3.2.2. Temperature

All 20 projections on mean annual maximum temperature (Figure 9a) and minimum temperature (Figure 9b) show significant increasing trends. On average, the increasing rates of the Central Valley regions (SAC, SJQ, and TUL) are fairly close to each other. The increasing rates of the coast regions (NC, SF, CC, and SC) and eastern regions (NL, SL, and CR) are slightly smaller and higher, respectively, compared to that of the Central Valley regions. Particularly, the median increasing rate in the coolest region, North Lahontan, is the highest among all regions in maximum temperature. This is mostly in line with what Figures 5 and 6 illustrate. In the historical period, both variables also exhibit increasing trends. However, for maximum temperature, only the trends for Central Coast, South Coast, San Francisco Bay, and South Lahontan regions are statistically significant at a significance level of 0.05. For minimum temperature, the trends of all regions except for Colorado River region are significant. Compared to historical trends, most projected trends have higher increasing rates. In general, the increasing trend is more significant in maximum temperature than in minimum temperature, implying that temperature range (difference between maximum and minimum temperature) is likely to increase. Comparing different regions, on average, the coastal regions (NC, SF, CC, and SC) tend to have the relatively smaller increasing rates while the eastern regions generally have the highest increasing rates. This is generally consistent with what has been observed in Figure 6.

**Figure 9.** Box-and-whisker plots of trend slopes of historical (1951–2013) and projected (2020–2099) mean annual: (**a**) maximum temperature (Tmax); and (**b**) minimum temperature (Tmin) (at significance level 0.05).

Similar to those observed on the annual scale (Figure 9), not all regions have significant trends in historical maximum temperature and minimum temperature on the seasonal scale (Figure 10a). Specifically, fall and winter minimum temperature exhibits no statistically significant changes for any region. Furthermore, the Tulare Lake region does not observe any significant trends in its maximum or minimum temperature in any season. Comparing two temperature variables, maximum temperature shows significant increasing trend in most cases while minimum temperature only exhibits significant warming in a couple of seasons (spring and summer) for a few regions. In contrast, mean projections on seasonal maximum temperature and minimum temperature show significant warming trends consistently for all regions (Figure 10b). Warming in summer and fall is more pronounced than warming in two other seasons. Looking at different regions, the eastern regions generally have the highest increasing trend while the coastal regions have the smallest amount of increasing rate. Particularly, the coolest region, North Lahontan, has the most significant increasing tendency in both maximum temperature and minimum temperature. The region has the highest seasonal warming rate in both maximum temperature (0.52 ◦C/decade) and minimum temperature (0.51 ◦C/decade) in summer. Those observations are largely in line with what Figure 7 shows.

**Figure 10.** Trend slopes of (**a**) historical (1951–2013) and (**b**) projected (2020–2099) mean seasonal maximum temperature (Tmax) and minimum temperature (Tmin) (at significance level 0.05). Different colors mean different trend slopes (per decade). White color indicates no significant trends.
