Robert S. Webb

Monsoon changes for 6000 years ago: Results of 18 simulations from the Paleoclimate Modeling Intercomparison Project (PMIP)

Amplification of the northern hemisphere seasonal cycle of insolation during the mid-Holocene causes a northward shift of the main regions of monsoon precipitation over Africa and India in all 18 simulations conducted for the Paleoclimate Modeling Intercomparison Project (PMIP). Differences among simulations are related to differences in model formulation. Despite qualitative agreement with paleoecological estimates of biome shifts, the magnitude of the monsoon increases over northern Africa are underestimated by all the models.

Mapping eastern North American vegetation change of the past 18 ka: No-analogs and the future

The method of modern analogs and an extensive data base of modern and fossil pollen data were used to generate a new series of paleovegetation maps for eastern North America spanning the past 18 ka. The maps illustrate the continuous nature of climate-induced vegetation change and the development, after about 10 ka, of modern regional vegetation patterns. Before the Holocene, vegetation biomes without modern analogs were widespread in response to climate conditions without modern analogs and, to a lesser extent, to the rapidity of climate change over the last glacial-interglacial transition. This geological perspective suggests that possible future climate changes could force similarly complex changes in natural vegetation, including the development of biomes without modern analogs.

Temporal and spatial patterns of Holocene dune activity on the Great Plains of North America: megadroughts and climate links

Abstract The Holocene record of eolian sand and loess deposition is reviewed for numerous presently stabilized dune fields on the Great Plains of North America. Dune field activity reflects decade-to-century-scale dominance of drought that exceeded historic conditions, with a growing season deficit of precipitation >25%. The largest dune fields, the Nebraska Sand Hills and ergs in eastern Colorado, Kansas and the Southern High Plains showed peak activity sometime between ca. 7 and 5 cal. ka. Loess deposition between ca. 10 and 4 cal. ka also signifies widespread aridity. Most dune fields exhibit evidence for one or more reactivation events sometime in the past 2 cal. ka; a number of localities register two events post 1 cal. ka, the latest potentially after 1400 AD. However, there is not a clear association of the latest dune remobilization events with up to 13 droughts in the past 2 cal. ka identified in dendroclimatic and lacustrine records. Periods of persistent drought are associated with a La Nina-dominated climate state, with cooling of sea surface temperatures in the tropical Pacific Ocean and later of the tropical Atlantic Ocean and the Gulf of Mexico that significantly weakens cyclogenesis over central North America. As drought proceeds, reduced soil moisture and vegetation cover would lessen evaporative cooling and increase surface temperatures. These surface changes strengthen the eastward expansion of a high-pressure ridge aloft and shift the jet stream northward, further enhancing continent-wide drought. Uncertainty persists if dune fields will reactivate in the future at a scale similar to the Holocene because of widespread irrigation, the lack of migratory bison herds, and the suppression of prairie fires, all of which enhance stabilization of dune fields in the Great Plains.

Climatic controls on interannual variability of precipitation δ18O: Simulated influence of temperature, precipitation amount, and vapor source region

We use an atmospheric GCM that incorporates stable isotopes and regional vapor source tracers in the hydrologic cycle to explore the relationship between interannual variability in climate and precipitation δ18O globally. On the basis of a 12-year simulation forced by observed sea surface temperatures (SSTs), we identify changes in the amount of precipitation and in the contributions of local and nearby vapor sources as the most important determinants of simulated interannual isotopic changes. The model simulates weak positive correlation between temperature and isotopic variability only in certain continental regions, mostly in the extratropics. Comparison with long observed records of isotopes and climate indicates that the model simulates realistic patterns of temperature-isotope correlation but may overestimate the isotopic influence of precipitation amount. Perturbations in circulation patterns that alter the transport and mixing of air masses at a site also change the relative contributions of vapor from different source regions. Simulated changes in vapor source regions are large, reaching ±10–15% of the total precipitation, and cause significant isotopic variability in nearly all grid cells. Our results suggest that shifts among vapor sources may provide an important control on the interannual isotopic variability observed in modern precipitation and paleoclimatic records. The isotopic variability simulated in this experiment results from the interaction of several aspects of climate. Interannual temperature variability generally involves circulation changes that alter air mass transport, vapor source regions, and condensation history; this advective mechanism may explain the relative weakness of temperature-isotope correlations in both the model and the observations.

Understanding uncertainties in future Colorado River streamflow

The Colorado River is the primary water source for more than 30 million people in the United States and Mexico. Recent studies that project streamf low changes in the Colorado River all project annual declines, but the magnitude of the projected decreases range from less than 10% to 45% by the mid-twenty-first century. To understand these differences, we address the questions the management community has raised: Why is there such a wide range of projections of impacts of future climate change on Colorado River streamflow, and how should this uncertainty be interpreted? We identify four major sources of disparities among studies that arise from both methodological and model differences. In order of importance, these are differences in 1) the global climate models (GCMs) and emission scenarios used; 2) the ability of land surface and atmospheric models to simulate properly the high-elevation runoff source areas; 3) the sensitivities of land surface hydrology models to precipitation and temperature changes; ...

The role of oceanic forcing in mid-Holocene Northern Hemisphere climatic change

Previous model investigations suggested that changes in orbital forcing and feedbacks associated with northward expansion of the boreal forest were both required to explain the full magnitude of enhanced high-latitude Northern Hemisphere summer warming at 6 ka, thus implying that biospheric feedbacks (decreased planetary albedo associated with forest expansion) may be large in the future [Foley et al., 1994; TEMPO (Testing Earth System Models with Paleo-Observations), 1996]. Before the magnitude of past biospheric and other feedbacks can be estimated with confidence, however, the role of realistic high-latitude oceanic forcing (i.e., sea surface temperature (SST) increases and sea ice reductions) should also be considered. Here we review existing paleoceanographic observations that suggest portions of the North Atlantic were up to 4°C warmer than today at 6 ka. We then combine our estimates of mid-Holocene North Atlantic SST and sea ice conditions with new climate model simulations to suggest that a significant portion of high-latitude summer warming at 6 ka in the Northern Hemisphere can be accounted for by altered orbital forcing, SSTs, and sea ice relative to today. Our results underscore the importance of incorporating realistic boundary conditions when using paleoenvironmental data to evaluate the climate systems sensitivity to altered forcing and suggest that high-latitude oceanic feedbacks were major contributors to enhanced high-latitude summer warming in the Northern Hemisphere at 6 ka.

Is a Transition to Semipermanent Drought Conditions Imminent in the U.S. Great Plains

AbstractHow Great Plains climate will respond under global warming continues to be a key unresolved question. There has been, for instance, considerable speculation that the Great Plains is embarking upon a period of increasing drought frequency and intensity that will lead to a semipermanent Dust Bowl in the coming decades. This view draws on a single line of inference of how climate change may affect surface water balance based on sensitivity of the Palmer drought severity index (PDSI). A different view foresees a more modest climate change impact on Great Plains surface moisture balances. This draws on direct lines of analysis using land surface models to predict runoff and soil moisture, the results of which do not reveal an ominous fate for the Great Plains. The authors’ study presents a parallel diagnosis of projected changes in drought as inferred from PDSI and soil moisture indicators in order to understand causes for such a disparity and to shed light on the uncertainties. PDSI is shown to be an ...

Modern pollen/climate calibration for southern South America

Abstract Using the response surface technique a modern pollen/climate calibration is presented for the mid-latitudes in southern South America between latitudes 39°S and 44°S, extending from the west coast in Chile across the Andes into the Patagonian steppe region. The climate parameters identified that relate to the present-day pollen distribution of the nine most important pollen types ( Nothofagus dombeyi -type, Nothofagus obliqua -type, Podocarpus , Weinmannia , Caldcluvia / Eucryphia -type, Cupressaceae, Myrtaceae, Misodendrum and steppe-types) are summer precipitation, summer and winter temperatures. Using the modern pollen/climate data set, three fossil pollen records (Mallin Aguado and Lago Moreno, east of the Andes, and Caunahue, west of the Andes) are interpreted in terms of these three seasonal climate parameters. Despite intervals of no-analog fossil pollen assemblages, the reconstruction shows similar climate trends on both sides of the Andes during the last 21 000 cal. yr BP (17 000 yr BP), with summer precipitation as low or markedly lower than today prior to 17 000 cal. yr BP (14 500 yr BP), as high or higher than today between 15 000 and 12 500 cal. yr BP (12 500 and 10 500 yr BP), and again lower than today between 12 500 and 8000 cal. yr BP (10 500 and 7200 yr BP). Summer and winter temperatures were low (or high) when summer precipitation was high (or low).

Understanding the mid-holocene climate

Abstract Paleoclimatic evidence suggests that during the mid-Holocene epoch (about 6000 yr ago) North America and North Africa were significantly drier and wetter, respectively, than at present. Modeling efforts to attribute these differences to changes in orbital parameters and greenhouse gas (GHG) levels have had limited success, especially over North America. In this study, the importance of a possibly cooler tropical Pacific Ocean during the epoch (akin to a permanent La Nina–like perturbation to the present climate) in causing these differences is emphasized. Systematic sets of atmospheric general circulation model experiments, with prescribed sea surface temperatures (SSTs) in the tropical Pacific basin and an interactive mixed layer ocean elsewhere, are performed. Given the inadequacies of current fully coupled climate models in simulating the tropical Pacific climate, this intermediate coupling model configuration is argued to be more suitable for quantifying the contributions of the altered orbit...

The Making of an Extreme Event: Putting the Pieces Together

We examine how physical factors spanning climate and weather contributed to record warmth over the central and eastern United States in March 2012, when daily temperature anomalies at many locations exceeded 20°C. Over this region, approximately 1°C warming in March temperatures has occurred since 1901. This long-term regional warming is an order of magnitude smaller than temperature anomalies observed during the event, indicating that most of the extreme warmth must be explained by other factors. Several lines of evidence strongly implicate natural variations as the primary cause for the extreme event. The 2012 temperature anomalies had a close analog in an exceptionally warm U.S. March occurring over 100 years earlier, providing observational evidence that an extreme event similar to March 2012 could be produced through natural variability alone. Coupled model forecasts and simulations forced by observed sea surface temperatures (SSTs) show that forcing from anomalous SSTs increased the probability of e...