You may have heard of the issue of sea-level rise, but you may not be clear about what is causing it, how much sea level is actually rising, and how sea-level rise is related to climate change. Find out more about the basics of global sea-level rise so you can speak confidently about it the next time the topic comes up at a planning meeting or with your friends. Better yet, start a conversation about it yourself!
When you hear the term sea-level rise it most often refers to the amount that seas are rising around the world. This is also known as Global Mean Sea-level Rise (GMSLR) and is the average amount that water levels are rising in all of the oceans on the planet, even though some areas are rising more than others. This is similar to saying that the average temperature on Earth is 16°C, even though we know some places are much colder and some are much warmer.
Want to learn more about the specifics behind measuring sea-level rise? Check out this great video from Minute Physics.
What is the first thing that comes to mind when you think about what is causing of sea-level rise? Probably melting ice – right? Melting land ice (glaciers, ice caps, ice sheets) is in fact one of the main causes of sea-level rise, but another less heard of cause is thermal expansion, which contributes even more to sea-level rise than melting land ice! Both however, are caused by an increase in Earth’s temperature.
Just as water takes up more space in a pot when it’s boiling than when it's first put on the stove as cold water, the oceans increase in volume as they heat up. With every degree Celsius of temperature increase, the ocean rises 0.2-0.6 m. This is called thermal expansion and means that the same amount of water in the ocean is taking up more space because it’s warmer. Now consider that the Earth’s ocean is taking in more than 90% of the heat from global warming and that thermal expansion doesn’t even include the additional water from melting land ice. Let that sink in!
Thermal ExpansionWarmer air and ocean temperatures also cause melting of land and sea ice. While melting sea ice, such as icebergs, does have an impact on the environment (think of the iconic image of polar bears losing their habitat), it doesn’t have a big impact on sea levels. This is because the floating ice more or less already displaces the same amount of water that is added to the ocean when it melts. Have you ever noticed this in a glass of water with ice? As the ice cubes melt, the level of water in your glass stays more or less the same. Melting land ice (mountain ice, ice caps, ice sheets) is what substantially contributes to a rise in sea level.
Together, thermal expansion and land ice melt (excluding the Greenland Ice Sheet and Antarctic Ice Sheet) account for 75% of sea-level rise globally.
One of the most recognized impacts of climate change is sea-level rise, which has the potential to cause considerable damage within coastal communities. Greenhouse gas (GHG) emissions, a well-known culprit of climate change, lead to rising air temperatures. As you learned in the previous section, a warmer Earth causes thermal expansion and melting of land ice, which in turn cause sea levels to rise.
Sea levels don’t respond instantly to warming. As oceans absorb heat there is a waiting period, or a lag-time, between when heat is absorbed and when the impact of that heat increase takes full effect. Think about the best month to go swimming in the ocean in Atlantic Canada - late August and early September are probably coming to mind right? This is because it takes the ocean all summer to respond to the warmer temperatures that we experience in June, July, and August. Or think about taking an ice cube out of the freezer. It doesn’t melt instantly, but it does melt eventually. These are both examples of lag-times and help explain how sea levels respond to global warming from greenhouse gases.
When scientists use the term locked-in, they are referring to the fact that once greenhouse gases enter the atmosphere we are locked in or committed to the impact of their warming on the Earth. This means that even if we cut all greenhouse gas emissions today we have already committed ourselves to a certain amount of continued warming and continued sea-level rise in the future.
Under the Paris Agreement, solidified in 2015 under the United Nations Framework Convention on Climate Change (UNFCCC), 197 countries have committed to enhanced action to address climate change including reducing GHG emissions and adapting to unavoidable climate impacts. The main goal of the agreement is to work together globally to keep global temperature rise well below 2°C (above pre-industrial levels) by the end of 2100, and secondly, to make an effort to keep the temperature increase lower than 1.5°C in the same time period. The Paris Agreement entered into force on November 4th, 2016, thirty days after the date on which at least 55 Parties to the Convention, accounting for at least 55% of total greenhouse gas emissions, deposited their instruments of ratification. On October 5th, 2016 Canada agreed to the ratification of the Paris Agreement and was key in surpassing the threshold needed for the Paris Agreement to be entered into force.
Scientists know what causes sea-level rise and have a really good idea of how that translates to rising seas. What is uncertain is how we, as a population, are going to BEHAVE. The amount of sea-level rise and the state of our climate really depends on our behaviour.
This is why scientists have different climate change projections ranging from low emission to high emission paths (also known as RCPs or Representative Concentration Pathways). Under a scenario with speedy, ambitious GHG mitigation (like RCP2.6), we can limit Global Sea-level Rise to 1 m during the next 500 years. With very minimal effort in mitigation (like RCP8.5), we could see a 1-3 m increase in global sea-level rise by 2300. Which path will we follow? The choice is ultimately up to you.
And while ambitious GHG mitigation may seem costly, consider that:
Less investment in adaptation
and damage to coastal infrastructure
The newest climate change scenarios are called Representative Concentration Pathways (RCPs) and are based on different pathways of GHG emissions. There are four pathways - RCP 2.6, RCP 4.5, RCP 6, and RCP 8.5 - which range from a low emission scenario (RCP 2.6) to a high emission scenario (RCP 8.5).
The larger the amount of GHG emissions we emit, the more likely we are to be on the high emission pathway. In other words, how we behave influences which path we follow.
Think of climate change projections like the chance of getting a cavity after a visit to the dentist. We ultimately know what leads to us getting a cavity: eating lots of sugar, not flossing, not brushing our teeth – essentially our behaviour.
When we leave the dentist we can follow three scenarios:
In other words, how we behave influences how likely we are to get a cavity.
This is the same when it comes to sea-level rise. We know what contributes to sea-level rise, but how we behave with GHG emissions will determine which path of sea-level rise we will follow: RCP 2.6, 4.5, 6, 8.5, or something even more extreme.
When we talk about sea-level rise, we often talk about 1) rates – how much sea levels are rising per year or 2) a specific amount that sea levels are expected to rise by a certain year. We can also talk about sea-level rise in terms of what has happened in the past and what we expect in the future.
In the past, between 1901-2010, sea levels have risen around the world at a rate of 1.7 mm/year, but between 1993 and 2010, sea levels have risen at a rate of 3.2 mm/year. This shows that today, sea levels are rising almost twice as fast as they did over the past 100 years.
Future sea-level rise rates and amounts are based on RCP scenarios. By the year 2100, global sea levels are expected to rise between 28-98 cm, with potential to reach well over 1 m.
Every 5-7 years, the Intergovernmental Panel on Climate Change (IPCC), produces a set of assessment reports detailing the most recent climate change findings from leading scientists around the world. The most recent set of reports, called Assessment Report 5 (AR5) were published in 2013-2014, and provide the latest, up-to-date information on global climate change impacts.
The IPCC Assessment Report 5, published in 2013/2014 is a result of the collaborative effort of 830 scientists from over 80 countries along with 1,000 contributing authors and 2,000 expert reviewers, assessing more than 30,000 scientific papers. The AR5 is over 4,800 pages long and is the most
comprehensive assessment of climate change ever undertaken.