Sea-level rise is not going to impact all of us equally. It’s fine to talk about sea-level rise globally, but what does it mean for you, in your community?
While global sea-level rise is the average amount that water levels are rising in all of the oceans on the planet, relative sea-level change is the specific amount of water level change that you are experiencing in your location - not a global average. Thermal expansion and melting land ice contribute to rising seas globally, but at the local level, other factors result in sea-level rise amounts that are often different from the global average. Depending on your local conditions, the rate of sea-level change in your location can be substantially different from the rate of sea-level change globally and in other locations. In fact, sea-level in Charlottetown is rising almost two times the rate of global sea-level rise each year and relative sea-level projections for Halifax for 2010-2100 (relative to 1986-2005) are 28% larger than global values for the same time interval.
So, when looking at how sea-level rise is going to impact YOUR community, focus on the local projections (relative sea-level change), not the global sea-level rise projections. You can see relative sea-level projections for small craft harbor communities across Canada with the Canadian Extreme Water Level Adaptation Tool (CAN-EWLAT).
Just as there are many factors that cause local weather to vary from place to place, there are also many factors that influence local sea level. In Canada, the biggest culprit of local differences in sea level is vertical land motion, followed by phenomena called sea-level fingerprinting, and regional oceanographic variability.
Vertical land motion is just what it says: the motion of land vertically. It might not seem like it when you stand on the ground outside, but on a Canada-wide scale, our land is flexible and has the ability to move up and down. During the last ice age, more than 15 000 years ago, Canada was covered in a massive glacier that was so heavy it gradually caused land in the center of the country to sink and land along the edges to rise (think of this as a bulge at the edge of the glacier). Picture a flexible ruler placed between two tables. As you put pressure on the middle, the center sinks and the edges rise, but as you remove the pressure, the center rises back up, the edges sink, and the ruler rebounds to its original position. This is currently happening in Canada except that it takes land a lot longer to rebound than a ruler. Once the glacier melted, the center of Canada began rising and the edges of Canada began sinking. This is called Global Isostatic Adjustment or GIA and is still happening today! Across Canada, places in Quebec are rising by 4 mm/year and the East Coast is sinking (or subsiding), with Nova Scotia sinking at a rate of 2 mm/year or approximately 20 cm/century.
Global isostatic adjustmentWhat does this mean for sea-level rise? Well, not only are sea levels rising in Atlantic Canada, but the land is also sinking. This not only means that sea levels in this region are rising more than the global average but also that the largest amounts of projected relative sea-level rise in Canada are going to occur in the Atlantic Provinces. In Halifax, approximately half of the experienced sea-level rise is due to global mean sea level and the other half is due to land subsidence.
Sea-level fingerprinting is a newer concept that explains how sea levels react when ice melting on land enters the sea. When you add more tap water to a glass, the level of your drink will rise, but the surface of your drink remains flat, meaning that the added water was distributed evenly throughout your glass. This is not the case with melting ice sheets and the ocean. When land ice melts and enters the ocean the added water does not get distributed evenly throughout the world’s oceans. This is because of the gravitational attraction between ice and water, which causes sea levels close to the melting source to fall and sea levels far away from the melting source to rise. So, as the Greenland Ice Sheet melts, places like the East Coast of Canada experience a reduction in sea-level rise and places like Brazil experience an increase in sea level rise. However, if the Antarctic Ice Sheet melts, the East Coast region of Canada will experience an increase in sea-level.
Regional oceanographic variability can be explained as the changes in sea level associated with changes of sea-water properties (i.e., fresher vs. saltier, warmer vs. colder) and movements of a body of sea water from one part of the ocean to another. Some of these include El Niño which can raise sea levels by tens of centimeters, ocean currents that can cause changes in sea levels of more than 1 m, and a weakening Gulf Stream which is projected to contribute 10-20 cm of sea-level rise by 2100. In the North Atlantic, near the end of the 20th century, the largest sea-level rise was seen north of the Gulf Stream due to the weakening of an ocean circulation called the Atlantic Meridional Overturning Circulation.
Everybody in Atlantic Canada has experienced an impact of sea-level rise or at the very least, knows someone who has. We are a region of coastal communities and the impacts of sea-level rise are already being felt. Inundation, erosion, flooding from storm surges, and salt water intrusion are the most well known impacts of sea-level rise and can be very damaging to coastal infrastructure. By the year 2020, it is estimated that the economic damages from sea-level rise in Canada will be around $1.6-5.4 billion EACH YEAR and by 2080, this amount could increase to $48.1 billion annually.
Inundation is the most direct effect of sea-level rise and is the permanent submergence of land due to a rise of the water levels in the ocean; creating a new, normal sea level. Simply explained, when you add water to a glass, the water level becomes higher and parts of the glass that weren’t under water before are now submerged. This is the new, normal level of water in your glass - that is of course, until you drink it. This becomes a bit more complex when we apply this analogy to the coast because the elevation and features along our coasts are so varied.
If you live on top of a 3 m cliff, a new normal water level of 2 m above the current levels will not submerge your property, even though you are right next to the ocean. However, if you live on low-lying land that has an elevation less than 1 m, a 2 m new normal water level is going to cause a lot of damage, even if you are 5 km from the edge of the ocean.
A new normal water level may not inundate your home, but it does bring high tides and storm surges closer to your property.
We often hear the term “1 in 100 year” storms or “1 in 100 year” floods, but what does this mean? The saying “1 in 100 year” means that every year there is a 1 in 100 chance of something happening. This is an estimate of the likelihood that an event will occur and is also called a recurrence interval or return period. It doesn’t mean that the event will happen once every 100 years; a “1 in 100 year” event can happen two years in a row. You could also look at a “1 in 50 year” event, or a “1 in 20 year” event, but scientists use a “1 in 100” year event because something that only has a 1 in 100 chance of happening every year is considered to be very impactful and damaging.
In Halifax, for example, an extreme water level event that currently has a recurrence interval of 50 years (1 in 50 chance of happening each year), will have a recurrence level of less than 2 years (1 in 2 chance of happening each year) by the year 2050 with a sea-level rise of 40 cm.
Saltwater intrusion is not as well known as coastal flooding and storm surges because it stealthily occurs when salt water seeps into the freshwater underground. This can impact our drinking water, forests, agricultural lands, and freshwater plant and animal species. Saltwater intrusion is caused by:
An increase in the demand of fresh groundwater drops the level of fresh water within the ground, allowing space for salt water to seep in. Unlike groundwater, there is no shortage of seawater. Secondly, an increase in sea levels causes displacement of ground water as the salt water moves further and further inland.
Coastal flooding and storm surge are two effects that we are very familiar with in Atlantic Canada and are most often caused by tropical storms and hurricanes (tropical cyclones) and nor’easters (extra-tropical cyclones) in this region. In Atlantic Canada the increase in sea level will impact more and more people along the coast.
Hurricane Juanresulted in$100 millionin damagesin PEI and Nova ScotiaHurricane Igorcaused up to$200 millionin damagesin NewfoundlandWhere inundation is the permanent submergence of land by seawater, coastal flooding is the temporary submergence of land by seawater and is often a result of storm surge. This is not the same as overland flooding which usually happens when rivers, lakes, and other freshwater overflow onto dry land.
Storm surge is a temporary increase in the height of the ocean from high winds and/or meteorological conditions, such as low atmospheric pressure, and is the difference between the water level observed during a storm and the level that the tide would normally be at if no storm was occurring. In strong storms with high winds, storm surge can cause a substantial amount of temporary rise in sea level. In 2003, Hurricane Juan caused a 1.63 m storm surge in Halifax and in Charlottetown the largest recorded storm surge, of 1.43 m, occurred in 1963. On very low-lying coasts, a small storm surge can cause water to reach quite far inland. An extreme example of this occurred in the Mackenzie Delta where a 1999 storm surge resulted in flooding that extended 30 km inland from the coast.
If we go back to the example of the house on top of a 3 m cliff in this situation, we can see that the house would probably have been safe with a 1.63 m storm surge on top of current water levels, however if the new normal water levels are 2 m above current levels, a 1.63 m storm surge amount would have substantial impacts on the property.
Storm surge is not connected to high tide, but if it occurs at the same time as a high tide, the water levels experienced during a storm can be much greater. Add seasonal variability of oceans and sea-level rise to the mix and you could see extreme water levels that can cause substantial damage to the coast. In January 2000, a major storm in Southeastern Newfoundland produced extreme water levels that caused damage 18 m above the average sea level.
Coastal erosion is a natural process and is the removal of rock and sediments by currents and wave action. In Atlantic Canada erosion rates across the region are expected to increase in most areas, but vary from province to province and between landscapes. However, of all of the Atlantic Provinces, Prince Edward Island is experiencing the highest rates, with an average of 28 cm of erosion per year. In some extreme cases, places in Atlantic Canada have experienced erosion rates of 10-15 m/year.
At current sea level, over the next 90 years,
1,000 residential homes, 8 barns, 7 gazebos, 42 garages and over 50km of roads are at risk from coastal erosion in PEI.
A rise in sea level doesn’t cause erosion directly but it does create deeper water levels along the coast, which allow for stronger waves with more eroding ability to reach the shore. It also brings these waves further inland to places where waves have not normally been able to reach in the past.
The rate of coastal erosion depends on many factors including the exposure of the coast to ongoing wind and waves, the strength of those waves, and the type of coastal landform. Landforms like beaches, dunes, and soft cliffs erode easily whereas rocky coasts and hard bedrock cliffs do not erode as easily, and in some cases, not at all. In New Brunswick, beaches, dunes, and salt marshes erode an average of 0.8 m/year and cliffs erode an average of 0.26 m/year. The main difference between the beach/dune erosion and cliff erosion is that when a cliff erodes, it loses the sediment forever, whereas when a beach/dune erode, the sediment can be stored offshore and often returns during calmer seas.
When erosion is happening in your community or on your property, it is obviously distressing, but what is not often understood about erosion is that it is a natural process and has been happening long before climate change and sea-level rise became issues. Erosion plays a very important role in supplying sand to other parts of the coastal ecosystem like beaches and dunes and erosion on some parts of the coast allows for other parts of the coast to replenish their lost sediment or build themselves up to respond to rising seas.