How to cool lakes while the world warms

Despite global warming, water temperatures in lakes may be lowered significantly by planting trees, research from York University shows. 

Clearwater Lake, a small lake on the outskirts of Sudbury, Ont., has cooled dramatically since the 1970s, in part because of a massive tree-planting program in the area, according to the study "Cooling lakes while the world warms," published in the January 2008 issue of Limnology and Oceanography.

"We wanted to find out why the lake was cooling despite regional climate warming," said lead author Andrew Tanentzap, a former York student who led the study with York biology Professor Norman Yan. Tanentzap graduated from York last year with an MSc in biology and is continuing his studies at Cambridge University, with a prestigious Gates Cambridge Scholarship. (See the June 25, 2007 issue of YFile.)

Left: Andrew Tanentzap

"What we discovered was that, over the years, the growth of all those trees lowered wind speeds in the area, which reduced mixing between warmer and cooler layers in the lake, allowing more of the bottom of the lake to stay cool."

More than eight million trees were planted in the Sudbury area from the mid-1970s to mid-1990s, to address massive deforestation due to sulphur dioxide emissions from Sudbury metal smelters, which had also acidified thousands of lakes.

"As Clearwater Lake recovered from acidification and the level of dissolved organic carbon increased, water clarity was reduced," said Yan. "That was a good thing because it meant less light could penetrate into the lake and warm it. However, I suspected reduced water clarity might not be entirely responsible for the lake cooling, so I asked Andrew to study the joint effects of wind speed and water clarity on the lake’s cooling pattern."

Ontario Ministry of the Environment data for Clearwater Lake from 1973 to 2001 reveals a profound pattern of deep water cooling in the lake. Daytime surface temperatures did not change over the nearly three decades, but bottom temperatures dropped by seven degrees, from 15 Celsius to 8 Celsius, and the cooler layer of water had grown substantially. This occurred despite a longer ice-free season, an important sign of global warming. However, at about the same time, local wind speeds had dropped dramatically, by 34 per cent between 1975 and 1995, and Yan wondered if there was a link.

Right: Norman Yan

Using a model developed by the University of Western Australia’s Centre for Water Research, Tanentzap was able to simulate lake mixing and heating processes in the lake. For expertise on wind, he looked down the hall at York to an international expert in another discipline, Professor Peter Taylor, in York’s Department of Earth and Space Science & Engineering, who specializes in atmospheric science.

By recomputing existing data, Taylor discovered that wind speeds at greater heights had not changed, but wind speeds at ground level had fallen substantially. This decrease in wind speed had reduced mixing between warm surface water and the cooler waters below, Tanentzap found. Tanentzap and Taylor’s study "On Sudbury-area wind speeds – a tale of forest regeneration," in the October 2007 issue of the Journal of Applied Meteorology and Climatology, demonstrates conclusively that reforestation in the Sudbury area was the major cause of reduced wind speed near ground level.

The reduction in water temperature in Clearwater Lake over the past 30 years or so has created habitat for lake trout, a clear indicator of the improvement in water quality since the 1970s. As important, Tanentzap’s study shows that it is necessary to understand all factors that can affect water clarity and wind speed in order to predict the effects of global warming on our lakes, said Yan. Taylor’s findings on reforestation and wind speed are equally important, with implications for many things ranging from the biology of lakes, to home heating costs and wind energy potential.