By Michael Carter
Bellevue’s weather has always followed familiar rhythms—wet, mild winters; relatively dry summers; bursts of spring growth; and crisp autumn transitions. But climate data and local observations show that those rhythms are changing in ways that affect our environment, health, and how we plan for the future.
Warming, Heat, and the Growing Number of Hot Days
Temperatures across the Pacific Northwest have been rising for decades. For Bellevue, that means more days exceeding 88 °F during summer and fall—days that were once rare are becoming part of our new normal. This shift is detailed in city projections showing that by mid-century, Bellevue could see 30–32 more days per year in which the heat is intense enough to strain health and infrastructure.
Changing Rainfall, Winter Storms, and Stream Flows
During roughly October through March, we still get most of our precipitation. But recent trends show increased intensity in certain winter storms—heavier rain, faster runoff, flashier stream flows. Meanwhile, earlier snowmelt in the Cascades is reducing summer water availability, which can lead to lower streamflows and water temperature highs that affect fish and aquatic life.
Shifts in Natural Rhythms: Phenology and Wetlands in Flux
“Phenology” refers to nature’s calendar—when trees leaf out, flowers bloom, or leaves fall. In Bellevue and greater King County, many plants are blooming earlier in the spring, and fall color is arriving sooner. These changes may seem small, but they’re signals of deeper ecological shifts. As different species adjust at different rates, timing mismatches can occur—for example, between pollinators and the plants they rely on.
Wetlands, too, follow seasonal cycles. Many are full and saturated through winter and spring, dry out in summer, then return with the fall rains. These cycles are essential—wetland soils alternately go through oxygen-poor (anoxic) and oxygen-rich (oxic) phases, which help process nutrients like nitrogen and protect water quality.
Smoke, Air Quality, and Health Impacts
Wildfire smoke has become a more frequent concern during late summer months. Paired with high heat and pollutant build-up, smoke can exacerbate breathing issues and reduce air quality even far from actual fires. Bellevue’s residents are increasingly exposed to these health risks, particularly vulnerable populations.
What Residents Can Do—and What’s Being Done
- Planting trees now helps mitigate heat islands—Bellevue’s tree canopy is part of a city-wide plan to preserve and expand urban forest cover.
- Water conservation during dry months ensures sufficient supply for both people and the environment, especially with earlier snowmelt reducing summer streamflow.
- City planning efforts are focusing on stormwater infrastructure upgrades, climate resilience, and reducing greenhouse gas emissions—Bellevue’s Sustainable Bellevue Plan lays out goals for both mitigation and adaptation.
- Following air quality alerts and minimizing outdoor exposure during smoke and heat events can protect individual health.
Bellevue’s climate is not static—it’s shifting, sometimes imperceptibly at first. But over time, the impacts grow. Observing seasonal changes, understanding trends, and taking steps both big and small are essential to keep our environment resilient, our communities healthy, and our city thriving.