Year In Review 2020 (v2)

Table 1. Top five warmest years by average temperature (°F).
Data source: NOAA/NCEI.

Rank

Arizona

California

1

2017

63.0

2014

61.4

2

2020

62.6

2015

60.8

3

2018

62.3

2020

60.6

4

2014

62.2

2017

60.3

5

2016

62.2

2016

60.1

1981-2010
Average

60.5

58.4

Table 2. Top five driest years by total precipitation (in.).
Data source: NOAA/NCEI.

Rank

Arizona

California

1

1956

6.04

2013

7.93

2

2020

6.63

1976

11.75

3

2002

6.97

2020

11.97

4

1950

7.00

1898

12.47

5

2009

7.03

1947

13.17

1981-2010
Average

12.58

23.12

Feature: Climate Change in the Southwest

The Fourth National Climate Assessment (NCA4, 2017) provides details on how our climate is changing, driven in large part by human activity. The report notes “heatwaves have become more frequent in the United States since the 1960s, while extreme cold temperatures and cold waves are less frequent. Recent record-setting hot years are projected to become common in the near future for the United States, as annual average temperatures continue to rise.” The annual average temperature across the Southwest has increased by 1.6 °F (Figure 5). Increasing temperatures threaten society at large and in particular across the Southwest, impacting hydrologic cycles, ecosystems, and quality-of-life issues.

Fig. 5. From the NCA4, Figure 6.1. “Observed changes in annual, winter, and summer temperature (°F). Changes are the difference between the average for present-day (1986–2016) and the average for the first half of the last century (1901–1960 for the contiguous United States, 1925–1960 for Alaska and Hawai‘i). Estimates are derived from the nClimDiv dataset.” Data source: NOAA/NCEI.

Projections of future climates are based on “representative concentration pathways” (RCPs), which are various scenarios used to estimate the magnitude of potential warming due to increasing amounts of greenhouse gases in the atmosphere. RCP scenarios are used in climate models to project potential future climate states. The primary source of climate model projection data is the Climate Model Intercomparison Project (CMIP), which “promote[s] a new set of coordinated climate model experiments”. The latest version, CMIP5, provides data for end users to study potential future impacts of various climate scenarios.

During 2020, Phoenix, AZ set a new record for most days in a year with a high temperature at-or-above 100 °F (110 °F) at 145 (53). To put this into a long-term context, temperature data from nineteen CMIP5 model members were obtained for Phoenix, AZ, using RCP scenarios 4.5 (low emission scenario) and 8.5 (high emission). Historical model forecast data were obtained back to 1950, with projected data obtained to 2100. For each member, historical data were bias-corrected to official observed data for Phoenix using a rank-sort method, with those corrections applied to projected data (Figures 6, 7).

Fig. 6. The number of days in a year with high temperatures in Phoenix, AZ at-or-above 100 °F. Red dots represent the number of days by year as observed at the NWS official climate station for Phoenix, whereas pink bars represent the given decadal average. Note that 2020 set a new record at 145 days, eclipsing the previous record of 143 days in 1989. The gray (black) bars represent the decadal average number of days with high temperatures at-or-above 100 °F in Phoenix, AZ from nineteen CMIP5 climate model projections for the low emission scenario RCP4.5 (high emission scenario RCP8.5). Vertical orange lines represent the 10th to 90th percentile range of values.

Fig. 7. Same as Figure X except for 110 °F.

The results show that the severity of temperatures experienced during 2020 are projected to be common within 20-40 years. Put another way, 2020 was a preview of what a typical summer in Phoenix, AZ will be like in 20-40 years. Climate projections become increasingly severe as the end of the century approaches.

Fig. 8. Historical average annual temperature for Phoenix, AZ (left), Yuma, AZ (center), and Imperial/El Centro, CA (right).  At 77.2 °F, 2020 was the second hottest on record (behind 77.3 °F in 2017).

Fig. 9. Average temperature percentiles by month across the Southwest. Data source: WestWide Drought Tracker/PRISM.

Fig. 10. Precipitation percentiles by month across the Southwest. Data source: WestWide Drought Tracker/PRISM.

Fig. 11. Spiral charts representing dry streaks at three local long-term climate stations. The distance from the center outward (length) represents duration while the position on the circle (angle) represents the time of year. The longest five dry streaks are labeled and any more than 100 days in 2020.

Feature: The Urban Heat Island

Fig. 12. Schematic of the Urban Heat Island effect across a community.

Image Source: Berkeley Lab Heat Island Group

The mechanisms which lead to the creation of an urban heat island (UHI) have been studied since the 18th century. All communities, regardless of size, generate an excess amount of heat through human activities and alterations to the built environment as compared to the surrounding or previously pristine conditions (Figure 12). As communities grow in size and population, the impacts also increase. This extra energy can impact the local climate in many ways, one of which is an increase in the air temperature. In general, daytime temperatures in urban areas are found to be slightly higher compared to the surrounding rural areas (with suburban location in between). This can be complicated in arid environments, as the added activity of irrigation of greenspaces within communities can cause humidity values to be higher than surrounding desert areas and more humid air requires more energy to warm than drier air. At night, the physical properties of the urban/suburban built environment (buildings, roads, parking lots, etc.) cause stored energy from the sun to be released much more slowly than the surrounding rural/undeveloped areas. The result is warmer nighttime temperatures within communities than the surrounding areas. For arid climates, the effect can be much more pronounced as dry desert air more readily cools at night. A cyclical effect can also develop, where abnormally warm overnight temperatures allow a “head start” for the next day.

Fig. 13. Satellite imagery of the Phoenix, AZ area in 1984 and 2016. Source: Google Earth

As early as 1921, urban effects on the local climate in Phoenix, Arizona were recognized. Analyzing winter-time minimum temperatures in the Salt River Valley, Gordon (1921) found that the weather station in Phoenix was warmer than expected for a low-valley location. He noted that the “Riverside Nursery is protected on the north by both the diversion of the cold air stream and by the city of Phoenix itself with its warming influence”, likely making him the first scientist to describe the Phoenix UHI. Since that time, the population has increased nearly 50-fold (1920 population of ~0.1M, 2019 population ~5.0M), resulting in an intensification and expansion of the UHI (Figure 13).

An analysis was completed comparing the long-term temperature records at the official Phoenix, AZ station (downtown from 1895 through 1953, then at Sky Harbor Airport) and surrounding stations. The surrounding stations were averaged into two groups - those within 30 miles of the official station (current location) and those more than 30 miles (Figure 14, right panel). Data for the three groups were then averaged by month/year and a rolling 30-year window was then applied (a standard time frame to consider for climatological purposes).

Fig. 14. High (right) and low (center) temperature analysis at the official record station for Phoenix, AZ (black), a group of urban stations (red), and a group of suburban/rural stations (blue). Current urban area shown as pink shading/gray outline. Station locations indicated on the right-panel map, color coded by group. High temperature trends between the three groups are similar for the past several decades (1991-2020) at 0.3-0.4 °F/decade. Low temperature trends at the official station (1.5 °F/decade) are similar to the urban group (1.3 °F/decade) and notably higher than the suburban/rural group (0.8 °F/decade), which is a change from earlier in the 20th century when trends were similar between all three.

For high temperatures, there has been a steady increase over the years (1991-2020) at the official station (urban core; Figure 14, left panel, black) of 0.4 °F/decade while the urban (red) and suburban/rural (blue) stations have warmed at 0.3 °F/decade. For low temperatures, the difference is more significant. Over the same 1991-2020 period, the official station has warmed 1.5 °F/decade (Figure 14, center panel, black) compared to 1.3 °F/decade at the urban stations (red) and 0.8 °F/decade at the suburban/rural stations (blue). It is evident that the rate of warming has accelerated in the past several decades at the urban stations though is showing some signs of leveling at the official station.