Klimamodelle müssen zunächst an der bekannten Klimavergangenheit kalibriert und getestet werden, bevor sie verlässliche Zukunftprognosen abgeben können. Eine besondere Herausforderung ist dabei das sogenannte mittlere Holozän, 8000-5000 Jahre vor heute, als es an vielen Stellen der Erde deutlich wärmer war als heute. Die Klimamodelle tun sich schwer, diese Wärme zu reproduzieren. Masakazu Yoshimori und Marina Suzuki haben sich das Problem in Climate of the Past Discussion vorgenommen und weisen darauf hin, dass dringend eine Lösung gefunden werden muss, um die Robustheit der Modelle zu garantieren.
Die bekannte Klimageschichte weicht noch immer signifikant von den Simulationsszenarien ab. Bereits vor knapp 30 Jahren forderte Mitchell (1990), dass die erfolgreiche Rückwärts-Kalibrierung der Klimamodelle eine unverzichtbare Voraussetzung ist, bevor die Modelle für Prognosen eingesetzt werden könnten. Des Weiteren empfehlen Masakazu Yoshimoriund Marina Suzuki, sich verstärkt auf die Klimaverstärker zu konzentrieren, die offenbar in ähnlicher Weise vor einigen Jahrtausenden wirkten wie heute, unabhängig davon, was letztendlich den Anstoß für die Erwärmung gab. Hier der Abstract des Papers:
The relevance of mid-Holocene Arctic warming to the future
There remain substantial uncertainties in future projections of Arctic climate change. Schmidt et al. (2014) demonstrated the potential to constrain these uncertainties using a combination of paleoclimate simulations and proxy data. They found a weak correlation between sea ice changes in the mid-Holocene (MH) and in future projections, relative to the modern period. Such an “emergent constraint” provides a powerful tool to directly reduce the range of uncertainty, provided that the necessary paleoenvironmental information is available. In the current study, we examine the relevance of Arctic warming in the past to the future through process understanding, rather than seeking a statistical relation. We conducted a surface energy balance analysis on 10 atmosphere and ocean general circulation models under the MH and future RCP4.5-scenario forcing. We found that many of the dominant processes that amplify Arctic warming from late autumn to winter are common between the two periods, despite the difference in the source of the forcing (insolation vs. greenhouse gases). We also quantified the contribution of individual processes to the inter-model variance in the surface temperature changes. The controlling term varies with the season, but the results suggest that the models’ representations of the surface albedo feedback, cloud greenhouse effect, turbulent surface heat fluxes, and indirect atmospheric stratification are important contributors. Based on the results for the Arctic warming mechanism obtained from this study, we conclude that proxy records of Arctic warming during the MH contain useful information that is relevant for understanding future Arctic climate change.
Auszug aus den Conclusions:
From this study, we conclude that proxy records of the Arctic warming for the MH contain useful information relevant to future Arctic climate change. The current disparity between the model and proxy 30 reconstructions is, therefore, of concern. The inclusion of dynamic vegetation feedback discussed in the previous section has a high priority. The relation between past and future climate is not due to a common forcing to the climate system but due to the feedbacks inherent in the climate system. Therefore, more effort should be made in seeking possible analogues between physical processes in the past and future climate, rather than in the past forcing. Our study supports the conclusion by Mitchell (1990) that it is a necessary condition for models to be able to reproduce the MH climate to produce reliable future projections, and we conclude that the evaluation of the models’ parameterization is embedded in the model validation exercise using proxy data.
