Back the the Futures: Integrated Scenarios’ Findings

(Image: David Rupp, “Evaluation of CMIP5 20th century climate simulations for the Pacific Northwest USA”, Journal of Geophysical Research: Atmospheres, VOL. 118) 

Temperature: End-of-century temperature projections resulting from Integrated Scenarios’ model runs range from a balmy 2° Fahrenheit uptick to a stifling 15° F surge, compared with the average temperatures for 1950 to 1999. But whether large or small, these spikes in the mercury are expected to have serious consequences for Northwest hydrology.

Precipitation Projections: The majority of Integrated Scenarios’ model runs are trending toward wetter winters and drier summers. However, only a handful of models project changes that are distinguishable from those measured in the last century. As a climate researcher might put it, natural variability still dominates the signal.

Snowpack: Due to warming temperatures, future precipitation in the Northwest is far more likely to fall as rain instead of as snow. This spells bad news for fish, farms, and city folk alike all of which rely on slow melting snow for their summer water needs.  

“Basically, what snow affords you is a free, natural reservoir, as opposed to a human constructed one,” explains Integrated Scenarios Bart Nijssen. “So we need to ask ‘what does it mean if that reservoir disappears?’”

According to Nijssen and his team’s modeling work, the Northwest’s spring snow will gradually become more and more depleted as this century progresses, with lower elevation watersheds being particularly hard hit by the rising temperatures. This earlier melt will mean larger stream flows in the winter—potentially leading to more flooding—and less soil moisture and increasingly diminishing natural reservoirs during the summer months.

Fire and Vegetation: By the middle of this century, all 20 Integrated Scenarios’ climate models agree: a warmer, drier Northwest will experience larger, more destructive, and more frequent forest fires. The project’s findings suggest these fires—which are expected to hit the western Northwest the hardest—will be so disruptive they could spark a kind of ecosystem regime change.

Under a warming climate the now-dominant evergreens, such as the Sitka spruce, will no longer be the best fit for the climate in much of the region’s coastal rainforests. Instead, deciduous and subtropical trees—now common along the California coast—will invade the evergreens’ territory. Large and destructive fires will aid the invaders—as will as pest outbreaks—making the takeover abrupt rather than gradual.

“The take home message,” says Bachelet, “is change in our rainforest region is not going to be smooth. It will be abrupt. It’s not if, but when. And it’s going to happen sooner rather than later.”

Here the historical period is shown in gray. Future projections are color-coded, yellow for the lower emissions scenario (RCP 4.5) and red for the high emissions scenario (RCP 8.5). The black line represents the multi-model average for each of the scenarios. Note: the scenarios diverge sharply at mid-century. (Image: ©David Rupp)
Pictured here are projections for summer precipitation under Integrated Scenarios’ two chosen emissions scenarios, the lower scenario (RCP 4.5), pictured in light blue, and the high emissions scenario (RCP 8.5), pictured in dark blue. The multi-model average—shown as the red wavering line—is represented by the climate model MIROC5, which was run using the high emissions scenario. MICRO5 was chosen as a representative model because it tracked closely with the multi-model average. The historical period is marked in gray. (Image: ©David Rupp)
As with the summer precipitation graph shown above, here the historical period is shown in gray, emissions scenarios are represented as differing colors of blue, and MIROC5 acts to represent the multi-model average. The trend noted by Integrated Scenarios’ researchers is one of wetter winter and drier summers. (Image: ©David Rupp)