Why oceans look blue, green or brown

Fly from the open ocean toward a river mouth and the water can seem to change character in minutes. Deep blue becomes green, then tan or brown. After a storm, a coastal plume may push that boundary kilometres offshore. None of those colours is simply painted onto the surface.

What we see is the small fraction of sunlight that survives a round trip: through the atmosphere, into the water, past whatever the water contains, and back toward an eye or sensor. Ocean colour is therefore less like a pigment sample and more like a report of what happened to light along the way.

Water itself selects blue

Sunlight contains many colours of visible light. Each corresponds to a range of wavelengths, the distances between neighbouring wave peaks. When it enters water, some is reflected at the surface, but much of it penetrates. Water molecules do not treat all wavelengths equally. They absorb red light more strongly than blue light. As the path through clear water lengthens, reds disappear first; the light that can remain and be scattered back toward an observer is weighted toward blue. NOAA describes the ocean as a sunlight filter.

This also explains why a glass of water does not look dramatically blue. The optical path through the glass is too short for the wavelength difference to become obvious. Across metres of clear water, the selective absorption accumulates.

Scattering matters too. Light can be redirected by water molecules and by tiny material in the water. The colour reaching us depends on both absorption, which removes photons from a beam, and scattering, which changes their direction. Oceanographers call the light emerging from the water water-leaving radiance. It is only a small part of everything a satellite sees above the sea.

Green can be a sign of life

The open ocean is not pure water. Near the sunlit surface live microscopic algae called phytoplankton. Their pigments, including the green pigment chlorophyll, absorb light for photosynthesis. This is the process by which they use light energy to make food. Chlorophyll absorbs strongly in blue and red parts of the visible spectrum while leaving a greater green contribution to escape. When phytoplankton are abundant, water that would otherwise look blue can shift toward green.

That is a useful clue, but colour is not a species label. Different organisms contain different pigment mixtures, cells scatter light, and minerals or dissolved substances may be present at the same time. A green patch may indicate high phytoplankton concentration, but a precise biological interpretation requires measurements across several wavelengths and comparison with observations in the water.

NASA's PACE programme studies the spectrum, the amount of light returning at different wavelengths. Its Ocean Color Instrument measures many narrow bands rather than only the three broad red, green and blue channels used in an ordinary photograph. Those observations help researchers distinguish optical patterns associated with phytoplankton, particles and dissolved material. NASA explains why PACE observes ocean ecology and colour together.

Brown water is not automatically dirty water

Rivers, waves and storms can keep mineral grains suspended. These particles scatter and reflect light efficiently, often raising the red and green portions of the returning signal and giving coastal water a tan, yellow-brown or muddy appearance. Shallow water may also reveal the colour of sand, rock, seagrass or coral beneath it because some light reaches the bottom and returns.

Another influence is coloured dissolved organic matter, abbreviated CDOM. It comes from decaying plants and other organic material washed from land or produced in water. CDOM absorbs shorter blue wavelengths more strongly and can make water look yellow, tea-coloured or dark brown even when there is little suspended mud.

These causes can overlap. After a storm, a river mouth where fresh and salt water mix, called an estuary, may contain mineral grains, CDOM and phytoplankton at the same time. That is why the interactive model below uses relative influences and an indicative swatch rather than pretending that one displayed colour uniquely identifies a substance.

Depth, bottom and angle change the answer

Colour is partly a property of the scene, not just the water sample. In deep water, the bottom contributes nothing because the light cannot complete the journey. In clear shallows, pale sand can make turquoise water; a dark seagrass bed can make the same water look green or nearly black. White foam scatters most visible wavelengths and can appear bright even above dark water.

The Sun's height, surface roughness and viewing angle also affect the path. Glitter from the surface can overwhelm the much weaker water-leaving signal. Clouds cast shadows. Light scattered by the atmosphere also contributes to the satellite signal. PACE uses polarimeter measurements to help estimate and remove that contribution before interpreting ocean colour. This step is called atmospheric correction. PACE explains how polarimeter measurements help correct for atmospheric light.

Turning colour into measurement

For centuries, sailors and naturalists compared water by eye. Spaceflight transformed that local observation into repeated global mapping. NASA's Coastal Zone Color Scanner, carried by Nimbus 7 from 1978, was the first spacecraft instrument devoted to ocean colour. It measured several spectral bands and demonstrated that patterns associated with chlorophyll and suspended material could be mapped from orbit. The Smithsonian records the instrument and its 1978–1986 operation.

Modern sensors measure the spectrum more finely, but they do not simply photograph “the amount of plankton.” Engineers calibrate detectors, remove atmospheric effects and apply algorithms that relate spectral signals to quantities such as chlorophyll concentration. Scientists then test those estimates against measurements from ships, buoys and autonomous instruments. The final map is an inference with uncertainty, not a direct chemical analysis of every pixel.

That distinction makes ocean colour more interesting, not less. A blue region of circulating ocean water, a green growth of algae and a brown plume spreading from a river are the visible outcomes of molecular absorption, microscopic life, geology, weather and viewing geometry. The sea is not one colour because it is never only water. The light returning from it has travelled through a story.