Earth System Diagnostics
On the Similarity and Apparent Cycles of Isotopic Variations in East Antarctic Snow-Pits
We explain why snow pits across different sites in East Antarctica show visually similar variations in water isotopes. We argue that the similarity and the apparent cycles of around 20 cm in the isotopic variations are the result of a seasonal cycle in isotopes, noise, for example from precipitation intermittency, and diffusion. The near constancy of the diffusion length across many ice-coring sites explains why the structure and cycle length is largely independent of the accumulation conditions.
Laepple, T., T. Münch, M. Casado, M. Hoerhold, A. Landais, and S. Kipfstuhl. 2017. “On the Similarity and Apparent Cycles of Isotopic Variations in East Antarctic Snow-Pits.” The Cryosphere Discuss. 2017 (September): 1–29. doi:10.5194/tc-2017-199.
Constraints on post-depositional isotope modifications in East Antarctic firn from analysing temporal changes of isotope profiles
The importance of post-depositional changes for the temperature interpretation of water isotopes is poorly constrained by observations. Here, for the first time, temporal isotope changes in the open-porous firn are directly analysed using a large array of shallow isotope profiles. By this, we can reject the possibility of post-depositional change beyond diffusion and densification as the cause of the discrepancy between isotope and local temperature variations at Kohnen Station, East Antarctica.
Münch, T., S. Kipfstuhl, J. Freitag, H. Meyer, and T. Laepple. 2017. “Constraints on Post-Depositional Isotope Modifications in East Antarctic Firn from Analysing Temporal Changes of Isotope Profiles.” The Cryosphere 11 (5): 2175–88. doi:10.5194/tc-11-2175-2017.
Flat Meridional Temperature Gradient in the Early Eocene in the Subsurface Rather than Surface Ocean
Knowledge of warmer-than-present climates in the geological past, such as the early Eocene, is of importance given their potential as an analogue for our future climate. In the marine realm, geological evidence suggests extremely warm sea surface temperatures at both poles. These subtropical temperatures in high latitudes and the associated flat latitudinal temperature gradient were not reproducible in climate model simulations, casting doubts on the skills of current models in simulating warm climates.
Here, we suggest that this problem is an artefact of the proxy interpretation. Using a novel approach of multiproxy variability comparison, we show that the current TEX86H interpretation overestimates past climate changes on all timescales. Our approach allows us to better constrain the TEX86H to temperature relationship and the origin of the temperature signal. We suggest that the early Eocene extreme polar warmth was overestimated and the flat temperature gradient did not reflect the sea surface, as proposed before, but the subsurface ocean.
Layering of surface snow and firn at Kohnen Station, Antarctica - noise or seasonal signal?
The density of firn is an important property for monitoring and modeling the ice sheet as well as to model the pore close-off and thus to interpret ice core-based greenhouse gas records. One feature, which is still in debate, is the potential existence of an annual cycle of firn density in low-accumulation regions. Several studies describe or assume seasonally successive density layers, horizontally evenly distributed, as seen in radar data. On the other hand, high-resolution density measurements on firn cores in Antarctica and Greenland showed no clear seasonal cycle in the top few meters. A major caveat of most existing snowpit and firn-core based studies is that they represent one vertical profile from a laterally heterogeneous density field. To overcome this, we created an extensive dataset of horizontal and vertical density data at Kohnen Station, Dronning Maud Land on the East Antarctic Plateau. We drilled and analyzed three 90 m long firn cores as well as 160 one meter long vertical profiles from two elongated snow trenches to obtain a two dimensional view of the density variations. The analysis of the 45 m wide and 1m deep density fields reveals a seasonal cycle in density. However, the seasonality is overprinted by strong stratigraphic noise, making it invisible when analyzing single firn cores. Our density dataset extends the view from the local ice-core perspective to a hundred meter scale and thus supports linking spatially integrating methods such as radar and seismic studies to ice and firn cores.
Laepple, Thomas, Maria Hörhold, Thomas Münch, Johannes Freitag, Anna Wegner, and Sepp Kipfstuhl. 2016. “Layering of Surface Snow and Firn at Kohnen Station, Antarctica – Noise or Seasonal Signal?” Journal of Geophysical Research: Earth Surface, January, 2016JF003919. doi:10.1002/2016JF003919.
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.
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.
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.
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.
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.
A model–data comparison of the Holocene global sea surface temperature evolution
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.
Reconciling discrepancies between Uk37 and Mg/Ca reconstructions of Holocene marine temperature variability
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. http://www.sciencedirect.com/science/article/pii/S0012821X13003221.