Evidence for climate warming and wetland drying in the North American Prairie Pothole Region

Background/Question/Methods

The Prairie Pothole Region (PPR) of North America provides valuable ecosystem goods and services in the form of biodiversity, groundwater recharge, and flood attenuation. WETLANDSCAPE is a simulation model built on twenty years of research, and over that time, WETLANDSCAPE and its predecessor WETSIM have demonstrated that the PPR is highly sensitive to climate change. The Cover Cycle Index (CCI) is the productivity metric used in WETLANDSCAPE to evaluate climate scenarios by tracking cover cycle switches among lake marsh, hemi-marsh, and dry marsh classes. Semipermanent wetlands that cycle through these classes are more dynamic and support higher biodiversity. WETLANDSCAPE has projected ecological effects under an approximate doubling of atmospheric CO₂ concentrations expected to occur between the years 2050-2100: an increase in air temperature of 2-4°C shifts the highest functioning climate of the PPR southeastward by several hundred kilometers, away from regions with high wetland density and toward largely agricultural regions devoid of functioning wetlands. Here, we adopt a hindcast approach to compare wetland conditions between two, 30-year periods (1946-75; 1976-2005) to determine if recent changes in climate have changed wetland functionality, and if modeled historic trends match those projected in earlier analyses.

Results/Conclusions

The hindcast analysis detected two main geographic shifts in CCI scores during the second half of the 20^th century. First, the climate of the second period in the western Canadian prairies was sufficiently drier and warmer to enlarge from 40.0-47.0 percent the area of the 1976-2005 map covered by the least productive CCI category. The area of the intermediate CCI category decreased correspondingly from 21.0 to 13.0 percent.  Second, the most productive area of the PPR did not change in areal extent but shifted eastward into northwestern Minnesota.  Areas in lighter colors bounding this swath had lower CCI scores because the climate was sub-optimal for wetland productivity:  too dry in the west and too wet in the east. The overall effect of the climate differential was to shift the most productive conditions eastward and to lower potential productivity in the west. The CCI remained unchanged at three, increased at five, and decreased at 11 of the 19 weather stations used in the analysis, indicating a widespread decline in climate favorability in the latter period. These results indicate that the simulated effects of climate warming are already underway and not just forecasted projections of latter 21^st century climate change.