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CGD 2009 Profiles in Science: Dr. Caspar Ammann
Summary of achievements
Caspar Ammann's research centers around the high-resolution climate of past centuries and millennia and how this information can help to understand what elements of future climate might be predictable as well as what potential environmental and ecological impacts are to be anticipated given various story lines of climate change scenarios.
Better natural climate forcing datasets are necessary for more realistic simulations of past centuries and millennia and for a quantitatively more thorough assessment of the cause of natural variability. The significantly increased number of ice core records from the polar ice sheets offer the opportunity of more thorough statistical description of the volcanic forcing, going from a single series to a description where every eruption is provided with a probability of that event having happened in the first place and where the magnitude can be described by a distribution. Such a probabilistic formulation of the forcings is scientifically consistent with the underlying goal of quantifying the link between the forcing and climate.
Climate model studies that investigate the effects of externally forced climate are being performed. Process studies of the effects of low and high latitude volcanic eruptions as well as simulations of solar irradiance/activity changes offer insight into the models ensemble response of the climate system. In collaboration with Post-Doc David Schneider, these simulations are also serving an interdisciplinary Arctic research collaborative that attempts to synthesize the climate record gained primarily from high-resolution lake records. Collaborations across ESSL are used to identify what model configurations are necessary to capture the necessary processes. Using a suite of simulations with increasing model complexity from the standard CCSM towards a more complete representation of the climate system of WACCM (vertical extent, coupled chemistry), climate response to the repeated solar cycle and to the injection of volcanic aerosol are investigated. Other simulations with the coupled CCSM-3 investigate the cause of global patterns of climate during Medieval times as well as the transition into the Little Ice Age. No coupled GCM has so far reproduced the full structure of climate anomalies that can be found in the proxy record. In collaboration with Nicholas Graham (HRC & Scripps) and a team led by Kim Cobb (Georgia Tech), simulations are being performed that focus on the tropical circulation and its possible response to radiative forcing through mid-latitude teleconnections.
Extending the instrumental record through the development of improved high resolution climate reconstructions is crucial if we are to understand the role of forced changes in climate at regional scales. A fundamental problem in identifying what part of climate variability is externally forced arises from the large uncertainty in existing reconstruction methods and series. Caspar's collaborations with a multi-institution team of paleoclimatologists and statisticians is developing a new way of reconstructing climate using Bayesian Hierarchical Models as the framework (http://www.cgd.ucar.edu/ccr/ammann/CMG/). This allows for a more complete exploitation of the available climate record through inclusion of records with extremely different characteristics as well as explicit physical constraints. Addtionally, the community program of the Paleoclimate Reconstruction (PR) Challenge (http://www.pages.unibe.ch/science/prchallenge/index.html) is setup to test the accuracy of regional reconstructions and to guide the paleo reconstruction communities in developing more adequate forward models for their proxies. Using climate model output, a systematic intercomparison of the existing reconstruction approaches is performed and a double-blind setup will allow the community to identify where the next efforts need to be put in.
Publications
Gao, C., A. Robock and C. Ammann. 2008: Volcanic forcing of climate over the past 1500 years: An improved ice core-based index for climate models. Journal of Geophysical Research-Atmospheres, 113, D23111, doi:10.1029/2008JD010239.
Figure 1: High resolution figure
Abstract: Understanding natural causes of climate change is vital to evaluate the relative impacts of human pollution and land surface modification on climate. We have investigated one of the most important natural causes of climate change, volcanic eruptions, by using 54 ice core records from both the Arctic and Antarctica. Our recently collected suite of ice core data, more than double the number of cores ever used before, reduces errors inherent in reconstructions based on a single or small number of cores, which enables us to obtain much higher accuracy in both detection of events and quantification of the radiative effects. We extracted volcanic deposition signals from each ice core record by applying a high-pass loess filter to the time series and examining peaks that exceed twice the 31-year running median absolute deviation. We then studied the spatial pattern of volcanic sulfate deposition on Greenland and Antarctica and combined this knowledge with a new understanding of stratospheric transport of volcanic aerosols to produce a forcing data set as a function of month, latitude, and altitude for the past 1500 years. We estimated the uncertainties associated with the choice of volcanic signal extraction criteria, ice core sulfate deposition to stratospheric loading calibration factor, and the season for the eruptions without a recorded month. We forced an energy balance climate model with this new volcanic forcing data set, together with solar and anthropogenic forcing, to simulate the large-scale temperature response. The results agree well with instrumental observations for the past 150 years and with proxy records for the entire period. Through better characterization of the natural causes of climate change, this new data set will lead to improved prediction of anthropogenic impacts on climate. The new data set of stratospheric sulfate injections from volcanic eruptions for the past 1500 years, as a function of latitude, altitude, and month, is available for download in a format suitable for forcing general circulation models of the climate system.
Figure caption: MAGICC-simulated NH temperature response (red curve) plotted on top of temperature reconstructions (shaded) and instrumental observations (black curve) (Figure 6.10 of IPCC [2007], used with permission). We plotted smoothed (31-year weighted mean) temperature anomalies with respect to the 1961–1990 mean.
Jones, P.D., K.R. Briffa, T.J. Osborn, J.M. Lough, T.D. van Ommen, B.M. Vinther, J. Luterbacher, E.R. Wahl, F.W. Zwiers, M.E. Mann, G.A. Schmidt, C.M. Ammann, B.M. Buckley, K.M. Cobb, J. Esper, H. Goosse, N. Graham, E. Jansen, T. Kiefer, C. Kull, M. Küttel, E. Mosley-Thompson, J.T. Overpeck, N. Riedwyl, M. Schulz, A.W. Tudhope, R. Villalba, H. Wanner, E. Wolff, and E. Xoplaki, 2009: High-resolution paleoclimatology of the last millennium: a review of current status and future prospects. Holocene, 19, 3-49, doi:10.1177/0959683608098952.
Figure 1: High resolution figure
Abstract: This review of late-Holocene palaeoclimatology represents the results from a PAGES/CLIVAR Intersection Panel meeting that took place in June 2006. The review is in three parts: the principal high-resolution proxy disciplines (trees, corals, ice cores and documentary evidence), emphasizing current issues in their use for climate reconstruction; the various approaches that have been adopted to combine multiple climate proxy records to provide estimates of past annual-to-decadal timescale Northern Hemisphere surface temperatures and other climate variables, such as large-scale circulation indices; and the forcing histories used in climate model simulations of the past millennium. We discuss the need to develop a framework through which current and new approaches to interpreting these proxy data may be rigorously assessed using pseudo-proxies derived from climate model runs, where the `answer' is known. The article concludes with a list of recommendations. First, more raw proxy data are required from the diverse disciplines and from more locations, as well as replication, for all proxy sources, of the basic raw measurements to improve absolute dating, and to better distinguish the proxy climate signal from noise. Second, more effort is required to improve the understanding of what individual proxies respond to, supported by more site measurements and process studies. These activities should also be mindful of the correlation structure of instrumental data, indicating which adjacent proxy records ought to be in agreement and which not. Third, large-scale climate reconstructions should be attempted using a wide variety of techniques, emphasizing those for which quantified errors can be estimated at specified timescales. Fourth, a greater use of climate model simulations is needed to guide the choice of reconstruction techniques (the pseudo-proxy concept) and possibly help determine where, given limited resources, future sampling should be concentrated.
Figure caption: The latitude–time evolution of zonal-mean forcing (W/², relative to the mean of the first 50 years, 1750–1799) diagnosed from the ALL250 simulation with HadCM3 described by Tett et al. (2007). This simulation was forced by variations in solar irradiance, volcanic aerosol loading, orbital forcing, land use, sulphate aerosol emissions and concentrations of well-mixed greenhouse gases and ozone. Values are smoothed in the time dimension (by a 5-yr low-pass filter) but not in the latitude dimension
Fan, F., M.E. Mann and C.M. Ammann 2009: Understanding Changes in the Asian Summer Monsoon over the Past Millennium: Insights from a Long-Term Coupled Model Simulation Journal of Climate, 22, 1736-1748, doi:10.1175/2008JCLI2336.1.
Figure 3: High resolution figure
Abstract: The Asian summer monsoon (ASM) and its variability were investigated over the past millennium through the analysis of a long-term simulation of the NCAR Climate System Model, version 1.4 (CSM 1.4) coupled model driven with estimated natural and anthropogenic radiative forcing during the period 850–1999. Analysis of the simulation results indicates that certain previously proposed mechanisms, such as warmer large-scale temperatures favoring a stronger monsoon through their effect on Eurasian snow cover, appear inconsistent with the mechanisms active in the simulation. Forced changes in tropical Pacific sea surface temperatures play an apparent role in the long-term changes in the ASM. Analyses of the simulation results suggest that the direct radiative effect of solar forcing variations on the ASM is quite weak and that dynamical responses may be far more important. Volcanic radiative forcing leads to a clearly detectable short-term reduction in the strength of the ASM. Comparisons with long-term proxy reconstructions of the ASM are attempted but are limited by the divergent behavior among different reconstructions as well as the limitations in the model’s coupled dynamics.
Figure caption: Analysis of long-term SASM precipitation variations in NCAR CSM1.4 simulation 850–1999. Shown is (top) leading EOF of precipitation field during the period 850–1999 and (bottom) corresponding PC. Precipitation PC is shown along with the IMI series of Fig. 2 for comparison.
Robock, A., C.M. Ammann, L. Oman, D. Shindell, S. Levis and G. Stenchikov. 2009: Did the Toba volcanic eruption of ~74 kBP produce widespread glaciation? J. Geophys. Res., 114, D10107, doi:10.1029/2008JD011652.
Figure 4: High resolution figure
Abstract: It has been suggested that the Toba volcanic eruption, approximately 74 ka B.P., was responsible for the extended cooling period and ice sheet advance immediately following it, but previous climate model simulations, using 100 times the amount of aerosols produced by the 1991 Mount Pinatubo eruption, have been unable to produce such a prolonged climate response. Here we conduct six additional climate model simulations with two different climate models, the National Center for Atmospheric Research Community Climate System Model 3.0 (CCSM3.0) and National Aeronautics and Space Administration Goddard Institute for Space Studies ModelE, in two different versions, to investigate additional mechanisms that may have enhanced and extended the forcing and response from such a large supervolcanic eruption. With CCSM3.0 we include a dynamic vegetation model to explicitly calculate the feedback of vegetation death on surface fluxes in response to the large initial reduction in transmitted light, precipitation, and temperature. With ModelE we explicitly calculate the effects of an eruption on stratospheric water vapor and model stratospheric chemistry feedbacks that might delay the conversion of SO2 into sulfate aerosols and prolong the lifetime and radiative forcing of the stratospheric aerosol cloud. To span the uncertainty in the amount of stratospheric injection of SO2, with CCSM3.0 we used 100 times the Pinatubo injection, and with ModelE we used 33, 100, 300, and 900 times the Pinatubo injection without interactive chemistry, and 300 times Pinatubo with interactive chemistry. Starting from a roughly present-day seasonal cycle of insolation, CO2 concentration, and vegetation, or with 6 ka B.P. conditions for CCSM3.0, none of the runs initiates glaciation. The CCSM3.0 run produced a maximum global cooling of 10 K and ModelE runs produced 8–17 K of cooling within the first years of the simulation, depending on the injection, but in all cases, the climate recovers over a few decades. Nevertheless, the "volcanic winter" following a supervolcano eruption of the size of Toba today would have devastating consequences for humanity and global ecosystems. These simulations support the theory that the Toba eruption indeed may have contributed to a genetic bottleneck.
Figure caption: Changes in monthly average global mean specific humidity for the ModelE 300 times Pinatubo simulation without interactive chemistry, showing large reductions of water vapor in the cold troposphere, but huge increases in the stratosphere, as the tropical tropopause cold trap is warmed by the volcanic aerosols in the lower stratosphere. The x axis is time in years.
Schneider, D.P., C.M. Ammann, B.L. Otto-Bliesner, and D.S. Kaufman . 2009: Climate response to large, high-latitude and low-latitude volcanic eruptions in the Community Climate System Model. Journal of Geophysical Research, doi:10.1029/2008JD011222, in press.
Figure 5: High resolution figure
Abstract: Explosive volcanism is known to be a leading natural cause of climate change. The second half of the 13th-century was likely the most volcanically perturbed half-century of the last 2000 years, though none of the major 13th-century eruptions have been clearly attributed to specific volcanoes. This period was in general a time of transition from the relatively warm Medieval period to the colder Little Ice Age, but available proxy records are insufficient on their own to clearly assess whether this transition is associated with volcanism. This context motivates our investigation of the climate system sensitivity to high- and low-latitude volcanism using the fully coupled NCAR Community Climate System Model (CCSM3). We evaluate two sets of ensemble simulations, each containing four volcanic pulses, with the first set representing them as a sequence of tropical eruptions, and the second representing eruptions occurring in the mid-high latitudes of both the Northern and Southern hemispheres. The short-term direct radiative impacts of tropical and high-latitude eruptions include significant cooling over the continents in summer, and cooling over regions of increased sea-ice concentration in Northern Hemisphere (NH) winter. A main dynamical impact of moderate tropical eruptions is a winter warming pattern across northern Eurasia. Furthermore, both ensembles show significant reductions in global precipitation, especially in the summer monsoon regions. The most important long-term impact is the cooling of the high-latitude NH produced by multiple tropical eruptions, suggesting that positive feedbacks associated with ice and snow cover could lead to long-term climate cooling in the Arctic.
Figure caption: Change in zonal-mean, seasonally averaged near-surface temperature for the tropical eruption scenario (left column) and the high-latitude scenario (right column). (A) JJA temperatures for tropical eruptions; (B) JJA temperatures for high-latitude eruptions; (C) DJF temperatures for tropical eruptions; (D) DJF temperatures for high-latitude eruptions.
Ammann, C.M., M.G. Genton and B. Li. 2009: Technical Note: Correcting for signal attenuation from noise: sharpening the focus on past climate. Climate of the Past, article.
Figure 6: High resolution figure
Abstract: Regression-based climate reconstructions scale one or more noisy proxy records against a (generally) short instrumental data series. Based on that relationship, the indirect information is then used to estimate that particular measure of climate back in time. A well-calibrated proxy record(s), if stationary in its relationship to the target, should faithfully preserve the mean amplitude of the climatic variable. However, it is well established in the statistical literature that traditional regression parameter estimation can lead to substantial amplitude attenuation if the predictors carry significant amounts of noise. This issue is known as "Measurement Error" (Fuller, 1987; Carroll et al., 2006). Climate proxies derived from tree-rings, ice cores, lake sediments, etc., are inherently noisy and thus all regression-based reconstructions could suffer from this problem. Some recent applications attempt to ward off amplitude attenuation, but implementations are often complex (Lee et al., 2008) or require additional information, e.g. from climate models (Hegerl et al., 2006, 2007). Here we explain the cause of the problem and propose an easy, generally applicable, data-driven strategy to effectively correct for attenuation (Fuller, 1987; Carroll et al., 2006), even at annual resolution. The impact is illustrated in the context of a Northern Hemisphere mean temperature reconstruction. An inescapable trade-off for achieving an unbiased reconstruction is an increase in variance, but for many climate applications the change in mean is a core interest.
Figure caption: CPS (blue) and multiple (red) regression reconstructions of NH mean temperature (10-year Gaussian smoothed results for visualization – high-resolution reconstructions are available in Supplementary Information, see http://www.clim-past-discuss.net/5/1645/2009/cpd-5-1645-2009-supplement.pdf) based on a network of twelve grid-points – comparable to Hegerl et al. (2007) – here subsampled from output of a coupled GCM (Ammann et al., 2007) where the true climate is known: (a) attenuated results from uncorrected OLS regression, and (b) the reconstructions from ACOLS.
Hughes, M.K. and C.M. Ammann. 2009: The future of the past—an earth system framework for high resolution paleoclimatology: editorial essay. Climatic Change, 94, 247-259doi:10.1007/s10584-009-9588-0.
Abstract: High-resolution paleoclimatology is the study of climate variability and change on interannual to multi-century time scales. Its primary focus is the past few millennia, a period lacking major shifts in external climate forcing and earth system configuration. Large arrays of proxy climate records derived from natural archives have been used to reconstruct aspects of climate in recent centuries. The main approaches used have been empirical and statistical, albeit informed by prior knowledge both of the physics of the climate, and of the processes imprinting climate information in the natural archives. We propose a new direction, in which emerging tools are used to formalize the combination of process knowledge and proxy climate records to better illuminate past climate variability on these time scales of great relevance to human concerns.
Kaufman, D.S., D.P. Schneider, N.P. McKay, C.M. Ammann, R.S. Bradley, K.R. Briffa, G.H. Miller, B.L. Otto-Bliesner, J.T. Overpeck, B.M. Vinther, and Arctic Lakes 2k Project Members. 2009: Recent warming reverses long-term Arctic cooling. Science, 325, 1236-1239, doi:10.1126/science.1173983.
Figure 7: High resolution figure
Abstract: The temperature history of the first millennium C.E. is sparsely documented, especially in the Arctic. We present a synthesis of decadally resolved proxy temperature records from poleward of 60°N covering the past 2000 years, which indicates that a pervasive cooling in progress 2000 years ago continued through the Middle Ages and into the Little Ice Age. A 2000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000.
Figure caption: Locations of the proxy climate records included in the synthesis. Map colors indicate trends in summer (JJA) temperature between 1958 and 2000 from the ERA-40 data series (34). Large and small symbols indicate records that extend back to 2000 years ago (2 ka) and to at least 1000 years ago, respectively. Site numbers are keyed to table S1.