Inhaltszusammenfassung

Volcanic ash clouds, injected into the stratosphere, can alter the Earth’s net radiation and initiate wide-spread surface cooling. Polar ash deposits from the last millennium indicate that past volcanic eruptions impacted global climate much more than any eruption since the onset of modern observations. Paleoclimatic evidence thus contributes to recognize the full range of associated atmospheric processes and potential threats of future eruptions. Additionally, climate reconstructions enable ...Volcanic ash clouds, injected into the stratosphere, can alter the Earth’s net radiation and initiate wide-spread surface cooling. Polar ash deposits from the last millennium indicate that past volcanic eruptions impacted global climate much more than any eruption since the onset of modern observations. Paleoclimatic evidence thus contributes to recognize the full range of associated atmospheric processes and potential threats of future eruptions. Additionally, climate reconstructions enable a validation of model simulations and their sensitivity to volcanic forcing. In this dissertation a new annually resolved temperature reconstruction for the Northern Hemisphere is developed to examine volcanic induced cooling events and to gain new insight in the history of volcanism. High-frequency variability in hemispheric temperature reconstructions is often based on tree-ring width (TRW) measurements, although this parameter was previously accused to be biased by biological memory. The analyzed TRW chronologies respond attenuated and delayed to abrupt temperature changes. Chronologies of maximum latewood density (MXD), in contrast, are more flexible and better suited for examining short-lived cooling events. For Europe the reconstructed temperature drop subsequent to volcanic eruptions is in good agreement with long instrumental records. Estimates of mean volcanic cooling are, however, very sensitive to the selection criteria for climatically relevant eruptions and the varying time lag between ash injection and climatic impact. Before extending the regional assessment of volcanic signals to a hemispheric scale, the spatial representativeness of the MXD network is assessed. Despite the comparatively small number of multi-centennial MXD chronologies, the ensemble mean can be calibrated successfully against extratropical summer temperatures of the Northern Hemisphere. The resulting reconstruction has an unprecedentedly accentuated high frequency signal over the past 1400 years and comprises a couple of distinct cooling events protruding beyond background year-to-year variability and longer term trends. Shortcomings in existing archives of volcanism motivate the application of a detection algorithm generating an independent record of climatically relevant eruptions. Designed for the characteristic volcanic cooling pattern and previously approved in a pseudoproxy environment, the algorithm picks up more than a dozen of these events in the MXD derived temperature reconstruction. Their dating generally agrees with historically documented eruptions or ash deposits from polar ice-cores. For some events, however, the exact timing is not in-phase proving that existing archives are either incomplete or imprecise. Deficiencies in ice-core derived reconstructions of volcanic activity propagate into the output of climate models forced with these datasets and explain some of the observed discrepancies between simulated and reconstructed volcanic cooling estimates. But the detection algorithm presented herein is capable to determine the most significant forcing events in both temperature estimates separately. Assessing the maximum cooling rate based on the detected events reveals a stronger and more persistent volcanic signal in simulated temperatures. Reconciling the ice-core derived reconstructions with the insights gained from paleoclimatology would improve the next generation of forcing datasets and last millennium model simulations. Besides facilitating data-model comparisons this could also expose more links between climate extremes and human history.» weiterlesen» einklappen

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DDC Sachgruppe:
Geowissenschaften