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There are many units by which to measure the impact of climate change: degrees of increasing temperature, feet of rising sea level, dollars needed to adapt to a warming world. But a group of scientists in California have put forth an intriguing new unit of measurement: kilometers per year.

Writing in a paper published Wednesday in Nature, scientists describe what they call the velocity of climate change, or more specifically, the speed of Earth's shifting climatic zones. As global temperature rises over the next century, the scientists argue, Earth's habitable climatic zones will start moving too, generally away from the Equator and toward the poles. That means many species of plants and animals will also have to move in order to survive. Whether or not they do will depend on several factors, but two of the most important are how fast a species can adjust its habitat range, and how quickly that range is moving out from under it.

Until now, ecologists have mostly focused on these factors as they affect individual species, but the new paper takes a more global view. By combining temperature projections on a very fine scale with global topographic maps, researchers have predicted change not for specific species, but for the climatic zones they need to keep up with.

Indeed, because global temperature is rising now, ecosystems are already on the move. "Once you explain it to people, it makes intuitive sense," says co-author David Ackerly, a University of California, Berkeley, biologist. "We know what it's like to drive north to escape the heat. It's concrete, rather than the abstractness of rising average temperatures."

More than intuitive, this new index could also prove very useful, especially to conservationists who work to keep species from extinction. While the average velocity of climate change may be a bit less than a half-kilometer per year worldwide, according to the paper, it can be significantly faster or slower depending on the local topography. In deserts and other flat areas, such as the Amazon basin, climatic zones will move faster, while hilly or mountainous terrain will slow things up. "In the Northern Hemisphere, for example," explains lead author Scott Loarie, "north-facing slopes tend to be cooler and wetter than south-facing slopes."

In short, opposite sides of a mountain may have different climates, even though they're close to each other. In areas with varied terrain including lots of hills, therefore, hospitable conditions might be available relatively nearby. "That was the unexpected message," says Loarie, an ecologist at the Carnegie Institution for Science at Stanford University. "There's lots of buffering capacity in heterogeneous landscapes."

According to the velocity maps that Loarie and his colleagues put together, only 8% of the world's national parks and other preserves will retain their current climate over the next century, compounding the problem of how to keep species from going extinct. One way to do that is simply to move them. But that's not only extraordinarily difficult, it can also backfire — just ask anyone in the southeastern U.S. about the inexorable advance of the imported invasive species the kudzu vine. "For some species on the brink of extinction, physically moving them might be our only option," says Loarie, "but setting aside connected, heterogeneous landscapes that allow natural movement will almost certainly be an better use of conservation dollars."

He and the other co-authors, including scientists at Climate Central in Palo Alto and the California Academy of Sciences in San Francisco, emphasize that their velocity maps are oversimplifications — at least so far. For one thing, they do not account for the unique characteristics of various species within a given ecosystem. Some species may have more tolerance for climate changes than others, and may not need to move as quickly; some species may be intolerant of change but unable to move. Other species may be sensitive to changes in rainfall, while still others responsive only to temperature — and changes in these weather patterns may not happen at the same rate. "The complexity is daunting," says Ackerly.

Nevertheless, while the climate-velocity concept is still crude, it's promising enough that Ackerly is collaborating with an organization called the Bay Area Open Space Council on habitat conservation strategies in central California. The new research informs one of the key challenges conservationists face: having only limited funds to buy up land, and, thus, having to spend wisely. "What we bring," says Ackerly, "is the ability to think about how topography might affect those decisions."

Michael D. Lemonick is the senior science writer at Climate Central.

• Michael D. Lemonick
• Whether species can survive climate change comes down, in part, to two factors: how fast it can adjust its habitat range, and how quickly that range is moving out from under it