Warming velocity

Definition

Warming velocity captures the modern warming trend at a place — how fast its annual mean temperature is increasing — using the linear regression slope of annual mean temperature on year, multiplied by 100 to convert per-year to per-century.

Source data

• NOAA NCEI nClimDiv — Annual mean temperature by climate division, 1895–present (1900–2024)

Steps

1. Attribute each ZCTA to its NCEI climate division using TIGER polygon intersection.
2. Pull annual mean temperature for years 1970 onward (modern warming era; pre-1970 trend is shallower and noisier).
3. Compute the OLS slope of temperature on year, then multiply by 100.
4. For places spanning multiple ZCTAs, take a population-weighted average of ZCTA-level slopes.
5. Buckets: cooling (<0), flat (0–1), warming (1–3), rapidly warming (3–5), accelerated warming (>5).

Known limitations

• nClimDiv is the most spatially smoothed long-record climate dataset; finer-grained variation within a division is not captured.
• Modern warming begins ~1970 in much of the US; pre-1970 slopes are different and less informative.
• Single-station urban-heat-island effects are not removed (a small effect at the climate-division level).
• Linear regression assumes constant rate; some places show accelerating warming better captured by polynomial fits — see related methodology.

Where this metric appears

Every place page, the Fastest-warming towns list, and the Climate Velocity Index.

Related methodology

• Climate stripes
• Climate Velocity Index

Cite this methodology

Commerce Institute. (2026). Warming velocity — Methodology. Retrieved from https://commerceinstitute.org/methodology/warming-velocity/.