In the first global tally of lake color, scientists examined over 85,000 lakes worldwide.
|Approximately one-third of the world's lakes are blue, but rising temperatures may cause this to change. PLUS IANWOOL/ISTOCK/GETTY IMAGES|
Climate change may cause some picturesque blue lakes to lose their blueness in the future.
According to the first global tally of lake color, roughly one-third of the world's lakes are blue. However, if average summer air temperatures rise by a few degrees, some of those crystal waters may turn a murky green or brown, according to the team's report published in Geophysical Research Letters on September 28.
The changing colors could influence how people use the water and provide information about the stability of lake ecosystems. Lake color is determined in part by what is in the water, but other factors such as water depth and surrounding land use also play a role. Green and brown lakes have more algae, sediment, and organic matter than blue lakes, according to Xiao Yang, a hydrologist at Southern Methodist University in Dallas.
Yang and colleagues analyzed the color of over 85,000 lakes around the world using satellite images from 2013 to 2020. Because storms and seasons can temporarily change the color of a lake, the researchers concentrated on the most common color observed for each lake over a seven-year period. The researchers also created an interactive online map for exploring the colors of these lakes.
Dina Leech, an aquatic ecologist at Longwood University in Farmville, Va., who was not involved in the study, calls the approach "super cool." These satellite data are "incredibly powerful."
The scientists then examined local climates at the time to see how they might be related to lake color around the world. Temperature and precipitation records for many small or remote bodies of water do not exist. Instead, the researchers relied on climate "hindcasts" calculated for every location on the planet and pieced together from sparse records.
Lakes in areas with average summer air temperatures below 19° Celsius were found to be more likely to be blue than lakes in warmer climates. However, up to 14% of the blue lakes studied are close to that threshold. If average summer temperatures rise another 3 degrees Celsius, which scientists believe is likely by the end of the century, those 3,800 lakes may turn green or brown (SN: 8/9/21). Warmer water encourages algae blooming, which changes the properties of the water, giving it a green-brown tint, according to Yang.
Extrapolating beyond this small sample of lakes is difficult. "We don't even know how many lakes there are in the world," says study coauthor and aquatic ecologist Catherine O'Reilly of Illinois State University in Normal. Many lakes are too small to be reliably detected by satellite, but tens of thousands of larger lakes may lose their blue hue, according to some estimates.
If some lakes lose their blue color, people will likely lose some of the resources they have come to value, according to O'Reilly. Lakes are frequently used for drinking water, food storage, and recreation. If the water becomes more clogged with algae, it may be unsuitable for play or more expensive to clean for drinking.
However, color changes do not necessarily indicate that the lakes are in poor health. "[Humans] don't value a lot of algae in a lake, but if you're a certain type of fish, you might be like 'this is great,'" says O'Reilly.
Lake color can indicate the stability of a lake's ecosystem, with shifting shades indicating changing conditions for aquatic life. The new study provides scientists with a baseline for assessing how climate change is affecting Earth's freshwater resources. Continued lake monitoring could aid scientists in detecting future changes.
"[The study] establishes a baseline against which future results can be compared," says Mike Pace, an aquatic ecologist at the University of Virginia in Charlottesville who was not involved in the research. "That, to me, is the study's great power."
X. Yang and colleagues The color of the world's lakes. Geophysical Research Letters, Vol. 49, e2022GL098925, September 28, 2022, doi:10.1029/2022GL098925.