Overview…
Sea level has been constantly changing throughout Earth’s history, but basically it goes like this…when things are hot, sea level is higher, when things are cold, sea level is lower. Changes in global temperature alter two things on Earth related to sea level:
1. How warm the oceans are – when you heat water it takes up more space and when you cool it, it takes up less. This is called thermal expansion. As the planet warms and the oceans absorb more heat, they literally take up more space and raise sea level.
2. How much ice melts – increasing temperatures will melt more ice and snow. When meltwater makes its way into the ocean it raises sea level. The majority of this ice and snow is found in the Polar Regions as ice sheets.
There are three major ice sheets:
1. Greenland – Potential 7.4 m sea level rise, current contribution 0.59 mm/yr.
2. West Antarctica – Potential 4.8 m sea level rise, current contribution 0.33 mm/yr.
3. East Antarctica – Potential 53 m sea level rise, current contribution -0.04 mm/yr.
The current total rate of sea level rise from all sources is between 2.8 and 3.6 mm/yr. This total is a combination of the discussed thermal expansion and ice sheets but also includes contributions from alpine glaciers around the world – the majority of which are retreating.
Currently thermal expansion and alpine glaciers are the main contributors to sea level rise, but as the polar regions warm, it is expected that the ice sheets will play a much greater role.
Due to increasing amounts of heat trapping carbon dioxide in the atmosphere we are warming the planet. But we have a choice how much warming we commit to.
With no reduction in emissions the rate of sea level rise in 2100 is expected to more than triple to between 8.0 and 16 mm/yr.
The situation is simple, the more warming we commit to, the more the ocean will expand, the more meltwater will reach the oceans, and the more sea levels will rise.
Current projections place sea level rise by 2100 between 0.28 and 0.98 m. The evidence suggests we can reduce this upper limit if we begin to cut carbon emissions as soon as possible.
Paris can be the start of global action to reduce carbon emissions and hence how fast sea levels will rise.
More detail…
Projections from process-based models of global mean sea level rise relative to 1986–2005.
How much sea level rises is entirely our choice. The RCP numbers stand for Representative Concentration Pathways – in other words how much greenhouse gases we emit this century.
The numbers 2.6, 4.5, 6.0 and 8.5 refer to the range of energy the Earth will receive by 2100. The amount of energy depends on how much more greenhouse gas is released.
The RCPs are consistent with a wide range of possible changes in future human greenhouse gas emissions.
RCP 2.6 assumes that global annual greenhouse gas emissions (measured in CO2-equivalents) peak between 2010-2020, with emissions declining substantially thereafter.
Emissions in RCP 4.5 peak around 2040, then decline. In RCP 6.0, emissions peak around 2080, and then decline.
In RCP 8.5, emissions continue to rise throughout the 21st century.
