Human emissions contribute to the Southwestern US precipitation decline

If you have been to the Southwestern US (SWUS), you know how precious every drop of water is. Summers are hot and dry, and the water resource of this region relies heavily on precipitation in winter and spring. When precipitation doesn’t come, drought can develop quickly and create remarkable socio-economic impacts. “Why has the SWUS precipitation declined since the 1980s?” becomes a puzzle to solve (Figure 1a). Kuo et al. (2025) revisited this attribution of post-1980 SWUS precipitation decline to internal decadal variability, and they showed that anthropogenic emissions have contributed to this decline. 

The SWUS precipitation variability is linked to the El Niño-Southern Oscillation (ENSO) teleconnection. Typically, the SWUS is expected to receive above-normal winter-spring precipitation during El Niño years and, vice versa, the SWUS receives less precipitation during La Niña years. This relationship of ENSO teleconnection in inter-annual time scales is also seen in decadal time scales (Figure 1b, 1c). Therefore, the post-1980 SWUS precipitation decline has been attributed to the La Niña-like decadal SST trend (Figure 1a). This post-1980 La Niña-like decadal SST trend has been treated as internal variability that just happened to be an unlucky scenario for the SWUS precipitation. 

Kuo et al. (2025) showed a systematic shift in the decadal ENSO-like teleconnection after 1980 using climate model large ensembles, which enable the separation of the anthropogenically forced component (e.g., driven by anthropogenic emissions) from different internal states (Figure 1d, 1e). This shift is especially visible in the El Niño-like case – the El Niño-like teleconnection under post-1980 anthropogenic emission differs from the pure El Niño-like internal decadal variability in the absence of anthropogenic emissions (comparing Figure 1b, 1d). 

With climate models, Kuo et al. (2025) tested the sensitivity of the cause of this shift in El Niño-like teleconnection by isolating individual potential drivers. They ran the climate model simulations that isolate (1) the external climate forcings (e.g., emissions of greenhouse gases, aerosols, etc) and (2) the trends in SST patterns.

The results in Kuo et al. (2025) identified the anthropogenic aerosols as one of the drivers of the North Pacific circulation change. Together with the uniform tropical warming, they shifted the SWUS precipitation trend toward a higher likelihood of drying after 1980. This study highlights the role of anthropogenic emissions in regional hydroclimate change – the post-1980 SWUS drought was more unavoidable than previously thought.