Last week’s issue of the Proceedings of the National Academy of Sciences included a compelling article by western fire researcher Tony Westerling and colleagues. The title, “Continued warming could transform Greater Yellowstone fire regimes by mid-21st century,” caught the attention of a lot of blogs and other media outlets.
Westerling and his co-authors modeled changing fire regimes in the Greater Yellowstone ecosystem by looking at past correlations between climate data and the size and occurrence of large fires, then projecting those trends forward to see how often fire could be expected to burn a given area under likely future climate conditions. Today the Greater Yellowstone area is dominated by conifer forests that are characterized by infrequent but severe fires. Every 100-300 years or so, major fires sweep through these ecosystems, killing a large proportion of trees in the affected area and starting the succession process over by providing shade-sensitive species with access to sunlight.
However, this new research suggests that an increase in temperatures of just a few degrees by mid-century could have profound effects both on patterns of fire in the Yellowstone area and on the ecosystems and species found there. All of the modeling results pointed to a more rapid fire cycle, with a given area burning every 30 years or less by 2050. As Westerling et al. point out, this kind of fire regime would also indicate a significant shift from the current mixed conifer forest type to something much different, something more like a dry woodland or unforested ecosystem. Such a complete shift in vegetation would obviously have dire impacts on many of the species that currently inhabit that area.
When we talk about helping ecosystems adapt to climate change, we often tend to imagine – and plan for – a gradual, almost imperceptible shift in conditions over long periods of time. But many scientists have shown that ecological systems can contain hidden transition points, thresholds beyond which rapid, extreme changes in ecosystem structure and function may be unavoidable and virtually irreversible. Westerling’s paper shows us is in quite vivid terms what ecological thresholds might look like on the ground, and it gives us a frightening glimpse of how soon climate change might start pushing us across these thresholds. How would we go about managing the transition to an unforested Yellowstone?