Showing papers by "Peter D. Ditlevsen published in 2010"

Tipping points: Early warning and wishful thinking

Abstract: [1] The causes for and possible predictions of rapid climate changes are poorly understood. The most pronounced changes observed, beside the glacial terminations, are the Dansgaard-Oeschger events. Present day general circulation climate models simulating glacial conditions are not capable of reproducing these rapid shifts. It is thus not known if they are due to bifurcations in the structural stability of the climate or if they are induced by stochastic fluctuations. By analyzing a high resolution ice core record we exclude the bifurcation scenario, which strongly suggests that they are noise induced and thus have very limited predictability.

Turbulence and Shell Models

Abstract: Preface 1. Introduction to turbulence 2. 2D turbulence and the atmosphere 3. Shell models 4. Scaling and symmetries 5. Chaotic dynamics 6. Helicity 7. Intermittency 8. Equilibrium statistical mechanics Appendix References Index.

Limitations of red noise in analysing Dansgaard-Oeschger events

Abstract: . During the last glacial period, climate records from the North Atlantic region exhibit a pronounced spectral component corresponding to a period of about 1470 years, which has attracted much attention. This spectral peak is closely related to the recurrence pattern of Dansgaard-Oeschger (DO) events. In previous studies a red noise random process, more precisely a first-order autoregressive (AR1) process, was used to evaluate the statistical significance of this peak, with a reported significance of more than 99%. Here we use a simple mechanistic two-state model of DO events, which itself was derived from a much more sophisticated ocean-atmosphere model of intermediate complexity, to numerically evaluate the spectral properties of random (i.e., solely noise-driven) events. This way we find that the power spectral density of random DO events differs fundamentally from a simple red noise random process. These results question the applicability of linear spectral analysis for estimating the statistical significance of highly non-linear processes such as DO events. More precisely, to enhance our scientific understanding about the trigger of DO events, we must not consider simple "straw men" as, for example, the AR1 random process, but rather test against realistic alternative descriptions.

Extension of stochastic resonance in the dynamics of ice ages

Abstract: Understanding the dynamics of ice ages has been a major challenge in climate research for more than a century. The cycles are thus attributed to the climatic response of the orbital changes in the incoming solar radiation to the Earth. However, these changes in the forcing are too small to explain the observed climate variations as simple linear responses, thus non-linear amplifications of the orbital forcing are necessary to account for the glacial cycles. Stochastic resonance was proposed by Benzi et al. [1] to describe this scenario. However, there are several shortcomings in the description of the glacial cycles as a simple stochastic resonance. In order to account for the non-periodic nature of the ice age cycles, especially the shift 1 million years ago from 41-kyr ice age cycles to the present approximately 100-kyr ice age cycles, a non-trivial extension of the notion of a stochastic resonance is needed.

Bifurcation structure and noise-assisted transitions in the Pleistocene glacial cycles

Abstract: [1] The glacial cycles are attributed to the climatic response of the orbital changes in the irradiance to the Earth. These changes in the forcing are too small to explain the observed climate variations as simple linear responses. Nonlinear amplifications of the orbital forcing are necessary to account for the glacial cycles. Here an empirical model of the nonlinear response is presented. From the model it is possible to assess the role of stochastic noise in comparison to the deterministic orbital forcing of the ice ages. The model is based on the bifurcation structure derived from the climate history. It indicates the dynamical origin of the mid-Pleistocene transition from the ‘‘41 ka world’’ to the ‘‘100 ka world.’’ The dominant forcing in the latter is still the 41 ka obliquity cycle, but the bifurcation structure of the climate system is changed. The model suggests that transitions between glacial and interglacial climate are assisted by internal stochastic noise in the period prior to the last five glacial cycles, while the last five cycles are deterministic responses to the orbital forcing.