Distinguishing Between Natural and Anthropogenic Part of Sea Level Trends

Abstract:

Detection and attribution of human influence on sea level rise are important topics that have not yet been explored in depth. From the perspective of assessing the contribution of human activities to climate changes, the sea level drivers can be partitioned in anthropogenic and natural forcing. In this study we try to answer the following two questions: (1) How large a sea level trend could be expected as result of natural internal variability? (2) Whether the sea level changes observed over the past century were natural in origin. We suppose that natural behavior of sea level consists of increases and decreases occurring with frequencies following a power law distribution and the monthly sea level records are power law long-term correlated time series. Then we search for the presence of unnatural external sea level trend by applying statistics of Lennartz and Bunde [2009]. We estimate the minimum anthropogenic sea level trend as a lower bound of statistically significant external sea level trend in the longest tide-gauge records worldwide. We apply this new method to distinguish between the trend-like natural oscillations and the external trends in the longest available sea level records and in global mean sea level reconstructions. The results show that the long-term persistence impacts strongly on sea level rise estimation. We provide statistical evidences that the observed sea level changes, at global and regional scales, are beyond its natural internal variability and cannot be explained without human influence. We found that sea level change during the past century contains an external component at 99% significance level in two thirds of the available longest tidal records worldwide. The anthropogenic sea level trend is about 1 mm/yr in global sea level reconstructions that is more than half of the total observed sea level trend during the XXth century, which is about 1.7 mm/yr. This work provides the first estimate of the minimal anthropogenic contribution to the observed sea level rise, which is crucial for developing climate adaptation strategies and for understanding human influence on the Earth climate.