Climate change and climate normals: What NOAA's fixed 1991-2020 baseline can and cannot show

Why a normal can't show a trend by itself

A normal averages 30 years into one number, so a warming inside the period is folded into it. To see change you need either successive normals (1981-2010 against 1991-2020) or years outside the period compared with it, which is what PlainClimate's recent-years tables do.

What recent years show

Across the places of 100,000 or more, the complete years 2021-2025 averaged +1.3°F against the normal at the median place; the largest departure was Brockton, MA (+4.0°F). Each place page carries its own five years.

What the long statewide records show

NOAA's statewide series reach back to 1895. Fitted over the whole record, the median state warmed 2.0°F per century, and in 44 states the warmest year came in 2010 or later. The pace is not steady: fitted from 1970 only, the median state warmed 5.1°F per century, and 45 of 49 states at least twice their whole-record rate.

What the records show

A warming baseline shows in the records before it shows in the normal. In the long NWS records of the 141 large cities, 25% of the monthly record highs were set in 2010 or later and 2% of the record lows. The extremes guide has the count by decade.

Why the 1991-2020 normal still matters

A normal is the agreed reference that forecasts, frost dates, heating and cooling degree days and building codes are set against, so "warmer than normal" always means warmer than 1991-2020. NOAA replaces the normals each decade; the 2001-2030 set is expected about 2031. Until then, a place whose recent years run warm, which is most of them (197 of 215 large places), has a normal that understates its present summers and overstates its winters' cold by roughly that departure.