CL1.2.6

We present a 50-member global monthly gridded Sea Surface Temperature (SST) and Sea Ice Concentration (SIC) dataset covering 850 years (1000–1849). The SST fields are based on an existing coarse-resolution ensemble of annual reconstructions and augmented with intra-annual and sub-grid scale variability, such that the annual means of the coarse resolution SST reconstructions are preserved. We utilize a large body of historical observational inputs from ICOADS (1780 – 1849) in an offline data assimilation approach.

Furthermore, the best sea ice analogs are selected based on a measure of similarity between subpolar and midlatitude SSTs of our reconstruction and HadISST SIC. The resulting SST and SIC fields will reflect a spatially and temporal consistent representation of the historical state of the ocean and are reconstructed to be used as forcing for AGCM simulations.

Reference:

Samakinwa, E., Valler, V., Hand, R. et al. An ensemble reconstruction of global monthly sea surface temperature and sea ice concentration 1000–1849. Sci Data 8, 261 (2021). https://doi.org/10.1038/s41597-021-01043-1

How to cite: Samakinwa, E., Valler, V., Hand, R., and Brönnimann, S.: Global monthly sea surface temperature and sea ice reconstruction for historical AGCM simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11664, https://doi.org/10.5194/egusphere-egu22-11664, 2022.

As they are not directly calibrated with meteorological series, underground temperature-depth profiles provide potentially useful constraints on past climate evolution. However, the global geothermal-climate dataset is spatially clustered and is based on measurements that span nearly 60 years.  Little information is available concerning uncertainties in measurements or site conditions which could impart non-climatic signals. Furthermore, the inversion for past temperatures is ill-posed meaning that solutions are non-unique and are sensitive to these uncertainties and to noise.

We developed a Bayesian hierarchical model to reconstruct climate from the global geothermal dataset. We employ a transdimensional formulation that tailors the inferred resolution of the temperature history in each location to the measurements. This avoids over-fitting through the inherent parsimony of Bayesian formulations. Additionally, we do not make any fixed assumptions about observational noise or a priori uncertainties. Instead, these are jointly inferred using a hierarchical setup.

When applied to 1012 profiles our method shows a long-term warming over the Northern Hemisphere in agreement with earlier studies. In the Southern Hemisphere recent warming follows an inferred period of stable temperatures from CE 1500-1800. Sensitivity tests show that these results are robust to choices of hyperpriors but that hard-wiring the level of observational noise influences the inferred amplitude of pre-20^th Century warming over the Northern Hemisphere.

How to cite: Hopcroft, P. and Gallagher, K.: Past hemispheric temperature variations from a Bayesian hierarchical analysis of the global geothermal dataset, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12595, https://doi.org/10.5194/egusphere-egu22-12595, 2022.

Annual-to-decadal variability in northern midlatitude temperature is predominantly dominated by the cold season. However, climate field reconstructions, which are essential for understanding the underlying mechanisms, are often based on tree rings. These mainly represent the growing season and allow limited insight on cold season effects. Plant and ice phenology data, on the other hand, are a rich source of cold season information that remains largely overlooked in climate reconstructions to date and could help to fill the seasonal gap. Here, we present Northern Hemispheric temperature field reconstructions for the extended cold season (October-to-May average) for 1701-1905 based entirely on phenological data. Time series of freezing and thawing dates of rivers together with a few early-spring plant observations covering a large area of the northern midlatitudes are used in a simple data assimilation framework.
The reconstructions allow a 320-yr perspective of climate variability and change of boreal cold season climate and unveil that the temperature of the northern midlatitude land areas exceeded the variability range of the 18th and 19th centuries by the 1940s, to which recent warming has added another 1.5 °C. We also find 5-10 year long sequences of cold northern midlatitude winters. The most prominent example lasted from 1808/9 to 1815/6. The conspicuously cooling during that period is associated with two volcanic eruptions (1808/9 and 1815), which caused cooling as a direct effect. The years between the eruptions are characterized by weak southwesterly atmospheric flow over the Atlantic-European sector in early winter. This lead to low Eurasian temperatures, which persisted into spring while the flow pattern did not. Twentieth century data and model simulations confirm this persistence and point to increased snow cover as a cause. This is consistent with independent information on Eurasian snow in the early 19th century.

How to cite: Burgdorf, A.-M., Brönnimann, S., Reichen, L., Franke, J., Hand, R., Valler, V., Samakinwa, E., Burgnara, Y., and Rutishauser, T.: Novel Cold Season Temperature Field Reconstructions for the Northern Midlatitudes from Phenological Data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9639, https://doi.org/10.5194/egusphere-egu22-9639, 2022.

This study conducted a comprehensive analysis of climate responses to volcanic eruptions occurred at different latitudes considering the last millennium volcanic eruptions available from Community Earth System Model ensemble simulations. Especially, we examine how different eruption latitudes induce the different responses in Arctic Oscillation (AO) with polar vortex and thereby exert different influences on northern Eurasian climate. We classify volcanic eruptions as tropical, northern and southern eruptions based on hemispheric aerosol loading ratios, which have different meridional structure of solar radiation perturbations and cause asymmetric climate response patterns between hemispheres, including tropospheric cooling and lower stratospheric warming. Volcanic eruptions found to cause stronger stratospheric polar vortex in both hemispheres with varying magnitudes depending on eruption latitudes. Following the tropical and southern eruptions, polar vortex enhancement is found in both hemispheric polar regions due to enhanced pole-to-equator temperature gradient and equatorward propagation of planetary waves. As a result of boreal winter averaged polar vortex enhancement, the tropical and southern eruptions found to cause more probability to occur at least the pentad strong polar vortex events during the boreal winter, which leads tropospheric westerly wind anomalies after a few days to the events. As a result, positive AO-like responses emerge at the lower troposphere. The positive AO induces surface air temperature warming as well as precipitation increase over the northern Eurasian continental regions. Following southern eruptions, the AO, Eurasian warming and wetting responses are much more extended to more southward (NH mid-latitudes) due to the more equatorward extended polar vortex variation. On the other hand, the Arctic polar vortex and the associated surface responses are only weakly influenced by the northern eruptions, in line with much poleward spread of volcanic aerosols and lesser equatorward extended planetary wave propagation in the lower stratosphere. These results suggest that while volcanic eruptions modulate surface climate by strengthening the polar vortex, their impacts are dependent on the eruption latitudes.

How to cite: Paik, S., Min, S.-K., Son, S.-W., An, S.-I., Kug, J.-S., and Yeh, S.-W.: Diverse Arctic Oscillation responses after volcanic eruptions at different latitudes during the last millennium, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10958, https://doi.org/10.5194/egusphere-egu22-10958, 2022.

The Arctic region is noted to be sensitive in its response to anthropogenic warming. The Greenland Ice Sheet is experiencing accelerated mass loss via surface melting and ice discharge. This freshwater input is likely to influence global heat distribution via the Atlantic Meridional Overturning Circulation (AMOC). To better understand past natural variations in this system, proxy reconstructions are required to give a longer-term perspective. Previous proxy studies have suggested that human-induced AMOC slowdown began as early as the nineteenth century. However, the lack of high temporal resolution data from the last millennium means that the role of meltwater discharge on the evolution of North Atlantic intermediate waters, especially during the Little Ice Age (LIA), remains unclear.

Here, we present both weathering and temperature records from deep-sea scleractinian corals collected from Southwest Greenland (Nuuk Trough). We analysed ²³⁴U/²³⁸U, rare earth elements with yttrium (REEY) and trace elements (Li/Mg temperature proxy) along with precise U-Th dating of corals. Samples were from 750 m and 1200 m water depth with ages spanning the last 1000 years. The study site is influenced by surface meltwater from the West Greenland Ice Sheet. It is also at the convergence point of shallow cold Arctic-sourced water and deeper warm Atlantic-sourced water, providing an ideal location for tracing AMOC variations.

Our coral data show West Greenland seawater δ²³⁴U has increased ~2‰ toward modern seawater value since the end of the LIA (1700 C.E.), suggesting an increase in subglacial physical weathering input. This is supported by our terrestrial discharge record from REEY data that indicates an increase in meltwater discharge since the end of the LIA. The temperature record shows a gradual cooling trend from 1600 to 1900 C.E. at 1200 m depth, followed by warming at 750m. We suggest that the temperature drop at intermediate depth is linked to a change in water mass structure, as the thermocline shallowed and colder, deeper waters expanded. Cooling at this depth is consistent with a weakened AMOC, with less penetration of warm Atlantic waters. Our findings highlight the complex interactions between glacial meltwater and intermediate water circulation in the last millennium.

How to cite: Sun, Y.-J., Robinson, L., Parkinson, I., Stewart, J., de Carvalho Ferreira, M. L., and Hendry, K.: Enhanced meltwater discharge and water mass evolution in Southwest Greenland since the end of the Little Ice Age, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12598, https://doi.org/10.5194/egusphere-egu22-12598, 2022.

The Atlantic Multidecadal Variability (AMV) modulates the North Atlantic surface ocean variability and affects decadal climate evolution up to the global scale; however, the underlying mechanisms of the AMV remains debated. We use a multi-model ensemble of transient past-millennium and unperturbed preindustrial control simulations contributing to the Paleoclimate Modelling Intercomparison Project - Phase 4 (PMIP4) to decompose the AMV signal into its internal and external components. The internal component of AMV exhibits no robust behavior across simulations during periods of major forcing such as strong volcanic eruptions, whereas the externally-forced component of AMV responds to volcanic eruptions with an immediate radiative cooling followed, in some simulations, by a sequence of damped multidecadal oscillations. This indicates that the intrinsic mechanism underlying the AMV is distinguishable from its response to external forcing. The internal component of AMV is tightly connected with the Atlantic meridional overturning circulation (AMOC) and controls the variations of AMV. The external component of AMV explains about 25% of the variance in the past millennium simulations, though less-consistency is found between models. Our results further indicate that the spatial imprint of external volcanic forcing on North Atlantic sea-surface temperatures differs from the surface pattern of the internal AMV contributing to the lack of robustness for the AMV pattern.

How to cite: Fang, S.-W., Khodri, M., Timmreck, C., Zanchettin, D., and Jungclaus, J.: Disentangling Internal and External Contribution to Atlantic Multidecadal Variability over Past Millennium, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9547, https://doi.org/10.5194/egusphere-egu22-9547, 2022.

Long-lived colonial cold-water corals have the potential to provide robust continuous archives of environmental change. These high-resolution records of the subsurface ocean are particularly valuable, especially at understudied intermediate water depths. Yet, to understand the anthropogenic impacts on the sub-surface ocean and better predict future changes, it is critical to establish the natural variation of temperature and circulation of the ocean system prior to the Industrial Revolution.

Here we combine temperature proxy and radiocarbon data from specimens of two taxa of cold-water coral that grew in intermediate water depths (~1500 m) in the tropical North Atlantic. In 2013, specimens of the bamboo coral Lepidisis spp. and scleractinian coral Enallopsammia rostrata were collected from sites currently situated in the boundary of North Atlantic Deep Water and Antarctic Intermediate Water to reconstruct the temperature and circulation history of the region. We demonstrate that bamboo corals can be used to reconstruct ambient seawater radiocarbon content when independently dated by organic node annual band counting. Radiocarbon was also analysed in Enallopsammia rostrata to develop age models for both the radial section and from discrete corallites (polyps) along a branch. Dating results show that this coral is about 500 years old, allowing us to generate a temperature record as far back as the Little Ice Age. Trace metal ratios were analysed along the growth axis of the coral, and the Li/Mg ratio was used as a temperature proxy. We find that the Li/Mg derived temperature of the most recent polyps is consistent with modern ambient temperature. The overall temperature record shows a general increasing trend since the Little Ice Age, while the radiocarbon record indicates no significant change until the late 20^th century. Combining these records allows us to reconstruct potential ocean circulation changes in the central tropical North Atlantic over last 500 years.

How to cite: Liu, Q., Robinson, L. F., Hendy, E., Chen, S.-Y. S., Stewart, J. A., Knowles, T., Li, T., and Samperiz Vizcaino, A.: Reconstruction of intermediate water temperature in the tropical North Atlantic since the Little Ice Age using cold-water corals, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5745, https://doi.org/10.5194/egusphere-egu22-5745, 2022.

Climate extremes over the mid-latitudes are driven by a combination of thermodynamical and dynamical factors. In Europe, the primary dynamical driver of summer climate extremes is the position of the jet stream over the Europe-North Atlantic (EU) region. In certain configurations, the EU jet creates a summer climate dipole between northwestern and southeastern Europe that can result in contrasting extreme weather conditions in the two regions. To study long-term variability in the EU jet configuration, as well as its potential impact on past climate extremes and human systems, we have reconstructed EU jet variability over the past 800+ years (1200-2005 CE). To accomplish this, we have combined five European tree-ring chronologies to reconstruct the July-August jet stream latitude for the EU domain (30°W - 40°E; EU JSL). Our reconstruction explains 40% of summer EU JSL variability over the instrumental period (1948-2005 CE) with strong skill.

We find that, over the past 800 years, opposite phases of EU JSL variability have consistently resulted in contrasting climate extremes, including heatwaves, droughts, floods, and wildfires, between northwestern Europe, specifically the British Isles, and southeastern Europe, specifically the Balkans and Italy. This EU JSL-driven summer climate dipole is captured in a network of historical documentary data that further document the societal impacts of EU JSL-related climate extremes on both sides of the dipole.

Our summer EU JSL reconstruction shows a century-long negative phase from ca. 1355-1450 CE, corresponding to anomalously wet and cool summers over the British Isles and dry and hot conditions over the Balkans. This negative phase is comparable to the recent (1970-present) EU JSL configuration. We also found a positive phase, with opposite summer climate dipole conditions, from ca. 1812-1861 CE. Our results thus suggest that the EU JSL has been a long-term primary driver of the European summer climate dipole, as well as of the associated climate extremes and societal impacts.

How to cite: Xu, G., Broadman, E., Meko, M., Klippel, L., Ludlow, F., Dorado-Liñan, I., Esper, J., and Trouet, V.: 800 years of summer European-North Atlantic jet stream variability and its impact on climate extremes and human systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3331, https://doi.org/10.5194/egusphere-egu22-3331, 2022.

Despite their pivotal role in climate research, direct instrumental records of meteorological variables are only available for the most recent part of climate history. Even in regions with longest tradition of weather measurements, such as central Europe, the existing series rarely comprise more than two centuries of reliable data. However, documentary sources, both quantitative and qualitative, can be employed to substantially extend the available records. Using the resulting multi-centennial data, previously unexplored features of climate system’s evolution can then be studied.

In this analysis, temporal variability in annual and seasonal series of temperature, precipitation and drought indices (Standardized Precipitation Index - SPI, Standardized Precipitation Evapotranspiration Index - SPEI, Z-index), pertaining to the territory of contemporary Czechia, has been studied over the 1501–2020 CE period. The series under investigation were reconstructed from multitude of Czech documentary data sources, combined with instrumental observations. Phenoclimatic temperature and SPEI reconstructions, derived from historical records of cereal and grape harvest dates, were also employed and compared to their documentary-based counterparts.

Statistical attribution analysis, utilizing multiple linear regression, confirmed the influence of covariates related to volcanic activity (prompting temporary temperature decreases, especially during summer) and the North Atlantic Oscillation (influential in all seasons except summer for all target variables) in the Czech climate reconstructions. Statistically significant components correlated with multidecadal variability in the northern Atlantic and northern Pacific (represented by multiproxy-reconstructed AMO and PDO indices) were identified in the Czech temperature and precipitation series as well as in all drought indices. Additionally, using wavelet and cross-wavelet analysis, notable oscillations shared by the AMO/PDO variations and the Czech climate series were found, particularly at periods of approximately 70–100 years.

How to cite: Mikšovský, J., Brázdil, R., Dobrovolný, P., Pišoft, P., Trnka, M., Možný, M., and Balek, J.: Temporal variability in central European climate reconstructions, 1501–2020 CE, and its attribution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4140, https://doi.org/10.5194/egusphere-egu22-4140, 2022.

The compound-specific signal of leaf wax n-alkanes can be used for reconstructing climatic and environmental changes. This work assesses the carbon and hydrogen isotopic compositions of sedimentary n-alkanes from a high-mountain lake record (Lake Peixão) in Serra da Estrela (Western Iberia, Portugal) over the last 2000 years.

Our interpretations are supported by the modern settings of the lake and the long-chain n-alkanes characterization of the modern vegetation in the lake’s watershed. The δ¹³C signal of long-chain n-alkanes (C₂₉ – C₃₃) suggests that terrestrial vegetation, dominated by C₃ heathlands and grass mosaics, did not change significantly since the last 2000 years. In contrast, δ¹³C signatures of C₂₅ and C₂₇, associated with aquatic plants inputs, reveal a more enriched and variable signal, suggesting these compounds as indicators of water availability in the studied area.  In this high-altitude setting, temperature significantly controls water availability and the δD signal of terrestrial plants (δD_terr), which shows major changes across the last 2000 years. In contrast, aquatic-derived n-alkanes (δD_aq) show a relatively constant and stable water source. These data enabled us to detect major climate shifts in the region and evaluate the role of the main drivers (solar activity and the North Atlantic Oscillation-NAO) in those long-term changes prior to intense human activities.

The Roman Period (0 – 500 AD) was relatively dry but shifting from a warm to a cold phase, under a predominant positive mode of NAO and a Grand solar maxima. From the Dark Ages until the Medieval Climatic Anomaly (500 – 1300 AD), the climate was generally mild and wet under a nonstationary mode of NAO and a gradual decrease in solar irradiance. The LIA (ca. 1350 – 1850 AD) was composed by two main phases: the first cold and wet followed by an extreme cold episode; both under a predominantly negative NAO mode. The extreme cooling (centered at 1700 AD), coincident with the Maunder Minimum, was driven by the southward displacement of the polar front, causing extended periods of ice cover on the lake. The climate became warm and dry since 1880 AD, coinciding decrease in vegetation capability to buffer the runoff energy and promote the observed high sedimentation rate during this period, probably due to increased anthropogenic impact.  This work also underlines the sensitive nature of the high-mountain lake ecosystems and contributes to the spatial coverage of paleoclimate studies in the Atlantic region of the Iberian Peninsula.

Acknowledgments

The authors are grateful to FCT (Fundação para a Ciência e a Tecnologia) for the financial support of this work through the projects: HOLMODRIVE—North Atlantic Atmospheric Patterns Influence on Western Iberia Climate: From the Late Glacial to the Present (PTDC/CTA-GEO/29029/2017). WarmWorld—Features, and lessons from Past Interglacials “warm periods” during the last 1.5 Ma (PTDC/CTA-GEO/29897/2017). RNS´s grant supported by Ultimatum—Understanding past climatic instabilities in the North Atlantic Region (IF/01489/2015) 

How to cite: N. Santos, R., Rodrigues, T., Schefuß, E., G.M.S. Cordeiro, L., Naughton, F., Oliveira, D., Hernández, A., and M. Ramos, A.: Climate and environmental changes over the last 2000 years in the Serra da Estrela, Portugal., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6632, https://doi.org/10.5194/egusphere-egu22-6632, 2022.

Since the beginning of this century, Europe has been experiencing severe drought events (2003, 2007, 2010, 2018 and 2019) which have had an adverse impacts on various sectors, such as agriculture, forestry, water management, health,and ecosystems. During the last few decades, projections of the impact of climate change on hydroclimatic extremes were often capable of reproducing changes in the characteristics of these extremes. Recently, the research interest has been extended to include reconstructions of hydro-climatic conditions, so as to provide historical context for present and future extremes. While there are available reconstructions of temperature, precipitation, drought indicators, or the 20th century runofffor Europe, multi-century annual runoff reconstructions are still lacking. In this study, we have used reconstructed precipitation and temperature data, Palmer Drought Severity Index and available observed runoff across fourteen European catchments in order to develop annual runoff reconstructions for the period 1500–2000 using two data-driven and one conceptual lumped hydrological model. The comparison to observed runoff data has shown a good match between the reconstructed and observed runoff and their characteristics, particularly deficit volumes. On the other hand, the validation of input precip-itation fields revealed an underestimation of the variance across most of Europe, which is propagated into the reconstructedrunoff series. The reconstructed runoff is available via figshare, an open source scientific data repository, under the DOIhttps://doi.org/10.6084/m9.figshare.15178107, (Sadaf et al., 2021).

How to cite: Nasreen, S., Vargas Godoy, M. R., Singh, U., Součková, M., Markonis, Y., Rakovec, O., Kumar, R., and Martin, H.: A 500-year annual runoff reconstruction for 14 selected European catchments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12894, https://doi.org/10.5194/egusphere-egu22-12894, 2022.

South east Australia experienced periods of multi-year droughts particularly within the last 2 millennia. However, given the limited evidence from smaller number of sites and scarcity of  quantitative, high-resolution climate records, it is largely unknown whether these droughts are a feature of climate through the Holocene and the extent to which they are experienced throughout the region. Where conditions are suitable, oxygen isotopes preserved in lake sediments are a useful tool for reconstructing past climate and environmental conditions. Here, we present preliminary results of a Holocene length record from Lake Surprise in western Victoria, from which we analysed δ¹⁸O of aquatic cellulose as a proxy for lake-water δ¹⁸O, complemented by organic carbon/nitrogen ratios, organic carbon isotopes and XRF (ITRAX) inferred elemental composition. Our interpretation of the palaeo-data is supported by ~3 monthly monitoring of water and sediment geochemistry to track the modern hydrology of the lake. Our preliminary results show a strong positive correlation between precipitation and sedimentary calcium (carbonate deposition) over the last 150 years, likely linked to changes in primary productivity. The aquatic cellulose δ¹⁸O record through Holocene is also correlated with carbonate concentration, reinforcing our interpretation of CaCO₃ deposition in the lake during wet periods. The cellulose δ¹⁸O record indicates a trend of gradually increasing aridity from early to late Holocene, with a notable extremely dry phase over the last 2 ka. Comparison of the cellulose δ¹⁸O record with high-resolution Holocene climate records indicates that multiple climate drivers such as ENSO intensification and Antarctic warming are strongly linked to increasing aridity of the region. Further work will focus on both increasing the resolution of the record to better identify the frequency and duration of key events and on quantifying natural hydroclimate variability, particularly via lake hydrologic modelling to better constrain the paleoclimate record.

How to cite: Dharmarathna, A., Tyler, J., Barr, C., Tibby, J., Jones, M., Ankor, M., Cadd, H., Gadd, P., Hua, Q., Child, D., Zawadski, A., Hotchkis, M., and Zolitschka, B.: Holocene climate variability in south east Australia; inferred from oxygen isotopes in sedimentary cellulose at Lake Surprise, Victoria., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10775, https://doi.org/10.5194/egusphere-egu22-10775, 2022.

Asten and McCracken (AGU 2021, paper H45Z-12) note strong ~85 and ~50 year periodicities in flood data and lake level data in NSW, south-east Australia.  We now compare data sets for the Brahmaputra River (latitude 25⁰N) 1800-2000CE with Lake George (latitude 35⁰S)  levels 1820-2020CE.  Both data sets show a pair of dominant spectral maxima at 80 and 50 year periods.

A study by Rao et al (2020) of observed and reconstructed discharge of the Brahmaputra shows only limited correlation of discharge rates with recorded floods.  We use a record of Oceanic Nino Index 1870-2021CE (McNoldy, 2021) to compare with floods and find that for time 1875-2010, 14 of 17 observed floods associate with La Nina events. However there were 27 La Nina events in this interval hence as a working hypothesis LaNina events are close to being a necessary condition (~82%) for floods but not a sole determinant. We use spectral analysis to locate multi-decadal natural cycles which also influence discharge levels and flood frequency.

The Brahmaputra discharge rate data extends back to year 1309CE (Rao et al, 2020).  The power spectrum   shows a series of strong maxima, especially at 242, 132, 90, &75year periods similar to those in ¹⁴C and ¹⁰Be records for the Holocene.  The entire record can be fitted using a model of 8 sinusoids, leaving only a 20% residual variance.  The model allows extrapolation of the discharge rate into the future and predicts an above-average discharge for years 1995-2040CE, peaking ~2020.  This predicted time-span of above-average discharge is based on natural frequencies embedded in the record and does not include any possible influences from 21^st-century global warming.  The prediction appears closer to the observed increase post-2000, than does a prediction based on CMIP5 models as provided in the Rao etal (2020) paper.

A further test of the efficacy of the discharge curve fitting method is provided by limiting the observed data to years 1309-1900CE, then projecting the model to 2200.  The projected curve from 1900 replicates the observed dry period 1950-1995 and validates the hypothesis that the dry period was not an unusual event but was part of the natural cycles as reconstructed since 1309.  The projected curve from 1900 also closely follows the model based on all data to 2010 in predicting the above-average discharge rates 1995-2040.

  As noted above both data sets show a pair of dominant spectral maxima at 80 and 50year periods. The similarity between the spectra invites a hypothesis that the long-period natural cycles at both locations have a common origin, possibly solar-related rather than being of local atmospheric/oceanic origin.  A key difference is that the phases of the spectral maxima are reversed for the two sites.  Physical mechanisms producing these dominant periods for the 19^th and 20^th centuries, and the phase difference between the northern and southern hemisphere sites are not yet known. They could be related to variations in solar insolation, cosmic-ray ionization of cloud cover, or mode changes in global ocean current systems driven by unknown external forcing.

How to cite: Asten, M. and McCracken, K.: The Gleissberg (~85 year) and other periodicities in the flood cycles of the Brahmaputra River past present and future; implications for possible global mechanisms, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8090, https://doi.org/10.5194/egusphere-egu22-8090, 2022.

By performing an ensemble of sensitivity experiments with the LOVECLIM model, we confirm the earlier results showing that temperature changes had a large influence on the land carbon cycle over the preindustrial Common Era. However, this process alone cannot explain the magnitude of the reconstructed atmospheric CO₂ and δ¹³CO₂ variations. In particular, even when the model is constrained to follow reconstructed temperature changes by data assimilation, and when applying relatively large values of the climate-carbon feedback parameter, it can only explain about 50% of the atmospheric CO₂ decrease between the 12^th and the 17^th century. We find that land use changes are likely responsible for most of the observed long term atmospheric CO₂ trend over the first millennium of the Common Era, and for up to 30 % of the decrease observed after 1600 CE. In addition, in our experiments, changes in southern hemisphere westerly winds induce slightly smaller changes in atmospheric CO₂ concentrations than those associated with land use change, and variations in δ¹³CO₂ of the same order of magnitude as the observed ones. Those wind changes also have a strong impact on the difference in ¹⁴C between the northern and southern hemisphere, presenting strong similarities with observed changes. Combining the effects of changes in temperature, land use and winds over the Southern Ocean provides a reasonable agreement with reconstructions for atmospheric CO₂ concentrations and δ¹³CO₂, especially for the low CO₂ values observed during the 17^th century. This underlines the important contribution of both land and ocean carbon processes. Nevertheless, some uncertainties remain on the origin of the relatively high CO₂ concentrations reconstructed during the 11^th and 16^th centuries.

How to cite: Goosse, H., Barriat, P.-Y., Brovkin, V., Klein, F., Meissner, K., Menviel, L., and Mouchet, A.: Contribution of climate variability, land-use and Southern Ocean dynamics to changes in atmospheric CO2 concentration over the past two millennia, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1525, https://doi.org/10.5194/egusphere-egu22-1525, 2022.