by The number of lakes on our planet has increased substantially in recent decades, according to a new study.
There has been a particular increase in the number of small lakes, which unfortunately emit large amounts of greenhouse gases, the researchers report.
The findings are of great importance for the Earth’s carbon account, global ecosystems and human access to water resources.
Bacteria and fungi that feed on dead plants and animals at the bottom of a lake emit large amounts of CO2, methane, nitrous oxide, and other gases. Some of these gases end up in the atmosphere.
This mechanism causes lakes to act as greenhouse gas factories. In fact, freshwater lakes probably account for 20% of all global CO2 emissions from fossil fuels into the Earth’s atmosphere.
Forecasts suggest that climate change will cause lakes to emit an increasing proportion of greenhouse gases in the future.
This is just one of the reasons why it is important to know how many and how big these lakes are, as well as how they develop. Until now, this information was unknown.
The researchers have now 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.
The survey shows that between 1984 and 2019, the area of global lake surfaces grew by more than 46,000 kmtwo (about 17,760 square miles), slightly more than the surface area of Denmark.
“There have been major and rapid changes with lakes in recent decades that affect greenhouse gas bills, as well as ecosystems and access to water resources. Among other things, our new knowledge about the extent and dynamics of the lakes allows us to better estimate their potential carbon emissions,” says Jing Tang, assistant professor in the department of biology at the University of Copenhagen and co-author of the study 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 (10^12 trillion) of carbon, which is equivalent to the increase in UK CO2 emissions in 2012. .
Increasingly smaller lakes (<1 kmtwo) have appeared since 1984. The number of these small lakes is especially significant according to the researchers, because they emit the most greenhouse gases relative to their size.
While small lakes represent only 15% of the total lake area, 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.
“Small lakes emit a disproportionate amount of greenhouse gases because they typically accumulate more organic matter, which turns into gases. And also, because they are usually superficial. This makes it easier for gases to reach the surface and rise into the atmosphere,” says Tang.
“At the same time, small lakes are much more sensitive to changes in climate and weather, as well as human disturbance. As a result, their sizes and the chemistry of the water fluctuate greatly. Therefore, while it is important to identify and map them, it is also more demanding. Fortunately, we have been able to justify that.”
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 lake surface, that is, artificial lakes. The other half is created mainly by melting glaciers or thawing permafrost.
The new data set offers a range of regional and global applications, according to the researchers.
“I have sent our new estimates of greenhouse gas emissions to the people responsible for calculating the global carbon budget, who are behind the UN IPCC climate reports. I hope they include them in the updated global emissions figures,” says Tang.
“In addition, the data set can be used to make better estimates of water resources in freshwater lakes and to better assess flood risk, as well as better lake management, because lake area also affects the biodiversity”.
Font: University of Copenhagen
