56%! The global ocean is becoming increasingly green and blue sphere | Aqua | Global
When people describe Earth's home, they always like to say it is a beautiful blue planet. This is because from space, the Earth, which accounts for 70% of the total area of the ocean, is a blue "water ball".
But in the future, the "blue ball" may become the "green ball" because researchers have found that over the past 20 years, more than half of the world's oceans have become greener due to climate change.
An image captured by NASA's Aqua satellite in 2004 showed an outbreak of Chlorella in the waters near Brittany, France, causing the sea to turn green. The study, published in Nature on July 12th, surprised scientists who had previously believed that more years of data needed to be collected to glimpse signs of climate change from the colors of the ocean.
"We are influencing ecosystems in unprecedented ways." The main author of the paper, marine and climate scientist B B. Cael said.
The color of the ocean can change due to various factors, such as the massive influx of nutrients from the depths of the ocean, causing explosive growth of phytoplankton, which in turn contain the green pigment chlorophyll.
Scientists can estimate the amount of chlorophyll by studying the wavelength of reflected light on the ocean surface, thereby understanding how many phytoplankton are present in the ocean.
In theory, the productivity of phytoplankton will change with the warming of seawater. However, the chlorophyll content in surface water changes significantly every year, making it difficult to distinguish between climate change and changes caused by significant natural fluctuations. Therefore, scientists believe that it may take up to 40 years of observation to discover any possible trends.
In addition, over time, many satellites have measured the color of the ocean, but each satellite's measurement method is slightly different, making it impossible to integrate data.
In this study, the Cael team decided to analyze data from the MODIS sensor on NASA's Aqua satellite. The Aqua satellite was launched in 2002 and is still orbiting the Earth.
The research team analyzed the trend data of seven different wavelengths of light from the ocean, without selecting a single wavelength of light. By analyzing MODIS data over the past 20 years, they can observe long-term changes in ocean color.
Researchers have found that 56% of the ocean surface has undergone significant changes, mainly concentrated in the sea area between 40 ° S and 40 ° N latitude. These tropical and subtropical waters have not experienced extreme seasonal changes, so the color changes of the ocean throughout the year are usually not significant, but because of this, small long-term changes are more pronounced there.
The intensity of ocean color changes depends on the wavelength of the measured light. Overall, as time goes on, the seawater becomes increasingly green.
In order to understand whether these changes are related to climate change, researchers compared the observed results with models that simulate how marine ecosystems respond to the increasing levels of greenhouse gases in the atmosphere, and found that the changes in the two matched.
So, what exactly makes the ocean greener? Cael believes that turning green may not be a direct impact of rising sea surface temperatures, as the observed areas of color change do not match those where temperatures generally rise.
The research team believes that one possibility is that this color shift is related to the distribution of nutrients in the ocean. As the surface seawater warms, stratification becomes more pronounced, making it more difficult for nutrients to rise to the surface. When there are fewer nutrients, smaller phytoplankton are more likely to survive. The ecological changes caused by changes in nutrient levels are ultimately reflected in the overall color changes of seawater.
This discovery has raised expectations for NASA's launch of the "Plankton, Aerosol, Cloud, and Marine Ecosystem" satellite in January 2024. The observation focus of this satellite is on the impact of climate change on ocean health, and it will observe ocean colors at more wavelengths than any other satellite in the past.
