Fidel González-Rouco

Chapter 1 Mediterranean climate variability over the last centuries: A review

Publisher Summary This chapter discusses a necessary task for assessing to which degree the industrial period is unusual against the background of pre-industrial climate variability. It is the reconstruction and interpretation of temporal and spatial patterns of climate in earlier centuries. There are distinct differences in the temporal resolution among the various proxies. Some of the proxy records are annually or even higher resolved and hence record year-by-year patterns of climate in past centuries. Several of the temperature reconstructions reveal that the late twentieth century warmth is unprecedented at hemispheric scales and is explained by anthropogenic, greenhouse gas (GHG) forcing. The chapter discusses the availability and potential of long, homogenized instrumental data, documentary, and natural proxies to reconstruct aspects of past climate at local- to regional-scales within the larger Mediterranean area, which includes climate extremes and the incidence of natural disasters. The chapter describes the role of external forcing, including natural and anthropogenic influences, and natural, internal variability in the coupled ocean–atmosphere system at subcontinental scale.

European summer temperatures since Roman times

The spatial context is criticalwhen assessing present-day climate anomalies, attributing them to potential forcings and making statements regarding their frequency and severity in a long-term perspective. Recent international initiatives have expanded the number of high-quality proxy-records and developed new statistical reconstruction methods. These advances allow more rigorous regional past temperature reconstructions and, in turn, the possibility of evaluating climate models on policy-relevant, spatiotemporal scales. Here we provide a new proxy-based, annually-resolved, spatial reconstruction of the European summer (June-August) temperature fields back to 755 CE based on Bayesian hierarchical modelling (BHM), together with estimates of the European mean temperature variation since 138 BCE based on BHM and composite-plus-scaling (CPS). Our reconstructions compare well with independent instrumental and proxy-based temperature estimates, but suggest a larger amplitude in summer temperature variability than previously reported. Both CPS and BHM reconstructions indicate that the mean 20th century European summer temperature was not significantly different from some earlier centuries, including the 1st, 2nd, 8th and 10th centuries CE. The 1st century (in BHM also the 10th century) may even have been slightly warmer than the 20th century, but the difference is not statistically significant. Comparing each 50 yr period with the 1951-2000 period reveals a similar pattern. Recent summers, however, have been unusually warm in the context of the last two millennia and there are no 30 yr periods in either reconstruction that exceed the mean average European summer temperature of the last 3 decades (1986-2015 CE). A comparison with an ensemble of climate model simulations suggests that the reconstructed European summer temperature variability over the period 850-2000 CE reflects changes in both internal variability and external forcing on multi-decadal time-scales. For pan-European temperatures we find slightly better agreement between the reconstruction and the model simulations with high-end estimates for total solar irradiance. Temperature differences between the medieval period, the recent period and the Little Ice Age are larger in the reconstructions than the simulations. This may indicate inflated variability of the reconstructions, a lack of sensitivity and processes to changes in external forcing on the simulated European climate and/or an underestimation of internal variability on centennial and longer time scales.

Testing the Mann et al. (1998) approach to paleoclimate reconstructions in the context of a 1000-yr control simulation with the ECHO-G coupled climate model

Statistical reconstructions of past climate variability based on climate indicators face several uncertainties: for instance, to what extent is the network of available proxy indicators dense enough for a meaningful estimation of past global temperatures?; can statistical models, calibrated with data at interannual timescales be used to estimate the low-frequency variability of the past climate?; and what is the influence of the limited spatial coverage of the instrumental records used to calibrate the statistical models? Possible answers to these questions are searched by applying the statistical method of Mann et al. to a long control climate simulation as a climate surrogate. The role of the proxy indicators is played by the temperature simulated by the model at selected grid points. It is found that generally a set of a few tens of climate indicators is enough to provide a meaningful estimation (resolved variance of about 30%) of the simulated global annual temperature at annual timescales. The reconstructions based on around 10 indicators are barely able to resolve 10% of the temperature variance. The skill of the regression model increases at lower frequencies, so that at timescales longer than 20 yr the explained variance may reach 65%. However, the reconstructions tend to underestimate some periods of global cooling that are associated with temperatures anomalies off the Antarctic coast and south of Greenland lasting for about 20 yr. Also, it is found that in one 100-yr period, the low-frequency behavior of the global temperature evolution is not well reproduced, the error being probably related to tropical dynamics. This analysis could be influenced by the lack of a realistic variability of external forcing in the simulation and also by the quality of simulated key variability modes, such as ENSO. Both factors can affect the largescale coherence of the temperature field and, therefore, the skill of the statistical models.

Climate evolution in the last five centuries simulated by an atmosphere-ocean model: global temperatures, the North Atlantic Oscillation and the Late Maunder Minimum

The main results of a transient climate simulation of the last 500 years with a coupled atmosphere-ocean model driven by estimated solar variability, volcanic activity and atmospheric concentrations of greenhouse gases are presented and compared with several empirical climate reconstructions. Along the last five centuries the climate model simulates a climate colder than mean 20th century conditions almost globally, and the degree of cooling is clearly larger than in most empirical reconstructions of global and North hemispheric near-surface air temperature (MANN et al., 1998; JONES et al., 1998). The simulated temperatures tend to agree more closely with the reconstruction of ESPER et al. (2002) based on extratropical tree-ring chronologies. The model simulates two clear minima of the global mean temperature around 1700 A.D. (the Late Maunder Minimum) and around 1820 A.D. (the Dalton Minimum). The temperature trends simulated after the recovery from these minima are as large as the observed warming in the 20th century. More detailed results concerning the simulated Late Maunder Minimum, together with a spatially resolved historical reconstruction of the temperature field in Europe, are presented. It is found that the broad patterns of temperature deviations are well captured by the model, with stronger cooling in Central and Eastern Europe and weaker cooling along the Atlantic coast. However, the model simulates an intense drop of air-temperature in the North Atlantic ocean, together with an extensive sea-ice cover south of Greenland and lower salinity in North Atlantic at high latitudes, reminiscent of the Great Salinity Anomaly. Also, during the Late Maunder Minimum the intensities of the Golf Stream and the Kuroshio are reduced. This weakening is consistent with a reduced wind-stress forcing upon the ocean surface.

Chapter 3 Relations between variability in the Mediterranean region and mid-latitude variability

Publisher Summary The Mediterranean climate is under the influence of both tropical and mid-latitude climate dynamics, being directly affected by continental and maritime air masses with significant origin differences. The peak of the winter season occurs between December and February, when the mid-latitude cyclone belt has usually reached its southernmost position. However, spring and autumn also contribute to a significant amount of precipitation. Being located at the southern limit of the North Atlantic storm tracks; the Mediterranean region is particularly sensitive to interannual shifts in the trajectories of mid-latitude cyclones that can lead to the remarkable anomalies of precipitation and, to a lesser extent, of temperature. Storm-track variability impacts primarily the western Mediterranean, but it hasa signature clearly detected in the eastern Mediterranean as well. The complex orography that characterizes most regions surrounding the Mediterranean basin can modulate and even distort climate anomaly patterns that otherwise would be geographically much more homogenous. Lack of water in winter and spring reflects in the crop yield. However, too much water in winter is harmful by drowning the seeds and retarding root development. The variability of precipitation plays a crucial role in the management of regional agriculture, in environment, in water resources and ecosystems, as well as social development and behavior.

A Review of 2000 Years of Paleoclimatic Evidence in the Mediterranean

The integration of climate information from instrumental data and documentary and natural archives; evidence of past human activity derived from historical, paleoecological, and archaeological records; and new climate modeling techniques promises major breakthroughs for our understanding of climate sensitivity, ecological processes, environmental response, and human impact. In this chapter, we review the availability and potential of instrumental data, less well-known written records, and terrestrial and marine natural proxy archives for climate in the Mediterranean region over the last 2000 years. We highlight the need to integrate these different proxy archives and the importance for multiproxy studies of disentangling complex relationships among climate, sea-level changes, fire, vegetation, and forests, as well as land use and other human impacts. Focusing on dating uncertainties, we address seasonality effects and other uncertainties in the different proxy records. We describe known and anticipated challenges posed by integrating multiple diverse proxies in high-resolution climate-variation reconstructions, including proxy limitations to robust reconstruction of the natural range of climate variability and problems specific to temporal scales from interannual to multicentennial. Finally, we highlight the potential of paleo models to contribute to climate reconstructions in the Mediterranean, by narrowing the range of climate-sensitivity estimates and by assimilating multiple proxies.

Disagreement between predictions of the future behavior of the Arctic oscillation as simulated in Two different climate models: Implications for global warming

Two global climate models (HadCM2 and ECHAM) forced with the same greenhouse-gas scenario (IS92a) are found to disagree in their simulated long-term trends of the intensity of the Arctic Oscillation (AO), an atmospheric circulation pattern of the Northern Hemisphere. The simulated AO trends are strongly dependent on the model and on the initial conditions of the simulations. The simulated winter temperature increase averaged over the Northern hemisphere is very similar in both models. However, the effect of the different AO trends on temperature causes clear differences in the predicted regional warming, which are reduced if the effects of the AO is linearly discounted. The uncertainty in the predictions of circulation changes has impacts on the estimation of regional temperature changes.

Comments on “Testing the Fidelity of Methods Used in Proxy-Based Reconstructions of Past Climate”

Abstract Mann et el. found that a version of the Regularized Expectation Maximization (RegEM) method to reconstruct the temperatures of the last millennium showed similar results to previous reconstructions in one of their earlier papers. They also tested the RegEM method in the surrogate climate of a simulation with the Climate System Model (CSM) and found no attenuation of the pseudoreconstructed centennial variability of the Northern Hemisphere mean temperature compared to the one simulated by the model. This is in contrast with the results by von Storch et al., who found, in a simulation with ECHO-G model, that the earlier Mann et al. method underestimates the centennial temperature variability of the Northern Hemisphere temperature. The newer paper by Mann et al. explains that this difference is in part due to the unrealistic character of the ECHO-G simulation. However, it is shown here that similar results to those of von Storch et al. are also found in an ECHO-G simulation that closely resembles th...

Wintertime Iberian Peninsula Precipitation Variability and its Relation to North Atlantic Atmospheric Circulation

To isolate the physical mechanism responsible for the relationship between North Atlantic large-scale circulation and precipitation amounts over Western Europe, a singular value decomposition (SVD) has been performed. This analysis is applied to a 30-winter dataset consisting of both monthly mean precipitation anomalies in Iberia and monthly mean 500 hPa geopotential height anomalies over the North Atlantic Ocean. The SVD analysis yields three significant pairs of patterns that account for 93% of the squared covariance between the two fields. This great amount of covariance indicates the strong coupling between large-scale circulation described by the 500 hPa height anomalies and the regional precipitation in the Iberian Peninsula.

Reconstruction of past Mediterranean climate

Mediterranean Climate Variability and Predictability (MEDCLIVAR; http://www.medclivar.eu) is a program that coordinates and promotes research on different aspects of Mediterranean climate. The main MEDCLIVAR goals include the reconstruction of past climate, describing patterns and mechanisms characterizing climate space-time variability, extremes at different time and space scales, coupled climate model/empirical reconstruction comparisons, seasonal forecasting, and the identification of the forcings responsible for the observed changes. The program has been endorsed by CLIVAR (Climate Variability and Predictability project) and is funded by the European Science Foundation. The main purpose of this first MEDCLIVAR workshop was to identify sources of early instrumental data and natural and documentary climate proxies that had not been previously explored and/or identified and that could be relevant for the reconstruction of the Mediterranean climate or weather extremes covering the past millennia. A key focus was on weather and climate information with high temporal (annual or higher) and spatial resolution as well as the potential to resolve past climate variability based on low-resolution proxies covering the past tens of thousands to hundreds of thousands of years.