More rainbows are in our future—and that’s a bad omen

One morning a few years ago, at home in the Manoa Valley on Oahu, scientist Kimberly Carlson looked out the window and saw a rainbow so bright and vibrant it took her breath away.

That wasn’t a shocker: Hawai’i is possibly the best place in the world to see rainbows today, and Manoa has particularly ideal conditions for vibrant bows: Frequent rain showers and sunlight.

But Carlson, now an environmental sciences professor now at New York University, realized she didn’t know the answer to a simple question: Would climate change affect Hawaii’s–and the whole planet’s–breathtaking rainbows? She asked her climate scientist colleagues the question, and they were so intrigued that they recruited a class of students to investigate. In November, they published their findings.

“Climate change is affecting rainbows–now we know that’s true,” says Carlson, the lead author on the paper, which used computer models to simulate future rainbow-ready conditions. As major weather patterns morph because of climate change, many parts of the world–particularly places nearer to the poles, like Alaska or Siberia, will get more rain–potentially adding dozens more rainbow-rich days by the end of the century. (See pictures of vibrant rainbows around the world. )

“But there’s also a flip side,” she warns. The Mediterranean, southern Africa, and even parts of tropical South America are forecast to get drier in the future and could lose a solid chunk of their rainbow-producing days by 2100. Although rainbows can often bring joy, they are also a sign of bigger problems around the world.

A recipe for rainbows

“Rainbows are like weeds–they’ll pop up wherever they can, small or large, bright or sometimes really puny,” says Raymond Lee Jr., an optics and meteorology expert at the U.S. Naval Academy in Annapolis (Maryland). Because their basic ingredients are simple and governed by simple physics, it’s easy to see why.

“The basic recipe for seeing any segment of the natural rainbow,” says Lee, “is sunlit rain.”

First, you need rain droplets–the bigger the better, says Lee, since smaller droplets reflect and refract incoming sunlight in a way that causes the outgoing light waves to interfere with each other, dampening the brightness of the bows. Then there must be direct sunlight that cuts through the atmosphere at an angle less than 42 degrees from the viewer’s eye, which is during the morning or afternoon in most parts of the world. The sky must be clear and not cloudy.

Carlson and her colleagues figured out they could essentially search for those exact conditions in climate models, however fleeting and ephemeral, and compare them with real rainbow observations to confirm that the models were accurately predicting bows.

They combed Flickr, a photo-sharing website, for any photos tagged “rainbow,” anywhere in the world, over about a 10-year period. They then matched these up with climate models that predicted where there would be rainbows. That is, the right amount of rain without deck-like overhead clouds at the right times of the year. The models matched most of the observations, so they could be used to predict future rainbows.

However, Lee, who was not involved in the study, pointed out that the models failed to predict rainbows during outbursts of heavy rain, exactly when physics would dictate the most spectacular bows appear because of the bigger droplet size.

More rainbows coming

A clear pattern emerged when the team spun the climate models forward to 2100 and searched for the same rainbow-likely conditions: Overall, a hotter Earth means more rainbows.

The planet will see just a slight increase on average–roughly four to five extra rainbow-days a year, on top of today’s average of 108 to 117, depending on which models you look at. The big gains are concentrated in a few areas.

“The models predict massive increases in rainbow days in Russia, Canada, Alaska, and in places that have super-high elevation, like the Himalayan plateau,” says Carlson–adding 30, 40, even 50 rainbow-possible days a year. But if a rainbow appears in a sky but no one is there, does it really happen? The world’s most populous and rainbow-rich areas, such as the Mediterranean, many islands, and the United States, are expected to receive fewer bows. Carlson states that future hotspots will not be where large numbers of people live now or where they will likely live in the future.

The changes are a result of larger patterns of climate change. In fact, these shifts highlight the greatest risks and dangers. The increases in the Arctic, for example, are likely to play out because water that used to fall from the sky as snow will more often trickle down as rain in a hotter future. And today’s rainbow-rich Amazon is predicted to get more frequently gripped by drought–both because the forest, which currently creates its own rain, will lose that superpower as it shrinks, and because planet-wide climate change is shifting and morphing the major weather patterns that cause tropical rainfall.

A secret tool

When Andrew Gettelman, a climate scientist at the Pacific Northwest National Laboratory, heard about Carlson’s paper, he emailed her immediately. He was working on a similar project but for a different purpose: To see how climate models worked.

Climate models are great at lots of things, but they still struggle to accurately reproduce some of the parts of the rainbow recipe: rain showers and cloud cover (which dictates whether the sun can peak through and instigate a rainbow). Gettelman states that “looking at the models’ ability to reproduce rainbows well can help you determine if the system is completely out of control.” He has seen models that accurately predicted rainbows, and they match Carlson’s results.

“What’s likely happening [in the future] is there are fewer clouds and slightly more rain, which means that you have more opportunities to see rainbows because clouds are going down,” he says. What the models, and this research, suggests is that “clouds are getting slightly less common, and slightly thinner, in the future.”

A rainbow-rich, less-cloudy future could therefore be a sign of deep planetary troubles, he stresses, because clouds, particularly low-elevation ones, help cool the planet by reflecting incoming sunlight.

What rainbows mean: Hope, horror, and more

In that sense, a more rainbow-filled future could actually align with the way rainbows are seen in many cultures around the world, where they have traditionally been a sign of danger or risk rather than a positive omen. (Learn how there are 12 kinds of rainbows. )

“Think about it: They’re often associated with extreme weather,” says Lee–an intense downpour, moving quickly across the landscape, and interspersed with sunlight, is often a sign of dangerously volatile conditions.

In some Aboriginal communities in Australia, rainbows represent the Rainbow Serpent, a creator, but also a destructive force. Its appearance in rainbow form is tied to the seasonal monsoon cycle, retreating during dry season, and resurfacing, often with avengeance during the rainy seasons.

The minor goddess Iris in Greece represents rainbows. She was often a messenger between gods or mortals, often carrying messages of war and conflict–or as an intermediary between Earth and the afterlife.

And in ancient Babylonia, many calamities, such as a famed leader’s defeat in 651 B.C., were preceded by dramatic rainbows, cementing their role as a dangerous portent.

In other places, such as Hawai’i where rainbows are almost woven into the landscape, The Native Hawaiians have many words to describe the arcs. Pulou is a rainbow arc that doesn’t touch the ground. Kahili is a vertical segment that reaches the sky above. Punakea is a bow made of tiny droplets that create a faint smear.

Though the models don’t show major changes to Hawai’is rainbow landscape just yet. Steven Businger, co-author of the paper, from the University of Hawaii Manoa, says that “the overall trend towards drought here in islands” is a concern.

Until that plays out, Businger will take this tiny silver lining. He said that climate change is often dealing with droughts and disasters. “But here’s this way to look at it that’s more ethereal.”