One morning a few years ago, at home in the Mānoa 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 Mānoa 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 posited the question to some climate scientist colleagues, and it intrigued them so much they enlisted a class full 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.
And though rainbows often instigate moments of joy like her own in Mānoa, more colorful skies are actually a signal of big problems across the globe.
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. That’s because their basic ingredients are common and governed by relatively straightforward physics.
“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. Finally, the sky must be showery rather than socked in with clouds.
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. Then they matched those up with places climate models predicted would have the right conditions for rainbows—the right amount of rain, without deck-like overhead clouds, at the right times of day and year. The models mostly matched the observations, meaning they could use them 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. But the big gains are concentrated in a few places.
“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 the sky and no one is there to see it, did it even happen? The most populated and rainbow-rich part of the world today, such as the Mediterranean and many islands, are forecasted to get less bows. Future hotspots are “not where big numbers of people live today, nor where they’re likely going to live in the future,” Carlson says.
The changes follow broader patterns of climate change; in fact, the shifts highlight some of the biggest 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 had been working on a similar project, but for a different purpose: to see how well climate models were working.
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). So “looking at whether the models reproduce rainbows well is a good way to see whether the system is totally out to lunch,” Gettelman says. So far, the models he has looked at have accurately predicted rainbow occurrences—and 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 cycles, retreating during the dry season and re-emerging, often with a vengeance, during the rainy season.
In Greece, the minor goddess Iris represents rainbows. She often served as a messenger between gods and mortals, usually carrying messages of war or conflict—or as a bridge 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, like Hawai’i, rainbows are practically woven into the landscape. Native Hawaiians have a wealth of different words to describe the arcs: Pūloʻu describes a rainbow arc that doesn’t touch the ground; kahili is a vertical fragment, shafting up to the sky above; and punakea is a bow that barely appears, made of tiny droplets that create only a faint smear of color.
Though the models don’t show major changes to Hawai’is rainbow landscape just yet. But “the overall trend is toward drought here in the islands,” says Steven Businger, a co-author of the paper from the University of Hawaii, Mānoa.
Until that plays out, Businger will take this tiny silver lining. “Climate change is usually dealing with droughts, disasters, destruction,” he says. “But here’s this way to look at it that’s more ethereal.”