Extended Data Fig. 6: Soil-moisture reconstruction skill.
From: Rapid intensification of the emerging southwestern North American megadrought in 2020–2021
Cross-validated reconstruction skill using (a) all tree-ring chronologies that cover 1700–1983 and (b) only the subset of chronologies that cover 800–1983 to reconstruct southwestern North America (SWNA) regionally averaged summer soil moisture anomalies (black: cross-validated reconstruction, red: observed, grey shading: 95% confidence intervals). Cross-validated time series represent out-of-sample estimates made by repeatedly recalibrating the reconstruction algorithm while withholding a decade of data at a time from the calibration period (1901–1983) and making reconstruction estimates for each withheld decade (the final period withheld was longer than a decade: 1971–1983). In (a and b), soil moisture anomalies are standardized relative to 1921–2000. (c) Comparison of the 22-year running means of the SWNA regionally averaged soil moisture reconstructions produced for (red) this study versus (blue) Williams et al.5. In (c), records are standardized relative to (blue) 800–2021 and (red) 800–2018. Dashed red and blue horizontal lines represent each reconstruction’s most negative 22-year mean anomaly, which in both reconstructions occurred in 1571–1592. R2 values in c indicate the squared correlation between the two reconstructions during 800–1983 at the annual and 22-year running mean timescales.
