Fig 6. State rankings by HAC-based t-statistics of mean temperature warming across U.S. states (1950–2021).

Sources: Own elaboration from PRISM state-level temperature series (see Sect 2). Map generated with MATLAB (R2024b, Mapping Toolbox); no additional permissions required beyond software citation. Colours represent the HAC-based t-statistic of the linear trend in mean temperature for each state. Cooler (bluer) shades indicate weaker or statistically insignificant warming signals (lower t-statistics), whereas warmer (green to yellow) shades indicate stronger statistical evidence of warming (higher t-statistics). Negative values, where present, correspond to cooling trends.

https://doi.org/10.1371/journal.pclm.0000808.g006

Fig 10. Warming typology across U.S. states (1950–2021).

Sources: Own elaboration from PRISM state-level temperature series (see Sect 2). Map generated with MATLAB (R2024b, Mapping Toolbox); no additional permissions required beyond software citation. States are classified into four categories (W0–W3) according to the estimated warming pattern, distinguishing mean warming, dispersion changes, and tail-specific effects.

https://doi.org/10.1371/journal.pclm.0000808.g010

The second figure presents the typology for all states and reveals some noteworthy patterns. The types are not uniformly distributed across the contiguous U.S., but rather concentrated in specific regions. For instance, Type W3 states are located along the West Coast, including California, Idaho, Nevada, Oregon, Washington, and Wyoming. This type of warming is characterized by a greater increase in higher temperatures than in lower temperatures, resulting in a positive trend in the iqr. This pattern corresponds to the West and parts of the Northwest climate zones.

In contrast, Type W2, characterized by a larger increase in lower quantiles relative to higher quantiles and a negative trend in the iqr, is primarily found in the Central North region, roughly corresponding to the Northern Rockies and Plains West NW Central area. This includes states such as Iowa, Minnesota, Montana, Nebraska, and North and South Dakota.

Finally, states in the Type W1 category, located in the Upper Midwest (East North Central), parts of the Northwest, and the Northeast, exhibit similar trends across all quantiles, with no statistically significant trend observed in the iqr.

Two final remarks. When aggregating data from all states, weighted by surface area, the overall trend for the contiguous U.S. can be classified as warming Type W1. (For the Globe as a whole, a W2 type is obtained (see [27]).) This indicates that the trends across the quantiles of the distribution are similar, with no statistically significant trend observed in the interquartile range (iqr). Additionally, there is evidence of acceleration in these trends. For instance, an analysis of typology over the period 1990–2021 reveals that the proportion of states experiencing warming has increased to 87%, with a notable rise in Type W3 states, which now represent 43% of the total. As regard to the average, it can also be seen that the number of states where it is significant increases as we approach the end of the sample.

In summary, while the typology derived from our methodology shares many similarities with the climate zones proposed by NCEI, it provides an alternative lens for identifying and testing the nuances of warming patterns. This alternative perspective may offer valuable insights into specific characteristics that might not be fully captured by traditional classifications.

https://journals.plos.org/climate/article?id=10.1371%2Fjournal.pclm.0000808