16 17 2 During the first half of the 19th century, several large tropical volcanic eruptions occurred 18 within less than three decades. Global climate effects of the 1815 Tambora eruption have 19 been investigated, but those of an eruption in 1808 whose source is unknown and the 20 eruptions in the 1820s and 1830s have received less attention. Here, we analyse the effect 21 of the sequence of eruptions in observations, global three-dimensional climate field 22 reconstructions, and coupled climate model simulations. All eruptions were followed by 23 substantial drops of summer temperature over the Northern Hemisphere land areas. In 24 addition to the direct radiative effect, which lasts 2-3 years, the simulated ocean-25 atmosphere heat exchange sustained cooling for several years following these eruptions, 26 affecting the slow components of the climate system. Africa was hit by two decades of 27 drought, global monsoons weakened, and the tracks of low-pressure systems over the 28 North Atlantic moved south. The low temperatures and increased precipitation in Europe 29 triggered the last phase of advance of Alpine glaciers. Only after the 1850s the transition 30 into the period of anthropogenic warming started. We conclude that the end of the Little 31 Ice Age was marked by the recovery from a sequence of volcanic eruptions, which makes it 32 difficult to define a single pre-industrial baseline. 33The period between around 1350 or 1450 and 1850 is often termed the "Little Ice Age" (LIA). 34In several regions the LIA was accompanied by glacier advances 1,2 . It might have been 35 initiated by volcanic eruptions 3 , but the relative contributions of solar and volcanic forcing 36 remain unclear. Given the regional differences 4 and uncertainties in the mechanism involved, 37 the onset of the LIA is still highly debated 5 . 38Importantly, the transition from the LIA into the period of anthropogenic warming is also not 39 well understood. After a rather warm phase around 1800, global climate cooled again in the 40 3 early 19th century 6 for several decades, accompanied by pronounced glacier advances in the 41 Alps. Recent work therefore dated the start of anthropogenic warming back to the early 19 th 42 century 7 . However, the fact that several major tropical volcanoes erupted between 1808 and 43 1835 (note that there is still large uncertainty -the 1808/09 eruption remains unknown 8 and 44 the attribution of the 1831 eruption has recently been questioned 9 ), including the well-studied 45 1815 Tambora eruption, makes the separation between volcanic and anthropogenic 46 contributions difficult. Based on attribution results, a small drop in greenhouse gas levels 47 109 ( Fig. 3a). According to both data sets the region remained dry during most of the first half of 110 the 19 th century. 111 We further analysed other monsoon regions and compared the palaeo-reanalysis with 112 independent observations 25,26 . We find a weakening of all India monsoon rainfall (Fig. 3b) 113 and of the strength of the Australian mons...
The construction of accurate age–depth relationships and a realistic assessment of their uncertainties is one of the fundamental prerequisites for comparing and correlating late Quaternary stratigraphical proxy records. Four widely used age–depth modelling routines – CLAM, OxCal, Bacon and Bchron – were tested using radiocarbon dates simulated from varved sediment stratigraphies. All methods produce mean age–depth models that are close to the true varve age, but the uncertainty estimation differs considerably among models. Age uncertainties are usually underestimated by CLAM, whereas age uncertainties produced by Bchron are often too large. With OxCal and Bacon, the setting of model-specific parameters influences the estimated uncertainties, which vary from too large to too small. The variability of sediment accumulation rates is underestimated by CLAM but overestimated by Bacon and Bchron. Bayesian age–depth models mainly improve the assessment of uncertainties of age–depth models.
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