Changes in water availability have a fundamental impact on ecosystem processes and societies at global and regional scales. Climate model projections for the 21st century indicate a trend towards increased atmospheric and hydrological aridity with rising greenhouse gas emissions. Regional water scarcity combined with diminishing groundwater reserves makes both wild and agricultural ecosystems sensitive to changes in water availability. As recent decades of anthropogenic warming have already pushed the global climate system beyond the limits of the historical record, paleoclimate archives, such as fossil pollen data, can offer valuable insights into the drivers of hydrological variability and the regional consequences of long-term drying trends.

New research combining machine learning-based water balance reconstructions with high-resolution climate simulations provides new insights into the patterns and drivers of moisture variability in mid-latitude eastern North America during the Holocene (the last approximately 12,000 years). These new results resolve previous inconsistencies between proxy records and climate models and show consistent spatiotemporal patterns of hydrological regime change in the Holocene.

Proxy data on the hydrological regime of eastern North America suggest severe and persistent multi-thousand-year droughts during the Holocene. Most of the Holocene was characterized by drier conditions than present-day conditions in most of the United States (except the Southwest) and southern Canada. Current climate models have often failed to accurately simulate these moisture variations in the Holocene, for example showing higher precipitation than present-day in mid-latitude North America during the early to mid-Holocene, in sharp contrast to the peak aridity indicated by proxy data.

The study reveals that the main spatiotemporal patterns of moisture in the Holocene can be adequately explained by a combination of two main factors: secular trend of summer insolation and deglaciation of the Laurentide Ice Sheet (LIS) and its impact on atmospheric circulation.

In the early Holocene (approximately 12,000 to 9,000 years ago), the presence of the extensive LIS ice sheet led to anticyclonic blocking that affected atmospheric circulation. This caused a significant cooling of summer temperatures and reduction of potential evapotranspiration (PET), which outweighed the reduction in precipitation, leading to relatively humid conditions in some areas (such as the Midwest) compared to the pre-industrial period. However, areas closer to the coast (such as the Northeast) were significantly drier during this period than they are today, likely due to moisture redirection. This east-west dipole in water balance in the early Holocene is consistent with model simulations and is associated with changes in summer sea level pressure.

When the LIS almost melted after 8,000 years before present, its influence on hydrological patterns diminished. Since that time, dry conditions in the mid-Holocene and their mitigation towards the present largely caused by long-term changes in orbital summer insolation. Warming and drying increased rapidly to a maximum drought around 8,000 years ago, where strong warming caused higher PET than today, while annual precipitation, despite a gradual increasing trend, remained almost as low as during the late deglaciation. Model simulations show that drying in the mid-Holocene was driven by high PET during significantly warmer summer periods. This means that The drought was caused by both higher PET due to warmer summer temperatures in response to orbital forcing, and clearly lower annual precipitation compared to pre-industrial conditions.The evolution of annual water balance curves in the simulations was dominantly shaped by the transition of PET from negative values 8,000 years ago to positive, but decreasing, values after overlapping a roughly linear increase in annual precipitation.

The key finding of the study is that despite different driving forces (natural in the Holocene vs. anthropogenic in the present and future), the drying mechanism in the mid-Holocene was similar to the drying trajectory predicted for mid-latitude North America this centuryThis future trajectory is also primarily driven by warming, which increases evapotranspiration.

Relatively dry conditions during most of the Holocene, as shown by both reconstructions and model simulations, suggest that the recent and modern climate is unusually wet, while drier conditions appear to have been the norm during most of the HoloceneAs future simulations predict that with rising greenhouse gas concentrations and global temperatures, midcontinental North America will experience increased precipitation but decreased available soil moisture due to increased evapotranspiration demand, it is likely that Reversal of the natural neoglacial cooling observed in recent millennia due to anthropogenic global warming may cause the return of midcontinental aridity in eastern North AmericaIn this context early to mid-Holocene serves as a reasonable natural analogue to understand the potential future impacts of climate change on the hydrological regime of the region. Spring