Outside of Alaska, Arizona and New Mexico are warming more quickly than anywhere else in the United States. You’d therefore expect that plants and animals in the Southwest would be especially vulnerable to the impacts of climate change, while those regions that are warming more slowly, like the Midwest, would would be less sensitive. Why then do many studies—like the one by Travis Belote and coauthors I grabbed the above figure from—come to roughly the opposite conclusion?
The answer has to do with a metric called climate velocity and the body of research related to it. Climate velocity attempts to quantify how far a species would have to travel in order to track shifts in the many dimensions of climate they are adapted to (temperature, precipitation, seasonality, and so on). The discrepancy between realized climate change and climate vulnerability is therefore largely driven by regional climatic gradients and topographic complexity. In a place like New Mexico—where you have tall mountains with Chihuahuan desert scrub on their foothills and wet(ish) spruce fir forest on their peaks—species may be able to move short horizontal distances upslope, instead of long horizontal distances across many bands of latitude. In a topographically and climatically compressed state like Missouri, the analogous climates of the future will be far away indeed.
No one would say these metrics are perfect. They are a planning tool, and a necessarily crude one. Climate velocity underestimates vulnerability in montane communities compared to a similar measure (“minimum cumulative exposure”) that accounts for climatic dissimilarity in low valleys between adjacent peaks. It doesn’t take into account the potential for species to evolve to cope with new conditions, or how that might vary systematically across space. (There is fairly little biology involved at all, actually, though the metric called “biotic velocity” attempts to deal with this shortcoming, if indirectly.) But they do provide managers with detailed information about which areas are most likely to retain most of their species, which areas are likely to lose many species, and which areas might be useful as corridors for species on the move.
The concept of climate velocity will no doubt have a significant role to play as Tribes, state and federal agencies, and private landowners collaborate to meet the goals of 30 by 30, as the push to protect 30% of U.S. lands and waters by 2030 is known. 30 by 30 has emerged as a key plank of Biden’s climate agenda, and provides an opportunity to broadly rethink how we approach land and biodiversity conservation in this country. Yet at the same time, the initiative’s slogan-ready appeal is a consequence of substantial ambiguity in its goals and the yardsticks we’ll use in trying to reach them. What, for instance, does “protected” actually mean?
Here, as usual, conservation science can help—but only so much. The Greater Gila may have a relatively low probability of entirely losing species and climatic zones, for example, but it is all but certain to undergo massive shifts in the the distribution and relative area of different vegetation types. In other words, a metric like climate velocity, paired with the goal of preventing regional extinctions, will suggest a drastically different course of action than the goal of making sure there are still extensive ponderosa forests east of Reserve, or still silverleaf oaks in the lower reaches of canyons in the San Mateos. In sorting out priorities, there will be no substitute for conversation.
(If you’re interested in just such a conversation, I’m chatting with Zach Wurtzebach from the Center for Large Landscape Conservation and Jon Horning and Madeleine Carey of WildEarth Guardians about 30 by 30 in a webinar this coming Wednesday, May 5th, 5:00 PM MDT. Details and registration are available here.)