Saturday, July 23, 2011

Sea-level rises are slowing, tidal gauge records show

From: The Australian

NSW Government advisor Phil Watson

Author of the NSW government's sea levels report, Phil Watson, at Terrigal beach on the NSW central coast yesterday. Picture: Dan Himbrechts Source: The Australian

ONE of Australia's foremost experts on the relationship between climate change and sea levels has written a peer-reviewed paper concluding that rises in sea levels are "decelerating".

The analysis, by NSW principal coastal specialist Phil Watson, calls into question one of the key criteria for large-scale inundation around the Australian coast by 2100 -- the assumption of an accelerating rise in sea levels because of climate change.

Based on century-long tide gauge records at Fremantle, Western Australia (from 1897 to present), Auckland Harbour in New Zealand (1903 to present), Fort Denison in Sydney Harbour (1914 to present) and Pilot Station at Newcastle (1925 to present), the analysis finds there was a "consistent trend of weak deceleration" from 1940 to 2000.

Mr Watson's findings, published in the Journal of Coastal Research this year and now attracting broader attention, supports a similar analysis of long-term tide gauges in the US earlier this year. Both raise questions about the CSIRO's sea-level predictions.

Climate change researcher Howard Brady, at Macquarie University, said yesterday the recent research meant sea levels rises accepted by the CSIRO were "already dead in the water as having no sound basis in probability".

"In all cases, it is clear that sea-level rise, although occurring, has been decelerating for at least the last half of the 20th century, and so the present trend would only produce sea level rise of around 15cm for the 21st century."

Dr Brady said the divergence between the sea-level trends from models and sea-level trends from the tide gauge records was now so great "it is clear there is a serious problem with the models".

"In a nutshell, this factual information means the high sea-level rises used as precautionary guidelines by the CSIRO in recent years are in essence ridiculous," he said. During the 20th century, there was a measurable global average rise in mean sea level of about 17cm (plus or minus 5cm).

But scientific projections, led by the Intergovernmental Panel on Climate Change, have suggested climate change will deliver a much greater global tide rise in mean sea level this century of 80-100cm.

The federal government has published a series of inundation maps based on the panel's predictions showing that large areas of Australia's capital cities, southeast Queensland and the NSW central coast will be under water by 2100.

Without acceleration in sea-level rises, the 20th-century trend of 1.7mm a year would produce a rise of about 0.15m by 2100.

Mr Watson's analysis of the four longest continuous Australian and New Zealand records is consistent with the findings of US researchers Robert Dean and James Houston, who analysed monthly averaged records for 57 tide gauges, covering periods of 60 to 156 years.

The US research concluded there was "no evidence to support positive acceleration over the 20th century as suggested by the IPCC, global climate change models and some researchers".

Mr Watson cautioned in his research and again yesterday that studies of a small number of northern hemisphere records spanning two or three centuries had found a small acceleration in sea-level rises. He said it was possible the rises could be subject to "climate-induced impacts projected to occur over this century".

Mr Watson's research finds that in the 1990s, when sea levels were attracting international attention, although the decadal rates of ocean rise were high, "they are not remarkable or unusual in the context of the historical record at each site over the 20th century".

"What we are seeing in all of the records is there are relatively high rates of sea-level rise evident post-1990, but those sorts of rates of rise have been witnessed at other times in the historical record," he said.

"What remains unknown is whether or not these rates are going to persist into the future and indeed increase."

He said further research was required, "to rationalise the difference between the acceleration trend evident in the global sea level time-series reconstructions (models) and the relatively consistent deceleration trend evident in the long-term Australasian tide gauge records".

With an estimated 710,000 Australian homes within 3km and below 6m elevation of the coast, accurate sea-level predictions are vital for planning in coastal areas anticipating predicted sea-level rises of almost a metre by 2100.