Lake ecosystems, and the organisms that live within them, are vulnerable to temperature change, including the increased occurrence of extreme heat and cold. However, very little is known about lake heatwaves—periods of extreme warm lake surface water temperature—and how they may change under global warming.
According to a new study by Bangor University and the European Space Agency, published in Geophysical Research Letters, human-induced climate change is responsible for an increase in heatwaves within freshwater lakes.
Lakes are a vital source of freshwater and provide valuable services such as drinking water, irrigation, and food, as well as being a key part of the natural environment. Very little is known about the affect climate change has had, but this pioneering research is the first step towards seeking to redress the negative effects of heatwaves.
The analysis focuses on the frequency of heatwaves from a sample of the world’s largest lakes over a 20-year period and models how often they will occur under climate change.
It was found that lakes have experienced a six-fold increased number of heatwaves since 1995. The ‘hot’ years of 2010, 2012 and 2016, showed a particular increase, with the greatest number of heatwaves (157) being recorded in the monitored lakes in 2016.
Further examination shows that 94% of the observed heatwaves categorised as ‘severe’, can at least in part, be attributed to climate change.
Whilst the satellite data and modelling for the 78 large lakes studied suggested that these lakes were two and five-times more likely to experience severe heatwaves in a 1.5°C and 3.5°C warmer world, the news for smaller lakes is far worse.
Looking at a global scale, and including smaller lakes, the model suggests that severe heatwaves will be 25 times more likely in a 3.5 warmer world.
What really stood out was the magnitude of human contribution. Most of the severe lake heatwaves we looked at had a significant anthropogenic imprint. And looking at how these heat waves will change in the future, the magnitude of change we expect to see in the coming decades was quite striking.
94% of severe heatwaves observed at lakes in recent decades can be attributed in part to climate change
The Great Lakes are a chain of deep freshwater lakes. With a combined area of around 244 000 sq km, the lakes represent the largest surface of freshwater in the world – covering an area exceeding that of the United Kingdom.
Holding around 23 000 cubic km of water, Lake Baikal is the largest freshwater lake by volume in the world.
Effects could be worse in smaller lakes
Lakes are highly complex ecosystems, and vary in their ecology, depending on their location.
Some common effects of greater heat will include increased algal blooms, which remove oxygen from the water and pose a serious threat to animals and humans that depend on lakes as a freshwater source. Warmer waters also mean more evaporation and less mixing, as the lake’s water becomes stratified with hot water on top and cooler water trapped below. Both issues can mean less oxygen, which can stress lake dwellers like fish who need to breathe.
At threat in the great lakes are organisms which are living at the extreme end of their temperature range.
Iestyn added: “Unlike humans, who can get into air conditioning or construct emergency shade, there’s no escape for aquatic organisms when they are exposed to these extreme temperatures.”
“We can expect that if these large bodies of water such as the Great Lakes are experiencing these heatwaves, then the effects must also be happening, and could even be worse in smaller shallower bodies of water,” he added.
The next step for the research team is to link the physics with the ecology to better understand the implication for freshwater lakes.
