The research, published in the Proceedings of the National Academy of Sciences (PNAS) on August 17, 2026, reconstructs environmental conditions in Yellowstone’s largest thermal area since the end of the last ice age. Whitlock et al. (2026) examined sediment records from five lakes and compared the findings with regional paleoclimatic simulations developed independently to investigate hydrothermal input, wildfire activity, vegetation, and lake conditions. Arsenic concentrations in lake sediments provide evidence for changes in hydrothermal input after the lakes formed. Hydrothermal contributions were generally higher during wetter intervals and lower during dry periods when reduced snow cover supplied less water to the hydrothermal circulation system. The timing and intensity of reconstructed input varied among individual lake sites. Water from precipitation and snowmelt enters the ground and circulates through Yellowstone’s heated subsurface. Changes in the availability of this water can alter the fluid supply to hydrothermal structures, making moisture availability an important factor in their activity. Between about 12,000 and 6,000 years ago, summer sunshine was about 8% greater than pre-industrial conditions. Regional paleoclimatic simulations reconstructed July–September temperatures 2.6–2.7°C (4.7–4.9°F) warmer, while effective humidity, defined as precipitation minus evaporation, was 36–54% lower. The vapor pressure deficit from July to September was 29 to 56% higher than the pre-industrial baseline. The vapor pressure deficit measures the difference between the amount of water vapor present in the air and the amount it can contain at saturation; Higher values correspond to a greater demand for atmospheric humidity. The reconstructed conditions were associated with increased wildfire activity preserved in the lake sediment charcoal record. The PNAS study identifies the interval between approximately 12,000 and 4,000 years ago as the interval of greatest wildfire activity in the reconstructed record of the Lower Geyser Basin. Charcoal particles deposited in lake sediments provide evidence of past fires, allowing researchers to examine variations in fire activity over thousands of years. The interval of greatest wildfire activity extended beyond the period of approximately 12,000 to 6,000 years used for the detailed temperature and humidity reconstruction. The study lakes were shallower and had lower nutrient levels during the warm, dry interval, based on evidence preserved in their sediments. Diatoms, microscopic algae whose remains accumulate at the bottom of lakes, provide information about historical water conditions and nutrient availability. The research combined pollen, charcoal, diatoms, and sedimentary arsenic and cesium to reconstruct vegetation, wildfires, aquatic conditions, and hydrothermal inputs. An independent research group independently developed high-resolution regional paleoclimatic simulations. The two streams of evidence allowed the researchers to compare preserved changes in the lakes with independently reconstructed temperature and humidity conditions. Simulations and sediment records identify deglaciation as a major influence on the early environmental history of the basin, followed by long-term variations in seasonal solar radiation associated with cyclic changes in Earth’s orbit. The variations contributed to changes in summer temperatures, moisture availability, and conditions associated with wildfires and hydrothermal inflow. Two of the five lakes in the study likely formed after hydrothermal explosions near the end of the last ice age, when glaciers were melting in the Yellowstone region. Their later sediment records preserve evidence of changes in the surrounding hydrothermal and terrestrial environments. Hydrothermal explosions can occur when pressure increases in a confined underground reservoir containing hot water and steam until surrounding rocks fracture. The expansion expels material and can create or enlarge a crater. The USGS identifies Pocket Basin in Lower Geyser Basin as an ancient hydrothermal explosion crater; is a separate geological example and is not identified here as one of the five study lakes. The study of the five lakes does not establish a recurrence interval or provide a forecast of future hydrothermal explosions. Their reconstruction of reduced hydrothermal input during dry intervals should not be interpreted as evidence of reduced magmatic hazard. The vegetation record shows comparatively little change in the composition and cover of lodgepole pine forests after their establishment approximately 12,800 and 11,000 years ago, despite recorded variations in hydrothermal input, wildfire activity, and lake conditions. Researchers attribute its relative stability in part to the area’s nutrient-poor rhyolitic soils, which limit the composition of the vegetation. Vegetation records from other parts of Yellowstone show more substantial changes in forest composition in different underlying rock types, where nutrient and moisture availability is greatest. References:1 Whitlock, C., Schiller, CM, Hostetler, SW, Hurwitz, S., Alt, M., Brown, SR, Harrison, LN, Alder, JR, Busch, K., Shelly, J., & McWethy, DB (2026). Postglacial ecosystem development of a hydrothermal landscape in Yellowstone National Park. Proceedings of the National Academy of Sciences, 123(35), e2613422123. https://doi.org/10.1073/pnas.26134221232 15,000 years of change in the Lower Yellowstone Geyser Basin – US Geological Survey – September 21, 2026
