ST. PAUL-Ice seems simple enough: Get water cold enough and it freezes.

True, but the science of ice is much more complex, especially when it is in real world bodies of water.

Scientists agree on a couple of things: No ice is fully safe and the thickness, and thus safety, of ice can vary greatly in a very short distance.

There is general agreement on one other fact, too. The ice season is getting shorter. One study shows a typical Upper Midwest lake will be covered with ice more than a week later than 150 years ago, and will be ice free nearly a week and a half earlier.

Statistics show that most ice-related deaths come early in the season when people are eager to venture onto lakes and rivers.

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"The most dangerous times are when you have the first ice form across the lake," said John Magnuson, a long-time University of Wisconsin lake expert.

But spring also presents dangers to people trying to use ice as long as possible.

Snow on lake and river ice often melts as the sun angle begins to increase, Magnuson said. The heat begins to penetrate the ice. That can cause ice to crystalize, which causes it to turn a dark blue that can signal danger.

Water from below may have moved up the crystals, weakening its structure.

The freeze-thaw cycle that soon will begin also affects ice integrity, causing cracks to fill with water; that water freezes when temperatures drop, expanding cracks.

Meteorologist Aaron Kennedy of the University of North Dakota said freezing and refreezing in the spring makes more dangerous ice, breaking down bonds between molecules.

Professor John Gulliver of the University of Minnesota said that as temperatures warm ice it "gets mushy. It doesn't really get thin. The first thing it does is get mushy."

Mushy ice breaks easier, he said.

The Minnesota Department of Natural Resources' revised recommendations for ice depth say a minimum of five inches is needed to walk on fresh, clear ice and 12 inches to 15 inches for heavier vehicles such as pickup trucks. However, the DNR's Lisa Dugan said those figures double if the ice is cloudy or snowy.

Gulliver said one problem with ice comes when a vehicle is driven on it.

"It creates a bow wave like you might have with a barge or a ship," he said.

The water wave under the ice can damage ice away from the vehicle's path, Gulliver said. "Eventually, the wave breaks the ice."

Added Kennedy: "Each vehicle that passes over the ice impacts that structural integrity."

Gulliver said many suggest speed limits of 16 miles per hour on small lakes, 25 mph on larger ones, but added, "I wouldn't go that fast regardless of where I am," saying he would keep speed down to 10 mph.

Extremely thick ice may prevent bow waves, Magnuson said.

New heavier ice houses do not create a problem themselves, Gulliver said, but if they are too close together they could. Experts suggest not parking vehicles next to heavy ice houses.

Kennedy works on snowmobile races and in preparing for them checks ice to make sure it is thick enough, setting up flags so riders can avoid areas that may not be safe.

But it is not just the ice itself that concerns him. Also of interest, he said, is whether "you have anything under that ice that is working to move heat away or move heat in."

For instance, a spring may add warmer water and currents may prevent ice forming as thick as it should for safety.

Kennedy said that in the springtime a southerly breeze can combine with more heat from the sun to thaw ice.

Spring rains also speed thawing.

Magnuson that if fish are right below ice, they bring warm water with them from lower levels and small holes may open through the ice. Small plant and animal life can emit methane, which also can force warmer water to contact the bottom of ice.

While most holes are small, large numbers of fish could make a hole "that would surprise somebody," Magnuson said.

The experts say lake and river ice are similar, although currents and other factors often present in rivers make them less predictable.

Shorter ice seasons have made even lake ice less predictable. Magnuson said in recent years state officials have banned heavy vehicles from some Wisconsin lakes and snowmobiles are not allowed unless they have floatation devices.

When it comes to climate change, the real problem may not be shorter ice seasons.

Gulliver said "what climate change has really done is given us warm spells and cold spells that are out of season. ... It is the variability in weather that has increased significantly." That means a warm spell in the middle of what otherwise is cold weather can weaken ice.