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SNYDER, C. W. Revised estimates of paleoclimate sensitivity over the past 800,000 years. 2019. Climatic Change. 156. 121–138.
Abstract | Link | Supplemental Methods
This study evaluates paleoclimate sensitivity over the past 800,000 years from proxy-based reconstructions of changes in global temperature, ice sheets and sea level, vegetation, dust, and greenhouse gases. This analysis uses statistical methods that are not biased by the variable (heteroscedastic) uncertainty in the reconstructions, and applies a Monte Carlo-style probabilistic framework to quantify several sources of measurement and structural uncertainty. Not addressing the heteroscedastic uncertainty would result in regression results that underestimate paleoclimate sensitivity by over 30%, and not using a probabilistic framework could underestimate the credible interval by fivefold. A comparison of changes in global temperature and changes in radiative forcing from greenhouse gases, ice sheets, dust, and vegetation over the past 800 kyr finds that the two are closely coupled across glacial cycles with a correlation of 0.81 (0.6 to 0.9, 95% credible interval). The variation of ΔT with ΔR over the past 800 kyr is non-linear, with lower correlation and lower responsiveness at colder temperatures. The paleoclimate sensitivity parameter estimates are 0.84 °C/W/m2 for interglacial periods and intermediate glacial climates and 0.53 °C/W/m2 for full glacial climates, 37% lower at the median. The estimates of S[GHG,LI,AE,VG] and the pattern of state dependence are similar across glacial cycles over the past 800 kyr. This analysis explicitly includes several sources of uncertainty and is still able to provide a strong upper bound for the paleoclimate sensitivity parameter for interglacial periods and intermediate glacial climates: over 1.5 °C/W/m2 is < 10% probability, 1.7 °C/W/m2 is < 5% probability, and over 1.9 °C/W/m2 is < 2.5% probability.
SNYDER, C. W. Evolution of global temperature over the past two million years. 2016, Nature. DOI: 10.1038/nature19798
Abstract | Link | Paper | Supplemental Methods
Data: Full Data | Temperature Reconstruction | Source data for Figures | Temperature Reconstruction Simulation Subset
Reconstructions of the Earth's past climate strongly influence our understanding of the dynamics and sensitivity of the climate system. Yet global temperature has been reconstructed for only a few isolated windows of time, and continuous reconstructions across glacial cycles remain elusive. Here I present a spatially weighted proxy reconstruction of global temperature over the past 2 million years estimated from a multi-proxy database of over 20,000 sea surface temperature point reconstructions. Global temperature gradually cooled until roughly 1.2 million years ago and cooling then stalled until the present. The cooling trend probably stalled before the beginning of the mid-Pleistocene transition (MPT), and pre-dated the increase in the maximum size of ice sheets around 900,000 years ago. Thus, global cooling may have been a pre-condition for, but probably is not the sole causal mechanism of, the shift to quasi-100,000-year glacial cycles at the MPT. Over the past 800,000 years, polar amplification has been stable over time, and global temperature and atmospheric greenhouse gas concentrations have been closely coupled across glacial cycles. A comparison of the new temperature reconstruction with radiative forcing from greenhouse gases estimates Earth system sensitivity of 9 degrees Celsius (7 to 13 degrees Celsius, 95% credible interval) change in GAST per doubling of atmospheric carbon dioxide over millennium timescales. This result suggests that stabilization at today's greenhouse gas levels may already commit the Earth to an eventual total warming of 5 degrees Celsius (3 to 7 degrees Celsius, 95% credible interval) over millennium timescales as ice sheets, vegetation and atmospheric dust continue to respond to global warming.
SNYDER, C. W. Bayesian hierarchical regression analysis of variations in sea surface temperature change over the past million years. 2016, Paleoceanography. DOI: 10.1002/2016PA002944
Abstract | Link | Paper | Supplemental Information | Full Data
Statistical challenges often preclude comparisons among different sea surface temperature (SST) reconstructions over the past million years. Inadequate consideration of uncertainty can result in misinterpretation, overconfidence, and biased conclusions. Here I apply Bayesian hierarchical regressions to analyze local SST responsiveness to climate changes for 54 SST reconstructions from across the globe over the past million years. I develop methods to account for multiple sources of uncertainty, including the quantification of uncertainty introduced from absolute dating into inter-record comparisons. The estimates of local SST responsiveness explain 64% (62% to 77%, 95% interval) of the total variation within each SST reconstruction with a single number. There is remarkable agreement between SST proxy methods, with the exception of Mg/Ca proxy methods estimating muted responses at high latitudes. The Indian Ocean exhibits a muted response in comparison to other oceans. I find a stable estimate of the proposed "universal curve" of change in local SST responsiveness to climate changes as a function of sin2(latitude) over the past 400,000 years: SST change at 45ºN/S is larger than the average tropical response by a factor of 1.9 (1.5 to 2.6, 95% interval) and explains 50% (35% to 58%, 95% interval) of the total variation between each SST reconstruction. These uncertainty and statistical methods are well-suited for application across paleoclimate and environmental data series inter-comparisons.
SNYDER, C. W., Mastrandrea, M. D., and Schneider, S. H. (2011) The Complex Dyanmics of the Climate System: Constraints on our Knowledge, Policy Implications and the Necessity of Systems Thinking. In Handbook of Philosophy of Science: Philosophy of Complex Systems, Hooker, C. (Ed.). Elsevier.
Abstract | Link
This chapter describes the contribution of complexity science to understanding of the climate system and the unique challenges its complex properties pose to climate predictions and policy analysis. First, it presents a brief exploration of the Earth's climate system through the lens of complexity science. Then, it introduces the data sources and modeling strategies that climate science uses to understand past behavior, to fingerprint causes of current climate changes, and to project future climate. The complex dynamics of the climate system constrain ability to gain knowledge about the climate system and add uncertainty to predictions of the impacts of human-induced climate change. It also investigates six case studies that illustrate the importance and development of key complexity themes in climate science: glacial-interglacial cycles, thermohaline ocean circulation, ice sheets, vegetation cover changes, extinction, and overshoot scenarios. In addition, it investigates the implications of the complexity of the Earth system for climate policy analysis. Assessments of the impacts of climate change are often disciplinary-based and not sufficiently integrative across important disciplinary subcomponents, producing misleading results that have potentially dangerous environmental consequences. The current framework of cost-benefit optimization is particularly flawed. Further, it describes how one should restructure climate policy analysis as an integrated assessment process, combining data and relationships from the physical, biological and social sciences, that includes robust assessments of potential risks within a vulnerability framework.
Mastrandrea, M. D., Tebaldi, C., SNYDER, C. W., and Schneider, S. H. (2011) Current and future impacts of extreme events in California. Climatic Change.
Abstract | Paper | CEC Report
In the next few decades, it is likely that California must face the challenge of coping with increased impacts from extreme events such as heat waves, wildfires, droughts, and floods. This study presents new projections of changes in the frequency and intensity of extreme events in the future across climate models, emissions scenarios, and downscaling methods, and for each California county. Consistent with other projections, this study finds significant increases in the frequency and magnitude of both high maximum and high minimum temperature extremes in many areas. For example, the frequency of extreme temperatures currently estimated to occur once every 100 years is projected to increase by at least ten-fold in many regions of California, even under a moderate emissions scenario. Under a higher emissions scenario, these temperatures are projected to occur close to annually in most regions. Also consistent with other projections, analyses of precipitation extremes fail to detect a significant signal of change, with inconsistent behavior when comparing simulations across different GCMs and different downscaling methods.
SNYDER, C. W. (2010) The value of paleoclimate research in our changing climate. Climatic Change, 100(3), 407-418.
Abstract | Paper
The paper by Etkin (2010) in this issue of Climatic Change reframes results from Antarctic ice cores in the context of systems theory, and comments on the utility of the past in predicting future climate change. Etkin's discussion touches on three fundamental questions that I will explore in the following springboard editorial essay.
1) What is the value of paleoclimate research for helping understand future climate changes?
2) What are key challenges in paleoclimate research?
3) Are we in a new state of the climate system completely different from the dynamics of the last million years?
Kurten, E. L., SNYDER, C., Iwata, T., and Vitousek, P. M. (2007) Morella cerifera Invasion and Nitrogen Cycling on a Lowland Hawaiian Lava Flow. Biological Invasions, DOI 10.1007/s10530-007-9101-5.
Abstract | Paper
Invasive plants that fix nitrogen can alter nutrient availability and thereby community dynamics and successional trajectories of native communities they colonize. Morella cerifera (Myricaceae) is a symbiotic nitrogen fixer originally from the southeastern U.S. that is colonizing native-dominated vegetation on a young lava flow near Hilo, Island of Hawai'i, where it increases total and biologically available soil nitrogen and increases foliar nitrogen concentrations in associated individuals of the native tree Metrosideros polymorpha. This invasion has the potential to alter the few remaining native-dominated lowland forest ecosystems in windward Hawai'i.