the air for a day was like smoking four or five cigarettes. That was true for smokers and nonsmokers, regardless of age, according to a mathematical comparison created by scientists at the University of California, Berkeley.

The summer's other climate turning point was Lake Superior's first known large algae bloom.

Starting in mid-August, anglers and kayakers from Superior to Cornucopia, Wis. were surprised to see the cold clear nearshore waters of the lake turn an icky green. Though a few small algae blooms have cropped up before in some spots, this one was unprecedented.

"The whole phenomenon surprised me," said Bob Sterner, director of the Large Lakes Observatory at the University of Minnesota Duluth.

These sometimes-toxic blue-green algae blooms have been on the increase across the Midwest in recent years, most noticeably in Lake Erie, where they have fouled Toledo's drinking water system.

Sterner said that while scientists don't completely understand the causes and frequency of blooms, they start with warmer water. And Lake Superior, he said, "is one of the fastest-warming lakes on earth."

In large part that's because warmer winters have greatly reduced the amount of ice cover every year, leaving the dark water exposed to the warming rays of the sun.

Then, earlier this summer, northern Minnesota and Wisconsin were hammered by some large storms that dropped up to 8 inches of rain over several days. That produced runoff that turned the water around Madeline Island and Ashland, Wis., the color of chocolate milk for days.

When that rainfall flowed through streams and storm sewers into the lake, it carried the second critical ingredient for algae blooms — nutrient pollution. That's primarily nitrogen and phosphorus from farms and lawns, domestic and wild animal waste, and decomposing vegetation from yards, streets and sidewalks.

It stewed for a few weeks, and then algae blossomed. Wind completed the formula by pushing the algae into shore.

"And that's where all the people are," Sterner said.

Sterner said scientists around the lake are starting to explore the problem and want to know if the lake has had other undocumented algae blooms in the past. They are also looking for the streams and other hot spots around the lake that may be delivering the largest share of nutrients after a rainfall. Because, he said, it would be a pity if the crystal waters of Lake Superior became more like Lake Erie's.

"Out of all the places on earth, Lake Superior is not where I expected this to be a problem" he said.

Green tomatoes, too

While it many not have felt like the hottest July or August ever, this summer is likely to set a record for warm nights in the Twin Cities.

"We're almost off the charts there," said Mark Seeley, a University of Minnesota meteorologist.

From June 1 to Aug. 23, nighttime temperatures averaged 65.3 degrees, a tie for first place with 2010. And September looks like it's going to be warmer than normal, meaning that this year could hit a record for nighttime highs, Seely said.

The same greenhouse gases that drive climate change also prevent warm air from escaping into the atmosphere at night.

That's not just a problem for people who like a good night's sleep. It's also a problem for people who like to grow their own tomatoes, which need cool nights to ripen.

"I've heard a lot of gardeners complaining," Seeley said. "We're going to have a lot of green tomatoes."