Annual Review of Earth and Planetary Sciences, 2007
To assess the significance of modern climate change, it is essential to place recent observed cha... more To assess the significance of modern climate change, it is essential to place recent observed changes in a longer-term context. This review assesses the evidence from both "proxy" climate data and theoretical climate model simulations with regard to the nature and causes of climate variability over a time interval spanning roughly the past two millennia. Evidence is reviewed for changes in temperature, drought, and atmospheric circulation over this timescale. Methods for reconstructing past climate from proxy data are reviewed and comparisons with the results of climate modeling studies are provided. The assessment provided affirms the role of natural (solar and volcanic) radiative forcing in past changes in large-scale mean temperature changes and in dynamical modes of climate variability such as the North Atlantic Oscillation (NAO) and El Niño/Southern Oscillation (ENSO) influencing large-scale climate. At hemispheric scales, late twentieth century warmth appears unprecedented in the context of at least the past 2000 years. This anomalous warmth can only be explained by modern anthropogenic forcing.
The fidelity of climate reconstructions employing covariance-based calibration techniques is test... more The fidelity of climate reconstructions employing covariance-based calibration techniques is tested with varying levels of sparseness of available data during intervals of relatively constant (stationary) and increasing (nonstationary) forcing. These tests employ a regularized expectation-maximization algorithm using surface temperature data from both the instrumental record and coupled ocean-atmosphere model integrations. The results indicate that if radiative forcing is relatively constant over a data-rich calibration period and increases over a data-sparse reconstruction period, the imputed temperatures in the reconstruction period may be biased and may underestimate the true temperature trend. However, if radiative forcing is stationary over a data-sparse reconstruction period and increases over a data-rich calibration period, the imputed values in the reconstruction period are nearly unbiased. These results indicate that using the data-rich part of the twentieth-century instrumental record (which contains an increasing temperature trend plausibly associated with increasing radiative forcing) for calibration does not significantly bias reconstructions of prior climate.
The POM-SAT model for comparing air and ground temperatures is based on the supposition that surf... more The POM-SAT model for comparing air and ground temperatures is based on the supposition that surface air temperature (SAT) records provide a good prediction of thermal transients in the shallow subsurface of the Earth. This model consists of two components, the forcing function and an initial condition, termed the pre-observational mean (POM). I explore the sensitivity of this model as a function of forcing periods at time scales appropriate for climate reconstructions. Synthetic models are designed to replicate comparisons between borehole temperatures contained in the global database of temperature profiles for climate reconstructions and gridded SAT data. I find that the root mean square (RMS) misfit between forcing functions and transient temperature profiles in the subsurface are sensitive to periods longer than about 50 years, are a maximum when the period and the 150-year time series are equal and then decreases for longer periods. The magnitude of the POM is a robust parameter for periods equal to or shorter than the length of this time series. At longer periods there is a tradeoff between the amplitude of the forcing function and the POM. These tests provide guidelines for assessing comparisons between air and ground temperatures at periods appropriate for climate reconstructions. The sensitivity of comparisons between the average Northern Hemisphere gridded SAT record and subsurface temperature-depth profile as a function of forcing period is assessed. This analysis indicates that the Northern Hemisphere extratropical average SAT and reduced temperature-depth profile are in good agreement. By adding modest heat to the subsurface at intermediate periods some improvement in misfit can be made, but this extra heat has negligible influence on the POM. The joint analysis of borehole temperatures and SAT records indicate warming of about 1.1 • C over the last 500 years, consistent with previous studies.
Zeitschrift Fur Angewandte Mathematik Und Physik, 1996
The plane elasticity problem of an infinite plate containing an elliptic inclusion is considered.... more The plane elasticity problem of an infinite plate containing an elliptic inclusion is considered. The Green's functions for a point force and/or a dislocation located outside the inclusion are derived. By using the complex potential approach of Muskhelishvili, the general solutions are obtained in a form of carefully selected functions plus an infinite series. The numerical convergence of the solutions is better than that of Stagni-Lizzio's solutions. The proposed solutions can also be applied to the case of a point force and dislocation acting at a point right on the interface.
Annual Review of Earth and Planetary Sciences, 2007
To assess the significance of modern climate change, it is essential to place recent observed cha... more To assess the significance of modern climate change, it is essential to place recent observed changes in a longer-term context. This review assesses the evidence from both "proxy" climate data and theoretical climate model simulations with regard to the nature and causes of climate variability over a time interval spanning roughly the past two millennia. Evidence is reviewed for changes in temperature, drought, and atmospheric circulation over this timescale. Methods for reconstructing past climate from proxy data are reviewed and comparisons with the results of climate modeling studies are provided. The assessment provided affirms the role of natural (solar and volcanic) radiative forcing in past changes in large-scale mean temperature changes and in dynamical modes of climate variability such as the North Atlantic Oscillation (NAO) and El Niño/Southern Oscillation (ENSO) influencing large-scale climate. At hemispheric scales, late twentieth century warmth appears unprecedented in the context of at least the past 2000 years. This anomalous warmth can only be explained by modern anthropogenic forcing.
The fidelity of climate reconstructions employing covariance-based calibration techniques is test... more The fidelity of climate reconstructions employing covariance-based calibration techniques is tested with varying levels of sparseness of available data during intervals of relatively constant (stationary) and increasing (nonstationary) forcing. These tests employ a regularized expectation-maximization algorithm using surface temperature data from both the instrumental record and coupled ocean-atmosphere model integrations. The results indicate that if radiative forcing is relatively constant over a data-rich calibration period and increases over a data-sparse reconstruction period, the imputed temperatures in the reconstruction period may be biased and may underestimate the true temperature trend. However, if radiative forcing is stationary over a data-sparse reconstruction period and increases over a data-rich calibration period, the imputed values in the reconstruction period are nearly unbiased. These results indicate that using the data-rich part of the twentieth-century instrumental record (which contains an increasing temperature trend plausibly associated with increasing radiative forcing) for calibration does not significantly bias reconstructions of prior climate.
The POM-SAT model for comparing air and ground temperatures is based on the supposition that surf... more The POM-SAT model for comparing air and ground temperatures is based on the supposition that surface air temperature (SAT) records provide a good prediction of thermal transients in the shallow subsurface of the Earth. This model consists of two components, the forcing function and an initial condition, termed the pre-observational mean (POM). I explore the sensitivity of this model as a function of forcing periods at time scales appropriate for climate reconstructions. Synthetic models are designed to replicate comparisons between borehole temperatures contained in the global database of temperature profiles for climate reconstructions and gridded SAT data. I find that the root mean square (RMS) misfit between forcing functions and transient temperature profiles in the subsurface are sensitive to periods longer than about 50 years, are a maximum when the period and the 150-year time series are equal and then decreases for longer periods. The magnitude of the POM is a robust parameter for periods equal to or shorter than the length of this time series. At longer periods there is a tradeoff between the amplitude of the forcing function and the POM. These tests provide guidelines for assessing comparisons between air and ground temperatures at periods appropriate for climate reconstructions. The sensitivity of comparisons between the average Northern Hemisphere gridded SAT record and subsurface temperature-depth profile as a function of forcing period is assessed. This analysis indicates that the Northern Hemisphere extratropical average SAT and reduced temperature-depth profile are in good agreement. By adding modest heat to the subsurface at intermediate periods some improvement in misfit can be made, but this extra heat has negligible influence on the POM. The joint analysis of borehole temperatures and SAT records indicate warming of about 1.1 • C over the last 500 years, consistent with previous studies.
Zeitschrift Fur Angewandte Mathematik Und Physik, 1996
The plane elasticity problem of an infinite plate containing an elliptic inclusion is considered.... more The plane elasticity problem of an infinite plate containing an elliptic inclusion is considered. The Green's functions for a point force and/or a dislocation located outside the inclusion are derived. By using the complex potential approach of Muskhelishvili, the general solutions are obtained in a form of carefully selected functions plus an infinite series. The numerical convergence of the solutions is better than that of Stagni-Lizzio's solutions. The proposed solutions can also be applied to the case of a point force and dislocation acting at a point right on the interface.
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