The context of megadrought, drought more severe than any we have experienced over the past 100 years, is assessed in this dissertation. A set of new climate reconstructions including drought, dustiness, and temperature from the south San Juan Mountains in southern Colorado is presented here and provides unforeseen insights into these unusual events. The global context of megadroughts is also analyzed using a network of reconstructions. The new drought record is from bristlecone tree-rings, spans the last 2000 years, and shows two periods with anomalous aridity and drought in the south San Juan Mountains. The later period corresponds with well-characterized medieval climate anomaly (MCA; 900-1400 AD) aridity in southwestern North America (henceforth the Southwest). The earlier interval coincides with the Roman Period (1-400 AD). A severe drought with, almost 50 consecutive years of below average tree-growth, occurs in the middle of the Roman Period during the 2nd century AD. Assessment of Roman and MCA droughts in the context of global climate reconstructions reveals that similar hemisphere scale circulation patterns during both intervals might have contributed to severe aridity in the Southwest. Next relationships between droughts and pluvials in western North America (henceforth the West) and global sea surface temperature (SST) patterns over the last 1100 years are examined. Several methods are used including teleconnection patterns imbedded in tree-ring reconstructed drought maps, and a global network of SST reconstructions. Teleconnection patterns during droughts and pluvials suggest that megadroughts and pluvials were likely forced in part by sequences of anomalous years in the Pacific and Atlantic Ocean, but the analyses also reveals contradictory results that may require new ways of understanding the relationship between SSTs and drought on long timescales. Next, returning to the south San Juan Mountains, we developed a new dust reconstruction from a lake sediment core. The reconstruction illustrates that dustiness has been an important component of Southwestern climate over the past 2941 years. The record shows high dust deposition in the past especially around 900 BC and during the MCA. High dust deposition before recent land use changes suggests that megadroughts or associated periods of aridity were widespread and severe enough to mobilize dust, perhaps resulting in further reductions to mountain snowpack and stream flow. Finally, a new biomarker based temperature reconstruction is presented. The reconstruction spans the last 2000 years and shows that the warmest temperatures during that interval occurred during the Roman Period and the MCA. The record suggests these periods were warmer than today, indicating the San Juan Mountains are a sensitive region to temperature change. Both past warm periods coincide with anomalous drought and dustiness, suggesting that temperature and dust may have acted as megadrought enhancing feedbacks. In summary, this dissertation helps characterize the timing and causes of southwest North American Megadroughts over the past 2000 years; separately addressing changes in moisture balance, dustiness, temperature, hemispheric circulation, and sea surface temperature forcing patterns during these unusual events.

The continuum of western North American hydroclimate during the last millennium is analyzed here using instrumental records, proxy data, and global climate model (GCM) simulations. We find that variance at long timescales (low frequencies) is generally more substantial than variance at short timescales (high frequencies). We find that local sources of autocorrelation (e.g., soil moisture storage) likely explain the tendency for variance to increase from monthly to interannual timescales, but that variance at longer timescales requires remote climate sources of variability. Our analysis of global climate model data indicates that at least one fully coupled GCM can reproduce the characteristics of the continuum on short (interannual) and long (multicentury) timescales, but that proxy spectra and GCM spectra disagree about the amount of variance present on intermediate (decadal to centennial) timescales. Since instrumental records, as well as multiple independent types of paleoclimate records, provide evidence that variance increases with timescale at these frequencies, and because numerical experiments indicate that local autocorrelation is not a likely source of variance at these timescales, we argue that climate model simulations underestimate the full range of low-frequency drought variability. Moreover, the models may also underestimate the risk of future megadroughts, which we attempt to quantify using a new method that combines frequency information from observational data with projections of 21st century hydroclimate. Our results indicate that the risk of a severe, decadal-scale drought during the coming century is at least 1-in-10 for most of the US Southwest, and may be as high as 1-in-3. These findings should be incorporated into adaptation and mitigation strategies to cope with regional climate variability and climate change.

Between 2000 and 2014, annual Colorado River flows averaged 19% below the 1906-1999 average, the worst 15-year drought on record. At least one-sixth to one-half (average at one-third) of this loss is due to unprecedented temperatures (0.9 degrees C above the 1906-1999 average), confirming model-based analysis that continued warming will likely further reduce flows. Whereas it is virtually certain that warming will continue with additional emissions of greenhouse gases to the atmosphere, there has been no observed trend toward greater precipitation in the Colorado Basin, nor are climate models in agreement that there should be a trend. Moreover, there is a significant risk of decadal and multidecadal drought in the coming century, indicating that any increase in mean precipitation will likely be offset during periods of prolonged drought. Recently published estimates of Colorado River flow sensitivity to temperature combined with a large number of recent climate model-based temperature projections indicate that continued business-as-usual warming will drive temperature-induced declines in river flow, conservatively -20% by midcentury and -35% by end-century, with support for losses exceeding -30% at midcentury and -55% at end-century. Precipitation increases may moderate these declines somewhat, but to date no such increases are evident and there is no model agreement on future precipitation changes. These results, combined with the increasing likelihood of prolonged drought in the river basin, suggest that future climate change impacts on the Colorado River flows will be much more serious than currently assumed, especially if substantial reductions in greenhouse gas emissions do not occur. Plain Language Summary Between 2000 and 2014, annual Colorado River flows averaged 19% below the 1906-1999 average, the worst 15-year drought on record. Approximately one-third of the flow loss is due to high temperatures now common in the basin, a result of human caused climate change. Previous comparable droughts were caused by a lack of precipitation, not high temperatures. As temperatures increase in the 21st century due to continued human emissions of greenhouse gasses, additional temperature-induced flow losses will occur. These losses may exceed 20% at mid-century and 35% at end-century. Additional precipitation may reduce these temperature-induced losses somewhat, but to date no precipitation increases have been noted and climate models do not agree that such increases will occur. These results suggest that future climate change impacts on the Colorado River will be greater than currently assumed. Reductions in greenhouse gas emissions will lead to lower future temperatures and hence less flow loss.