Comments on GWRC Draft Climate Change Strategy
de Lange, W. P. (2015). Comments on GWRC Draft Climate Change Strategy (Report). Commission report for CPU Inc.
Permanent Research Commons link: https://hdl.handle.net/10289/9662
Considering the sea level projections adopted by the DCCS, the key points are: a) Due to vertical land movements, the magnitude of relative sea level changes around the coast of the Wellington region varies significantly, and at centennial scales the effects of a major earthquake and the cumulative effect of slow-slip events are likely to dominate over the effects of global absolute sea level changes. b) It is evident that historic absolute sea level changes observed at Wellington do not agree with estimated historic global absolute sea level changes, or with CMIP5 projections for the available period of overlap this Century. Therefore, it is unlikely that projections of future global absolute sea levels provide a useful estimate of future sea levels in Wellington. c) Relative sea level changes at Wellington are not tracking either the CMIP5 projections for absolute sea level rise, or the MfE guidelines for planning purposes. 1. This is predominantly due to the lack of any statistically significant acceleration, which is an underlying assumption in both the projections and the guidelines. 2. Further, the CMIP5 models do not account for regional-scale variability in the processes driving sea level changes. It is clear that major ocean sub-basins experience different sea level changes at different times, which do not accord with the global average modelled by the sea level projections. Within the sub-basins there are also significant variations. This variability indicates that an anthropic sea level signal is unlike to be detectable at Wellington this Century. 3. Finally, it is clear that the relative contributions of the different components of sea level rise have been changing over the last few decades, which means the processes driving sea level changes are different to those assumed by the projections. d) As identified by the NIWA report on sea level trends and variability for the Wellington, it is unlikely that sea level rise will abruptly accelerate to the rates required to achieve the MfE guidelines. Therefore, the MfE guidelines are an over-estimate of potential sea level rise over the next century, and the values specified should be considered very unlikely. Considering the climate extreme projections adopted by the DCCS, the key points are: a) The projected changes in the frequency and magnitude of extreme wind and precipitation events are smaller than the natural variability of these events at any specific location or between locations within the Wellington region. This is due to the effects of local topography and the scale of the systems associated with extreme events. The projected changes are very unlikely to be detectable during this Century. b) The climate for the Wellington Region is strongly influenced by sea surface temperatures, the local topography, and a range of climate oscillations including ENSO, SAM and PDO. None of these is adequately incorporated into CMIP5 projections (and even less so in earlier projections). It is very unlikely that projections of global mean surface air temperature will provide any useful estimates of future climate for specific locations in the Wellington Region. c) The MfE guidelines utilise downscaled climate projections produced for the IPCC TAR. Apart from being more than a decade out of date, the methodology used was identified by the IPCC TAR as being flawed. The downscaled projections are also provided as a regional “average”, which is very unlikely to provide any useful estimate of future climate for any specific location. d) There appears to be fundamental disagreement over the relative influence of key climate oscillations on the climate of the Wellington Region, particularly the relative affects of ENSO, SAM and PDO on extreme events. Without a better understanding of the influence of these on the present climate of the Wellington Region, it is difficult to accept any projections based on assumed changes to their behaviour in the future. e) There is strong evidence that the CMIP5 models have over-projected future temperature changes, although there is on-going disagreement as to why this has occurred. The same problems are also evident to the earlier models used to produce the MfE guidelines. Until the discrepancies between the out-of-sample observations and model projections are resolved it would be imprudent to rely on model projections for planning purposes.
This was a report commissioned by CRU Inc to review the science underpinning the Greater Wellington Regional Council Draft Climate Change Strategy. © 2015 copyright by the author.