Climate change, causing oceans to become more green

Posted on 16 July, 2023 by benyamin chahkandi

Highlights; 

• More than half of the world's oceans have become greener over the past 20 years, likely due to global warming. This was determined by analyzing 20 years of data on ocean color from NASA's Aqua satellite.
• The greening indicates increased phytoplankton and algae in the surface waters. Phytoplankton contain chlorophyll which makes the ocean appear greener.
• The tropical and subtropical waters between 40°S and 40°N latitude changed the most in color. These areas don't vary much seasonally, so long-term color changes are more visible.
• The observed ocean color changes matched those predicted by a model simulating effects of increased greenhouse gases. However, the causes are still uncertain.
• One hypothesis is that surface warming is increasing ocean stratification and reducing nutrient mixing, favoring smaller phytoplankton species and altering ecosystems.
• The findings highlight the need for hyperspectral monitoring of ocean color, like the upcoming NASA PACE mission, to better understand ecological impacts of color changes.

In the last 20 years, more than half of the world's oceans have turned green, most likely as a result of global warming. Scientists expected to need many more years of data before they could detect signals of climate change in the colour of the oceans, so their discovery—reported today in Nature1—came as a surprise.

According to lead author B. B. Cael, an ocean and climate scientist from the National Oceanography Centre in Southampton, UK, "we are affecting the ecosystem in a way that we haven't seen before."

The ocean's hue can change for a variety of causes, such as when nutrients rise from its depths and support massive phytoplankton blooms that are rich in the chlorophyll-containing green pigment. Scientists can determine how much chlorophyll is there and consequently how many living things like phytoplankton and algae are present by analysing the wavelengths of sunlight reflected off the ocean's surface. Theoretically, when ocean waters warm due to climate change, biological production should vary.

However, because the amount of chlorophyll in surface waters can vary significantly from year to year, it can be difficult to distinguish between changes brought on by climate change and large-scale natural fluctuations. Up to 40 years of data were anticipated by scientists to identify any trends2.

The fact that different satellites have recorded ocean colour in different ways over time, making it impossible to combine the data, is another complicated element. The MODIS sensor on NASA's Aqua satellite, which was deployed in 2002 and is currently orbiting the Earth well past its predicted six-year lifetime, was chosen by Cael's team to be the source of data analysis. Instead than focusing only on the one wavelength used to detect chlorophyll, the researchers examined seven distinct wavelengths of ocean light for patterns. For a very long time, Cael has believed that using the entire colour spectrum will improve things.

The researchers were able to detect long-term changes in ocean colour using two decades of MODIS data. 56% of the world's ocean surface showed noticeable changes, primarily in the regions between latitudes 40° S and 40° N. Because the areas don't suffer harsh seasons, these tropical and subtropical waters often don't vary greatly in hue throughout the year; therefore, tiny long-term changes are more noticeable there, according to Cael.

The measured light's wavelength affects the colour change's intensity. The seas are generally turning greener with time.

The researchers compared the data to the outcomes of a model3 that simulated how marine ecosystems may react to rising amounts of greenhouse gases in the atmosphere to see if the shifts could be connected to climate change. The model's predictions and the observed changes agreed.

various green hues
What is causing the waters to grow greener is the question at hand. Because the regions where the colour change was seen do not coincide with places where temperatures have usually increased, Cael believes that it is unlikely to be a direct result of rising sea surface temperatures. The distribution of nutrients in the water may play a role in the change, as one theory suggests. The stratification of the ocean's upper layers increases with warming surface waters, making it more difficult for nutrients to climb to the surface. Smaller phytoplankton have a higher chance of surviving when there are fewer nutrients, therefore changes in nutrient levels may cause changes in the ecosystem, which will be reflected in variations in the overall colour of the water.

The finding raises anticipation for NASA's Plankton, Aerosol, Cloud, and Ocean Ecosystem (PACE) satellite, which is set to launch on its next major mission to track ocean colour. PACE, which is scheduled to launch in January 2024, has the ability to monitor ocean colour in many more wavelengths than any other satellite to date, a feature known as "hyperspectral."

Ivona Cetini, an oceanographer at NASA's Goddard Space Flight Centre in Greenbelt, Maryland, who works on PACE, believes that all of this "definitely confirms the need for global hyperspectral missions like PACE." In the coming years, the spacecraft "should allow us to understand the ecological implications of the observed trends in ocean ecosystem structure."

source: www.nature.com/articles/d41586-023-02262-9