How Will Sea Level Rise Affect Antigonish?

Sea level is rising on the shores of Antigonish county and is certain to continue. But how much and with what impacts? In this post we try to answer these questions, drawing on global and local sources.

Rates of relative sea level rise due to glacial isostatic adjustment (from The State of Nova Scotia's Coast Report)
Rates of relative sea level rise due to glacial isostatic adjustment (from The State of Nova Scotia’s Coast Report)

Even without climate change, the land is sinking in this area and will fall another 17 cm by 2100 (20 cm/century).  Melting glaciers and polar ice caps add to the volume of the ocean; and as it warms, it expands. The Intergovernmental Panel on Climate Change (IPCC) says these processes will result in a further global average sea level rise of between 28 and 98 cm by the end of this century, depending on our ability to control greenhouse gas emissions. Even the “best case” scenario studied by the IPCC suggests that climate change will result in additional rise of 44 cm, give or take 15 cm.  (See the IPCC projections here). Adding the 17 cm due to sinking land, and allowing for uncertainty, we would expect sea level rise of between 45 cm and 78 cm by 2100.

That is the best case. On the high side it could be over a metre (115 cm). As bad as this sounds, it is not the whole story.  First, it does not take into account the chance that marine-based sectors of the antarctic ice sheet will melt.  These are attached to Antarctica but extend offshore and are supported by the sea bed. The melting of this ice is not well understood by scientists so the IPCC does not include it in any of its scenarios. They do say, however, that if it does occur, there is “medium confidence that this additional contribution would not exceed several tenths of a metre of sea level rise during the 21st century”.  So, roughly, all things considered, in Antigonish we are looking at between ½ and 1½ metres of sea level rise in this century.

Second, these estimates are for the average sea level.  Another concern is what will happen during exceptionally high tides, especially when these occur during the storms that are expected to become more frequent and more extreme as the climate changes.

Third, there is still a lot of uncertainty about the upper limit of this estimated range. James Hansen and colleagues published a paper in July this year which the Washington Post covered under the headline “The world’s most famous climate scientist just outlined an alarming scenario for our planet’s future.” In the paper, an all-star lineup of climate scientists predict that the contribution of melting ice sheets in Greenland and Antarctica will be greater than is generally appreciated and that even if we are able to limit global mean temperature increase to less that 2°C, we could still see “at least several metres” of global mean sea level rise. They argue that because of the “extreme sensitivity of sea level to ocean warming and the devastating economic and humanitarian impacts of a multi-meter sea level rise”, this, rather than the 2°C limit, should determine our allowable planetary greenhouse gas emissions. [UPDATE, March 22, 2016: This paper, published online before peer review has now been published in a peer reviewed version, with little change.]

What physical evidence is there of these processes at work so far in Antigonish and what are the expected future impacts?  Tim Webster and others, in a report released in 2010, described their use of airborne light detection and ranging (LiDAR) technology, tidal gauge data, and analysis of aerial photography from 1971 to 2007 to look at flood and erosion risk due to sea level rise in Antigonish County.

Some of their results include the following.
1. The aerial photography series suggests that although Pomquet beach is growing, other beaches (at Cape Jack and Mahoney’s) are in retreat.
2. Using historical hourly tide gauge data from the nearest location (Pictou), and assuming that the rate of sea level rise due to climate change (50 cm) and land subsidence (20 cm) totals 70 cm, the authors calculate that a 2.27 m storm-related flood level (which occurred on December 30, 1993) “had a 65% probability of occurring within 19 years and a 99% probability of occurring within 45.5 years”. This is based on historical storm surge data so would underestimate actual risk if future storms are more severe or frequent than past storms. Nor does it include the potential “several tenths of a metre” of SLR from the marine antarctic ice sheet.
3. Inundation maps (using the same conservative assumptions) show the effects of the 2.27 m storm event with the addition of 70 cm for expected sea level rise over the next century. To see these maps go to the report.

A report on coastal planning in Antigonish County, written by Amber Nicol in 2006, identifies specific hazards related to sea level rise and climate change. Apart from the effects of a rise in mean sea level, the expected reduction in winter sea ice would reduce protection against erosion during powerful winter storms. Possible effects of such storms include overwash and breaching of barrier beaches.  Because it is growing, Pomquet beach may be less vulnerable to storms than the beaches protecting Antigonish and Tracadie Harbours. Nicol notes that breaches have recently occurred at the entrance to Antigonish Harbour at Mahoney’s beach on Boxing Day in 2004, and on the western side of Tracadie Harbour prior to October 2000.  If protected salt marshes become more open to the ocean, the implications for biodiversity, habitat preservation, and shoreline integrity are potentially serious.

To conclude, there is strong consensus that we should expect the mean sea level to rise between ½ and 1½ metres this century, a consequence of the combined effects of sinking land and rising global mean sea level due to climate change. With more frequent and severe storms, the probable effects include accelerated shoreline erosion, damage to property and infrastructure, and reduced ecosystem resilience. But if some credible projections are accurate, and the actual rise in global sea level is several metres within the same time frame, the unprecedented loss of property and habitat in this part of Nova Scotia may seem trivial in comparison with the catastrophic consequences for many of the largest population centres on the planet.

by Jay Ross

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