Species latitudinal range shifts are tracking high climate change signal-to-noise ratios

The way in which climate change is experienced by species will be highly variable, due both to the spatial and temporal heterogeneity of climate change as well as differences in species’ sensitivity to such changes. While previous methods for addressing variation in climate exposure have considered metrics of climate change speed or direction (e.g., climate velocity), these metrics do not account for historical climate variability. Species have evolved to accommodate historical variability in climate, and we expect range shifts to be more likely in areas where anthropogenic climate change exceeds the bounds of unforced climate variability. We assessed the historical relationship between climatic variability, the strength of the anthropogenic climate change signal, and observed terrestrial latitudinal species range shifts. We accounted for directional change in climate variables (signal) relative to natural variability (noise) using signal to noise ratios (SNR) for ecologically relevant temperature metrics and used linear regression to understand the relationship between SNR and observed species range shifts. We also looked at projected mid-century SNRs calculated from a large ensemble Global Climate Model to identify places where strong SNRs may emerge and where we may therefore anticipate significant range shifts. We found that as the climate becomes increasingly different from historical climate averages (i.e., as SNR increases) species shift faster on latitudinal leading edges. This is true independent of the magnitude of temperature change. The effects of SNR are more variable on trailing edges and within species ranges. Most range shift studies to date have taken place in North America and Europe, which are not the places that have experienced the highest SNR. While SNR is projected to increase globally by mid-century, increases will be highest in tropical areas. These areas might therefore be hotspots of future range shifts. It is important to consider historical variability experienced by the species in addition to absolute temperature change, especially for latitudinal leading edges. We discuss implications for studying future range shifts and range shift management.