Back from EGU 2016

We just returned from a very successful EGU2016 conference after presenting talks about radiocarbon-based age-model uncertainty, the role of paleoclimate reconstruction biases on model-data comparisons, the spatial scaling of temperature variability, reconstructing the temperature variability on the Antarctic plateau, and posters about glacial climate variability and the signal contained in Holocene proxy data.

Assessing performance and seasonal bias of pollen-based climate reconstructions in a perfect model world

Reconstructions of summer, winter or annual mean temperatures based on the species composition of bio-indicators such as pollen, foraminifera or chironomids are routinely used in climate model-proxy data comparison studies. Most reconstruction algorithms exploit the joint distribution of modern spatial climate and species distribution for the development of the reconstructions. They rely on the assumption that environmental variables other than those reconstructed should not be important, or that their relationship with the reconstructed variable(s) should be the same in the past as in the modern spatial calibration dataset. Here we test the implications of this assumption on Holocene pollen based reconstructions in an ideal earth system model world, in which climate and vegetation are known at all times. We show that correlations between climate variables in the modern climate/vegetation relationship are systematically extended into the reconstructions. Summer temperatures, the most prominent driving variable for modeled vegetation change in the Northern Hemisphere, are accurately reconstructed. However, the amplitude of the winter and mean annual temperature cooling between the mid-Holocene and present day is overestimated, and similar to the summer trend in magnitude. Our results indicate that reconstructions of multiple climate variables from the same bio-indicator dataset should be treated with caution. Expert knowledge on the eco-physiological drivers of the proxies, and statistical methods that go beyond the cross-validation on modern calibration datasets are crucial to avoid misinterpretation.

A two-dimensional view of water isotopes in Antarctic firn

In low-accumulation regions, the reliability of δ18O-derived temperature signals from ice cores within the Holocene is unclear, primarily due to the small climate changes relative to the intrinsic noise of the isotopic signal. In order to learn about the representativity of single ice cores and to optimize future ice-core-based climate reconstructions, we studied the stable-water isotope composition of firn at Kohnen station, Dronning Maud Land, Antarctica. Analysing δ18O in two 50m long snow trenches allowed us to create an unprecedented, two-dimensional image characterizing the isotopic variations from the centimeter to the hundred-meter scale. Our results show seasonal layering of the isotopic composition but also high horizontal isotopic variability caused by local stratigraphic noise. Based on the horizontal and vertical structure of the isotopic variations, we derive a statistical noise model which successfully explains the trench data. The model further allows to determine an upper bound for the reliability of climate reconstructions conducted in our study region on seasonal to inter-annual time scales, depending on the number and the spacing of the cores taken.

Warmer and wetter or warmer and dryer? Observed versus simulated covariability of Holocene temperature and rainfall in Asia

Temperatures in Asia, and globally, are very likely to increase with greenhouse gas emissions, but future projections of rainfall are far more uncertain. Here we have investigated the linkage between temperature and precipitation in Asia on interannual to multicentennial timescales using instrumental data, late Holocene paleoclimate proxy data and climate model simulations.
We find that the relationship between temperature and precipitation is timescale-dependent. While on annual to decadal timescales, negative correlations dominate and thus cool summers tend to be rainy summers, on longer timescales precipitation and temperature are positively correlated; cool centuries tend to be dryer centuries in monsoonal Asia. In contrast, the analyzed CMIP5/PMIP3 climate model simulations show a negative correlation between precipitation and temperature on all timescales. Although many uncertainties exist in the interpretation of the proxy data, there is consistency between them and the instrumental evidence. This, and the persistence of the result across independent proxy datasets, allowed us to suggest that the climate model simulations might be considerably overestimating the short-term negative associations between regional rainfall and temperature. Furthermore, they may lack forcing or variability that could create long-term positive relationships between them.

Rehfeld, K., and T. Laepple (2016), Warmer and wetter or warmer and dryer? Observed versus simulated covariability of Holocene temperature and rainfall in Asia, Earth and Planetary Science Letters, 436, 1–9, doi:10.1016/j.epsl.2015.12.020.

Glacial cooling as inferred from marine temperature proxies TEXH86 and UK’37

How much colder was the Last Glacial Maximum relative to present? This question has been pondered upon by paleoclimatologists since the 70s, mostly by means of proxies that are preserved in sediments. Because proxies are not direct measurement of the climatic parameters of interest, their robustness needs to be constrained, typically by using other proxies. In this study, we take a multi-site, multi-proxy approach in tackling the age-old scientific question of LGM cooling. Unlike previous studies, we limit our analysis to proxies measured in tandem on the same sediment cores to allow for a direct comparison. The mean LGM cooling inferred from an increasingly applied archaeal-lipid based proxy, TEX86H, is approximately twice as strong as that inferred from a better constrained alkenone-based proxy UK’37. Although differences in recording season and water depth are usually invoked to explain proxy divergence in single site reconstructions, we find it an unlikely explanation for the systematic proxy discrepancy observed here, judging from the seasonal and water column structure of LGM cooling in the PMIP3/CMIP5 simulations. Instead, it is likely due to a fundamental bias in the TEX86H calibration due to the unconstrained archaeal habitat depth and the export mechanism of the lipids.

Ho, Sze Ling, und Thomas Laepple. „Glacial cooling as inferred from marine temperature proxies TEXH86 and UK′37“. Earth and Planetary Science Letters 409 (1. Januar 2015): 15–22. doi:10.1016/j.epsl.2014.10.033.

Long-term winter warming trend in the Siberian Arctic during the mid- to late Holocene

Relative to the past 2,000 years, the Arctic region has warmed significantly over the past few decades. However, the evolution of Arctic temperatures during the rest of the Holocene is less clear. Proxy reconstructions, suggest a long-term cooling trend throughout the mid- to late Holocene  whereas climate model simulations show only minor changes or even warming . Here we present a record of the oxygen isotope composition of permafrost ice wedges from the Lena River Delta in the Siberian Arctic. The isotope values, which reflect winter season temperatures, became progressively more enriched over the past 7,000 years, reaching unprecedented levels in the past five decades. This warming trend during the mid- to late Holocene is in opposition to the cooling seen in other proxy records. However, most of these existing proxy records are biased towards summer temperatures. We argue that the opposing trends are related to the seasonally different orbital forcing over this interval. Furthermore, our reconstructed trend as well as the recent maximum are consistent with the greenhouse gas forcing and climate model simulations, thus reconciling differing estimates of Arctic and northern high-latitude temperature evolution during the Holocene.

Meyer, Hanno, Thomas Opel, Thomas Laepple, Alexander Yu Dereviagin, Kirstin Hoffmann, und Martin Werner. „Long-Term Winter Warming Trend in the Siberian Arctic during the Mid- to Late Holocene“. Nature Geoscience 8, Nr. 2 (Februar 2015): 122–25. doi:10.1038/ngeo2349.

Back from Antarctica

ECUS Phd student Thomas Münch and AWI glaciologist Sepp Kipfstuhl successfully finished their field campaign at Kohnen station on the Antarctic Plateau. By sampling 3m deep and 50m long trenches of Antarctic firn, they could obtain a likely unprecedented dataset of the lateral variability of density and isotopes at a low accumulation site. The new data will allow detecting potential post- depositional processes influencing the water isotopes and will provide the empirical basis for statistical modeling of the stratigraphic noise influencing proxy records.

Ocean surface temperature variability: Large model–data differences at decadal and longer periods

Determining magnitudes of sea surface temperature variability is important for attributing past and predicting future changes in climate, and generally requires the use of proxies to constrain multidecadal and longer timescales of variability. We report a multiproxy estimate of sea surface temperature variability that is consistent between proxy types and with instrumental estimates but strongly diverges from climate model simulations toward longer timescales. At millennial timescales, model−data discrepancies reach two orders of magnitude in the tropics, indicating substantial problems with models or proxies, or both, and highlighting a need to better determine the variability of sea surface temperatures.

Laepple Thomas, und Peter Huybers. „Ocean Surface Temperature Variability: Large Model–data Differences at Decadal and Longer Periods“. Proceedings of the National Academy of Sciences 111, Nr. 47 (25. November 2014): 16682–87. doi:10.1073/pnas.1412077111.

Global and regional variability in marine surface temperatures

Accurate representation of the spread in predictions of future climate is, arguably, as important as correctly predicting a central value. Comparison against observed variability is one means of evaluating the skill of general circulation models in simulating the spread of plausible temperatures. In this study we report on a systematic comparison of sea surface temperature variability between instruments and model simulations. Novel is that we account for data inhomogeneities and uncertainties in comparing with simulations. In comparison against the CMIP5 ensemble of simulations, global-average SST variability is consistent between models and observations, but regional SST observations show systematically greater variability at decadal and longer timescales. These results suggest that models underestimate intrinsic variability and may help explain why few simulations reproduce the observed trend in global warming over the past 15 years. Given the short instrumental record, there is some complication inherent to inferring variability during an interval containing substantial trends in global temperature. Thus, the logical next step will be to use of paleo-data (e.g. Laepple and Huybers 2013, see below) to extend the model-data comparison and to include intervals prior to this last century.

T.Laepple and P. Huybers, Global and regional variability in marine surface temperatures, Geophysical   Research Letters, in press, Early View, doi: 10.1002/2014GL059345, 2014

Appraisal of TEX86 and TEX86L thermometries in subpolar and polar regions

Due to a lack of correlation between TEX86 values and atlas SST in a limited number of surface sediment samples in the Arctic, the application of TEX86 paleothermometer in polar and subpolar regions has always been thought to be uncertain (e.g. Kim et al. 2010). By putting together existing data sets and newly analyzed 160 sediment core-top data, we show that there is in fact a good correlation between the proxy values and the overlaying water temperatures in the Southern Ocean and the North Pacific, demonstrating the applicability of the TEX86 paleothermometer in these regions. Whilst the TEX86-derived estimates fit well with annual mean SST, the estimates derived from TEX86L, i.e. the recommended index for application in polar and subpolar regions, are closer to summer SSTs. Since both indices are based on the same source organisms, the index difference cannot be attributed to seasonality, suggesting the need of further work in scrutinizing the definition of the GDGT-based indices.

Ho, Sze Ling, Gesine Mollenhauer, Susanne Fietz, Alfredo Martínez-Garcia, Frank Lamy, Gemma Rueda, Konstanze Schipper, u. a. „Appraisal of TEX86 and thermometries in subpolar and polar regions“. Geochimica et Cosmochimica Acta 131 (15. April 2014): 213–26. doi:10.1016/j.gca.2014.01.001.

Similarity estimators for irregular and age-uncertain time series

In this paper we investigate similarity estimators that could be suitable for the quantitative investigation of linear and nonlinear dependencies in proxy time series. We test the efficiency of Pearson correlation, mutual information and event synchronization in estimating coupling strength and coupling lag numerically, using ensembles of synthetic stalagmites with short, autocorrelated, linear and nonlinearly coupled proxy time series, and in the application to real stalagmite time series. While time series irregularity effects can be alleviated using the presented adapted estimators, age uncertainty increasing beyond 5% of the time series length makes it impossible to identify coupling lag or coupling strength. Mutual information and event synchronization show strengths in the analysis of nonlinear phenomena. A toolbox with functions to analyze paleoclimatic time series for dependencies is included.

Rehfeld, K., und J. Kurths. „Similarity estimators for irregular and age-uncertain time series“. Clim. Past 10, Nr. 1 (16. Januar 2014): 107–22. doi:10.5194/cp-10-107-2014.

Impurity controlled densification: new model for stratified polar firn

Based on our findings of a significant impact of impurities of the firn densification in Antarctica and Greenland (see below), we developed a firn densification model, which includes the impurity effect. The excellent agreement of simulated and measured high-resolution density profiles confirms the new approach. The new models allow us for the first time to simulate the density layering in firn down to the firn-ice transition and thus will allow for a more quantitative understanding of the air entrapment in ice-cores.

Freitag, J., Kipfstuhl S., Laepple T and Wilhelms F. Impurity controlled densification: new model for stratified polar firn, Journal of Glaciology, 59 (218), pp. 1163-1169 . doi: 10.3189/2013JoG13J042

Core-scale radioscopic imaging: a new method reveals density– calcium link in Antarctic firn

Understanding the densification of polar firn is crucial for the interpretation of gases trapped in the ice and air-based temperature proxies and dating techniques.  Recently (Hoerhold et al., 2012), we could demonstrate a significant impact of impurities on the densification. However, the effect in Antarctica was less clear, mainly because of the lower signal/noise ratio in the data.

With the development of a new technique to measure firn density and its application to a low-accumulation Antarctic ice-core, this work shows that the link between calcium and density is equally strong in Antarctica as in Greenland and thus seems to be a universal feature of the densification of polar firn.

Freitag, J., Kipfstuhl S. and Laepple T. Core-scale radioscopic imaging: a new method reveals density– calcium link in Antarctic firn,  J. Glaciol., 59 (218), pp. 1009-1014, 2013

A model–data comparison of the Holocene global sea surface temperature evolution

Significant discrepancies exist between Holocene Mg/Ca and Uk37 proxies of sea surface temperature variability.  Using a statistical model of the proxy sampling processes and noise sources, these discrepancies can be understood and corrected. We thus provide a consistent estimate of regional sea surface temperature variability on centennial to millenial time-scales.

Laepple, Thomas, and Peter Huybers. 2013. “Reconciling Discrepancies Between Uk37 and Mg/Ca Reconstructions of Holocene Marine Temperature Variability.” Earth and Planetary Science Letters. doi:10.1016/j.epsl.2013.06.006.

Reconciling discrepancies between Uk37 and Mg/Ca reconstructions of Holocene marine temperature variability

A systematic comparison of marine Holocene temperature trends from two independent temperature proxies (Uk37 and Mg/Ca) and a range of climate models shows, that the simulated trends are significantly lower than the reconstructed ones.  Accounting for seasonality or depth changes in the proxy does not resolve this discrepancy. This provides a challenge for the interpretation of proxy data as well as for the model sensitivity to orbital forcing.

Lohmann, G., M. Pfeiffer, T. Laepple, G. Leduc, and J.-H. Kim. 2013. “A Model–data Comparison of the Holocene Global Sea Surface Temperature Evolution.” Clim. Past 9 (4) (August 6): 1807–1839. doi:10.5194/cp-9-1807-2013.