P. Ditlevsen - Academia.edu (original) (raw)

Papers by P. Ditlevsen

Current Climate Change Reports, 2016

Over the last decade, our understanding of climate sensitivity has improved considerably. The cli... more Over the last decade, our understanding of climate sensitivity has improved considerably. The climate system shows variability on many timescales, is subject to non-stationary forcing and it is most likely out of equilibrium with the changes in the radiative forcing. Slow and fast feedbacks complicate the interpretation of geological records as feedback strengths vary over time. In the geological past, the forcing timescales were different than at present, suggesting that the response may have behaved This article is part of the Topical Collection on Constraints on Climate Sensitivity This work arose from a workshop on Mathematical approaches to palaeo-climate sensitivity in November 2015. A.v.d.H., H.D., and R.v.d.W. initiated the workshop and led the drafting of this study together with R.

Climate of the Past, 2010

During the last glacial period, climate records from the North Atlantic region exhibit a pronounc... more 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.

Nature, 2013

A new Greenland ice core, the NEEM core, reaches back to the last interglacial, the Eemian, 130-1... more A new Greenland ice core, the NEEM core, reaches back to the last interglacial, the Eemian, 130-115 ka (thousand years before year 1950), a period warmer than our current interglacial, and documents the Greenland climate and ice sheet response to Arctic warming. Here, the stratigraphy of the deep NEEM ice, spanning the major part of the Eemian, is reconstructed from folded ice using globally-homogenous parameters known from dated Greenland and Antarctic ice core records. The warmest Greenland surface temperatures found at NEEM are 8±4 o C warmer than the mean of the last millennium) are estimated from water stable isotopes at the onset of the Eemian (126 ka), followed by a gradual cooling trend very likely driven by summer insolation. During the Eemian the thickness of the NW Greenland ice sheet decreased by 400±250 m reaching surface elevations of 130±300 m lower than the present at 122 ka. The findings show a modest response of the Greenland ice sheet to the significant warming in the early Eemian. In addition, the ice core data at NEEM reveal significant surface melt during the Eemian. During the exceptional heat over Greenland in July 2012 melt layers formed at NEEM. With additional warming surface melt might become more common in the future.

Since their first description, Dansgaard-Oeschger (DO) events have fascinated palaeoclimatologist... more Since their first description, Dansgaard-Oeschger (DO) events have fascinated palaeoclimatologists in a very special way. Almost regularly occurring, rapid warming and cooling transitions appear to be of importance not only in the past, but also for the future long-term climate evolution. Hence, strong interest has arised in a physical understanding of the processes that lead to the occurrence of such events. In parallel, statistics has provided first insights into the question whether or not DO events may be originated in certain classes of simple stochastic processes. The state-of-the-art in corresponding research is that first-order auto-regressive processes can be ruled out C754

Stochastics and Dynamics, 2011

Here we present a simple stochastic threshold model consisting of a deterministic slowly decaying... more Here we present a simple stochastic threshold model consisting of a deterministic slowly decaying term and a fast stochastic noise term. The process shows a pseudo-resonance, in the sense that for small and large intensities of the noise the signal is irregular and the distribution of threshold crossings is broad, while for a tuned intermediate value of noise intensity, the signal becomes quasi-periodic and the distribution of threshold crossings is narrow. The mechanism captured by the model might be relevant for explaining apparent quasi-periodicity of observed climatic variations where no internal or external periodicities can be identified.

Paleoceanography, 2011

Various climatic processes are thought to evolve as rapid, shift-like events, which points at the... more Various climatic processes are thought to evolve as rapid, shift-like events, which points at the presence of nonlinear dynamics. Time series analysis of nonlinear processes, however, is not trivial, for example, because of the difficulty in coming up with a realistic random process as a viable null hypothesis. In this methodology paper we construct a basic two-parameter process of shift-like excursions in an excitable system with a threshold. We demonstrate that this stochastic process, in comparison with a specific one-parameter process, can better reproduce main features of the waiting time histogram of abrupt glacial climate events, the Dansgaard-Oeschger events, as seen in two paleoclimatic proxy records, the North Greenland Ice core Project (NGRIP) ice core and the Sofular stalagmite d 18 O records. We use the two-parameter process to test some arguments that were proposed in the ongoing discussion of a possible solar role in triggering Dansgaard-Oeschger events. Using our approach, we suggest for future studies to generate time series of random events which can serve as a more plausible null hypothesis for Monte Carlo based statistical tests on the regularity of shift-like processes such as Dansgaard-Oeschger events.

Annals of Glaciology, 2002

Rapid climate changes during the last glacial period were first observed in ice-core records (Dan... more Rapid climate changes during the last glacial period were first observed in ice-core records (Dansgaard and others, 1982). These shifts between interstadials, called Dansgaard–Oeschger (D-O) events, and stadials or deep glaciation were later seen in Atlantic sediment records (Bond and others, 1993), pointing to the ocean circulation as a strong component in the dynamics of these shifts (Wright and Stocker, 1991). the interstadial states are observed to have a characteristic ``sawtooth’’ shape, indicating a gradual drift of the stable interstadial state toward the stable stadial state. In order to contrast the two climate states, we have separated the δ18O signal from the Greenland Icecore Project ice core into periods corresponding to the two states. the climate variability in the two different climatic states is different (Johnsen and others, 1997). We find that the standard deviation is significantly larger in the stadial than in the interstadial state. Both states are found to ha...

North Atlantic climate during glacial times was characterized by large-amplitude switchings, the ... more North Atlantic climate during glacial times was characterized by large-amplitude switchings, the Dansgaard-Oeschger (DO) events, with an apparent tendency to recur preferably in multiples of about 1470 years. Recent work interpreted these intervals as resulting from a subharmonic response of a highly nonlinear system to quasi-periodic solar forcing plus noise. This hypothesis was challenged as inconsistent with the observed variability in the phase relation between proxies of solar activity and Greenland climate. Here we reject the claim of inconsistency by showing that this phase variability is a robust, generic feature of the nonlinear dynamics of DO events, as described by a model. This variability is expected from the fact that the events are threshold crossing events, resulting from a cooperative process between the periodic forcing and the noise. This process produces a fluctuating phase relation with the periodic forcing, consistent with proxies of solar activity and Greenlan...

Abstract. During the last glacial period, climate records from the North Atlantic region exhibit ... more 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 ...

Climate of the Past Discussions

The significance of the apparent 1470 years cycle in the recurrence of the Dansgaard-Oeschger (DO... more The significance of the apparent 1470 years cycle in the recurrence of the Dansgaard-Oeschger (DO) events, observed in the Greenland ice cores, is debated. Here we present statistical significance tests of this periodicity. The detection of a periodicity relies strongly on the accuracy of the dating of the DO events. Here we use both the new NGRIP GICC05 time scale based on multi-parameter annual layer counting and the GISP2 time scale where the periodicity is most pronounced. For the NGRIP dating the recurrence times are indistinguishable from a random occurrence. This is also the case for the GISP2 dating, except in the case where the DO9 event is omitted from the record. Whether or not the record shows a truly periodic beating has strong implications for identifying the underlying cause. If the recurrence is periodic it suggests an external cause. If the recurrence of DO events is not periodic it points to triggering mechanisms internal to the climate system being manifested at the millennial timescale. this analysis we use both the GISP2 (Meese/Sowers) and the NGRIP (GICC05) time

Introduction and abstract. It has almost become a standard in stochasticmechanics applications of... more Introduction and abstract. It has almost become a standard in stochasticmechanics applications of stochastic di#erential equations that the driving forcesare modeled as Gaussian white noises, that is, as scalar or vector Brownian motionincrements. However, this modeling may not always lead to responses thatcomply well with observed data. In particular the tails of the observed responsedistributions may even for linear systems

Climate Dynamics, 2015

The version presented here may differ from the published version. If citing, you are advised to c... more The version presented here may differ from the published version. If citing, you are advised to consult the published version for pagination, volume/issue and date of publication Noname manuscript No.

Physical review. B, Condensed matter, Jan 15, 1989

Climate of the Past Discussions, 2007

From a detailed analysis of marine and terrestrial aerosol tracers in the NGRIP ice core we ident... more From a detailed analysis of marine and terrestrial aerosol tracers in the NGRIP ice core we identified two distinct glacial atmospheric flow patterns. The climate transition from Marine Isotope Stage 5 (MIS 5) to MIS 4, at approximately 75 kyr BP, marks a shift between two different atmospheric flow regimes. Before this transition, during MIS 5d-a, the state of atmospheric flow was alternating between the two modes of different flow patterns, while a more persistent flow pattern was prevailing through the glacial period afterwards. These changes are accompanied by strong changes in an independent Greenland ice core proxy, namely the deuterium excess from the GRIP ice core, reflecting changes in the hydrological cycle and moisture source temperatures as well. The changes in atmospheric flow pattern are correlated with changed extent of ice-rafted detritus (IRD) deposition in the North Atlantic, indicating that the state of the atmospheric flow was highly sensitive to the waxing and waning of the Laurentide ice sheet.