One hundred years of climate change in Mexico

number of stations is insufficient to depict the complex spatial patterns of mean precipitation [47]; however, for some months (i.e., from January to May) the ratio signal was acceptable. Therefore, the precipitation surface for period t1-1940 must be used with caution. The monthly average RTMSE for temperatures was below 0.6˚C and below 10 mm for precipitation of all three-climate periods. We found differences between the three time periods in the deseasoned climatic surfaces of monthly precipitation (Wilks’ Lambda = 0.295, Fdf = 24,178478 = 6249.079, P < 0.001), maxi- mum temperature (Wilks’ Lambda = 0.419, Fdf = 24,178478 = 4050.582, P < 0.001) and minimum temperature (Wilks’ Lambda = 0.398, Fdf = 24,178478 = 4346.174, P < 0.001) averaged across all Mexico. The Pettitt test showed that the number of weather stations holding inhomogeneities varied among the three climatic variables and biogeographic provinces. For maximum temperature, 899 stations out of 4152 (21.7%) presented inhomogeneities, ranging from 10.6–28.2% among provinces (S2 Table in S1 File). These numbers were higher for minimum temperature, for which 36.2% of 5079 presented inhomogeneities. For some provinces inhomogeneities occurred in more than 40% of the stations (S3 Table in S1 File). Inhomogeneities were lower for precipitation (9.4% of 5239 weather stations, ranging from 4–19% among provinces) (S4 Table in S1 File). The breakpoint years–which refers to the estimated year in which an abrupt transition occurred in the time series–were more frequent during the second half of t2-1970

and the first half of t3-2000 in all three climatic variables and across all provinces (S1, S2 and S3 Figs in S1 File). Changes between time periods The magnitude of change in annual mean temperature (Bio 01) and precipitation (Bio 12) dif- fered between the first and second time periods and between the second and third time peri- ods. At the country level, the mean annual temperature decreased, on average, 0.32˚C from t1- 1940 to t2-1970, and increased 0.22˚C from t2-1970 to t3-2000 (Table 2). This tendency was also consistent across regions, but the Neotropics experienced the greatest change between t1- 1940 to t2-1970, (-0.64˚C), while the Nearctic did so between t2-1970 and t3-2000 (-0.3˚C; Table 2). The largest increases (>1.5˚C) from t2-1970 to t3-2000 were found in the desert mountains of northern Mexico and Baja California (S4 Fig in S1 File). Additionally, extreme cooling and warming increased in magnitude since t2-1970 (Table 2). Across Mexico, maximum and minimum temperatures have increased since t2-1970 in most provinces and for almost all seasons, even when at the beginning of the century maxi- mum temperature decreased mainly in the southern provinces of the country (Fig 3). Between t1-1940 and t2-1970, Baja California and Yucata´n provinces suffered the greatest changes, with mean annual temperature decreases ranging from almost 2˚C to almost 4˚C in the summer (Fig 3). Between t2-1970 and t3-2000, there was a tendency of temperature increase in the northern provinces with no consistent change observed in the southern ones (Figs 3 and 5).

Within seasons, the pattern was similar to the changes observed in the mean annual tempera- ture (Table 2), as we observed an increase in temperatures throughout periods, particularly for the second half of the 20th century, although the changes in minimum temperature were not consistent between periods. This happened mainly in the biogeographic provinces of Depre- sio´n del Balsas, Oaxaca, Sierra Madre del Sur, Oaxaca, and Socunusco. Moreover, the increase observed in the minimum temperature in winter (DJF) and spring (MAM) between t2-1970 to t3-2000 was much larger in the northern provinces than elsewhere in the country (Fig 3). In general, across Mexico, the total area where positive changes in precipitation occurred during the century is larger than the area with negative changes. However, the magnitude of PLOS ONE Historical climate change in Mexico PLOS ONE | https://doi.org/10.1371/journal.pone.0209808 July 16, 2020 8 / 19