Historical and projected climate change for Grand Canyon National Park and surrounding areas

Abstract

Globally, anthropogenic climate change is one of the greatest threats to resources in protected areas. This report examines historical and projected climate change across the Greater Grand Canyon Landscape (GGCL), including Grand Canyon National Park. Grand Canyon National Park warmed significantly from 1895-2020 (annual mean increase of 1.89? F/century), with temperatures increasing at a faster rate from 1970-2020 (6.31? F/century). Warming occurred at all elevations and seasons across the GGCL, but rates differed spatially. Average annual total precipitation within Grand Canyon National Park did not change significantly over either period examined (1895-2020; 1970-2020). A variety of changes in the region of Grand Canyon National Park have been detected and attributed, at least in part, to anthropogenic climate change, including reduced soil moisture (and associated drought), reduced Colorado River flow, doubling of the area burned by wildfire across the western United States, reduced regeneration of low-elevation ponderosa pine and Douglas-fir as well as pinyon pine and juniper populations, northward shifts in many bird species distributions and declines of bird species occupancy in the Mojave Desert, and reduced bumble bee species richness and abundance (key pollinators). To help managers understand and plan around a range of plausible future climates, we present two plausible but contrasting climate futures for the Greater Grand Canyon Landscape, characterized at mid-century (2040-2069) and late-century (2070-2099). Examining multiple plausible futures avoids over-optimizing management strategies for a single projected future that may not occur. Overarching patterns that emerged from both climate futures include additional warming (average, as well as extreme temperatures), seasonal increases in extreme precipitation events, fewer freezing days and days with snow, and higher moisture deficit (a correlate with landscape dryness, conditions conducive to fire, and vegetation stress). The selected climate futures differed in terms of 1) the degree of warming, 2) whether winter precipitation increases or decreases, 3) whether annual precipitation increases or stays similar, 4) whether drought conditions increase or decrease, and 5) whether runoff increases or decreases. Runoff is projected to occur earlier under both climate futures and is projected to exhibit a more episodic pattern. Based on a literature review, projected changes to the physical, ecological, and cultural resource domains of the region resulting from anthropogenic climate change include: ? Increasing drought risk and aridification ? Reduced Colorado River flow ? Reduced groundwater infiltration ? Decreasing runoff (from snow or rain) in the spring, summer, and fall, and increasing runoff in the winter ? Increasing occurrence of large fires ? Increasing invasive grasses in the Mojave Desert ecosystems west of the park, providing more fuel for wildfire ? Exacerbated post-fire erosion and sediment in Grand Canyon watersheds ? Increased episodes of drought-induced tree mortality ? Upslope shifts of the elevational zones of pinyon-juniper woodland, ponderosa pine forest, and spruce-fir forest, as well as increases in non-forest areas and aboveground biomass declines ? Reduced abundance of riparian vegetation that tolerates water inundation ? Increasing invasive plant distribution and abundance, favoring their establishment and productivity ? Colonization of the GGCL by some bird species and extirpation of others ? Increasing non-native fish populations relative to native fishes ? Declining butterfly populations ? Increasing temperatures will increase visitation, especially during winter and shoulder seasons ? Exacerbation of existing threats to archeological resources, cultural landscapes, and historic structures, as well as emergent vulnerabilities related to climate change One goal of this work is to support the Resource Stewardship Strategy (RSS) process that Grand Canyon National Park plans to undertake. We anticipate that connecting the climate changes described here to the climate sensitivities of resources within the park will play a critical role in setting goals and strategies during development of the RSS, as well as proactively adapting to anticipated changes.