Researchers bolted temperature loggers to 53 utility poles in South Tacoma and got usable data from 46, the rest lost to vandalism, theft or other damage. Readings taken at the same moment differed by 2.57 degrees Celsius on average, and full tree canopy within 10 meters was modeled to make a location 1 degree cooler than one with none.

Across one low-tree part of South Tacoma, Washington, temperatures recorded at the same time varied among locations by 2.57 °C on average, with much wider gaps at the extremes. The measurements came from the same neighborhood during the same summer. So why the difference?

We are not clinicians, epidemiologists, or heat-safety specialists, and nothing here is medical or personal-safety advice. This is our reading of one study. Its patterns are broad, population-level ones specific to a single Tacoma neighborhood, not a prescription for any individual reader or family.

What the researchers put on the poles

The work came out of the Greening Research in Tacoma project, a collaboration involving The Nature Conservancy, the University of Washington, the City of Tacoma and the Tacoma Tree Foundation. A team led by ecologist Ailene K. Ettinger set up a dense temperature network in South Tacoma, an area with about 9 percent tree cover, compared with about 20 percent citywide.

The instruments were small temperature loggers placed inside solar-radiation shields and mounted on 53 utility poles along sidewalks. They recorded hourly measurements through the summer of 2022. Not every sensor survived. The paper reports that “Vandalism, theft, and other damage to temperature loggers resulted in 46 utility poles where temperature data were available during summer 2022.”

So what explains the gap between sidewalks?

The strongest single predictor was nearby tree canopy. As tree cover within 10 meters of a logger increased, the temperature at that location decreased. In the model, the relationship was linear, with cooling of about 0.01 °C for every additional percentage point of nearby canopy.

Add that up and you get the headline figure, with an important caveat: it is a modeled prediction, not a measured guarantee. The authors write that “increasing from no tree cover to 100% tree cover at a given location would lead to a predicted drop in temperature of 1.0 °C at that location.” This is what the model predicts for the South Tacoma data, not a rule that full tree cover reduces the temperature by exactly one degree on every street.

A comparable study identified by the researchers was conducted in Madison, Wisconsin. There, going from zero to full tree cover within 10 meters corresponded to a daytime decrease of about 0.7 °C. The Tacoma authors therefore keep the geography attached to their conclusion: “every unit of added tree canopy cover can help to reduce local air temperature on hot days in South Tacoma.”

Why a couple of degrees is not trivial

A degree or two can matter when it pushes temperatures across a safety threshold.

The authors report that “the probability of crossing a temperature based human health threshold is two to five times greater in places with no trees versus full tree cover within 10 m.” The size of the modeled effect varied with elevation and the temperature threshold, but locations without canopy had a higher estimated probability of exceeding the thresholds. For context, the region’s June 2021 heat event resulted in nearly 1,000 deaths.

What we take from a study this small and this local

This was one observational field study, conducted in one neighborhood over one summer, using 46 usable logger locations. It found an association between nearby trees and lower air temperature but does not by itself establish causation.

What makes the study interesting is its fine scale. Many city heat maps work in broad geographic units. This network measured conditions pole by pole, closer to how someone experiences a walk through a neighborhood. Ettinger, as quote by The Nature Conservancy, described the value of that approach this way: “It’s only with fine-scale data from within the communities themselves that we can start to answer questions like: When we plant trees, how do people experience that?”

The equity issue is difficult to miss. The study area had less than half Tacoma’s citywide tree cover, and about 31 percent of residents lived below the poverty line, roughly triple the county rate. If tree canopy provides protection at the scale of a sidewalk, where a city preserves and plants trees can influence who benefits from cooler local conditions.

Extreme heat is a real health risk. If you or someone you look after is vulnerable to it, please consult local health authorities and public-health guidance for advice on staying safe.