REGINA - If you ever wanted to visit a pristine Arctic lake, you're probably about 100 years too late.

A study involving 13 universities and research institutes, including two from Canada, has found nitrogen resulting from human activities has polluted remote lakes throughout the northern hemisphere for more than a century.

Simply put, burning fossil fuels and using agricultural fertilizers increases the amount of nitrogen in the atmosphere. The gas finds its way into lakes through rain or snow.

"Normally you'd think (gases would) be diluted in the vast quantity of atmosphere. There just wouldn't be a clearly measurable signature," said Peter Leavitt, a professor at the University of Regina, who was part of the study.

"And that's what's so surprising about these studies. Even in sites that are thousands of kilometres away from everything, we're seeing evidence of this atmospheric pollution, when there's just nothing around there in any direction that could have caused it."

The scientists, who also included researchers from the University of Alberta, measured the chemical composition of bottom deposits in 33 lakes from Colorado to the High Arctic and as far away as Norway.

Leavitt, who is the Canada Research Chair in Environmental Change and Society at his university, said the project was like aquatic archeology.

The scientists shoved a plastic tube into the mud at the bottom of a lake and pulled it out. They divided the tube into sections and estimated the age of each section by looking at the radioactive particles that occur naturally in the atmosphere.

Newer material is at the top and older material at the bottom.

Leavitt said more than three-quarters of the lakes showed signs of human-made nitrogen.

"The interesting wrinkle here is most of the lakes, particularly the ones in Canada, well up north of 60 (degrees latitude), are all thousands of kilometres from the nearest source of pollution," he said.

"It's not that somebody walked over and dumped nitrogen into the lakes."

Most previous studies point have looked at local sources of pollution.

One study found Regina's Wascana Creek has nitrogen levels over the maximums set by the World Health Organization. Another found Lake Winnipeg has dangerously high levels of nitrogen and phosphorus, which produce blue-green algae blooms so large they are visible from space.

But that's not necessarily a surprise given their nearness to populated areas.

"This is quite a different story," Leavitt said. "This is saying that humans have had effects for over a 100 years in some of the most remote ecosystems on the planet."

Leavitt also said if the changes in remote lakes were due to climate or local effects, there would have been a difference in when the nitrogen started to show up. That's not the case.

"The other piece of this that's important is that the timing is virtually identical in all the lakes -- about 110 years ago...just after the second industrial revolution. That's when you see the signature of the change in all the sites," he said.

"They're all doing it at exactly the same time, which is consistent with the atmosphere fundamentally changing."

Researchers warn such global nitrogen pollution may interact with climate change to produce a double whammy that could alter remote lakes in ways not seen in the last 10,000 years.

The input of nitrogen into the lakes changes the microbes that regulate greenhouse gas emissions, take gases out of the atmosphere and cycle nutrients. Leavitt calls them the "the building blocks of the food web in the lake."

"We don't know that it's changed to this point, but certainly we're seeing evidence that...the foundation of the food web is starting to change. And usually when you change a foundation, like in an earthquake, when the foundation starts shaking, it's not good news."

Nitrogen is a vital nutrient for life. Farmers apply fertilizers containing it to bolster food crops.

The U.S. Environmental Protection Agency says humans already have doubled the rate of nitrogen released into the biosphere since 1950. Leavitt warns that level will only increase as the world's population grows.

"We're going to have three billion more people on the planet within about 50 years. We want to feed them, we have the moral imperative of feeding them and we're going to do it by slathering on more and more levels of fertilizer," Leavitt said.

"The best prognosis, the best forecast if you want, of that nitrogen use is that it's going to double again from present values in the next 50 years, so this signature is really just the start of what is almost certain to follow."

The study appears in Friday's issue of the journal Science.