Papers by Kristoffer Rypdal
Lovejoy, Sarlis, and Varotsos (LSV) replicate a test in our comment by using a different time ser... more Lovejoy, Sarlis, and Varotsos (LSV) replicate a test in our comment by using a different time series representing internal variability, and conclude that this test rejects the linear response hypthesis in the ZebiakCane model. This time series is the first 195 yr of the volcanic response record, during which there was no volcanic forcing. We demonstrate that the short length of this record creates large finite size uncertainties which render their result statistically insignificant. We also comment on some passages in their reply about physical paradigms, and on faulty statistical reasoning and apparent self-contradictions in L&V’s writings. 1 Reply to “Introduction” As a motivation for conduction these tests in the ZC model LSV write: “To situate the debate, recall that whereas at short enough time scales, when external forcings are 5 small enough, then theoretically we may expect the atmospheric response to be approximately linear, however, at long enough time scales, due to tempe...
Objective: To estimate the weekly excess all-cause mortality in Norway and Sweden, and to estimat... more Objective: To estimate the weekly excess all-cause mortality in Norway and Sweden, and to estimate the years of life lost (YLL) attributed to COVID-19 in Sweden and the significance of mortality displacement. Methods: We found expected mortality by taking the declining trend and the seasonality in mortality into account. From the excess mortality in Sweden in 2019/20, we estimated the YLL attributed to COVID-19 using the life expectancy in different age groups. We adjusted this estimate for possible displacement using an auto-regressive model for the year-to-year variations in excess mortality. Results: We found that excess all-cause mortality over the epidemic year (July to July) 2019/20 was 517 (95%CI -12, 1074) in Norway and 4329 (3331, 5325) in Sweden. There were reported 255 COVID-19 related deaths in Norway, and 5741 in Sweden, that year. During the epidemic period March 11 – November 11, there were 6247 reported COVID-19 deaths and 5517 (4701, 6330) excess deaths in Sweden. T...
The concept of multiple scaling regimes in temperature time series is examined, with emphasis on ... more The concept of multiple scaling regimes in temperature time series is examined, with emphasis on the question whether or not a mono-scaling model with one single scaling regime can be rejected from observation data from the Holocene. A model for internal variability with only one regime is simpler, and allows more certain predictions on time scales of centuries when combined with existing knowledge of radiative forcing. Our analysis of spectra from stable isotope ratios from 5 Greenland and Antarctica ice cores shows that a scale break around centennial time scales is evident for the last glacial period, but not for the Holocene. Spectra from a number of late Holocene multiproxy temperature reconstructions, and one from the entire Holocene, have also been analysed, without identifying a significant scale break. Our results indicate that a single-regime scaling climate noise, with some non-scaling fluctuations on millennial time scale superposed, cannot be rejected 10 as a null model...
This comment from L&V contains no substantiated arguments which invalidate anything in our commen... more This comment from L&V contains no substantiated arguments which invalidate anything in our comment article or in our first reply. It is a lengthy collection of unsubstantiated and erroneous claims which obscures the real issue, which is: Have L&V presented valid tests which prove that the temperature response in the climate models is inconsistent with a linear response model? L&V are in denial about the most obvious facts. One of these is that adding a noise to a intermittent signal will reduce the intermittency. The claim of theirs that is most relevant to the linearity issue is that the statistical uncertainties are so large that they overshadow the intermittency-reducing effect of curved structure-function plots and internal variability. This assertion is unsubstantiated, false, and bizarre. If statistical errors were this important they would invalidate all L&V’s results from their original paper, and we would have used it against them. Moreover, in our previous reply, a code wa...
We have been asked to help the editor make a final decision about publication of this paper in ES... more We have been asked to help the editor make a final decision about publication of this paper in ESD. Our general position is that the issue is important, and although we find many weaknesses in this paper, we have no desire to prevent it from being published, provided our comments are published as part of the public discussion. We find similar weaknesses in other of Shaun Lovejoy's publications, and it may be better that these issues are publicly discussed. Thus, our recommendation is that the editor reads our comment and use it as a background for the decision. In the original manuscript the analysis leading to the subadditivity in the ZC-model was based on an incorrect neglect of a cross-term leading to Eq. ( ). The authors were asked to use the full expression and correct the result in Fig. according to this. In their reply the authors acknowledge the error, state that they have corrected it in Fig. , but claim that the correction does not alter the conclusion. However, the actual revision they have made is rather disturbing. In the text the incorrect Eq. ( ) and the incorrect argument for it is maintained, and so is the conclusion that the "theoretical additive fluctuation level is reduced by a factor ≈ 2.5." Then they add the following paragraph: "It should be noted that the latter holds assuming independence (pink curve in Fig. ) of the solar and volcanic forcing. For comparison, the purple curve in Fig. illustrates the results obtained when analyzing the series constructed by directly summing the two response series (instead of assuming statistical independence). It is clearly seen that the basic result still holds but it is a little less strong (a factor of ≈ 2). The reason for the difference is that
International Journal of Environmental Research and Public Health, 2020
As of November 2020, the number of COVID-19 cases was increasing rapidly in many countries. In Eu... more As of November 2020, the number of COVID-19 cases was increasing rapidly in many countries. In Europe, the virus spread slowed considerably in the late spring due to strict lockdown, but a second wave of the pandemic grew throughout the fall. In this study, we first reconstruct the time evolution of the effective reproduction numbers R(t) for each country by integrating the equations of the classic Susceptible-Infectious-Recovered (SIR) model. We cluster countries based on the estimated R(t) through a suitable time series dissimilarity. The clustering result suggests that simple dynamical mechanisms determine how countries respond to changes in COVID-19 case counts. Inspired by these results, we extend the simple SIR model for disease spread to include a social response to explain the number X(t) of new confirmed daily cases. In particular, we characterize the social response with a first-order model that depends on three parameters ν1,ν2,ν3. The parameter ν1 describes the effect of...
BackgroundTo suppress the COVID-19 outbreak, the Norwegian government closed all schools on March... more BackgroundTo suppress the COVID-19 outbreak, the Norwegian government closed all schools on March 13, 2020. The kindergartens reopened on April 20, and the schools on April 27 and May 11 of 2020. The effect of these measures is largely unknown since the role of children in the spread of the SARS-CoV-2 virus is still unclear. There are only a few studies of school closures as a separate intervention to other social distancing measures, and little research exists on the effect of school opening during a pandemic.ObjectiveThis study aimed to model the effect of opening kindergartens and the schools in Norway in terms of a change in the reproduction number (R). A secondary objective was to assess if we can use the estimated R after school openings to infer the rates of transmission between children in schools.MethodsWe used an individual-based model (IBM) to assess the reopening of kindergartens and schools in two Norwegian cities, Oslo, the Norwegian capital, with a population of appro...
Earth System Dynamics, 2014
Northern Hemisphere (NH) temperature records from a palaeoclimate reconstruction and a number of ... more Northern Hemisphere (NH) temperature records from a palaeoclimate reconstruction and a number of millennium-long climate model experiments are investigated for long-range memory (LRM). The models are two Earth system models and two atmosphere-ocean general circulation models. The periodogram, detrended fluctuation analysis and wavelet variance analysis are applied to examine scaling properties and to estimate a scaling exponent of the temperature records. A simple linear model for the climate response to external forcing is also applied to the reconstruction and the forced climate model runs, and then compared to unforced control runs to extract the LRM generated by internal dynamics of the climate system. The climate models show strong persistent scaling with power spectral densities of the form S(f ) ∼ f -β with 0.8 < β < 1 on timescales from years to several centuries. This is somewhat stronger persistence than found in the reconstruction (β ≈ 0.7). We find no indication that LRM found in these model runs is induced by external forcing, which suggests that LRM on sub-decadal to century time scales in NH mean temperatures is a property of the internal dynamics of the climate system. Reconstructed and instrumental sea surface temperature records for a local site, Reykjanes Ridge, are also studied, showing that strong persistence is found also for local ocean temperature.
Earth System Dynamics Discussions, 2014
Northern Hemisphere (NH) temperature records from a reconstruction and a number of millennium-lon... more Northern Hemisphere (NH) temperature records from a reconstruction and a number of millennium-long climate model experiments are investigated for long-range memory (LRM). The models are two Earth system models and two atmospheric-ocean general circulation models. The periodogram, detrended fluctuation analysis and wavelet variance analysis are applied to examine scaling properties and to estimate a scaling exponent of the temperature records. A simple linear model for the climate response to external forcing is also applied to the reconstruction and the forced climate model runs, and then compared to unforced control runs to extract the LRM generated by internal dynamics of the climate system. With one exception the climate models show strong persistent scaling with power spectral densities of the form <i>S(f)</i> ~…
Journal of Geophysical Research: Atmospheres, 2013
The paper explores the hypothesis that the temporal global temperature response can be modeled as... more The paper explores the hypothesis that the temporal global temperature response can be modeled as a long‐range memory (LRM) stochastic process characterized by a Hurst exponent 0.5
The use of long-range memory models as a description of the noise in Earth surface temperatures h... more The use of long-range memory models as a description of the noise in Earth surface temperatures has increased the recent years, and as a measure of the persistence for such time series we have the Hurst exponent. It is known that sea surface temperatures are more persistent than land temperatures, and that global temperatures are more persistent than local temperatures. We also know that the persistence is higher for lower latitudes than for higher latitudes. My results confirm these observations, and in addition they reveal what the Hurst exponents are for spatial scales between local and global. This is done by performing spatial averaging over gridded temperature data to obtain new time series in more coarse-grained grid boxes. To find an explanation for the increase in Hurst exponent that is seen when increasing the spatial scale, I have studied how the autocovariance function for a large grid box depends on the spatial cross-covariances within the box. If these are strong compared to the autocovariances in that area they will have an impact on the Hurst exponent. Scale free long-range memory models are found to give a good description for global temperature and many of the local temperatures on time scales from a few months to ten years. The largest deviations are observed in the eastern equatorial Pacific where ENSO is a very dominating process.
Physical Review Letters, 2010
We re-examine observational evidence presented in support of the hypothesis of a sun-climate comp... more We re-examine observational evidence presented in support of the hypothesis of a sun-climate complexity linking by N. Scafetta and B. West, Phys. Rev. Lett. 90, 24 (2003). The original analysis concluded that the integrated solar flare index and the global temperature anomaly both follow Lévy-walk statistics with the same waiting-time exponent µ. However, this analysis did not account for trends in the signal, cannot deal correctly with infinite variance processes (Lévy flights), and crucial information about the stochastic properties of the signals is lost by considering only the second moment. The Lévy-walk signatures and common waiting-time exponent µ ≈ 2.1 found for the two signals are essentially a result of failure to eliminate the effects of trends. Our analysis shows that properly detrended, integrated solar flare index is well described as an uncorrelated Lévy flight, while the detrended, integrated temperature anomaly record is consistent with a persistent fractional Brownian motion. These very different stochastic properties of the solar and climate records do not support the hypothesis of a sun-climate complexity linking.
Physica A: Statistical Mechanics and its Applications, 2013
Maximum likelihood estimation applied to high-frequency data allows us to quantify intermittency ... more Maximum likelihood estimation applied to high-frequency data allows us to quantify intermittency in the fluctuations of asset prices. From time records as short as one month these methods permit extraction of a meaningful intermittency parameter λ characterising the degree of volatility clustering of asset prices. We can therefore study the time evolution of volatility clustering and test the statistical significance of this variability. By analysing data from the Oslo Stock Exchange, and comparing the results with the investment grade spread, we find that the estimates of λ are lower at times of high market uncertainty.
Earth System Dynamics Discussions, 2015
We show that in order to have a scaling description of the climate system that is not inherently ... more We show that in order to have a scaling description of the climate system that is not inherently non-stationary, the rapid shifts between stadial and interstadial conditions during the last glaciation cannot be included in the scaling law. The same is true for the shifts between the glacial and interglacial states in the quaternary climate. When these events are omitted from a scaling analysis we find that the climate noise is consistent with a 1/<i>f</i> law on time scales from months to 10<sup>5</sup> years.
Earth System Dynamics Discussions, Jul 13, 2016
Lovejoy and Varotsos (2016) (L&V) analyse the temperature response to solar, volcanic, and solar ... more Lovejoy and Varotsos (2016) (L&V) analyse the temperature response to solar, volcanic, and solar plus volcanic forcing in the Zebiak-Cane (ZC) model, and to solar and solar plus volcanic forcing in the Goddard Institute for Space Studies (GISS) E2-R model. By using a simple wavelet filtering technique they conclude that the responses in the ZC model combine subadditively on timescales from 50 to 1000 years. Nonlinear response on shorter timescales is claimed by analysis of intermittencies in the forcing and the temperature signal for both models. The analysis of additivity in the ZC model suffers from a confusing presentation of results based on an invalid approximation, and from ignoring the effect of internal variability. We present tests without this approximation which are not able to detect nonlinearity in the response, even without accounting for internal variability. We also demonstrate that internal variability will appear as subadditivity if it is not accounted for. L&V's analysis of intermittencies is based on a mathematical result stating that the intermittencies of forcing and response are the same if the response is linear. We argue that there are at least three different factors that may invalidate the application of this result for these data. It is valid only for a power-law response function; it assumes power-law scaling of structure functions of forcing as well as temperature signal; and the internal variability, which is strong at least on the short timescales, will exert an influence on temperature intermittence which is independent of the forcing. We demonstrate by a synthetic example that the differences in intermittencies observed by L&V easily can be accounted for by these effects under the assumption of a linear response. Our conclusion is that the analysis performed by L&V does not present valid evidence for a detectable nonlinear response in the global temperature in these climate models.
Past Global Changes Magazine, 2017
Earth's global surface temperature shows variability on an extended range of temporal scales ... more Earth's global surface temperature shows variability on an extended range of temporal scales and satisfies an emergent scaling symmetry. Recent studies indicate that scale invariance is not only a feature of the observed temperature fluctuations, but an inherent property of the temperature response to radiative forcing, and a principle that links the fast and slow climate responses. It provides a bridge between the decadal- and centennial-scale fluctuations in the instrumental temperature record, and the millennial-scale equilibration following perturbations in the radiative balance. In particular, the emergent scale invariance makes it possible to infer equilibrium climate sensitivity (ECS) from the observed relation between radiative forcing and global temperature in the instrumental era. This is verified in ensembles of Earth system models (ESMs), where the inferred values of ECS correlate strongly to estimates from idealized model runs. For the range of forcing data explored...
Earth System Dynamics Discussions, 2016
Lovejoy and Varotsos (L&V) analyse the temperature response to solar, volcanic, and solar plus vo... more Lovejoy and Varotsos (L&V) analyse the temperature response to solar, volcanic, and solar plus volcanic, forcing in the Zebiak-Cane (ZC) model, and to solar and solar plus volcanic forcing in the GISS-E2-R model. By a simple wavelet filtering technique they conclude that the responses in the ZC model combine subadditively on time scales from 50 to 1000 yr. Nonlinear response on shorter time scales is claimed by analysis of intermittencies in the forcing and the temperature signal for both models. The analysis of additivity in the ZC model suffers from a confusing presentation of results based on an invalid approximation, and from ignoring the effect of internal variability. We present a test without this approximation which is not able to reject the linear response hypothesis, even without accounting for internal variability. We also demonstrate that internal variability will appear as subadditivity if it is not accounted for. The analysis of intermittencies is based on a mathematic...
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Papers by Kristoffer Rypdal