As one of the last remaining large, nearly intact ecosystems in the northern temperate zone of the world, the Greater Yellowstone Area (GYA) lays claims to many superlatives. For the angler, notably, it is arguably flyfishing central and the headwaters of the west, giving rise to the Green, Madison/Gallatin, Snake, Wind River/Big Horn, and the Yellowstone rivers. And it’s the focus of the recently released Greater Yellowstone Climate Assessment.
Instead of being bound by the political and jurisdictional boundaries that typically define such efforts, the Greater Yellowstone Climate Assessment is the first of its kind to consider climate change impacts at the ecosystem-scale, focusing on the six major watersheds within the region.
Authored by researchers at the US Geological Survey, Montana State University, University of Wyoming, and others, the collaborative report brings the best available science to the issue of climate change with a focus on water. The assessment explores past trends in climate change, current conditions, and projected future changes, taking the often abstract and distant concepts associated with climate science and bringing them into focus at the regional level.
The following table summarizes data on temperature, snowfall, and peak stream flow for the 1950-2018 historical period and projected changes to 2100 by hydrological unit, i.e., watersheds.
Some Findings
Temperature
GYA average temperature for 2001-2020 is probably as high or higher than any period in the last 20,000 years, and likely higher than previous glacial and interglacial periods in the last 800,000 yr. Meteorological records since 1950, averaged across the GYA, show that mean annual temperature in the GYA has increased by 2.3°F (1.3°C) at a rate of 0.35°F (0.20°C) per decade. [high confidence].
Precipitation
The geologic record indicates that decade-long periods of low precipitation have occurred in the past 1200 yr. These dry periods were times of reduced snowpack, more fires, lower streamflow, and establishment of trees above present tree line. Since 1950, average precipitation across the GYA has not changed significantly and remains near 15.9 inches (40.5 cm) with year-to-year variability of 2.2 inches (5.6 cm) [high confidence]. Precipitation has increased in spring and fall, by 17-23% in April and May, and 42% in October. It has declined by 17% in June and 11% in July. [high confidence].
As climate has warmed, mean annual snowfall in the GYA has declined. Much of the snowfall decline has occurred in spring when warming was greatest. [high confidence]. Measurable snowfall has become rare in June and September as the snow-free season has lengthened.
Streamflow and runoff
Streamflow records in the GYA since the early 20th century allow comparison of current trends to past events such as the 1930s Dust Bowl drought. Since 1925, the timing of peak flow has shifted earlier in the year by 8 days (range of 1-15 days in the watersheds), extending the length of the water-limited warm season. [high confidence].
The shift in the timing of peak streamflow since 1970 has been approaching the early timing that occurred during the 1930s Dust Bowl drought. The recent shift, however, is caused by rising spring temperatures that melt snow earlier, whereas during the Dust Bowl drought it was caused by a year-round decline in precipitation.
In selected free-flowing rivers within the GYA, annual flows since the mid-20th century have decreased by 3-11%, spring flows have increased by 30-80%, and summer and fall minimum flows have declined by 10-40% (table below).
Monthly mean streamflow in free-flowing rivers in the GA from 1985-2018 (left column) and the percent changes from the 1950-1984 average (right column).
The data show a future where significant changes will be experienced at mid- and high elevations where runoff from snowmelt increases in spring (March through May) and decreases in summer (June through August).
Stakeholder Concerns
The assessment explores present and future implications for agriculture, energy, wildfire, and winter recreation in the region. Interviews with stakeholders yielded important insights into the climate realities faced by local communities with the following key findings emerging from these conversations:
Water issues are at the core of climate change impacts in the GYA. Communities and managers will continue to face challenges like drought and shifts in seasonal water cycles in the future.
Participants’ understanding of and response to climate change is driven more by their background (stakeholder group) than their location (watershed).
A pressing need exists for a climate information hub that is comprehensive, collaborative, accessible, and useful to experts and the public alike.
For the most part, meaningful policy to address and adapt to climate change is lacking in the GYA.
By addressing water issues like availability and quality in future climate adaptation work, we stand to have positive impacts on myriad other conditions including wildlife habitat, fisheries health, and the economy of local communities.
The assessment documents changes that have been observed and projects what the changes may be in the future. Now, rather than spending time arguing about the origins of climate change or who to blame, we can hopefully shift to what can be done to respond, to build climate resiliency in fisheries management, water use, and agricultural practices.