Rising temperatures to make oceans bluer and greener

newquay in cornwall
Newquay in Cornwall. The north Atlantic is among the oceans expected to change colour if sea temperatures rise. Photograph: Ian Woolcock/Getty Images/iStockphoto

Scientists say effects of global warming on ‘phytoplankton’ will intensify the colours

— The blues and greens of the ocean will become even bluer and greener by the end of the century as a result of global warming, scientists have found.

Researchers say the colour changes are down to the effect of climate change on populations of tiny water-dwelling organisms, known as phytoplankton, that convert sunlight into chemical energy through photosynthesis, as well as effects on levels of other colourful components of the oceans.

“In the same way that plants on land are green, phytoplankton are green as well, so the amount and different types of phytoplankton affect the colour of the ocean surface,” said Dr Anna Hickman, co-author of the research from the school of ocean and earth science at the University of Southampton.

Being able to detect changes in levels of such organisms is important, Hickman added. “We are interested in phytoplankton because they are tiny marine plants, they contribute about half of global photosynthesis, they are the base of the marine food web.”

Writing in the journal Nature Communications, Hickman and colleagues from the UK and US report how they came to their conclusions by using a computer model that predicts how factors such as temperature, ocean currents and ocean acidity affects the growth and types of phytoplankton in the water, as well as levels of other coloured organic matter and detritus. However, in an important twist to previous studies, they also explored how such changes would affect the absorption and reflection of light at the ocean surface.

marine diatom cells
Marine diatom cells (Rhizosolenia setigera), which are an important group of phytoplankton. Photograph: Karl Bruun/AP

 

The results reveal that if the temperature of the global sea surface rises by 3°C (5.4°F) by 2100 – as expected from the current scenario – the colour of more than half the oceans, including the north Atlantic, will change.

Hickman said: “Crudely speaking, where the water is currently quite blue because the phytoplankton [have a] relatively low biomass, you are going to see the water getting more blue, and where the ocean is relatively more green because the biomass is higher, you are going to see [it] getting [greener].”

That, the authors say, reflects shifts in phytoplankton populations, and knock-on effects, that stem from factors including a retreat of sea ice near the poles, rising temperatures and less mixing of waters.

“Looking at just the colour of the ocean, and how that is going to change in the future by monitoring it from satellites, is actually going to give us an early warning signal of changes in the phytoplankton,” said Hickman.

Importantly, she said, the shift in reflectance of blue/green light appeared to give an earlier indication of changes to phytoplankton than estimates of the amount of chlorophyll present, a measure currently used to monitor phytoplankton levels.

That, the researchers said, was probably down to a number of factors, including that shifts in ocean colour take into account not only changes in the overall amount of phytoplankton – which can vary dramatically, for example with the season – but also changes in the species present, an important consideration since different types of phytoplankton use chlorophyll yet absorb slightly different wavelengths of light.

Hickman said this was helpful, as using satellite data to examine specific changes in blue/green light can reveal changes in the phytoplankton population. This can then provide a more accurate reflection of the climate trend than calculating chlorophyll levels from this data, which only gives a sense of the amount of phytoplankton.

However, it seems artists need not worry about upgrading their palettes. “I would probably predict that the colour change isn’t going to be perceptible to the naked eye,” said Hickman.


by Nicola Davis | The Guardian