ARE SEA LEVELS RISING?

SEA LEVEL RISE

Sea level rise is a contentious topic but we have gone through the most up to date scientific literature to provide you with an accurate depiction of the state of our oceans. The first step in addressing sea level rise is understanding the mechanisms which drive it.

CURRENT SEA LEVEL RISE TRENDS

Global mean sea level (GMSL) has increased by between 21-24 cm since 1880, and has increase by 9.38cm since 1993 (NASA [1], 2019). Sea level rise of 3.2mm/year is an often quoted amount of sea level rise occurring each year but this is just the average rate which took place between 1997 to 2012. Prior to 1997, between 1993 and 1997 the rate of sea level rise was equal to 1.5mm/year -54% lower than than this average. Post 2012, between 2013-2017 the rate of sea level rise was equal to 5mm/yr, which is 56% higher than this average. The acceleration of the rate of sea level rise is not linear but exponential and this pattern of growth is unlikely to slow in the foreseeable future.

HISTORIC SEA LEVELS TRENDS

Global sea levels have always fluctuated, however they have been uncharacteristically stable over the last 8,000 years, covering most of recorded human history. The current sea level is relatively high compared to the last 5 million years as seen in the sea level chart of the last 800,000 years. The substantial sea level fluctuations which are in the order of 100m are associated with glacial (ice age) cycles. The sea level observed during the most recent glacial maximum which occurred around 20,000 years ago was roughly 120m below todays level.

The driver for the glacial cycles and the associated sea level rise fluctuations is the Milankovitch cycle which describes episodic variations in the earth’s orbit around the sun, with the cycle occurring roughly every 100,000 years. These variations in orbit have a significant impact on the amount of solar energy absorbed by the earth’s atmosphere and consequently on global temperatures. Glacial periods occur during phases of the Milankovitch cycle in which the earth is exposed to lower amounts of solar radiation, and conversely, interglacial periods occur during phases when the earth is exposed to higher amounts of solar radiation. We are currently poised to enter into another glacial period however due to human induced climate change this is very unlikely to occur in the foreseeable future.

MELTING OF THE ANTARCTIC AND GREENLAND ICE SHEETS

Increases in global temperatures are causing the Antarctic and Greenland ice sheets to melt, the rate of which is accelerating as the global temperature rise. There are complex physics principles surrounding the melt of ice sheets which makes it very hard to accurately predict ice sheet melting rates. A number of positive feedback mechanisms occur within the melting process which result in accelerating melting rates.

One such feedback mechanism is the reduced albedo associated with ice sheet melt. Albedo refers to the amount of solar radiation that is reflected away from the earth’s surface. The white surface of ice sheets make them great at reflecting solar radiation and hence are associated with a high albedo. This stops a large amount of solar radiation being absorbed by the earth’s surface. As the ice sheet melts its size is reduced resulting in more sunlight hitting the water and land once covered by the ice sheet. These exposed surfaces have a much lower albedo than an ice sheet, resulting in more solar radiation being absorbed by the earth and an increase in the rate of global warming.

Glacial lubrication is another positive feedback mechanism which is associated with ice sheet melt. Glacial lubrication refers to the reduction in friction between the ice sheet and the land underneath as a result of melt water flowing underneath. The higher the melt rate the greater the volume of water flowing under the glacier which results in the glacier moving at a faster rate towards the sea.

MELTING OF GLACIERS

Glaciers naturally melt slightly each summer with winter snow fall usually sufficient to replace this melt. A reduction in snowfall attributed to changing weather patterns and warming will result in a net negative loss of volume from the glacier into the oceans.

THERMAL EXPANSION

As the oceans increase in temperature, the water molecules expand due to a phenomenon known as thermal expansion. This leads to an increase in the volume of the world’s oceans as the warmer water molecules take up more space than they did previously.

WHAT IS THE FUTURE FOR SEA LEVEL?

Carbon dioxide (CO₂) levels within the atmosphere have been fluctuating between roughly 200ppm and 300ppm for the last 400,000 years. The current atmospheric CO₂ level is above 400 parts per million (ppm), a level which has not been exceeded for the last 3 million years. The last time CO₂ was at this level was during the Pliocene Epoch between 2.6 to 5 million years ago when sea levels are estimated to have been around 15m higher than they are today.

The ocean currently absorbs around 40% of global carbon dioxide emissions each year. However as the ocean becomes warmer its capacity to absorb CO₂ decreases and the rate at which it is absorbed by the atmosphere increases.

The correlation between temperature, sea level and CO₂ is significant as seen in the graph of these variables over the last 400,000 years. The peaks and troughs are associated with interglacial and glacial periods respectively. The current CO₂ levels and predicted future temperatures are well outside of the upper ranges of the last 400,000 years. Based on this relationship it is reasonable to assume that sea levels will also exceed these upper bounds some time in the future.

The current CO₂ levels in our atmosphere are unprecedented at 400ppm and the rates of warming which are forecast are breaking new territory. The uncertainty surrounding the future of greenhouse gas emissions and the complex physics surrounding the melt of ice sheets means that it is very hard to predict future sea level rise accurately. To allow for this uncertainty global climate models are run with different scenarios which aim to cover the widely accepted range of possible future emissions and ice melt scenarios. There are various emission scenarios which cover anything from a swift reduction in global fossil fuel use, to scenarios where their use grows unabated. Similarly there are various ice sheet scenarios comprised of different ice sheet melt rates for Antarctica and Greenland.

The most recent and notable sea level rise projections were published by the National Oceanic and Atmospheric Administration of America (NOAA) in 2017. These projections build on previous work published by the Intergovernmental Panel on Climate Change (IPCC) in 2013. The 2017 sea level rise projections are broken down into various scenarios (low - extreme) which reflect the relative amount of sea level rise associated with each scenario. Understanding the uncertainty in future sea levels allows property owners to accurately asses their exposure to risk and allows sufficient time to plan how best to mange their asset.

For information on climate change, global warming and CO₂ emissions head to our Climate Change Information Blog Post.