More is not always better: delta-downscaling climate model outputs from 30 to 5 min resolution has minimal impact on coherence with Late Quaternary proxies

<p>Both proxies and models provide key resources to explore how palaeoenvironmental changes may have impacted diverse biotic communities and cultural processes. While proxies are thought to provide the “gold standard” in reconstructing the local environment, they only provide point estimates for a limited number of locations. On the other hand, models have the potential to afford more extensive and standardized geographic coverage of multiple bioclimatic variables. A key decision when using model output is the appropriate geographic resolution to adopt; models are coarse scale, in the order of several arc degrees, and so their outputs are usually downscaled to a higher resolution. Most publicly available model time series have been downscaled to 30 or 60 arcmin, but it is unclear whether such resolution is sufficient for certain applications like species distribution models or whether this may homogenize environments and mask the spatial variability that is often the primary subject of analysis. Here, we explore the impact of increasing the resolution of model output from 30 to 5 arcmin using the delta-downscaling method, which interpolates and applies the long-term difference between past and present model datasets to a higher-resolution grid of observed present-day climate. We seek to determine to what extent further downscaling captures climatic trends at the site level through direct comparison with proxy reconstructions, evaluating different versions of the output from the HadCM3 Global Circulation model for annual temperature, mean temperature of July, and annual precipitation against a large empirical dataset of pollen-based reconstructions from across the Northern Hemisphere. Our results demonstrate that models tend to provide broadly similar accounts of past climate to that obtained from proxy reconstructions, with coherence tending to decline with age and at higher altitudes. However, our results imply that using the delta method to downscale to a very fine resolution has a minimal net effect on the coherence of model output with pollen records in most cases. Optimal spatial resolution is therefore likely to be highly dependent on specific research contexts and questions, with careful consideration required regarding the trade-off between highlighting local-scale variations and increasing potential error via unreliable interpolation.</p>