For the last 24 years, the Southwest United States has been caught in the driest megadrought since 800 C.E.
The megadrought is driven by several factors, primarily high temperatures. Since 2000, every state in the Southwest has experienced increasing average temperatures, with some states experiencing temperatures two degrees Fahrenheit warmer than average. Increased temperatures have shortened snow seasons and decreased the moisture in the soil, exacerbating drought.
The megadrought has sparked concerns about the future of water resources in the Southwest, particularly regarding the Colorado River. The management of the Colorado River has been a pervasive issue among the seven states — Arizona, California, Colorado, Nevada, New Mexico, Utah and Wyoming — and tribal territories relying on the basin.
As resources become increasingly scarce, legal battles and conflicts are likely to occur, emphasizing the need to understand the state of Southwestern water resources now to improve future management decisions.
In the Southwest, snowpack in mountainous regions is critical for supplying water in the spring and summer. Across the Western United States, spring snowpack has declined by nearly 20 percent on average from 1955 to 2020. Snowpack, along with other precipitation patterns, is impacted by both internal variability and anthropogenic change.
Internal variability, also known as natural variability, refers to naturally occurring changes in weather patterns. Ocean-atmosphere oscillation, seasonal variation and changes in atmospheric patterns are all examples of internal variability.
Anthropogenic change refers to changes in the climate that are driven by human activities. This variation — including greenhouse gas emissions — would not naturally occur without human involvement.
Researchers from Cornell’s Department of Earth and Atmospheric Sciences have been working on a multi-institutional grant project estimating the effect of anthropogenic changes on snow water resources in the Upper Colorado River Basin.
The Upper Colorado River Basin supplies more than 90 percent of the water supply for the entire Colorado River Basin. To understand the local processes affecting snow water resources in the basin, the project will combine an innovative climate model framework with hydrological models created by researchers at the University of Colorado Boulder.
According to Ankur Dixit, a postdoctoral associate in earth and atmospheric sciences, the ensemble models use outputs from global climate models. Ensemble models allow researchers to run multiple simulations of climate data with different initial conditions to isolate the effects of climate change.
Due to the richness of the data, the researchers narrowed their dataset down to four different ensemble members. The models were selected if they showed equivalent extreme trends for precipitation, temperature and snow water.
Using dynamical downscaling, the researchers are able to extrapolate global climate data to understand climate patterns at regional or local scales. The data was downscaled using the Weather Research and Forecasting Model.
This framework generated a large dataset at the 100, 45, 9 and 3 kilometer resolutions, which allows the researchers to understand variations and uncertainty of estimates at different scales.
This dataset may allow researchers to understand the past and current impact of anthropogenic change and internal variability on snow water resources. This information can help them identify the emergence of anthropogenic warming signals in snow water resource metrics in the basin.
The emergence of an anthropogenic warming signal refers to the point in time when effects of human-driven changes exceed internal variability.
“For now, the range of internal variability is stronger than climate change,” Dixit said. “But if we don’t do anything about it, what would be the time period when this climate change would surpass that range and become the most dominant factor?”
Understanding the emergence of the climate change signal is “very critical information” for decision-making, particularly for water resource management and operation of dams, according to Dixit.
“When you still have internal variable variability as a dominant component, you are hopeful because this internal variability is not irreversible. If it’s going down, it’s going to come up after some time,” Dixit said. “But if it’s your climate change signal, we just consider this irreversible. It’s not going to come down, so those changes would be kind of permanent.”
The project is ongoing and the results from the climate models will be integrated with hydrological models in the future. These results will be translated into policies that will improve water resource management across the southwest.
“Climate, hydrology and stakeholders, those are the basic pillars of the project,” Dixit said.
Taylor Rijos can be reached at [email protected].