NoTricksZone | 1 Aug 2016
It is widely assumed that sea levels have been rising in recent decades largely in response to anthropogenic global warming. However, due to the inherently large contribution of natural oscillatory influences on sea level fluctuations, this assumption lacks substantiation. Instead, natural factors or internal variability override the detection of an anthropogenic signal and may instead largely explain the patterns in sea level rise in large regions of the global oceans.
Scientists who have recently attempted to detect an anthropogenic signal in regional sea level rise trends have had to admit that there is “no observable sea-level effect of anthropogenic global warming,” or that the “sea level rise pattern does not correspond to externally forced anthropogenic sea level signal,” and that sea level “trends are still within the range of long-term internal decadal variability.”
Below are highlighted summaries from 4 peer-reviewed scientific papers published within the last few months.
For the convenience of the readers, our basic results are shown in Figure 1. We identified five individual oscillations (upper panel), including a sea-level amplitude of 70 mm (top–bottom [t-b]) of the 18.6-year oscillation caused by the lunar nodal oscillation (LNO) … Together with a general sea-level rise of 1.18 mm/y, the sum of these five sea-level oscillations constitutes a reconstructed or theoretical sea-level curve of the eastern North Sea to the central Baltic Sea (Figure 1, lower panel), which correlates very well with the observed sea-level changes of the 160-year period (1849–2009), from which 26 long tide gauge time series are available from the eastern North Sea to the central Baltic Sea. Such identification of oscillators and general trends over 160 years would be of great importance for distinguishing long-term, natural developments from possible, more recent anthropogenic sea-level changes. However, we found that a possible candidate for such anthropogenic development, i.e. the large sea-level rise after 1970, is completely contained by the found small residuals, long-term oscillators, and general trend. Thus, we found that there is (yet) no observable sea-level effect of anthropogenic global warming in the world’s best recorded region.
Building up on the relationship between thermocline and sea level in the tropical region, we show that most of the observed sea level spatial trend pattern in the tropical Pacific can be explained by the wind driven vertical thermocline movement. By performing detection and attribution study on sea level spatial trend patterns in the tropical Pacific and attempting to eliminate signal corresponding to the main internal climate mode, we further show that the remaining residual sea level trend pattern does not correspond to externally forced anthropogenic sea level signal. In addition, we also suggest that satellite altimetry measurement may not still be accurate enough to detect the anthropogenic signal in the 20-year tropical Pacific sea level trends.
The tropical Pacific has featured some remarkable trends during the recent decades such as an unprecedented strengthening of the Trade Winds, a strong cooling of sea surface temperatures (SST) in the eastern and central part, thereby slowing global warming and strengthening the zonal SST gradient, and highly asymmetric sea level trends with an accelerated rise relative to the global average in the western and a drop in the eastern part. These trends have been linked to an anomalously strong Pacific Walker Circulation, the major zonal atmospheric overturning cell in the tropical Pacific sector, but the origin of the strengthening is controversial. Here we address the question as to whether the recent decadal trends in the tropical Pacific atmosphere-ocean system are within the range of internal variability, as simulated in long unforced integrations of global climate models. We show that the recent trends are still within the range of long-term internal decadal variability.
The observed 20th century sea level rise represents one of the major consequences of anthropogenic climate change. However, superimposed on any anthropogenic trend there are also considerable decadal to centennial signals linked to intrinsic natural variability in the climate system. … Gravitational effects and ocean dynamics further lead to regionally varying imprints of low frequency variability. In the Arctic, for instance, the causal uncertainties are even up to 8 times larger than previously thought. This result is consistent with recent findings that beside the anthropogenic signature, a non-negligible fraction of the observed 20th century sea level rise still represents a response to pre-industrial natural climate variations such as the Little Ice Age.