Abrupt shifts in precipitation regimes, also known as precipitation whiplash, often refer to a dry (wet) extreme immediately following a wet (dry) extreme with no break in the normal precipitation regimes. The decreasing shift time between the two precipitation extremes poses little time for human preparedness to adapt to the extreme circumstance, which can exacerbate the individual impacts of the drought or pluvial, and thus result in cascaded heavy impacts on the ecosystem and human society adversely. Precipitation variability is projected to increase in the 21st Century on daily-to-multiyear time scales with distinct regional characteristics in a warmer world, which could further increase the vulnerability of the ecosystem to changes in precipitation. 

Intensification of hydrological cycle, and subsequent changes in precipitation variability and wet and dry spells stems from the thermodynamic and dynamic responses to global climate change under various forcings involved. Greenhouse gases and aerosol emissions are two primary anthropogenic activities that show warming and cooling effects of climate change, respectively. Given that in the coming decades, greenhouse gases will increase while aerosol emissions will decrease significantly worldwide, it is of extreme potential to understand the effects of greenhouse gases and aerosol forcings on precipitation whiplash. Our efforts attempt to disentangle potentially competing influences on changes in precipitation whiplash from regional to global scales and improve the understanding of their future changes.

• Our research objectives

Using various precipitation datasets obtained from observations, reanalyses, a large multi-model ensemble (CMIP6), and a large single-model ensemble (CESM-LENS), we characterize the global occurrence frequency, transition duration and intensity of precipitation whiplash over the historical (1979-2019) period on the regional and global scales. We also analyze projected future changes in whiplash under the scenario of the RCP8.5 emissions using simulations and assess the influence of individual anthropogenic forcings using CESM-XLENS.

• Will precipitation whiplash become more severe in the future?

Both observations and simulations show increases in the global mean frequency of whiplash events since the late 1990s. By the end of 21^st Century, the projected global (land) mean frequency of whiplash will be 2.56±0.16 (3.43±0.22) times compared to the current period, accompanied by increasingly rapid and intense transitions between wet and dry extremes. The polar and monsoon regions are projected to experience the most dramatic increases in whiplash frequency, with more than 196% increase in monsoon regions located on the west of the Pacific. Changes in precipitation whiplash occurrences are concurrent with changes in precipitation totals, despite modest spatial heterogeneity of covarying trends in precipitation totals and whiplash. The rate of change in precipitation whiplash is much greater than precipitation totals when the globe is becoming simultaneously wetter and more variable.

• How will anthropogenic forcings affect precipitation whiplash?

Our findings suggest that anthropogenic signals of global mean (land mean) dry-to-wet and wet-to-dry whiplash features emerge in 2028 (2017) and 2033 (2017), respectively. Greenhouse gas emissions will bring about a substantial increase in risk of precipitation whiplash occurrences (55±4% up to 2079) with a drastic response in monsoon regions over the west of the Pacific (>50%) and polar regions (>120%), despite opposing influences of industrial aerosol emissions on decreasing occurrences, given greenhouse gas emissions are projected to sharply outpace industrial aerosol emissions after the 2020s.

Although humans are adapting to regional regimes of scarce or excess precipitation, this adaptation will be complicated by ongoing changes in precipitation regimes. This study is primarily concerned with the superposition of sub-seasonally to seasonally persistent and widespread dry or wet extremes that have resulted in a double whammy to agricultural yields, water quality, and safety of life and property. The more frequent, intense and rapid dry-wet transitions resulting from the increasing temporal variability of sub-seasonal-scale precipitation will further challenge water resource management and disaster prevention for human society. Recognizing the all-around risk induced by more volatile precipitation and more whiplash under multiple timescales is the first step to develop a resilient coupled natural and human system in a warming climate.