OPINION: Are sea levels rising? Victoria University's Prof James Renwick responds to last weekend's post by climate change skeptics Bryan Leyland and Victoria University lecturer Willem de Lange — which attracted a record number of comments.
(This op-ed was submitted in response to Evidence doesn't support rapid future sea level rise by Waikato University senior lecturer Willem de Lange and Bryan Leyland - Editor)
The last time atmospheric carbon dioxide (CO2) levels were as high as they are now, about 400 parts per million (ppm), was roughly three million years ago.
At that time, sea levels were 10-20 metres higher than they are at present. The geological record also suggests that the Antarctic ice shelves and large parts of its ice sheets have waxed and waned considerably over time and that 400ppm CO2 may be an upper limit for the existence of the Ross Ice Shelf and much of the West Antarctic ice sheet. No climate models are needed to infer this information because it is written into the surface of the earth itself.
The recent Royal Society of New Zealand report on the implications of climate change and the earlier report on sea level rise referred to by authors Bryan Leyland and Willem de Lange are based on the scientific findings summarised in the Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC) and reflect the weight of evidence in the peer-reviewed scientific literature.
Those findings, endorsed by science academies worldwide, show that sea level rise of at least another 30cm this century is virtually certain and that the rise in sea level by 2100 may be up to one metre or more. This range represents the uncertainty in predicting the rate of the rise, associated largely with uncertainties in future emissions of greenhouse gases.
Messrs Leyland and de Lange refer to only a small sub-set of the available evidence considered by the IPCC, this subset being those at the bottom end of the full spectrum of studies. Through the cumulative scientific literature, it is well-established that global average sea levels rose at around 1.7mm/year during the 20th century and are presently rising at around 3.3mm/year, based on a combination of tide-gauge records and satellite measurements.
Where satellite and tide-gauge measurements overlap (approximately the past 20 years), the agreement on the rate of sea level rise is excellent. There is no unexplained discrepancy. Based on the known physics of ice melt and thermal expansion of ocean water, the rate of sea level rise is bound to keep increasing as greenhouse gas concentrations rise.
With regard to the disappearing islands in the Solomon Islands, the authors of the study upon which the news report was based went to great lengths to identify the exact causes of the loss of land. They demonstrated conclusively that sea level rise was the main cause and not land subsidence.
When the best evidence of total eventual sea level rise is so high (over 10m with CO2 continuing above 400 ppm), debates about the present rate of rise, while perhaps important to the short-term confidence of coastal property owners, distract from our single greatest responsibility to future generations.
Professor James Renwick is the chairman of the Royal Society of New Zealand’s Climate Change Implications for New Zealand panel and professor in the School of Geography, Environment and Earth Sciences, Victoria University of Wellington
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