More than one-third of hundreds of 25-square-km and above lakes in the world have become clearer over the past 20 years, according to a new study published online in the journal Scientific Data.
About 36 percent of the world's 1,049 large lakes and reservoirs saw their colors shifting from the yellow end to the blue end of the color spectrum, which generally means increasing clarity. They are concentrated in cold regions, such as the Qinghai-Tibet Plateau, northern Europe, northern North America and southern South America.
While 8 percent of these lakes, scattered all over the world, saw their colors shifting from the blue end to the yellow end between 2000 and 2018, implying decreasing clarity.
The study was based on remote sensing satellite images and conducted by scientists from the Aerospace Information Research Institute under the Chinese Academy of Sciences.
Lead researcher Zhang Bing said the study used a water color index to show the color variation and transparency change in lakes.
Water colour observation is based on the fact that clear water appears blue while turbid water turns green and/or yellow with increased levels of suspended sediment.
Compared with ocean water, lake water is complex and affected more by the surrounding environment and human activities.
Scientists believe global lakes have experienced dramatic changes in water quality under the impact of urbanization, population growth and climate change in recent decades, but there is a lack of consistent water color data.
The new study will rectify the data shortage, said Zhang. It offers a dataset for the spatial patterns and long-term change trends of water color worldwide since 2000.
His team has had previous successes in long-term water transparency observation of China's large lakes.
Zhang said further research is needed to find the reasons behind the color changes. They speculate that the chief cause would be global climate change.
"Rising temperatures brought on more glacial meltwater and precipitation in cold regions, leading to the increased volume of large lakes," Zhang told Xinhua. "If the increased amount of water does not bring in too much sediment or nutrients, the lakes will become clearer."
According to the study, the lake water data will be used to find the drivers of global and regional lake color change and the interaction mechanisms between water color, climate change and human activities. (Xinhua)
