This study investigates the historical and future variability of and transitions between precipitation extremes in the Midwest using state-of-the-art climate models in CMIP6. We find significantly increased magnitude of precipitation extremes and increased frequency of transitions, which could have substantial socio-economic and environmental impacts in the Midwest. Similar results about future wet and dry extremes have been documented in recent studies of the CMIP6 projections (e.g., Akinsanola et al. 2020; Cook et al. 2020). Observation-based assessments have suggested that increased precipitation variability and systematic warming have important implications for flood risk and conjunctive water management (Hamlet and Lettenmaier 2007). The projected increase in annual maximum SPI suggests heavy precipitation is expected to be more intense, possibly leading to increased flood risk and issues with excessively wet soils (Scoccimarro and Gualdi 2020; Byun et al. 2019). Due to the large area of agricultural land in the Midwest, the increased heavy precipitation is also likely to drive more nutrient runoff to surface water bodies (Coffey et al. 2018; Motew et al. 2018), and augment long-standing soil erosion issues in the region (Thaler et al. 2021). The increased flood risk also poses challenges for the drainage system in urban areas (Yazdanfar and Sharma 2015), particularly those with undersized systems and/or combined sewer overflows.
Meanwhile, the projected decrease in annual minimum SPI indicates that future dry conditions will get drier, exposing the agricultural regions to potential economic losses due to drought (Ukkola et al. 2020). Irrigation has often been proposed as a climate adaptation strategy to improve crop resilience to future changes in drought risk (e.g., Li et al. 2020). However, along with being cost-prohibitive, widespread adoption of irrigation, especially in the currently majority non-irrigated agricultural lands in the Midwest, could exacerbate water supply issues for municipal or commercial use in times of prolonged drought. Soil and water conservation strategies are also vital adaptation measures for Midwest agriculture, and are becoming increasingly important to boost resilience to drought and reduce soil erosion and nutrient runoff from increasing precipitation intensity.
It is important to note that the sequencing of precipitation extremes can greatly determine the magnitude of associated impacts. For example, a 30-day dry extreme that follows a prolonged wet period will have less socio-economic impacts than the same extreme following a near-normal or prolonged dry period. Similarly, conditions preceding wet extremes such as soil moisture conditions, reservoir levels, and streamflow can greatly impact the extent of flood damage associated with wet extremes. Although it is beyond the scope of this study, further impact-focused research is necessary to better recognize and communicate the implications of changing Midwest precipitation extremes for drought and flood impact preparedness, adaptation, and management.
We also find more frequent transitions of precipitation extremes, particularly transitions from wet spring to dry summer in the Midwest. Observation-based studies also document a wetting trend during the early growing season and a drying trend during the late growing season in the Midwest (Dai et al. 2015). Such a transition would seriously impair crop production, especially for the rainfed crops, which dominate agriculture in the eastern Midwest. Excess precipitation and flooding in spring can cause widespread planting delays for both commodity and annual specialty crops, soil compaction, poor seed germination, higher fungus, and bacterial disease incidence, and lead to issues with nutrient loss and soil erosion (Rao and Yi, 2003; Kleinman et al. 2006). Concurrently, even 30- or 90-day drought, if aligned with crop pollination and/or grain/fruit formation periods, can lead to yield decreases or crop failure (Westcott et al. 2005; Rippey et al. 2015). The results of this study show projected increases in the speed of transitions between extremely wet and dry conditions, suggesting overall less time for preparation and management of the hazard impacts.
Like other precipitation extreme studies (such as Akinsanola et al. 2020, Srivastava et al. 2020), this study is based on climate projections from state-of-the-art climate models in CMIP6. However, we need to acknowledge certain limitations of the current analysis. First, the dry bias in the central US has been a long-standing issue in CMIP5 and CMIP6 models (Al-Yaari et al. 2019; Srivastava et al. 2020). The SPI-based analysis can somewhat avoid the influence of the mean bias, but associated uncertainty in precipitation distribution may still affect the identified precipitation extremes (Pierce et al. 2015). Second, the springtime extreme precipitation over the central US is primarily controlled by mesoscale convective systems (Feng et al. 2016). However, current CMIP6 models still are not able to resolve these mesoscale convective systems due to their coarse spatial resolutions (Ridder et al. 2021). Therefore, it is necessary to investigate the impacts of bias correction and high-resolution dynamical downscaling on the transitions of precipitation extremes. Meanwhile, although dynamical downscaling can provide more useful information for regional impact studies due to its higher spatial resolution, uncertainties related to regional climate models (RCMs) cannot be ignored. Our previous study found that different RCMs with the same GCM boundary conditions can lead to opposite changes in precipitation extremes (Chen and Ford 2021). Some studies even show worse performance in dynamic downscaling than GCMs (e.g., Mishra et al. 2018). Moreover, coarse-resolution (e.g., 12–50 km) and high-resolution (e.g., convection-permitting resolution, < 5 km) RCMs may lead to inconsistent rainfall intensity (Kendon et al. 2017). Therefore, when dynamically downscaled CMIP6 climate data becomes available, it will be worthwhile to evaluate the added value of downscaling in precipitation extremes compared to the GCMs.
It is expected that the intensity of heavy precipitation would scale with the change in air temperature (Held and Soden 2006). Therefore, we see significantly increased annual maximum SPI with the greatest increase in the SSP585 scenario, which corresponds to the greatest temperature increase (Cook et al. 2020). Meanwhile, a warmer atmosphere would take longer to replenish its moisture between storms (Shiu et al. 2012), potentially leading to longer dry spells and intensified drought conditions. However, identifying the mechanisms that result in projected more frequent and rapid transitions of precipitation extremes will be a focal point in our future work.
Despite the potential future risks of intensified precipitation extremes and more frequent transitions over the Midwest, we note considerable differences in the projected changes among different scenarios. The projected increase in magnitude and frequency of precipitation extreme transitions can be largely avoided under a lower-emission scenario (Figs. 5–8). Aligning with previous literature that has explored the impacts of 0.5°C less global warming on climate extremes (such as Zhang et al. 2018; King and Karoly 2017; Hoegh-Guldberg et al. 2018), this study highlights the importance of climate mitigation efforts in reducing the risks of extreme events in the Midwest.
In summary, this study investigates the projected changes in transitions of precipitation extremes in the Midwest using climate simulations from 17 CMIP6 models. Two SPI-based metrics, intra-annual variability and transition adopted from Ford et al. (2021), are used to quantify the magnitude, duration, and frequency of transitions between wet and dry extremes. The evaluation with the observation-based precipitation dataset suggests the multimodel ensemble median of CMIP6 can reasonably represent the spatial patterns of the SPI extremes and transitions during the historical period. For instance, using 30-day SPI, which depicts short-term (e.g., monthly) precipitation variability, we see greater intra-annual variability and higher frequency of transitions in the eastern half of the Midwest, especially in the Great Lakes region. With longer SPI intervals, which represent longer-term (e.g., seasonal) precipitation variability, the northern areas exhibit greater magnitude and shorter duration of intra-annual variability, and higher frequency of transitions.
Climate projections suggest significantly intensified wet extremes across the Midwest by the end of the century, with a greater increase in the north and the Great Lakes region. The short-term SPI also shows intensified dry extremes over the western half of the Midwest. Consequently, there is significantly increased intra-annual variability in most of the areas in the Midwest compared to the historical period. Meanwhile, a warming climate also leads to more frequent and rapid transitions between the wet and dry extreme events, especially over the Great Lakes regions and the northern states. Seasonality analysis further reveals that more frequent transitions from a wet spring to a dry summer (or from a dry fall to a wet winter/spring) will occur in the Midwest. The difference among three scenarios, including SSP585, SSP245, and SSP126, indicates that the intensified precipitation extremes and accelerated transitions can be greatly alleviated under a lower emission scenario, and highlights the importance of effective climate action in the long-term development of climate-vulnerable regions in the Midwest.