3.1 Changes in temperature extremes
Figure 3 shows the projected changes in TXx, which depicts the intensity of hot extremes. Both CMIP5 and the raw NA-CORDEX suggest an overall increase in TXx throughout the country at the 1.5°C and 2.0°C warming levels. The intensified TXx is stronger in CMIP5 than that in the raw NA-CORDEX. Due to the observed cooling in summer temperature over the Midwest (Mueller et al. 2015), observation-based bias corrections lead to less warming in TXx in the bias-corrected NA-CORDEX compared to the raw NA-CORDEX (Figure 3c,f), but the increase in TXx is even stronger in other regions. Effects of 0.5 °C less global warming on TXx are shown in Figures 3g-i. Despite the discrepancies in the projected changes (Figures 3a-f), all three datasets show significant decreases in TXx with 0.5°C less global warming. The averaged decrease in TXx across the country is -0.8°C, -0.8°C, -0.7°C based on the CMIP5, raw NA-CORDEX, and bias-corrected NA-CORDEX, respectively. Therefore, the dynamic downscaling or bias correction does not play a substantial role in the average changes in hot extreme intensity.
Figures 4a-c show the projected changes in TX90p, which describes the frequency of hot extremes. Under the 2°C global warming, there is a significant increase in TX90p, especially over the southern US. However, the CMIP5 models suggest a stronger increase in the southeast, while the bias-corrected NA-CORDEX suggests a stronger increase in the southwest. From the 2°C warming to 1.5°C warming, the hot extremes become less frequent, especially over the southeastern US (Figures 4d-f). Despite the slight regional difference, the three datasets show similar average changes in TX90p (about -3%), indicating nearly eleven fewer days annually with daily maximum temperature above the 90th percentile if there is 0.5°C less warming. The reduced TXx and TX90p suggest that the intensity and frequency of hot extremes would be reduced if the global mean temperature increase is limited 1.5°C. Consequently, impacts from 0.5°C less warming include reduced extreme heat exposure and likely more overall positive human health outcomes. This finding is particularly important as exposure risk to extreme heat in the U.S. is disproportionately high among groups with the most limited adaptive capacity (Guirguis et al. 2018; Madrigano et al. 2018; Voelkel et al. 2018). Extreme heat is also a serious health risk for agricultural workers in the U.S. (Culp and Tonelli, 2019), a group that often does not have access to proper healthcare (Magaña and Hovey, 2003; Hoerster et al. 2011).
The changes in intensity and frequency of cold extremes (TNn and TN10p) are shown in Figure 5. Under the 2°C global warming, TNn shows a much larger change than the change in TXx. The increase in the minima of daily minimum temperature exceeds 3°C in most of the areas. The three datasets show a good agreement, but NA-CORDEX presents more spatial details of the changes. Meanwhile, the frequency of cold extremes decreases significantly, with the greatest reduction over the western US. The magnitude of the reduced frequency in cold extremes is relatively small compared with increased frequency in hot extremes (Figures 4a-c). With 0.5°C less global warming, there are reduced changes in TNn and TN10p. Over the contiguous US, the warming in TNn would decrease by 1.0°C in CMIP5 and 1.2°C in NA-CORDEX. The average increase in TN10p is 1.2% based on the three datasets, indicating about four more days annually with daily minimum temperature below the 10th percentile if there is 0.5°C less warming. As global warming will make cold extremes less intense and less frequent, the 0.5°C less warming will increase the intensity and frequency of cold extreme events. Therefore, there can be increased risks of cold-related mortality for human beings and frost damage for plants.
3.2 Changes in precipitation extremes
Figure 6 shows the projected changes in two indices of heavy precipitation (Rx5day and R10mm). The former is the annual maximum 5-day rainfall, potentially related to extreme precipitation events in some regions; the latter is the number of heavy precipitation days (Zhang et al. 2011). The index Rx5day increases in most of the areas on the 2.0°C warming level, suggesting intensified precipitation in a warming climate, especially for the heavy precipitation events. The three datasets exhibit a good agreement in increased Rx5day, but only bias-corrected NA-CORDEX shows an evident decrease in limited areas in Texas and New Mexico. The two NA-CORDEX datasets show a greater increase in Rx5day over the mountainous regions in the western US than CMIP5, which is also documented in previous downscaling studies (e.g., Meyer and Jin 2017). The intensification of precipitation events will generally be reduced if there is 0.5°C less warming, with a stronger reduction over the central US. The average Rx5day would decrease by 3% across the country. If examining the changes in actual precipitation amount (in mm, shown in Figure S1), the heavy precipitation intensity can be reduced by up to 10 mm/5-day, especially over the Midwest.
The projected changes in the frequency of heavy precipitation (R10mm) are shown in Figures 6g-l. Under the 2.0°C global warming climate, both CMIP5 and NA-CORDEX present more frequent heavy precipitation in the North but less frequent events in the South. However, NA-CORDEX shows decreased R10mm in large areas of the southern US, and bias correction introduces a less increase (or even a decrease) in R10mm over the northwestern US compared to the raw CORDEX. The 0.5°C less warming does not exert consistent effects on R10mm in many regions of the country except for the Northwest and the northern Plains, where there are 1~1.5 fewer days with heavy precipitation annually. The changes in Rx5day and R10mm suggest that there is high confidence that intensity and frequency of heavy precipitation will increase under a warming climate, especially over the northern US. Heavy precipitation is linked to both flash and riverine flooding, therefore reduction of heavy precipitation frequency from 0.5°C less warming, given no change in development, floodplain management, and policy, will decrease flood risk. Reduction of heavy precipitation, particularly in the Midwest and northern Plains regions, decreases top soil erosion and nutrient runoff from agricultural fields, thereby maintaining good soil health and surface water quality (Morton et al. 2015).
Figure 7a-f shows the changes in dry spell. Under a warming climate, the dry spell will become significantly longer in the Northwest, along the southern border, and in the central US. The bias-corrected NA-CORDEX projects significantly extended dry spells over greater areas in the southern US compared to the other two datasets. However, the response of dry spell length to 0.5°C less warming varies spatially, and models show less agreement within and among the datasets in most of the regions (shown as not robust in Figures 7d-f). Only over the West, the dry spell is significantly reduced, by approximately three days, with 0.5°C less warming.
The projected changes in drought frequency are shown in Figures 7g-i, which exhibits similar spatial patterns as the dry spell changes. Under the 2.0°C global warming, drought events become fewer in the majority of the northern US due to the increased precipitation (Figure 6), and droughts become more frequent over the West and Southwest. The three datasets agree on the general locations of the increased drought events, but CMIP5 shows the smallest changes and the bias-corrected NA-CORDEX shows the strongest increase. For instance, the bias-corrected NA-CORDEX indicates that drought frequency would increase by over 20% in the Southwest, which accounts for more than two months with SPI below -0.8 every year, while the increased drought based on CMIP5 is just over one month. The difference between CMIP5 and bias-corrected NA-CORDEX implies downscaling and bias correction can significantly influence the simulated precipitation variability over those regions (also shown in Figure 6), and its mechanism needs to be further investigated in future studies. With 0.5°C less global warming, robust reductions in drought frequency are only found in limited areas of the Southwest. The drying trend in the western US under a warming climate is also found in previous studies (Zhao and Dai 2017; Naumann et al. 2018). This can be associated with the wet-get-wetter and dry-get-drier pattern or the thermodynamic contribution of global warming (Chou et al. 2013). The drying can also be associated with mean zonal moisture advection due to seasonally dependent changes in land-sea moisture contrast over the west US under warming (Dong et al. 2019). Therefore, less warming can potentially reduce the dry spells over these regions. The changes in CDD and SPI suggest there are likely increased risks posed by meteorological drought in the western and southwestern US under a warming climate, resulting in substantial impacts on the ecosystems, agriculture, and energy infrastructure. Although the reduced drought hazard by limiting global warming to 1.5°C is only found in very limited areas, considering the elevated water demand induced by global temperature increases (Wang et al. 2016), the 0.5°C less warming can still potentially reduce water stress of the natural and agricultural environment.
3.3 Regional Variability
Figure 8 summarizes the influence of 0.5°C less warming on all the temperature indices in the nine climate regions. We use boxplots to demonstrate the robustness of their changes in individual regions. The three datasets generally agree on the significant changes in the four temperature indices over all the regions, but there is an inter-model spread within each dataset. Regionally, there are greater changes in TX90p in Ohio Valley, Southwest, South, and Southeast, and greater changes in TNn in the northern US (including Northwest, Northern Rockies and Plains, Northeast, Upper Midwest, and Ohio Valley).
Compared to the temperature extremes, the changes in precipitation exhibit less robustness and greater inter-model spread (Figure 9). This result is consistent with the IPCC report and other modeling studies (Sillmann et al. 2013). Decreased Rx5day is found in Northwest, Northern Rockies and Plains, Upper Midwest, and Ohio Valley. There is a robust decrease in Rx10mm in the Northwest, Northern Rockies, and Plains, and a robust decline in CDD only in the West. Uncertainties in the estimated changes exist in other regions. For instance, CMIP5 and the raw NA-CORDEX agree on the reduced drought length in the Ohio Valley with 0.5°C less warming, but more than half of bias-corrected NA-CORDEX models show an opposite change. Furthermore, no robust changes are found in drought frequency over the nine regions. As shown in Figures 7j-l, significant influence of the 0.5°C less warming on drought frequency only appears in some areas of California and Arizona, and does not remain in the regional averages.
To better illustrate how future intensification of climate extremes can be avoided by 0.5°C less warming, we use Equation 1 to quantify the avoided intensification relative to changes under the 2.0°C global warming (Table 3). The actual values in avoided change are shown in Table S1. If future warming is limited from 2.0°C to 1.5°C, the projected intensification in hot extremes will be reduced by 32~46% in intensity and 35~42% in frequency across the country. The changes in the intensity of heavy precipitation can also be significantly limited by 23~41% in regions such as the North, South, and Southeast. However, impacts on the frequency of heavy precipitation and drought duration are only evident in limited areas. For instance, under the 2.0°C global warming, there is more frequent heavy precipitation in the Northwest and prolonged dry spell in the West (Figure 7). The 0.5°C less warming would limit the intensification of these hazards by 28% and 35%, respectively.
3.4 Seasonal Variability
Additionally, considering the seasonality of trends in precipitation, we examine the changes in Rx5day in different seasons (Figure 10). Under the 2.0°C warming, all the datasets suggest a robust increase in Rx5day in most areas of the contiguous US during winter and spring except for some regions in the Southwest and South. This is consistent with the annual changes in Rx5day (Figure 6 a-c). During summer and fall, CMIP5 shows increased Rx5day in the eastern US and parts of the western US, and no evident change is found in the central US. NA-CORDEX partially agrees with CMIP5. However, in biased-corrected NA-CORDEX, there is a robust decrease in Rx5day in the South.
With the 0.5˚C less warming, all the datasets show that the increased Rx5day during winter and spring can be greatly avoided in the northern and central US (Figure S2). This spatial pattern is consistent with the changes at the annual scale (Figure 6d-f). This suggests the projected (or avoided) changes in annual heavy precipitation intensity are more contributed by winter and spring. Similar seasonal contributions are also found in R10mm (not shown). The greater increase in winter/spring extreme precipitation can be explained by Clausius–Clapeyron relationship, in which changes in extreme precipitation are largely determined by increases in temperature (Wehner 2013), or the thermodynamic contribution to winter extratropical cyclones (Akinsanola et al. 2020; Yettella and Kay 2017). The seasonal contributions agree with previous studies on seasonal changes in extreme precipitation events (Wehner 2013; Singh et al. 2013; Janssen et al. 2016; Ning et al. 2015; Akinsanola et al. 2020). As wet extremes in spring can harm agricultural production through delayed and extended planting periods (Urban et al. 2015), the avoided increase in wet extremes would exert positive impacts on agriculture in the northern and central US. Concurrently, reductions in extreme precipitation has tangible effects on the frequency and intensity of flash flooding and riverine flooding in both urban and rural areas.