彩民宝典

Island nations could be forgiven for feeling slighted. They already face the brunt of the effects of climate change: Rising sea levels, dwindling resources, threats to infrastructure and economic foundations. But to add insult to injury, thousands of these islands are too small to be accounted for in the global climate models (GCMs) used by scientists to measure the effects of climate change.听

In a new study published in the journal听Nature Climate Change, a new way of modeling the effects of climate change on islands shows that previous analyses underestimated the number of islands that would become substantially more arid by mid century鈥�73 percent, up from an estimate of 50 percent. That leaves the population of those islands鈥攁pproximately 18 million people鈥攊n the position of being what CIRES Fellow Kris Karnauskas, the paper's lead author, and his coauthors refer to as 鈥渃omputationally disenfranchised.鈥�

It also means that what鈥檚 known about the effects of climate change on islands鈥� freshwater systems may have been woefully incomplete. GCMs show 50 percent of all small islands becoming wetter and 50 percent becoming drier, as far as rainfall goes. But those models by themselves don't take into account what happens on these unaccounted-for islands and, in fact, Karnauskas and his coauthors found that 73 percent of islands will actually become more dry as a result of increased evaporation.听

鈥淚slands are already dealing with sea level rise,鈥� says Karnauskas, also a professor of atmospheric and oceanic sciences at the 彩民宝典. 鈥淏ut this shows that any rainwater they have is also vulnerable. The atmosphere is getting thirstier, and would like more of that freshwater back.鈥�

The problem stems from the fact that GCMs aren鈥檛 all that fine-grained. These models divide the planet into a grid and each grid box is approximately 240 km by 210 km. That鈥檚 a pretty big space and if there鈥檚 a tiny island鈥攐r even an island chain like French Polynesia鈥攁lone in one of those grid boxes, it makes it impractical to include them in the model.听

鈥淭hink of pixels,鈥� says Karnauskas. 鈥淚f they鈥檙e too big to resolve the freckles on someone鈥檚 nose, you won鈥檛 be able to see those freckles. You have to have super fine pixels to resolve it, and frankly that鈥檚 not what global climate models were designed to do.鈥�

The 鈥減ixels鈥� of the GCMs are too big and scientists don鈥檛 have the computer resources yet to do something on a more refined scale. Take, for example, an island like Easter Island, which is听3,512 kilometres听off the coast of Chile in the South Pacific. Easter Island is small and it鈥檚 the only spot of land in its GCM grid box. Essentially, it鈥檚 a freckle and the GCM can鈥檛 get down to that level of detail. So, in the current GCMs, Easter Island doesn鈥檛 exist鈥攖hat whole grid square is just considered open ocean.

That鈥檚 the case with islands all over the globe and it鈥檚 a real problem when it comes to knowing what climate change will do to islands鈥� freshwater supplies. Unlike continents or larger islands, the effects of climate change on freshwater for these smaller, isolated islands aren鈥檛 being calculated.听

Paper after paper in my field show changes in drought or aridity,鈥� says Karnauskas. 鈥淏ut my eye always looks at the maps and graphs in those papers and I wonder why we can't see islands. Using models, it turns out, is much less straightforward for islands than for places where there are big chunks of land.鈥�

To understand how climate change will affect freshwater, scientists have to understand what鈥檚 happening with precipitation听and听evaporation. The first part is easier: Current GCMs can tell you all about precipitation over land or over the ocean. Even in a grid square like the one that鈥檚 home to Easter Island, they can estimate how much precipitation is likely falling from the sky.

But evaporation is another matter. When it comes to those same small islands, the models don鈥檛 show how much water is evaporating because those islands don鈥檛 exist in the models鈥攊t鈥檚 all ocean there. Nor can it be calculated using the amount evaporating off the ocean, as ocean evaporation follows different physical principles than water evaporating off land. Without knowing how much water is evaporating off these islands, there鈥檚 been no way to know exactly how the freshwater supplies are being affected. So Karnauskas and his former colleagues from the Woods Hole Institute in Massachusetts developed a way to get the information needed to know what鈥檚 happening on islands.

Karnauskas draws a diagram of a cube on a white board. 鈥淭his is a 3-D picture of an ocean grid cell,鈥� he explains. 鈥淪ay there鈥檚 an island in here. The climate model doesn鈥檛 have the island but let鈥檚 go to the location where there听ought听to be an island and use the information on the model atmosphere from directly over that cell.鈥澨�

Essentially, they鈥檙e looking at the climate听above听the surface of the island to make an approximation of the island鈥檚 actual climate. They can do this because many of the islands are so small that climate above the island isn鈥檛 much different from climate above the ocean, especially averaged over a day or longer. That鈥檚 been verified even on islands as large as Maui, where data from weather stations at airports shows surprisingly little difference from data from weather stations moored hundreds of kilometers offshore.

鈥淲e called it the blind pig test,鈥� explains Karnauskas with a grin. 鈥淚f you were a blind pig flying in this area, would you know there was an island here? Could you feel a difference in the heat or the humidity?鈥澨�

A 鈥渟uccessful鈥� blind pig test means you can鈥檛 tell if you鈥檙e over land or over ocean. If that鈥檚 the case, scientists don鈥檛 need to know anything from the land itself to predict evaporation; they just need to know what鈥檚 happening in the atmosphere right near the surface. From that information, and some tools borrowed from the engineering field, they can glean how much water is evaporating and, thus, get a more accurate picture of the ratio of precipitation to evaporation in a particular area.

Karnauskas sees this work as extremely important, both for understanding climate change in these regions and in considering human health and safety. A vast majority of the people living on these remote island rely on rainwater as the source of their drinking water. And for those that already have health issues due to water quality, increased pressure on freshwater systems will only exacerbate the problem. Already someone from the Cook Islands, an archipelago in the South Pacific Ocean, saw mention of his research online and reached out for more details.听

鈥淭here鈥檚 an opportunity to get important information out there,鈥� Karnauskas says. 鈥淭his is a framework to provide more accurate information on what to expect.鈥�

Contact:
碍谤颈蝉听碍补谤苍补耻蝉办补蝉,听303-735-4395
kristopher.karnauskas@colorado.edu
Laura听Krantz, CIRES communications,听303-492-1790
laura.krantz@colorado.edu
Katy听Human, CIRES communications,听303-735-0196听
kathleen.human@colorado.edu