What if snow were black?

Snow is white and sparkly. On a sunny day, it can be very bright.

What if snow were black instead? Would it still stay frozen? Let's imagine it.

On a sunny winter day, fresh snow can be so bright that it is hard to look at. That brightness is sunlight bouncing back off the snow.

What if snow were black? In this thought experiment, you can follow three paths: why dark snow melts faster, how much sunlight different surfaces reflect, and what happens in real mountains when dust or soot darkens the snow.

Clean snow is highly reflective, and that reflectivity helps it persist: energy that is reflected cannot warm or melt the snowpack. Scientists describe this with albedo, a number from 0 to 1.

This thought experiment imagines snow with an albedo close to zero. Nothing about falling snow would change, only its color. The question is what that single change would do to how fast snow melts, how long it lasts and what happens to the ice beneath it. Real dark particles on snow give measured evidence; the fully black snow is reasoning, labeled as ideas.

Sunlight looks white, but it has all the colors in it.

Snow bounces almost all the light back. When all the colors bounce back together, we see white.

Sunlight is a mix of all the colors. Most things soak up some colors and bounce back others, and that gives them their color. Clean snow bounces back nearly all the visible light, without picking any color in particular, so it looks white.

The National Snow and Ice Data Center explains that the amount of sunlight snow reflects is called its albedo. Very deep snow can even look blue, because light that travels far into it loses some of its red.

NSIDC explains that most natural materials absorb some sunlight, which gives them color, whereas clean snow reflects most visible light without preference for any wavelength, so it appears white. Light that penetrates deep snow travels a long path, and the snow absorbs more red, leaving the blue tints seen in deep holes and ice.

How much sunlight a snowpack reflects is its albedo. Thick sea ice covered with snow reflects as much as 90 percent of incoming solar radiation. As snow melts and ponds form, albedo drops, the surface absorbs more energy, and melting speeds up: a feedback that links color directly to melt.

Let's pretend all the snow is black.

It might soak up sunshine and warm up. Snowmen might melt fast! Sledding days might be shorter. This is only pretend.

Now imagine snow falling black instead of white. On the first sunny day, it might soak up the sunshine instead of bouncing it away. It might warm up and melt much faster.

Snow might vanish from hills weeks earlier in spring, so rivers fed by melting snow might rise sooner and run low later in summer. Glaciers might lose their bright snowy covering earlier each year. These are ideas, worked out from what scientists see when dust or soot darkens real snow.

With albedo near zero, a snowpack might absorb almost all incoming sunlight instead of reflecting most of it, sharply increasing the energy available for warming and melting. Melt might begin earlier and proceed faster, shortening seasonal snow cover.

Knock-on effects might follow. Snow-fed rivers might peak earlier in spring. Glaciers might lose their protective snow cover sooner in the year, exposing ice to sunlight and warm air for longer, the same mechanism that a NASA-led study linked to the 19th-century retreat of Alpine glaciers. Because these are extrapolations far beyond measured cases, all of them are ideas, not predictions.

Dark things soak up sunshine. They get warm.

That is why a dark road feels hot on bare feet on a sunny day. Dark snow would get warm too.

Light colors reflect sunlight; dark colors absorb it. The sunlight that is absorbed turns into heat. NSIDC points out that you already know this: on a hot summer day, people avoid walking barefoot on dark blacktop.

Scientists measure reflection with albedo. A value of 1 means a surface reflects all the sunlight, and 0 means it absorbs it all. Snow-covered ice reflects up to 90 percent of sunlight, sea ice 50 to 70 percent, and the dark ocean only about 6 percent.

NSIDC defines albedo as the unitless fraction of incoming solar energy a surface reflects, mostly in visible light: 0 is a perfect absorber, 1 a perfect reflector. Absorbed energy heats the surface or, for ice and snow, melts it. Typical values: open ocean about 0.06, bare sea ice about 0.5 to 0.7, snow-covered thick sea ice up to about 0.9, melt ponds about 0.4 to 0.5.

Black carbon, NASA notes, is the strongest sunlight-absorbing particle in the atmosphere. NSIDC notes that dust lofted high enough can travel hundreds or even thousands of miles, carrying eroded soil, debris from construction, grazing and farming, and fine ash from wildfires onto distant snow.

Wind can blow dust onto snow in the mountains. The snow turns brownish.

Dusty snow melts weeks sooner than clean snow!

Real snow sometimes gets darker. In the San Juan Mountains of Colorado, wind carries dust from dry lands onto the snow. Studies found that a dusty layer can make snow disappear 21 to 51 days earlier in spring.

Soot matters too. A NASA-led study found strong evidence that soot from coal burning in industrial Europe darkened snow on Alpine glaciers. Between 1860 and 1930, large glaciers in the Alps retreated by nearly a kilometer on average, even though temperatures were dropping.

NSIDC reports that in southwestern Colorado's San Juan Mountains, a surface dust layer can shorten snow-cover duration by 21 to 51 days, whereas a 2 to 4°C rise in air temperature would shorten it by only 5 to 18 days. Buried dust layers re-emerge as clean snow above them melts.

A 2013 study in the Proceedings of the National Academy of Sciences, led by Thomas Painter of NASA's Jet Propulsion Laboratory, combined ice cores, records and a glacier model. Large Alpine valley glaciers retreated by nearly 1 km on average between 1860 and 1930 despite cooling; only when the darkening effect of industrial black carbon was included did the model match the record.