For nearly 50 years, there’s been consistent data from Trout Lake in Vilas County. There’s also less consistent data in the summer months going back to the 1920s.
That long term data shows lake temperature gradually increasing from the 1920s to the early 2000s.
In the last 25 years, it’s been warming faster with the lake now about 1 degree Celsius warmer in the summer months than it was 100 years ago.
There is consistent data for Lake Mendota in Madison going back 150 years that shows the same trend.
“It's very clear that temperatures are warmer in the summer. The July and August average temperatures are statistically, significantly higher than they were in the early 80s, and even further back from that time, like than like 100 years ago, they're warmer,” said UW Trout Lake Station limnologist Carol Warden.
There’s less long-term data for September and October, but the data going back to the 80s shows, water temperatures on Trout Lake are also increasing in the Fall.
“When you go for a late September swim thinking you're going to be a cold-water dipper, that might not be the case anymore for some of us,” said Warden.
One degree might not sound like a lot, but as Warden points out people are able to feel the difference in temperatures when treading water in lakes just between our feet and upper body.
“So certainly, all the other biological things that actually spend their whole lives in the lakes are going to be able to pick up on those changes as well, and the thing about it too is the trajectory seems to be in only one direction,” said Warden.
The planet’s 10 warmest years on record have all occurred since 2015.
As human-driven climate change increases air temperatures, Warden says the lake temperatures will rise with them.
We’re already seeing some of the impacts.
The number of days of ice on the lakes have dropped by about 10 days since 1982.
Warden says bigger impacts will be on cold water species who will dive deeper into the lakes in search of colder water.
“That bottom part of the lake is going to stay cold. No matter what, there will be cold water down there. However, you need oxygen too, so that's where those confounding factors come in,” said Warden. “If you have a lot of algae growth, once those algae start dying and sinking to the bottom, that oxygen is going to get used up by bacteria eating those algae.”
Warden puts increased algae growth among the top of the list for impacts as lake water warms. Heat is a major factor in algae blooms.
She also reminds people that while things like blue-green algae can get a bad rap because of the negative health impacts on people and pets, it’s an essential part of lake ecology.
“If you want to go to a fish fry, you got to have some algae to start that process, kind of thing,” said Warden. “It's always about when it gets out of balance. So even if it gets warmer up here and our nutrients are in check, we shouldn't have an issue with imbalance.”
Limiting shoreline development is one of the best ways to reduce nutrient runoff into lakes.
“I like to challenge people to think about the one thing or two things that you can offer the lake, and the easiest one that we can do as riparian landowners with lakefront property is to keep that shoreline natural,” said Warden. “It's one of the best ways that we can promote healthy water, a healthy environment, a stable shoreline, esthetic beauty, all these great things.”
