Projected Temperature and Precipitation Extremes for Tucson, Arizona: Using Stakeholder-Defined Criteria for Adaptation Planning

It has often been stated that climate change will be experienced by citizens through changes in extreme events. The city of Tucson, Arizona currently experiences a variety of extremes including: intense precipitation and flooding from summer thunderstorms or multi-day winter fronts; episodes of multiple days of extreme heat; and episodes of multiple days (or even months) without measurable precipitation. As the city prepares its climate change adaptation plan, changes in these extremes are of importance to operational managers in city and county departments, natural resource managers stewarding lands adjacent to the city, and citizens concerned with the sustainability of the city.

In order to better address these concerns and take our climate projections to the next level, we focused in on the extreme events that are most critical and relevant for the community. To define extremes, we surveyed key city and county employees, resource managers, and members of the community climate change committee. We analyzed the survey responses to identify critical thresholds for temperature and precipitation events and then tailored our modeling work to identify projected changes with respect to those recommended thresholds.

We analyzed statistically and dynamically downscaled climate model projections of extreme heat, cold, and precipitation, based on high (A2) and low (B1) SRES emissions scenarios, for both the city and the wider region. We compare results from the two downscaling methods. We also compared results for the average of gridcells that comprise Tucson's city footprint with results for a larger region, which is affected by key climate phenomena, such as the El Nino-Southern Oscillation and the North American Monsoon. For Tucson, models project annual average temperature increases of 2.8°F (2021-2050), 4.5°F (2041-2070), and 7.7°F (2071-2099) (A2). We find that maximum temperatures above the user-defined threshold of 100°F will increase by an average of ~42 days per year; this is likely to affect public health (through increased heat stress) as well as increase water and electricity demand. For the winter season, models project a decrease of ~4 days with temperatures below freezing. Annual regional precipitation is projected to decrease in the last half of the century, and spring precipitation is projected to decrease in all time periods, regardless of emissions scenario. Days with zero precipitation increase by ~6 days/year, which has implications for water use and for fire danger in the forested area to the north of the city. Stakeholders defined two extreme high precipitation thresholds: precipitation greater than 0.5 inches per day, and precipitation greater than 1.0 inches per day. High precipitation is of particular concern to floodplain managers, given several instances of widespread flooding during the last 30 years. Both dynamically and statistically downscaled projections show slight increases in extreme high precipitation amounts, but increases were not outside of the average of the historic range.

Using stakeholder input to tailor the classification of extreme events, makes those results more meaningful to city and county departments and municipal operational agencies. User-defined thresholds allow climate adaptation planners to more easily communicate climate change projections to city managers, local decision makers, and concerned citizens. Building off critical thresholds that are already known to represent potential climate- and weather-related vulnerabilities in the community, should make it easier for participants in scenario planning to visualize future threats and to articulate adaptation strategies and the feasibility of implementing strategies.