Holocene relative sea-level changes and vertical movements along the Italian and Istrian coastlines

Published and new data exist for relative sea-level change for 105 locations (127 samples) during the late Holocene, along the Italian (and Istrian) coasts. These data, compared with predictions (derived from two different models associated with the last glacial cycle) allowed the calculation of the tectonic vertical movements. They are based on precise measures of geomorphological and archaeological markers between 0.4 and 12.6 ka cal. BP, sampled at elevations between +7 and −51 m. In order to decipher the broad pattern of Holocene tectonic vertical movements along the Italian coastline, these data were compared with predicted sea-level curves using the most recent models published for the Mediterranean sea. Tectonic rates varied from −4.85 mm/a, in a core at Sybaris, to 5 mm/a, in the volcanic areas of Pozzuoli and Pantelleria. New MIS 5.5 (125 ka) data, mostly from the Venetian plain, are reported. In particular the depth of the base of MIS 5.5 paralic deposits found in four cores near Venezia provides a mean subsidence of 0.62 mm/a. New, precise mass spectrometer U-Th analyses on Cladocora layers from the bottom of a long core (named ENEA), indicate older ages (195.7 ± 1.6 and 161.2 ± 1.2 ka, respectively), relative to the published MIS 5.5 ages, which were based on alpha-counting U-Th data.Instrumental data obtained from tide gauges and repeated levelling measurements from the NE Adriatic and Sicily are also considered. These methods have one great advantage with respect to continuous GPS measurements and the satellite altimetric observations, in that a much greater time span is available. Although the altimetric measurements are available for 16 years, and the GPS for less than a decade, repeated levelling lines cover up to 50 years and tide gauge observations in some cases to 100 years or more. The greater time span allows for more stable differential rate estimates. The repeated levelling shows that the plain east of Mestre is subsiding (to −4 mm/a). The Messina tidal gauge demonstrates a total coseismic and post-seismic subsidence of 77 cm associated with the event of 1908, the post-seismic phase lasting for at least 13 years. The Reggio Calabria tidal station points to an uplift of this station relative to Palermo in the order of 1–2 mm/a.     

Influence of glacial isostatic adjustment upon current sea level variations in the Mediterranean

Present-day sea level variations in the Mediterranean depend on various factors, including recent climatic forcing, tectonic activity, anthropogenic effects, and glacio-isostatic adjustment. The latter is governed by mantle rheology and the spatio–temporal distribution of the late-Pleistocene ice sheets and it is expected to produce a long-wavelength pattern of sea level variations across the Mediterranean, mostly determined by the response of the solid earth and of the geoid to loading effects of melt water since the end of deglaciation. Modeling glacio-isostatic effects in this region is necessary for a correct interpretation of tide gauge and GPS time-series, and thereby to constrain both the present-day climate-related sea level rise and regional or local geological, tectonic and human-driven displacements. By an exhaustive exploration of the parameter space of mantle rheology and ice sheet chronologies, in this work we outline upper and lower bounds on the current rate of sea level variation associated with glacial isostatic adjustment in the Mediterranean. This may contribute to a full assessment of coastal vulnerability by sea level rise on a regional and local scale.     

Glacio–isostatic Adjustment in the Po Plain and in the Northern Adriatic Region

Vertical movements in the Po plain (northern Italy) are controlled by natural and anthropogenic effects. Since Italy is located in the far–field of the former late Pleistocene ice sheets, isostatic deformations are primarily driven by melt water loading and represent a major component of long–term natural movements across the entire Mediterranean. In addition to far–field sources, here we consider the isostatic effects of melting of the nearby Würm Alpine ice–sheet, suggesting that it is possible to put bounds on its maximum thickness, extent and chronology by Holocene relative sea level observations from the northern Adriatic. Using various plausible ice models, and adopting a viscosity profile that matches Holocene relative sea level observations in the Mediterranean, we find that melting of the Alpine ice sheet is always responsible for upward movements in the Po plain, currently at rates of ~ 0.5 mm/yr. When both far– and near–field sources are considered, the rate of sea level change in the Venetian Lagoon for the most reasonable mantle rheology and melting chronology is negative, i.e., opposite to that attributed to human activity and recent climatic variations. However, its amplitude (fractions of mm/yr) is small compared to the secular signal observed by tide gauges (~2 mm/yr), which makes glacial isostasy a second–order mechanism of sea level variation in this region.     

Analysis of Summer Ozone Observations at a High Mountain Site in Central Italy (Campo Imperatore, 2388 m a.s.l.)

Tropospheric ozone (O₃) is an important atmospheric pollutant and climate forcer. The Mediterranean basin is a hot-spot region in terms of short-term O₃ distribution, with frequent episodes of high tropospheric O₃, especially during summer. To improve the characterisation of summer O₃ variability in the Mediterranean area, during the period 6–27 August 2009 an experimental campaign was conducted at Campo Imperatore, Mt Portella (CMP), a high mountain site (2,388 m a.s.l.) located in the central Italian Apennines. As deduced from analysis of atmospheric circulation, the measurement site was significantly affected by air masses originating over the Mediterranean basin, which affected the measurement site for 32 % of the time. Analysis of average values and diurnal and day-to-day variability revealed that CMP O₃ observations (average value 60.0 ± 5.1 ppbv) were comparable with measurements at other European mountain stations, indicating a prevalent effect of meteorological conditions and atmospheric transport on the synoptic scale. In fact, only a small “reverse” diurnal variation typically characterises diurnal O₃ variability because of local thermal wind circulation, which sporadically favours transport of air masses rich in O₃ from the foothill regions. Statistical analysis of five-day back-trajectory ensembles indicates that synoptic-scale air-mass transport from the Mediterranean Sea usually results in decreasing O₃ concentrations at CMP, whereas the highest hourly O₃ values are mostly associated with air masses from central continental Europe, eastern Europe, and northern Italy. High O₃ concentrations are also related to downward air-mass transport from higher altitudes. Comparison of in-situ O₃ variability with tropospheric O₃ satellite-based measurements reveals similar features of the two data sets. Together with the results from back-trajectory analysis, this indicates that CMP measurements might usefully improve characterisation of broad-scale O₃ variability over the central Mediterranean basin.     

The first multi-model ensemble of regional climate simulations at kilometer-scale resolution part 2: historical and future simulations of precipitation

This paper presents the first multi-model ensemble of 10-year, “convection-permitting” kilometer-scale regional climate model (RCM) scenario simulations downscaled from selected CMIP5 GCM projections for historical and end of century time slices. The technique is to first downscale the CMIP5 GCM projections to an intermediate 12–15 km resolution grid using RCMs, and then use these fields to downscale further to the kilometer scale. The aim of the paper is to provide an overview of the representation of the precipitation characteristics and their projected changes over the greater Alpine domain within a Coordinated Regional Climate Downscaling Experiment Flagship Pilot Study and the European Climate Prediction system project, tasked with investigating convective processes at the kilometer scale. An ensemble of 12 simulations performed by different research groups around Europe is analyzed. The simulations are evaluated through comparison with high resolution observations while the complementary ensemble of 12 km resolution driving models is used as a benchmark to evaluate the added value of the convection-permitting ensemble. The results show that the kilometer-scale ensemble is able to improve the representation of fine scale details of mean daily, wet-day/hour frequency, wet-day/hour intensity and heavy precipitation on a seasonal scale, reducing uncertainty over some regions. It also improves the representation of the summer diurnal cycle, showing more realistic onset and peak of convection. The kilometer-scale ensemble refines and enhances the projected patterns of change from the coarser resolution simulations and even modifies the sign of the precipitation intensity change and heavy precipitation over some regions. The convection permitting simulations also show larger changes for all indices over the diurnal cycle, also suggesting a change in the duration of convection over some regions. A larger positive change of frequency of heavy to severe precipitation is found. The results are encouraging towards the use of convection-permitting model ensembles to produce robust assessments of the local impacts of future climate change.

     

Bounds on the Time–history and Holocene Mass Budget of Antarctica from Sea–level Records in SE Tunisia

Solving the sea–level equation for a Maxwell Earth, we analyze the sensitivity of Holocene sea–level records in SE Tunisia to the time–history of remote ice sheets. Assuming that mantle viscosity increases moderately with depth, we find that in this region the sea–level variations driven by the Northern Hemisphere ice sheets cancel, so that the late–Holocene sea–level high–stand suggested by the geological record merely reflects the melting history of Antarctica. New insight into the history of this ice sheet is obtained analyzing the information contained in a revised set of relative sea–level observations for sites across the Mediterranean covering the last 8 kyrs. From a trial–and–error misfit analysis, it holds true that in this region the match between model predictions and observations improves when the volume of water released from Antarctica is well below the value imposed by the ICE3G chronology and when a sudden meltwater pulse is allowed between 8 and 7 kyrs before present, corresponding to the epoch of the catastrophic rise event known as CRE3.