Lukas Brunner

Institute for Atmospheric and Climate Science | ETH Zurich

This Page is no longer maintained please visit my new Homepage at lukasbrunner.github.io

About

I'm a senior scientist in the Climate Physics group at ETH Zurich working in the frame of the Horizon 2020 project EUCP. My research focuses on the investigation of uncertainties in European climate model projections. I currently mainly work with CMIP5 & 6 on global and regional scale. I also have some expertise in the analysis of atmospheric blocking and I'm passionate about science to public communication and open science.

In my free time I volunteer as board member and president for the Club Alpbach Vorarlberg. We are a non-profit dedicated to enabling dialog between countries, generations, and disciplines. Every year we award scholarships for the European Forum Alpbach to young people with ties to Vorarlberg.



Publications

Google Scholar

2021

Hegerl, G., A. P. Ballinger, B. Booth, L. F. Borchert, L. Brunner, M. Donat, F. Doblas-Reyes, G. Harris, J. Lowe, R. Mahmood, J. Mignot, J. Murphy, D. Swingedouw, and A. Weisheimer (2021): Toward Consistent Observational Constraints in Climate Predictions and Projections, Front. Clim., 3, DOI 10.3389/fclim.2021.678109
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Sperna Weiland, F. C., Visser, R. D., Greve, P., Bisselink, B., Brunner, L., & Weerts, A. H. (2021): Estimating Regionalized Hydrological Impacts of Climate Change Over Europe by Performance-Based Weighting of CORDEX Projections. Front. Water, 3, DOI 10.3389/frwa.2021.713537
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2020

Brunner, L., A. G. Pendergrass, F. Lehner, A. L. Merrifield, R. Lorenz, and R. Knutti (2020): Reduced global warming from CMIP6 projections when weighting models by performance and independence. Earth Syst. Dynam., 11, 995-1012, DOI: 10.5194/esd-11-995-2020
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Brunner, L., C. McSweeney, A. P. Ballinger, D. J. Befort, M. Benassi, B. Booth, E. Coppola, H. de Vries, G. Harris, G. C. Hegerl, R. Knutti, G. Lenderink, J. Lowe, R. Nogherotto, C. O'Reilly, S. Qasmi, A. Ribes, P. Stocchi, and S. Undorf (2020): Comparing methods to constrain future European climate projections using a consistent framework. J. Climate, 33, 8671-8692, DOI: 10.1175/JCLI-D-19-0953.1
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Merrifield, A. L., L. Brunner, R. Lorenz, I. Medhaug, and R. Knutti (2020): An investigation of weighting schemes suitable for incorporating large ensembles into multi-model ensembles. Earth Syst. Dynam., 11, 807-834, DOI: 10.5194/esd-11-807-2020
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Lehner, F., C. Deser, N. Maher, J. Marotzke, E. Fischer, L. Brunner, R. Knutti, and E. Hawkins (2020): Partitioning climate projection uncertainty with multiple Large Ensembles and CMIP5/6. Earth Syst. Dynam. 11, 491-508, DOI: 10.5194/esd-11-491-2020
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2019

Brunner, L., R. Lorenz, M. Zumwald, R. Knutti (2019): Quantifying uncertainty in European climate projections using combined performance-independence weighting. Eniron. Res. Lett. 14(12), DOI: 10.1088/1748-9326/ab492f
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2018

Brunner, L., N. Schaller, J. Anstey, J. Sillmann, A. K. Steiner (2018): Dependence of present and future European temperature extremes on the location of atmospheric blocking. Geophys. Res. Lett. 45, DOI: 10.1029/2018GL077837
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Unterberger, C., L. Brunner, S. Nabernegg, K. Steininger, A. K. Steiner, E. Stabentheiner, S. Monschein, and H. Truhetz (2018): Spring frost risk for regional apple production under a warmer climate. PLoS ONE 13, DOI: 10.1371/journal.pone.0200201
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2017

Brunner, L. and A. K. Steiner (2017): A global perspective on atmospheric blocking using GPS radio occultation – one decade of observations. Atmos. Meas. Tech., DOI: 10.5194/amt-10-4727-2017
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Brunner, L., G. C. Hegerl, A. K. Steiner (2017): Connecting atmospheric blocking to European temperature extremes in spring. J. Climate 30.2, pp. 585-594, DOI: 10.1175/JCLI-D-16-0518.1
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2016

Brunner, L., A. K. Steiner, B. Scherllin-Pirscher, and M. W. Jury (2016): Exploring atmospheric blocking with GPS radio occultation observations, Atmos. Chem. Phys. 16.7, 4593-4604, DOI: 10.5194/acp-16-4593-2016
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Other publications

Brunner L., M. Hauser, R. Lorenz, and U. Beyerle (2020): The ETH Zurich CMIP6 next generation archive: technical documentation. DOI: 10.5281/zenodo.3734128
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Brunner L. (2018): A new perspective on atmopsheric blocking from observations – detection, analysis, and impacts (PhD thesis). Wegener Center Verlag Graz, Scientific Report No. 76-2018, ISBN: 978-3-9504501-3-2
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Mohankumar, S. E. P., K. Mintz-Woo, M. Damert, L. Brunner, and J. Eise (2018): Blogging Climate Change: A Case Study, In: Addressing the Challenges in Communication Climate Change Across Various Audiences DOI: 10.1007/978-3-319-98294-6
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Brunner L. (2014): Stratospheric ozone and temperature evolution over the past decades (Master's thesis). Wegener Center Verlag Graz, Scientific Report No. 59-2014, ISBN: 978-3-9503608-6-8
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Research

Human influence on the climate system is clear. This is evident from the increasing greenhouse gas concentrations in the atmosphere, positive radiative forcing, observed warming, and understanding of the climate system. (IPCC)

A motivation of my research for laypeople

I work in the frame of a EU Horizon 2020 project called European Climate Prediction (EUCP) system. The project aims at providing reliable information about future European climate, which can be used by decision-makers.

Part I: uncertainties in climate projections

While the warming of the climate system through the emission of man-made greenhouse gases is clear from observations and models, projections of the future climate face a range of uncertainties. In the following I'm going to introduce some of the most important sources of uncertainty and how my research can help to reduce them.

Internal variability stems from the fact that the Earth's climate is an extremely complex and chaotic system. The animation below shows global mean temperature for the years 1900 to 2005 from one selected climate model. Ignoring a possible trend for now, the (seemingly random) fluctuations are referred to as internal variability and constitute one important source of uncertainty. In fact, internal variability cannot really be reduced since it is part of the climate system (however, one could potentially select temporal and spatial scales with lower natural variability – more about that later)
Deser at al. 2012a (paywall), Deser at al. 2012b, Martel et al. 2018



Scenario uncertainty arises because climate scientists can't make predictions about decisions taken by politics and society. Will we continue to emit greenhouse gases as we did in the past? Or will we manage to reach the goals set by the Paris Agreement, hence limiting global warming to well below 2°C? In order to be able to make statements about the future climate we use projections based on different paths our society (read: our emissions) might take. The animation below shows the possible development of global mean temperature between 2006 and 2100 based on three such assumptions.
IPCC 2007, Moss et al. 2010 (paywall), IPCC 2010, IPCC 2017



Model uncertainty accounts for the fact that climate models can never completely reproduce reality (I've already mentioned that the climate system is extremely complex). Models therefore use approximations to represent certain processes. Different models use different approximations and therefore yield slightly different results. The animation below consecutively adds more models for each of the three scenarios, resulting in a representation of uncertainty due to imperfect modelling.
Smith et al. 2002 (paywall), Knutti 2008, Rowlands et al. 2012 (paywall),



Wrap-up part I: I introduced the three main sources of uncertainty we face when predicting Earth's future climate. The animation below puts it all together and shows the relative contributions from each of the three sources to the total uncertainty. As can be seen, the contributions are not constant over time. For the near future internal variability still plays an important role. Model uncertainty is most important on an intermediate time scale, while predictions for the end of the century are dominated by the scenario uncertainty.



Based on data from the ETH CMIP-5 archive. Figure 1 shows historical annual mean, global mean temperature anomalies from the CNRM-CM5 model. Figure 2 additionally shows the RCPs 2.6, 4.5, and 8.5. Figure 3 shows up to 28 models. Figure 4 shows the CNRM-CM5 model as a representation for internal variability, the standard deviation from the multi-model mean as a representation for model uncertainty, and the three RCPs as a representation for scenario uncertainty. The small figure inside figure 4 shows the relative contributions from the three sources of uncertainty based on a method proposed by Hawkins and Sutton 2009

Part II: Introducing a model democracy

Recap: In the last part I introduced the three main sources of uncertainty in projections of future climate - now let's discuss how climate scientists deal with them. When it comes to the possible emission paths, climate scientists, generally speaking, do not like to speculate about the probability of the scenarios (even though it has recently been argued that we should) and normally make analysis conditional to a given scenario. Internal variability, in turn, can be quantified, isolated, and investigated using different methods (such as so-called large model ensembles; more about model ensembles later) but it can not really be reduced as mentioned before. So, in my work I focus on model uncertainty as it is often considered to have the largest potential for reduction.

Model uncertainty is closely connected to the forced response which is basically the reaction of the climate system to the external (external from the perspective of the climate system) forcing applied to it by anthropogenic emissions. For example, we can't know exactly how much the climate system will warm for a given amount of future CO2 emissions (i.e., forcing). Or, if we turn it around, we can't know exactly how much more CO2 we can emit while still staying below a certain level of warming (this is the concept of carbon budgets). So in either case there is some uncertainty involved, which is reflected in the spread between estimates from the different models and the more we can (justifiably) reduce this uncertainty the better we know what to expect.

While in the last part I looked at global temperature, let us now consider something more relevant for decision makers (and maybe also the reader): here I show Central European summer temperature change (relative to 1995-2014) until the middle of the century. For simplicity I focus on the high emission path only (effectively disregarding scenario uncertainty). Most of the internal variability is removed by using a 20-year running mean as well as averaging over the region. In the figure below model after model is added (highlighted in red). You might notice that some models are represented by several lines. These are the ensemble members of a model mentioned earlier; they only differ due to the remaining internal variability (again more about that later). In total I here use 10 different models in 29 realisations.



How do we get an estimate of uncertainty from all these lines? Well, the simplest thing is to just calculate statistics giving every run one vote in what has been called model democracy (paywall). There are several potential shortcomings of such an approach but it is still widely used because it is simple and easy to understand. In the figure below I replace the individual estimates of Central European Climate by some probability ranges. Doing so one important question that arises is: Do these ranges represent actual probabilities in the real climate system or, in other words, do we actually have a 50% chance of experiencing a warming within the 50% range? I will look into this question in more detail in the next parts.



Wrap-up part II: It is clear from all models that more greenhouse gases lead to more warming, highlighting the need for rapid reductions in emission in order to close the gap to what is needed for the Paris agreement goals. Due to the complexity of the system different climate models give slightly different estimates of future changes (based on a given emission path). The simplest way of calculating the most likely warming and the related uncertainty around it is to assume model democracy and give each model one vote.
The figure below summarises the steps from the individual model estimates to a distribution of 20-year climatological change by the middle of the century (2041-2060). It focuses on mean change and two uncertainty ranges: 66% and 90% or what the the IPCC calls the likely and very likely ranges, respectively.



PS. You might wonder why the model uncertainty is so small around 2010. That is just a property of the anomaly calculation I've chose here: the change is with respect to 1995-2014, which means that approximately in the middle of this period each model has to cross the zero-line by design, restricting the spread.

PPS. You might also think that 2060 minus 2041 is really 19 and not 20 years. This is a subtle detail but when talking about years its more intuitive to count both the start- and end-year. If you consider, for example, the period 2041-2042 it becomes clear immediately that these are two years, namely 2041 and 2042. (I once even had to try and convince an Austrian quality newspaper about this but failed...)

Code

A global blocking detection algorithm

During my PhD I implemented a blocking detection algorithm in Python (3.x). It enables the classification of atmospheric blocking based on global geopotential height fields and following different definitions from the literature. A detailed description can be found in section 3.2 (page 25f) of my PhD-thesis.

The code is freely available under a MIT license on GitHub: https://github.com/lukasbrunner/blocking

Climate model Weighting by Independence and Performance

The Climate model Weighting by Independence and Performance (ClimWIP) package is based on earlier work by people from my research group (mainly Reto Knutti, Jan Sedláček, and Ruth Lorenz). Ruth and I have implemented the current version in pure Python (>3.6) and it is freely available under a GPLv3.0 on GitHub: https://github.com/lukasbrunner/ClimWIP

Address

Dr. Lukas Brunner

lukas.brunner@env.ethz.ch
CHN N 16.1
Institute for Atmospheric and Climate Science
ETH Zurich
Universitaetstrasse 16
8092 Zurich, Switzerland

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