For more than a decade, scientists have argued about the warmth of the current interglaciation.Was the warmth of the preindustrial late Holocene natural in origin, the result of orbital changes that had not yet driven the system into a new glacial state? Or was it in considerable degree the result of humans intervening in the climate system through greenhouse gas emissions from early agriculture? Here we summarize new evidence that moves this debate forward by testing both hypotheses. By comparing late Holocene responses to those that occurred during previous interglaciations (in section 2), we assess whether the late Holocene responses look different (and thus anthropogenic) or similar (and thus natural). This comparison reveals anomalous (anthropogenic) signals. In section 3, we review paleoecological and archaeological syntheses that provide ground truth evidence on early anthropogenic releases of greenhouse gases. The available data document large early anthropogenic emissions consistent with the anthropogenic ice core anomalies, but more information is needed to constrain their size. A final section compares natural and anthropogenic interpretations of the δ 13 C trend in ice core CO 2 .
Here we identify and analyze proxy data interpreted to reflect hydro-climatic variability over the last 10000 years from the Mediterranean region to: 1) outline millennial and multicentennial scale trends and 2) identify regional patterns of hydro-climatic variability. A total of 47 lake, cave and marine records were transformed to z-scores to allow direct comparisons between sites, put on a common timescale and binned into 200-year time slices. Six different regions were identified based on numerical and spatial analyzes of z-scores: S Iberia and Maghreb, N Iberia, Italy, the Balkans, Turkey, and the Levant, and the overall hydro-climate history of each region was reconstructed. N Iberia is largely decoupled from the five other regions throughout the Holocene. Wetter conditions occur in the five other regions between 8500 to 6100 yrs BP. After 6000 yrs BP climate oscillated until around 3000±300 yrs BP, which seems to have been the overall driest period in the Eastern Mediterranean and North Africa. In contrast, Italy and N Iberia seem to have remained wetter during this period. In addition, non-metric multidimensional scaling (nMDS) was applied to 18 long, continuous climate z-score records that span the majority of the Holocene. nMDS axes 1 and 2 illustrate the main trends in the z-score data. The first axis captures a long-term development of drier condition in the Mediterranean from 7900 to 3700 yrs BP. Rapid shifts occur in nMDS axis 2 at 6700 to 6300 BP, 4500 to 4300 BP, and 3500 to 3300 BP indicating centennial-scale climate change. Our synthesis highlights a dominant south/east vs north/west Mediterranean hydro-climate dipole throughout the Holocene and therefore confirms that there was no single climate trajectory characterizing the whole Mediterranean basin during the last ten millennia.
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