More than half of the world’s oceans have become greener in the past 20 years, probably because of global warming. The discovery, reported in Nature, is surprising because scientists thought they would need many more years of data before they could spot signs of climate change in the colour of the oceans.

Satellite data shows that over the last 20 years, colour shifts from blue to green have occurred over 56 per cent of the world’s oceans. The changes are particularly evident in tropical regions near the equator.

“We are affecting the ecosystem in a way that we haven’t seen before,” says lead author B. B. Cael, an ocean and climate scientist at the National Oceanography Centre in Southampton, UK.

The ocean can change colour for many reasons, such as when nutrients well up from its depths and feed enormous blooms of phytoplankton, which contain the green pigment chlorophyll. By studying the wavelengths of sunlight reflected off the ocean’s surface, scientists can estimate how much chlorophyll there is and thus how many living organisms such as phytoplankton and algae are present. In theory, biological productivity should change as ocean waters become warmer with climate change.

But the amount of chlorophyll in surface waters can vary markedly from year to year, making it hard to differentiate any changes induced by climate change from the big natural swings. Scientists thought it might take up to 40 years of observations to spot any trends.

Another complicating factor is that numerous satellites have measured ocean colour over time, and each did so in a slightly different way, so the data cannot be combined. Cael’s team decided to analyse data from MODIS, a sensor aboard NASA’s Aqua satellite, which was launched in 2002 and is still orbiting Earth, far surpassing its anticipated six-year lifetime.

To see if the shifts could be linked to climate change, the researchers compared the observations to the results of a model that simulated how marine ecosystems might respond to increasing levels of greenhouse gases in the atmosphere. The observed changes matched those in the model.

Shades of green

Now, the question is what is turning the oceans greener. It’s probably not a direct effect of increasing sea surface temperatures, Cael says, because the areas where colour change was observed do not match up with those where temperatures have generally risen. One possibility is that the shift might have something to do with how nutrients are distributed in the ocean. As surface waters warm, the upper layers of the ocean become more stratified, making it harder for nutrients to rise to the surface. When there are fewer nutrients, smaller phytoplankton are better at surviving than larger ones, and so changes in nutrient levels could lead to changes in the ecosystem that are reflected in changes in the water’s overall colour.

But this is just one idea; the researchers can’t yet say exactly why the changes are happening. “The reason we care about the colour is because the colour tells us something about what’s happening in the ecosystem,” Cael says.

Coverage of this story can be read in the Daily Mail, EuroNews, Guardian, Grist and Time. The full peer-reviewed paper in Nature can be read here.