When rising temperatures brewed up a perfect storm of excessive rainfall and extreme heat in the summer of 2014, the fallout hit home. The impacts were felt by at least half a million people in Ohio when a super-bloom of cyanobacteria, a toxic blue-green algae, shut down drinking water supplies for several days.

The algae feasted on fertilizers washed in from farm fields, and the warming may have pushed the lake past an environmental tipping point. In warming waters, the cyanobacteria thrived and spread in a slimy green froth across hundreds of square miles. Schools, restaurants and other public facilities shut down after contamination was detected in Toledo's water intake system, and local stores quickly ran out of bottled water. In the summer of 2015, the algae spread again.

Similar blooms in Lake Erie were common in the 1960s through the 1980s and then diminished, thanks to better pollution controls. But the outbreaks have increased once again in the past decade, according to NOAA.

Scientists already know that the rapid warming of lakes will increase the number and intensity of such blooms, as well as other disturbances like oxygen-deprived dead zones. A new study published June 23 in the journal Scientific Reports helps identify which lakes are most likely to be affected, information that can help people and communities develop adaptation plans amid climate change.

Based on an evaluation of 20 years (1991-2011) of data, the study concluded that deep lakes in cold climates—including the Great Lakes— are most at risk from global warming. As the buildup of greenhouse gases continues, some lakes have already passed climate tipping points. Many others could breach critical temperature thresholds in the next few decades.

Deep lakes are warming fast because they can store heat for a long time, which enables it to build up. By contrast, smaller, shallower lakes are also warming, but can cool down relatively quickly when temperatures drop. Lakes in cold climates are strongly affected by global warming as their ice cover melts earlier each year, allowing the sun's heat to penetrate the water much earlier in the spring.

Lead author Iestyn Woolway, from the University of Reading, said one of the biggest concerns is how lakes will respond to extreme heat events, as well as large, stair-step changes like the sudden upturn in global temperatures that started in the 1980s. Some lakes may be able to respond to gradual changes in temperature, but research shows that big shifts have already hit lakes hard, he said.

"The rapid temperature increase observed in lakes over the past 100 years could cause widespread damage to the ecosystem," Woolway said. "In terms of temperature thresholds that could 'tip' an ecosystem, the most likely to have significant impact would be 4 degrees Celsius."

When lake surface temperatures increase by more than that early in the spring, it disrupts the natural cycling, or upwelling, of deep, cold water to the surface. That persistent layering leads to oxygen-starved in the depths. Such dead zones are set to increase by 25 percent through the end of the century.

Illinois State University researcher Catherine O'Reilly said there's no doubt that climate change will shift lakes into new regimes.

"These tipping points or thresholds are likely created not just by climate change itself, but by interactions between climate change and other factors, such as climate oscillations like El Niño ... But now, each year is a new record-setting year, and because the climate itself does not return to normal, the lake is never able to recover completely," she said.

Knowing which lakes are likely to be affected and when it will happen will help communities prepare for the impacts, said Rensselaer Polytechnic Institute researcher Kevin Rose. Beyond cutting global greenhouse gas emissions and limiting warming, scientists emphasize that maintaining healthy ecosystems are the best way to make them resilient to climate change and other impacts. For fish and other freshwater organisms, the key to survival may be mobility.

"If there are fewer barriers to dispersal among lakes, like, dams, and road culverts, then fish and other species can move among water bodies to find better habitat. Removing barriers might help species maintain their preferred temperature ranges," he said.

Most mitigation measures would be based on site-specific conditions, Woolway said. Increasing monitoring would help detect temperature shifts in real-time to track the potential for any ecological disturbances.

"Creating climate sanctuaries would certainly help some threatened species and limit the impact of non-native species from dominating a system," he concluded.