Many aspects of climate change are related to the net energy imbalance at top of Earth's atmosphere. This measures the difference between incoming solar radiation and outgoing radiation from emission and reflected solar radiation; the net imbalance must be reflected in changes to global energy storage. While the net imbalance has been measured by satellites in recent decades and constrained by ocean heat-content estimates from Argo floats, long-term changes are poorly constrained, even in the 20th century. Here we test the hypothesis that the planetary energy imbalance can be estimated from observations, and proxies, of surface temperature using data assimilation over the Common Era.

We present and evaluate a method for reconstructing outgoing radiation at the top of the atmosphere from assimilation of surface temperature observations. The method is first tested during the satellite era by assimilating HadCRUT surface temperature observations using prior estimates drawn from historical climate-model simulations outside the validation period. Results show higher skill in the reconstructions compared to AMIP simulations for interannual variability in both the outgoing infrared, and reflected solar radiation fields, when compared to CERES measurements. Over longer periods of time, the effective radiative forcing (ERF) from changing greenhouse gases and aerosols must be considered since ERF is, by definition, the radiative response independent of surface temperature changes. Adding estimates of ERF from climate simulations to the reconstructed radiation fields shows good agreement with AMIP simulations over the historical period. We extend the method to the Common Era using PAGES2K temperature-sensitive proxies and the Last Millennium Reanalysis assimilation framework.

How to cite: Hakim, G., Black, A., and Armour, K.: Reconstructing Earth Energy Imbalance over the Common Era, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16901, https://doi.org/10.5194/egusphere-egu23-16901, 2023.

Global changes have been found in simulated and reconstructed temperatures during the Last Millennium (LM), mainly associated to changes in the Earth’s energy balance like those during the periods of the Medieval Climate Anomaly (MCA; ca. 950-1250 CE) and the Little Ice Age (LIA; ca. 1450-1850 CE), respectively characterized by warmer and cooler conditions over many regions. Even if the impact of these changes in the hydroclimate is not fully understood, evidences of coordinated changes in the hydroclimate of distant regions can be also found in reconstructions from areas of North America, Europe and the Mediterranean basin, South America, Eastern Africa and Monsoon Asia. The timing of such changes suggests a link to the MCA and the LIA, while other areas like Southeast Asia and the Indo-Pacific also show coordinated changes in the hydroclimate of the LM, but without a patent link to the periods of MCA and LIA.

To better assess these coordinated changes in the hydroclimate of distant regions, and whether they also extend to other periods of the LM different from the MCA and the LIA, evidences from reconstructions and simulations have been compiled, including: a compilation of 92 reconstructions reporting changes from wetter to drier or from drier to wetter conditions in the transition from MCA to LIA; reconstruction-based products like the Drought Atlases for Europe (OWDA), North America (NADA), Asia (MADA), Mexico (MXDA) and Eastern Australia and New Zealand (ANZDA); hybrid products like the Paleo Hydrodynamics Data Assimilation product (PHYDA) and the Last Millennium Reanalysis (LMR); and model simulations from the Community Earth System Model - Last Millennium Ensemble (CESM-LME) and the Coupled Model Intercomparison Project Phase 5 (CMIP5) and 6 (CMIP6).

The comparison between proxy-based datasets and model simulations has allowed to obtain robust conclusions regarding the behavior of the hydroclimate of different regions and to deeply analyze the impact of external forcing and internal variability on hydroclimate changes. In tropical areas, changes have been mainly associated to alterations in the position and intensity of the Intertropical Convergence Zone (ITCZ), while in extratropical areas the alteration of variability modes like the Northern (NAM) and Southern Annular Modes (SAM) may have had a major role in the response of hydroclimate to changes in external forcing.

How to cite: Roldán, P., González-Rouco, J. F., Smerdon, J., and García-Pereira, F.: Global changes in the hydroclimate of the Last Millennium: Evidences from reconstructions and simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3587, https://doi.org/10.5194/egusphere-egu23-3587, 2023.

Proxy records (corals, marine sediments, etc.) documenting the last 2000 years (2K) provide evidences for the wide range of natural variability not captured by recent direct observations. Assessing climate models ability to reproduce such natural variations is crucial to understand climate sensitivity and impacts of future climate change. The length of record is relatively short for investigating slow climate features, especially when considering coupled ocean-atmosphere variability. In order to extend the information contained in proxies from the locations and times to which they pertain, additional information is needed to create a climate field reconstruction. Paleoclimate data assimilation offers a powerful way to extend the instrumental period and better characterize the decadal to secular natural ocean variability by optimally combining the physics described by climate models with information from available observations while taking into account their uncertainties. Here we present a new Proxy Data Assimilation product based on a sequential importance resampling particle filter (PF-SIR) that uses Linear Inverse Modeling as an emulator of GCMs, providing dynamical ocean memory and improving the reconstruction of low-frequency climate variability. The climate reconstructions include robust uncertainty quantification and a set of physically consistent spatial fields useful for dynamical inquiry beyond what is feasible from proxies or climate models alone. We use these new results to explore low-frequency aspects of main coupled variability modes and provide some constrains on climate model simulations for the last millennium.

How to cite: Jebri, B. and Khodri, M.: Large ensemble particle filter for proxy-based spatial reconstructions of the last 2000-years climate variability, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-654, https://doi.org/10.5194/egusphere-egu23-654, 2023.

The Modern Era Reanalysis (ModE-RA) extends current re-analysis back until the year 1420 CE at monthly resolution. It combines our understanding of physics coming from an ensemble of atmospheric model simulations with all available direct and indirect climate observations of monthly to annual resolution. A 20-member ensemble of atmospheric model simulations (ModE-Sim) driven by an ensemble of external forcings and an ensemble of new sea surface temperature reconstructions serves as a prior estimate of the possible climate states at each assimilation time step. After the entire simulations were completed, we assimilated multiple data sources using an offline Kalman filtering technique. We include up to ~100000 monthly to annual observations per year. These consist of thousands of existing and newly digitised instrumental measurements of temperature, precipitation, wet days per months, and pressure, including measurements made on ships over the ocean and in harbours. Earliest instrumental station data go back to the year 1658. Additionally, we collected and digitised climate information from historical documents, including phenological data. These are especially valuable for the autumn, winter and spring season and go back to the year 1420. Finally, we assimilate annually resolved climate proxies. The vast majority are tree-ring observations, which represent growing season conditions on the continents. In otherwise data sparse regions, we supplemented ice and coral data at high latitudes and the tropical oceans, respectively. ModE-RA offers especially  insides into interannual to multidecadal variability such as phases of accelerated warming, monsoon strength or subtropical droughts as well as rare events such as volcanic eruptions. 

How to cite: Franke, J., Veronika, V., Hand, R., Samakinwa, E., Burgdorf, A.-M., Lundstad, E., Brugnara, Y., Hövel, L., and Brönnimann, S.: ModE-RA - the Modern Era Reanalysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-386, https://doi.org/10.5194/egusphere-egu23-386, 2023.

Stable water isotopes are widely used to reconstruct past temperature and precipitation variations in many regions of the Earth. While the general physical processes of how isotope variations are controlled by temperature and water amounts is well understood, quantifying past isotope changes as a proxy for temperature and precipitation is much more difficult. An explicit simulation of water isotopes in current Earth system models is one possibility to approach this problem.
In this study we present new results of a transient fully-coupled simulation of the climate of the last two millennia, using the isotope-enabled model MPI-ESM-wiso. We analyze long-term trends, variability and extremes of the simulated isotope changes in precipitation, compare model results to available data compilations (e.g. PAGES Iso2k) and quantify the relations with associated temperature and hydrological changes. Our analyses show that for many regions of the Earth, the surface warming starting with the beginning of the industrialization period is clearly imprinted in the oxygen-18 isotope signal of precipitation, e.g., over Greenland, North America, and the Atlantic Ocean. However, in some regions recent temperature, precipitation and isotope changes seem to be decoupled, e.g., over West Antarctica, Oceania, and the Indian Ocean. In our presentation, we will discuss this regional varying imprint of the Anthropocene on the water isotope signal in detail. Further analyses focus on changes in isotope variability in different regions over the last two millennia, as well as the possible detection of past climate extremes in different isotope records.

How to cite: Werner, M. and Cauquoin, A.: Climate trends, variability and extremes recorded by water isotopes during the last two millennia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13169, https://doi.org/10.5194/egusphere-egu23-13169, 2023.

Almost all of the Central European temperature reconstructions covering the last two millennia reflect summer rather than mean annual air or cold season temperatures. To address the seasonal bias, we developed highly resolved 2000 year-long calcite isotope (δ¹⁸O) and fluid inclusion water isotope (δ²O and δ¹⁸O) records from a stalagmite from Milandre Cave in the Swiss Jura Mountains. Present-day climate in this region is strongly influenced by westerly air masses, making it an ideal site to record climate variability in the North Atlantic and European realm. Calibration of the Milandre Cave isotope records with historical and observational temperature and isotope data enables us to reconstruct mean annual air temperatures.  Our new temperature reconstruction shows temperature variations of approximately 2°C during the past two millennia, the temperature difference between the warmest decade of the Medieval Climate Anomaly (950–1250 CE) and the coldest decade of the Little Ice Age (1400–1700 CE) is around ~1.7°C. In general, higher central European temperatures were reconstructed in the periods 450–600 CE and 1000–1150 CE, and relatively low temperatures were recorded in the intervals 650–900 CE and 1350–1700 CE. Modelled cold season temperatures for the past millennium compare remarkably well with our reconstruction, and confirm the importance of both solar and internal forcing on Central European temperature.

How to cite: Fleitmann, D., Häuselmann, A. D., Cheng, H., Leuenberger, M., and Affolter, S.: Central European temperature variations over the past two millennia recorded in a stalagmite from western Switzerland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13374, https://doi.org/10.5194/egusphere-egu23-13374, 2023.

The project ROPEWALK, funded by the AP Møller Mærsk Fund, is a joint initiative of the Danish National Archive and the Danish Meteorological Institute over the period 2023-2026. The aim of the project is to digitize and transcribe all weather observations in ship journals and logbooks stored in the Danish National Archive.

The collection in the archive is remarkable for several reasons. A huge amount of data (more than 750 shelf metres) is stored, beginning as early as the 1680s. With the exception of the Napoleonic wars and the Danish state bankruptcy in 1814, the data is complete. In particular, there were no losses during the Second World War.

In the archive, logbooks from Danish ships over large parts of Northern Hemisphere are found. Of particular interest are observations from two regions, the Øresund and Greenland.

In connection with the Sound duties which every ship passing the sound or belts had to pay between 1426 and 1857, weather observations were made on board of war ships placed at strategic locations near Copenhagen, Helsingør and Nyborg. These ships had to ensure that no one passed without paying the duties. Probably for practical reasons, weather observations were tabulated as early as the first half of the 18th century. In several cases, observations were conducted every time the ship bell was struck, resulting in 48 observations in the course of one day. The early part of the logbook collection is from the Little Ice Age, and numerous ice observations in the Danish waters have been preserved.

The other group of logbooks which are of particular interest are from voyages to the colonies, in particular to (western) Greenland. The Greenlandic Trade Company had a monopoly for commerce with Greenland for nearly 200 years, and foreign ships would not be allowed to call a port. These "Greenland Voyages" were conducted several times per year. In many cases, detailed sea ice observations have been made.

The original logbooks are being scanned by the National Archive in highest possible resolution. The scans will then be transcribed by means of machine learning. This is possible, since the political system in Denmark was absolutistic between 1660 and 1848, and logbooks from different periods resemble each other much more than is the case for the nautical heritage in other seafaring nations. Where this is not possible, the data will be transcribed with the help of volunteers. For the oldest logbooks, which are in free text rather than in tabular form, we could locate older transcriptions which are much easier to read tahn the original data, either by machine or manually.

All transcribed data will be made publicly available. They can be used for future research or as input for reanalysis projects. We will present first results of our analysis.

How to cite: Stendel, M. and Kronegh, A. J.: ROPEWALK (Rescuing Old data with People's Efforts: Weather and climate Archives from LogbooK records) - a digitization project for three centuries of weather observations on board of Danish ships, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3407, https://doi.org/10.5194/egusphere-egu23-3407, 2023.

Wine is a particularly climate-sensitive crop for which long documentary series are available in much of Europe. Most studies of climate–wine growth relationships have hitherto utilised grape harvest date series. We instead investigate the climatic signature, and impact from volcanic forcing, on two long annual wine harvest quantity records from the Moselle Valley, in present-day Luxembourg, close to the latitudinal limit of commercial wine agriculture in Europe. A strong positive relationship between reconstructed temperature, especially summer temperature, and wine production quantities is found in our data that extend from the mid-fifteenth century until the end of the eighteenth century. Extremely cold or wet years show clear imprints on the wine harvest quantities. Large declines in the wine harvest – sometimes even total harvest failures – occurred after volcanic forcing events. The most significant decline in wine production quantities occurred during the first year after a volcanic forcing event. However, persistent wine production decline is evident over several consecutive years following larger volcanic forcing events. We compare the signature of Moselle Valley wine production declines following volcanic forcing events with corresponding tree-ring growth declines in different regions of Europe. It is evident that wine production shows a stronger and more distinct downturn following volcanic forcing events, even more minor ones, than tree-ring growth does in central Europe. Possible mechanisms for this behaviour are explored and discussed.

How to cite: Charpentier Ljungqvist, F., Christiansen, B., Schneider, L., and Thejll, P.: Strong volcanic and climatic shocks on early modern wine production, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2148, https://doi.org/10.5194/egusphere-egu23-2148, 2023.

For the region of central Scandinavia, there exists high-quality tree-ring based temperature reconstructions that cover the last 1200 years. This paper utilizes climate reconstructions in tandem with historical sources to estimate the relationships between climatic variability and agriculture in central Scandinavia between the 16th and 20^th centuries, with a specific focus on temperatures, harvests and agricultural dates. Using historical sources on agriculture importantly allows for evaluation of different climate reconstructions before the era of instrumental measurements. Thus, this paper not only sheds light on the societal relevance of past and present climatic change and variability, but also provides insights into the climate of the past.

While historical harvest dates in many parts of Europe have been employed to reconstruct growing season temperatures, this paper also includes virtually unique long time-series of sowing and hay-cutting dates going back to the late 17^th and 18^th century, respectively, and shows how these type of agricultural dates also had significant relationships with temperature variability in central Scandinavia. Furthermore, agriculture in this region, located in the northern boreal zone, was heavily constrained by growing season temperatures, and harvests exhibits much clearer relationships with climatic variability compared with less marginal regions of Europe. Climate-harvest relationships are studied at a local level, allowing for spatial anlaysis of micro-climatological differences within the larger regional context.

How to cite: Skoglund, M.: Agroclimatic relationships in Central Scandinavia, c. 1560–1920, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3969, https://doi.org/10.5194/egusphere-egu23-3969, 2023.

Daily atmospheric conditions before 1900 have been rarely investigated due to the limited availability of instrumental meteorological records. The documentary evidence is an alternative source that archives the atmospheric state. In Japan, the Historical Weather Database (HWDB, accessible by: http://tk2-202-10627.vs.sakura.ne.jp) provides descriptive daily weather information recorded in diaries at many stations since the 1660s. We utilize data assimilation to achieve high-temporal reconstructions by optimally combining observations with climate model forecasts. This study reconstructs daily weather conditions in the 1810s by assimilating diary weather information for the first time. We first categorize the descriptive records into “sunny”, “cloudy”, and “rainy”, and then assimilate these diary-based weather categories into the Global Spectral Model (GSM) through a local ensemble transform Kalman filter (LETKF) scheme. The reconstructed precipitation corresponds well with the daily synoptic pattern illustrated by documentary evidence in Japan. In a single-day case in August, 80% of non-assimilated diary categories are consistent with precipitation results. The atmospheric characteristics are also well reproduced in the Meiyu-Baiu season. Our results show better accuracy than the Twentieth Century Reanalysis (20CR) dataset due to their weak constraint in the Japan region. In addition, the Tambora eruption in April 1815 was among the largest in recent history, leading to the temperature decrease in Europe in the following year, commonly known as the “Year Without a Summer”. In our results, the surface air temperature anomaly indicates significant cooling also occurred in Japan in the summer of 1816, demonstrating the climate response to the Tambora eruption. This study shows the capability of diary data assimilation to reproduce daily atmospheric conditions, providing the basis to understand the cause of short-term variability in the past climate.

How to cite: Wang, X., Toride, K., Ichino, M., and Yoshimura, K.: Atmospheric Analysis in the 1810s by Assimilating Diary-based Weather Category, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12593, https://doi.org/10.5194/egusphere-egu23-12593, 2023.