An uncertainty distribution is developed to describe areal-average infiltration at Yucca Mountain averaged over the next million years. The distribution uses Infiltration Tabulator for Yucca Mountain (ITYM) model results to estimate uncertainty in net infiltration given decadal-average climate, driven with two independent estimates for potential future climatic sequences. Both future climatic sequences use orbital mechanics linked to paleoclimatic proxies to estimate future climate states, but one uses primarily biotic indicators and the other uses primarily abiotic indicators to link paleoclimate to orbital stage. Abiotic indicators suggest that present-day climate variation by latitude in the Great Basin is reasonably analogous to glacial-scale climate variation at a single location once orographic, lake effect, and insolation influences are accounted for. In the southern Sierra Nevada, abiotic indicators suggest that mean annual precipitation was 1.7-1.9 times larger than present and mean annual temperature was 5.6-7 degrees C cooler than present at the last glacial maximum. The influence of decadal to millennial-scale variability is estimated using the 8000 year Methuselah Walk tree-ring record, which suggests that even large inferred fluctuations over the Holocene only modestly increase 8000-year-average infiltration. Combining the future-climate projections with the infiltration model uncertainty distribution yields approximately lognormal uncertainty distributions for projected million-year areal-average net infiltration, with a mean of 41 mm/yr and coefficient of variation of 0.81. Even if anthropogenic effects significantly alter climatic patterns relative to past climate cycles, the projected anthropogenic period is sufficiently brief that million-year averages are not likely to be substantially influenced.
