Showing papers by "Kevin J. Anchukaitis published in 2012"

Tree rings and volcanic cooling

Abstract: To the Editor — In their Letter, Mann and colleagues1 claim to have identified a discrepancy between the degree of volcanic cooling in climate model simulations and the analogous cooling indicated in a tree-ring-based Northern Hemisphere temperature reconstruction2, and attribute it to a putative temporary cessation of tree growth at some sites near the temperature limit for growth. They argue that this growth cessation would lead to missing rings in cool years, thus resulting in underestimation of cooling in the tree-ring record. This suggestion implies that periods of volcanic cooling could result in widespread chronological errors in tree-ring-based temperature reconstructions1,3. Mann and colleagues base their conclusions solely on the evidence of a tree-ring-growth model. Here we point to several factors that challenge this hypothesis of missing tree rings; specifically, we highlight problems in their implementation of the tree-ring model used1, a lack of consideration of uncertainty in the amplitude and spatial pattern of volcanic forcing and associated climate responses, and a lack of any empirical evidence for misdating of treering chronologies. Several aspects of their tree-ringgrowth simulations are erroneous. First, they use an algorithm that has not been tested for its ability to reflect actual observations (Supplementary Fig. 1), even though established growth models, such as the Vaganov–Shashkin model4,5, are available. They rely on a minimum growth temperature threshold of 10 °C that is incompatible with real-world observations. This condition is rarely met in regions near the limit of tree growth, where ring formation demonstrably occurs well below this temperature: there is abundant empirical evidence that the temperature limit for tree-ring formation is around 5 °C (refs 6,7). Mann and colleagues arbitrarily and without justification require 26 days with temperatures above their unrealistic threshold for ring formation. Their resulting growing season becomes unusually short, at 50–60 days rather than the more commonly observed 70–137 days4,7. Furthermore, they use a quadratic function to describe growth that has no basis in observation or theory, and they ignore any daylength and moisture constraints on growth. These assumptions all bias Mann and colleagues’ tree-growth model results1 towards erroneously producing missing tree rings. Reconstructing simulated temperatures in the same manner as Mann and colleagues, but using a well-tested tree-ring growth model5 and realistic parameters provides no support for their hypothesis (Fig. 1). Instead we find good agreement between summertime temperatures reconstructed from pseudoproxies and those simulated with a climate model (CSM1.4)8 (Fig. 1a), for the whole record as well as in specific years following major volcanic eruptions (Fig. 1b–d). Mann and colleagues’ principal result arises from their failure to select a realistic minimum temperature for growth, use actual treering chronology locations and recognize Tree rings and volcanic cooling

A Long-Term Perspective on a Modern Drought in the American Southeast

Abstract: The depth of the 2006‐9 drought in the humid, southeastern US left several metropolitan areas with only a 60‐120 day water supply. To put the region’s recent drought variability in a long-term perspective, a dense and diverse tree-ring network—including the first records throughout the Apalachicola‐Chattahoochee‐Flint river basin—is used to reconstruct drought from 1665 to 2010 CE. The network accounts for up to 58.1% of the annual variance in warm-season drought during the 20th century and captures wet eras during the middle to late 20th century. The reconstruction shows that the recent droughts are not unprecedented over the last 346 years. Indeed, droughts of extended duration occurred more frequently between 1696 and 1820. Our results indicate that the era in which local and state water supply decisions were developed and the period of instrumental data upon which it is based are amongst the wettest since at least 1665. Given continued growth and subsequent industrial, agricultural and metropolitan demand throughout the southeast, insights from paleohydroclimate records suggest that the threat of water-related conflict in the region has potential to grow more intense in the decades to come.

Pre‐Columbian deforestation as an amplifier of drought in Mesoamerica

Abstract: Droughts in pre-Columbian Mesoamerica caused significant societal disruptions during the Late Classic and Post-Classic Periods. While the primary causes of these droughts are still debated, it has been speculated that they may be linked to extensive deforestation associated with high population densities during these intervals. Here we show that pre-Columbian deforestation would have biased the climate in Mesoamerica towards a drier mean state, amplifying drought in the region. In climate model simulations using a pre-Columbian land cover reconstruction, annual precipitation decreases by 5%-15% throughout southern Mexico and the Yucatan compared to simulations using either natural forest cover or forest regrowth associated with population declines after 1500 C. E. These changes are driven primarily by large reductions (10%-20%) in precipitation during the late summer wet season (August-September). When compared to precipitation changes estimated to have occurred during the Maya collapse, our results suggest that deforestation could account for up to sixty percent of the mean drying during this interval. Many regions previously deforested in the pre-Columbian era are now under dense forest cover, indicating potential future climate impacts should tropical deforestation of these areas accelerate. Citation: Cook, B. I., K. J. Anchukaitis, J. O. Kaplan, M. J. Puma, M. Kelley, and D. Gueyffier (2012), Pre-Columbian deforestation as an amplifier of drought in Mesoamerica, Geophys. Res. Lett., 39, L16706, doi:10.1029/2012GL052565.

Snow cover and precipitation impacts on dry season streamflow in the Lower Mekong Basin

Abstract: Climate change impacts on dry season streamflow in the Mekong River are relatively understudied, despite the fact that water availability during this time is critically important for agricultural and ecological systems. Analyses of two gauging stations (Vientiane and Kratie) in the Lower Mekong Basin (LMB) show significant positive correlations between dry season (March through May, MAM) discharge and upper basin snow cover and local precipitation. Using snow cover, precipitation, and upstream discharge as predictors, we develop skillful regression models for MAM streamflow at Vientiane and Kratie, and force these models with output from a suite of general circulation model (GCM) experiments for the twentieth and twenty-first centuries. The GCM simulations predict divergent trends in snow cover (decreasing) and precipitation (increasing) over the twenty-first century, driving overall negligible long-term trends in dry season streamflow. Our study demonstrates how future changes in dry season streamflow in the LMB will depend on changes in snow cover and precipitation, factors that will need to be considered when assessing the full basin response to other climatic and non-climatic drivers.