This is the second part of a three-part series by Caitlyn Kennedy, science writer for NOAA’s Climate Program Office, on how climate change is expected to impact Boulder’s water supply. Read part one here.
A few blocks from Ellinghouse’s office, climate scientist Joel Smith works for Stratus Consulting, an environmental consulting group. A coordinating lead author of the Intergovernmental Panel on Climate Change’s Fifth Assessment Report, much of his career has been focused on assessments of future climate change at the national and international level. But as one of Boulder’s water customers, Smith is acutely aware that climate change could have significant consequences for his community’s water resources.
In 2005, Smith approached Ellinghouse with a proposal to participate in an in-depth analysis of vulnerabilities of Boulder’s water supply to climate change. Smith and Ellinghouse would be collaborating with an expert team of climate scientists, including contributors from AMEC, the University of Colorado, the National Center for Atmospheric Research, and the National Oceanic and Atmospheric Administration.
Smith and the rest of the research team wanted to develop ways of building bridges between different types of climate data—representing the past and the future, the global and the local scale—that do not mesh easily with each other. They planned to combine projected changes in temperature and precipitation from a range of climate change scenarios with the paleoclimate record of climate variability in the Boulder Creek basin.
The research team generated simulations from 21 global climate models (the same models used by the Intergovernmental Panel on Climate Change) to project the wide range of possible climate changes in the central Rocky Mountains. Different models are based on slightly different assumptions about the physics that describe the climate system and how the different parts of the system are connected to one another.
All of the models agreed that temperatures would increase in the future, but they were inconveniently split on the question of precipitation. About half predicted more precipitation, and half predicted less. The scientists continued their analysis with four models whose projections spanned a range of possible precipitation outcomes—from wetter to drier.
Using each of the four models, the team projected how temperature and precipitation in the Boulder region might change by the years 2030 and 2070. Because no one knows what new technologies may be developed or what steps countries may take to reduce greenhouse gas emissions in the future, the team conducted three simulations with each of the four models, varying the future concentration of greenhouse gases from low to high. Higher emissions would be expected to lead to more significant changes in climate, including temperature and precipitation, in a shorter amount of time.
Laying the Past onto the Future
It’s at this stage of the process that attempts to use global climate models for local planning can easily fall apart. The models reflect large-scale changes in temperature and precipitation, which must then be translated into local conditions, in this case annual stream flows. To be useful to water managers like Ellinghouse, annual stream flow estimates must be broken down even further into monthly or even weekly estimates to evaluate specific effects on local water systems.
To give Ellinghouse the information she needed, Smith and his colleagues used tree-ring data to extend the existing Boulder Creek paleoclimate record into a 437-year history of stream flow extending back to 1566. The team statistically matched paleoclimate streamflows with similar streamflows in the modern record, and then used the modern observations of temperature and precipitation to reconstruct a corresponding long-term record of climate variability.
Finally, to factor in climate change, the team combined the temperature and precipitation changes from the global climate models with the historical temperature and precipitation record. They used the combined record to generate altered streamflow sequences for Boulder Creek that reflect what would be experienced if the long-term climate variability of the past repeated in a warmer future.
Even after overcoming the challenge of hybridizing past and future climate information, however, the study still isn’t finished from Ellinghouse’s perspective.
“It’s not enough to say, ‘Annual stream flows will be lower or higher under this climate scenario,’” Ellinghouse explains. “For the information to be really applicable to water management, what we need to know is: what does that do to our water yields and our water rights?”
Ellinghouse worked with Lee Rozaklis to feed the hybridized streamflow data into what she calls her ‘what if?’ model —the computerized version of Boulder Creek and the city water system that she uses to analyze water supplies and demands. This water management model simulates how natural and human factors—like precipitation, evaporation, population growth, lawn grass and agricultural irrigation, diversion points, reservoirs, and local water rights—affect Boulder’s ability to meet its water demand.
Check in tomorrow for part-three, the final installment.