Small lakes are rising in number, and why is it bad for climate?

While large lakes on every continent are drastically shrinking and drying up by leaps and bounds, smaller ones are springing up elsewhere. They consist of artificial reservoirs to collect water and also lagoons created by the melting of permafrost or glaciers, due to climate change. It is these smaller, shallow lakes that are also contributing to global warming with the emissions they cause.

Between 1984 and 2019, the surface of lakes worldwide grew by more than 46,000 square kilometres, which is equivalent to more than half of Andalusia, increasing its greenhouse gas emissions.

Scientists at the University of Copenhagen (Denmark) and other universities have prepared a more accurate and detailed map of the world’s lakes than ever before.

The researchers mapped 3.4 million lakes and their evolution over the past four decades using high-resolution satellite imagery combined with artificial intelligence.

Greenhouse gases

‘There have been major and rapid changes in lakes in recent decades that affect greenhouse gas accounts, as well as ecosystems and access to water resources. This new knowledge about the extent and dynamics of lakes allows us to better estimate its carbon emissions potential,’ explains Jing Tang, assistant professor in the Department of Biology and co-author of the study, which was just published in Nature Communications.

According to the study’s calculations, the annual increase in CO2 emissions from the lakes over the period is 4.8 teragrams (10 to the power of 12 trillion) of carbon, which is equivalent to the increase in UK CO2 emissions in 2012.

Since 1984 smaller and smaller lakes (<1 km2) have appeared. This fact is especially important according to the researchers because they are the ones that emit the most significant amount of greenhouse gases in relation to their size. Although small lakes represent only 15% of the total area of the planet, they account for 25% of CO2 emissions and 37% of methane. In addition, they also contribute 45% and 59% of the net increases in CO2 and CH4 emissions from the lake, respectively, over the period 1984-2019.

Since 1984, a large number of lakes with an area of ​​less than one square kilometre have appeared, and these small freshwater bodies are the ones that emit the most greenhouse gases in relation to their size. While small lakes account for only 15% of the planet’s total lake surface, they account for 25% CO₂ emissions and 37% methane.

They accumulate more organic matter

“Small lakes emit a disproportionate amount of greenhouse gases, because they normally accumulate more organic matter, a matter that turns into gases. And also, because they tend to be shallow. This makes it easier for gases to reach the surface and rise into the atmosphere’ Jing Tang explains, assistant professor in the Department of Biology and co-author of the study.

‘At the same time, small lakes are much more sensitive to changes in weather and climate, as well as a human disturbance. As a result, their sizes and water chemistry fluctuate greatly. So while it is important to identify and map them, is also more demanding. Fortunately, we have been able to justify this”, he adds in a statement.

The mapping also reveals that there are two main reasons for the appearance of many new lakes on Earth: climate change and human activities. Reservoirs account for more than half of the increase in the lake area, that is, artificial lakes. The other half is created mainly by melting glaciers or thawing permafrost.

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