Twentieth Century Hydroclimate Changes Consistent with Human Influence
This study compared predicted and real-world soil moisture data to look for human influences on global drought patterns in the 20th century. Climate models predict that a human "fingerprint" - a global pattern of regional drying and wetting characteristic of the climate response to greenhouse gases - should be visible early in the 1900s and increase over time as emissions increased. Using observational data and data reconstructed from tree rings, the researchers found that the real-world data began to align with the fingerprint within the first half of the 20th century.
This study is the first to provide historical evidence connecting human-generated emissions and drought at near-global scales.
Using CMIP5 models, we obtained a “fingerprint” of greenhouse gases on the Palmer Drought Severity Index (PDSI) and performed a formal detection and attribution analysis for three periods:
- 1900-1949: The signal is detectable: highly unusual to have arisen from internal variability (which we estimate from pre-1850 GDA data and CMIP5 piControl simulations. It is attributable to the collection of external forcings included in the CMIP5 simulations.
- 1950-1975: The signal is negative and unlikely to have arisen from internal variability alone. It suggests a possible role for aerosol forcing, although the fingerprint of aerosols remains poorly understood due to uncertainties in aerosol emissions, radiative forcing, and hydroclimate response.
- 1980-2017: The signal is once again positive, but has not yet re-emerged from the “noise” of naturally forced variability. This may be related to recent “hiatus” conditions characterized by anomalously cool equatorial Pacific SSTs and relatively muted warming.