Climate Change’s Impact on Ocean Colour

A study titled “Global climate-change trends detected in indicators of ocean ecology” has been published in the journal Nature.

Contents

Highlights

The ocean’s colour has undergone significant change over the last 20 years due to human-induced climate change.
More than 56% of the world’s oceans have experienced a shift in colour, surpassing the total land area on Earth.
Tropical oceans near the Equator have been most affected, with their waters turning greener.

The colour shift is observed in areas experiencing ocean stratification. Ocean stratification is the natural separation of the ocean’s water into horizontal layers based on density.
The features of ocean stratification include a warmer, lighter, and less salty nutrient-poor water layer on top of a heavier, colder, and saltier nutrient-rich water layer.
Normally, ocean ecosystems, currents, wind, and tides mix these layers, creating smoothed temperature and salinity transitions.
However, due to climate change, increased stratification has made it harder for these layers to mix, leading to several consequences:
- Decreased ability to absorb atmospheric CO2.
- Improper mixing of absorbed oxygen with cooler layers below, leading to depleted oxygen levels and threatening marine life’s survival.
- Hampered movement of nutrients from lower layers to the surface.
- Low nutrient conditions, favoring smaller plankton and altering the composition of the plankton community.

The change in ocean colour may not directly impact marine life but serves as an important indicator of the marine ecosystem’s state.
It reflects the state of flux in marine ecosystems and the potential risk of going completely out of balance in the future.

Oceans generally appear blue or navy blue due to the absorption and scattering of light.
When sunlight falls on deep and clear water, longer wavelength parts of the spectrum, such as red, yellow, and green, are absorbed by the water, while shorter wavelength parts, such as blue and violet, are reflected back, leading to the blue colour.
Different colours are observed when water is not clean or deep. For example, dead leaves and sediments can make Atlantic Ocean waters along Argentina’s coastline brownish, while the presence of phytoplankton in the upper surface of water makes it appear green in other parts.

The study’s data on ocean colour has been collected using the MODIS (Moderate Resolution Imaging Spectroradiometer) imaging equipment onboard the Aqua satellite of NASA since 2002.