A 2000-year history of forest disturbance in southern Pacific Costa Rica: pollen, spore, and charcoal evidence from Laguna Santa Elena

Paleoecological records reveal information about past changes in vegetation composition and disturbance in tropical ecosystems. These environmental histories have direct bearing on modem ecological studies and the management of protected areas, and provide information about the interaction between prehistoric peoples and their environment. In this study, I reconstruct prehistoric and historic forest disturbance and vegetation change from southern Pacific Costa Rica, in the vicinity of the Las Cruces Biological Station and the La Amistad International Park and Biosphere Reserve. Pollen and charcoal in sediments from Laguna Santa Elena reveal a nearly continuous record of human alteration of these tropical forests over the past two millennia. The basal portion of the core shows nearly intact premontane forests approximately 1800 cal. yr. BP, although there is evidence of human presence on the landscape in the form of maize pollen and charcoal. Clearing for agriculture resulted in the dominance of disturbance taxa in the watershed beginning at least 1400 cal. yr. BP. The pollen record reveals only one possible, brief hiatus from human occupation of the watershed, although secondary succession began to occur in the Laguna Santa Elena watershed prior to that time, starting about 700 cal. yr. BP. Three eruptions of nearby Volcan Bani at approximately 580, 1080, and 1440 cal. yr. BP apparently had little effect on the prehistoric populations in the immediate vicinity of the lake. Historic and modem land clearance has perpetuated a vegetation assemblage of disturbance and successional taxa. The pollen and charcoal records from Laguna Santa Elena demonstrate that this region of Costa Rica has a long history of forest clearance, which has implications for science and conservation in nearby ecological reserves and research stations, as well as for ongoing archaeological investigations in the region

How unusual is the 2012–2014 California drought?

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 41 (2014): 9017–9023, doi:10.1002/2014GL062433.For the past three years (2012–2014), California has experienced the most severe drought conditions in its last century. But how unusual is this event? Here we use two paleoclimate reconstructions of drought and precipitation for Central and Southern California to place this current event in the context of the last millennium. We demonstrate that while 3 year periods of persistent below-average soil moisture are not uncommon, the current event is the most severe drought in the last 1200 years, with single year (2014) and accumulated moisture deficits worse than any previous continuous span of dry years. Tree ring chronologies extended through the 2014 growing season reveal that precipitation during the drought has been anomalously low but not outside the range of natural variability. The current California drought is exceptionally severe in the context of at least the last millennium and is driven by reduced though not unprecedented precipitation and record high temperatures.D. G. was supported by a NOAA Climate and Global Change Postdoctoral Fellowship. Both authors also acknowledge the Woods Hole Oceanographic Institution Academic Program Office for support of this research.2015-06-3

Identifying coherent spatiotemporal modes in time-uncertain proxy paleoclimate records

Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Climate Dynamics 41 (2013): 1291-1306, doi:10.1007/s00382-012-1483-0.High-resolution sedimentary paleoclimate proxy records offer the potential to expand the detection and analysis of decadal- to centennial-scale climate variability during recent millennia, particularly within regions where traditional high-resolution proxies may be short, sparse, or absent. However, time uncertainty in these records potentially limits a straightforward objective identification of broad-scale patterns of climate variability. Here, we describe a procedure for identifying common patterns of spatiotemporal variability from time uncertain sedimentary records. This approach, which we term Monte Carlo Empirical Orthogonal Function (MCEOF) analysis, uses iterative age modeling and eigendecomposition of proxy time series to isolate common regional patterns and estimate uncertainties. As a test case, we apply this procedure to a diverse set of time-uncertain lacustrine proxy records from East Africa. We also perform a pseudoproxy experiment using climate model output to examine the ability of the method to extract shared anomalies given known signals. We discuss the advantages and disadvantages of our approach, including possible extensions of the technique.JET acknowledges the UCAR Climate and Global Change Postdoctoral Fellowship for support.2014-08-2

Pattern-based Downscaling of Snowpack Variability in the Western United States

The decline in snowpack across the western United States is one of the most pressing threats posed by climate change to regional economies and livelihoods. Earth system models are important tools for exploring past and future snowpack variability, yet their coarse spatial resolutions distort local topography and bias spatial patterns of accumulation and ablation. Here, we explore pattern-based statistical downscaling for spatially-continuous interannual snowpack estimates. We find that a few leading patterns capture the majority of snowpack variability across the western US in observations, reanalyses, and free-running simulations. Pattern-based downscaling methods yield accurate, high resolution maps that correct mean and variance biases in domain-wide simulated snowpack. Methods that use large-scale patterns as both predictors and predictands perform better than those that do not and all are superior to an interpolation-based “delta change” approach. These findings suggest that pattern-based methods are appropriate for downscaling interannual snowpack variability and that using physically meaningful large-scale patterns is more important than the details of any particular downscaling method

Multi-scale drought and ocean–atmosphere variability in monsoon Asia

© The Author(s), 2015. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Environmental Research Letters 10 (2015): 074010, doi:10.1088/1748-9326/10/7/074010.Spatially extensive and persistent drought episodes have repeatedly influenced human history, including the 'Strange Parallels' drought event in monsoon Asia during the mid-18th century. Here we explore the dynamics of sustained monsoon failure using observed and tree-ring reconstructed drought patterns and a 1300-year pre-industrial community earth system model control run. Both modern observational and climate model drought patterns during years with extremely weakened South Asian monsoon resemble those reconstructed for the Strange Parallels drought. Model analysis reveals that this pattern arises during boreal spring over Southeast Asia, with decreased precipitation and moisture flux, while related summertime climate anomalies are confined to the Indian subcontinent. Years with simulated South Asian drying exhibit canonical El Niño conditions over the Pacific and associated shifts in the Walker circulation. In contrast, multi-year drought periods, resembling those sustained during the Strange Parallels drought, feature anomalous Pacific warming around the dateline, typical of El Niño Modoki events.This work was performed with support and funding from the Significant Opportunities in Atmospheric Research and Sciences Program (NSF AGS-1120459), WHOI Academic Programs Office Funds, and NSF AGS-1338734, AGS-1203704, and AGS-1304245

Detection and removal of disturbance trends in tree-ring series for dendroclimatology

The authors thank The Carnegie Trust for the Universities of Scotland for providing funding for Miloš Rydval’s Ph.D. The Scottish pine network expansion has been an ongoing task since 2006, and funding must be acknowledged to the following projects: EU project “Millennium” (017008-2), Leverhulme Trust project “RELiC: Reconstructing 8000 years of Environmental and Landscape change in the Cairngorms (F/00 268/BG)”, and the NERC project “SCOT2K: Reconstructing 2000 years of Scottish climate from tree rings (NE/K003097/1)”. Kevin J. Anchukaitis was also supported by a grant for the US National Science Foundation (ARC-0902051).Nonclimatic disturbance events are an integral element in the history of forests. Although the identification of the occurrence and duration of such events may help to understand environmental history and landscape change, from a dendroclimatic perspective, disturbance can obscure the climate signal in tree rings. However, existing detrending methods are unable to remove disturbance trends without affecting the retention of long-term climate trends. Here, we address this issue by using a novel method for the detection and removal of disturbance events in tree-ring width data to assess their spatiotemporal occurrence in a network of Scots pine (Pinus sylvestris L.) trees from Scotland. Disturbance trends “superimposed” on the tree-ring record are removed before detrending and the climate signals in the precorrection and postcorrection chronologies are evaluated using regional climate data, proxy system model simulations, and maximum latewood density (MXD) data. Analysis of subregional chronologies from the West Highlands and the Cairngorms in the east reveals a higher intensity and more systematic disturbance history in the western subregion, likely a result of extensive timber exploitation. The method improves the climate signal in the two subregional chronologies, particularly in the more disturbed western sites. Our application of this method demonstrates that it is possible to minimise the effects of disturbance in tree-ring width chronologies to enhance the climate signal.PostprintPeer reviewe

Multidecadal variability in East African hydroclimate controlled by the Indian Ocean

The recent decades-long decline in East African rainfall suggests that multidecadal variability is an important component of the climate of this vulnerable region. Prior work based on analysing the instrumental record implicates both Indian and Pacific ocean sea surface temperatures (SSTs) as possible drivers of East African multidecadal climate variability, but the short length of the instrumental record precludes a full elucidation of the underlying physical mechanisms. Here we show that on timescales beyond the decadal, the Indian Ocean drives East African rainfall variability by altering the local Walker circulation, whereas the influence of the Pacific Ocean is minimal. Our results, based on proxy indicators of relative moisture balance for the past millennium paired with long control simulations from coupled climate models, reveal that moist conditions in coastal East Africa are associated with cool SSTs (and related descending circulation) in the eastern Indian Ocean and ascending circulation over East Africa. The most prominent event identified in the proxy record—a coastal pluvial from 1680 to 1765—occurred when Indo-Pacific warm pool SSTs reached their minimum values of the past millennium. Taken together, the proxy and model evidence suggests that Indian Ocean SSTs are the primary influence on East African rainfall over multidecadal and perhaps longer timescales

Forced and unforced variability of twentieth century North American droughts and pluvials

Research on the forcing of drought and pluvial events over North America is dominated by general circulation model experiments that often have operational limitations (e.g., computational expense, ability to simulate relevant processes, etc). We use a statistically based modeling approach to investigate sea surface temperature (SST) forcing of the twentieth century pluvial (1905-1917) and drought (1932-1939, 1948-1957, 1998-2002) events. A principal component (PC) analysis of Palmer Drought Severity Index (PDSI) from the North American Drought Atlas separates the drought variability into five leading modes accounting for 62% of the underlying variance. Over the full period spanning these events (1900-2005), the first three PCs significantly correlate with SSTs in the equatorial Pacific (PC 1), North Pacific (PC 2), and North Atlantic (PC 3), with spatial patterns (as defined by the empirical orthogonal functions) consistent with our understanding of North American drought responses to SST forcing. We use a large ensemble statistical modeling approach to determine how successfully we can reproduce these drought/pluvial events using these three modes of variability. Using Pacific forcing only (PCs 1-2), we are able to reproduce the 1948-1957 drought and 1905-1917 pluvial above a 95% random noise threshold in over 90% of the ensemble members; the addition of Atlantic forcing (PCs 1-2-3) provides only marginal improvement. For the 1998-2002 drought, Pacific forcing reproduces the drought above noise in over 65% of the ensemble members, with the addition of Atlantic forcing increasing the number passing to over 80%. The severity of the drought, however, is underestimated in the ensemble median, suggesting this drought intensity can only be achieved through internal variability or other processes. Pacific only forcing does a poor job of reproducing the 1932-1939 drought pattern in the ensemble median, and less than one third of ensemble members exceed the noise threshold (28%). Inclusion of Atlantic forcing improves the ensemble median drought pattern and nearly doubles the number of ensemble members passing the noise threshold (52%). Even with the inclusion of Atlantic forcing, the intensity of the simulated 1932-1939 drought is muted, and the drought itself extends too far into the southwest and southern Great Plains. To an even greater extent than the 1998-2002 drought, these results suggest much of the variance in the 1932-1939 drought is dependent on processes other than SST forcing. This study highlights the importance of internal noise and non SST processes for hydroclimatic variability over North America, complementing existing research using general circulation models

Pluvials, droughts, the Mongol Empire, and modern Mongolia

Although many studies have associated the demise of complex societies with deteriorating climate, few have investigated the connection between an ameliorating environment, surplus resources, energy, and the rise of empires. The 13th-century Mongol Empire was the largest contiguous land empire in world history. Although drought has been proposed as one factor that spurred these conquests, no high-resolution moisture data are available during the rapid development of the Mongol Empire. Here we present a 1,112-y tree-ring reconstruction of warm-season water balance derived from Siberian pine (Pinus sibirica) trees in central Mongolia. Our reconstruction accounts for 56% of the variability in the regional water balance and is significantly correlated with steppe productivity across central Mongolia. In combination with a gridded temperature reconstruction, our results indicate that the regional climate during the conquests of Chinggis Khan’s (Genghis Khan’s) 13th-century Mongol Empire was warm and persistently wet. This period, characterized by 15 consecutive years of above-average moisture in central Mongolia and coinciding with the rise of Chinggis Khan, is unprecedented over the last 1,112 y. We propose that these climate conditions promoted high grassland productivity and favored the formation of Mongol political and military power. Tree-ring and meteorological data also suggest that the early 21st-century drought in central Mongolia was the hottest drought in the last 1,112 y, consistent with projections of warming over Inner Asia. Future warming may overwhelm increases in precipitation leading to similar heat droughts, with potentially severe consequences for modern Mongolia