This article is part of the 4sea Project.
The global climate has been changing throughout our planet’s history and as a result various ice ages with warmer and colder periods have occurred. The oceans play a significant role in the global climate system and are therefore greatly affected by disturbances and changes in the system. In addition to larger global changes in the sea level during long periods of time, the sea level can also change on a local level in matter of less than a few hours during strong storm surges and tides.
It is therefore normal for the sea level to change occasionally for shorter time periods as well as on a longer time scale, but for the past century, and more precisely for the past few decades, the global rate has been increasing abnormally fast. Scientific evidence suggests that the ongoing increase in the global mean temperature, mainly caused by human induced greenhouse gas emissions, is behind this increase.1 The world oceans have absorbed most of the excess heat in the climate system and have consequently been warming up at an increasing rate. As the water warms up, it expands and takes up more space, which leads to sea-level rise. In addition to the thermal expansion, the warmer temperatures result in the melting of land-based glaciers and ice sheets, whose meltwater ends up in the oceans that contribute to the rise.2
The global mean sea level remained relatively stable for 2000 to 3000 years until the end of the 19th century.3 The average global sea level has then risen by approximately 0.19 meters for the past century with an average annual increase of around 1.7 mm according to the Fifth Assessment report (AR5) of the Intergovernmental Panel on Climate Change (IPCC). For the past few decades the annual rate has been as high as 3.2 mm, and is only expected to increase throughout the 21st century.2 However, the problem is not as simple as that. The complexity lies in the fact that the sea level is not changing uniformly around the world. Depending on many different local factors, sea level rise might be significantly higher or lower in some areas compared to the global average. These differences can depend on regional ocean currents and therefore the temperature of the sea, salinity differences, the shape of the coastal area, withdrawal of fossil fuels or groundwater, erosion as well as subsidence or alternatively post-glacial rebound caused by the compressive force of former glaciers during the last glacial period.4
So why should we worry about sea-level rise? 8 out of the 10 largest cities in the world are located close to the sea and up to 40 % of the global population live within 100 km from the coastline.5 This amount is only expected to rise throughout the 21st century as more people move from rural to more central and urban areas. Human settlement has always gravitated towards coastlines because of the importance of shipping, trading as well as for the usually more moderate climate that occurs closer to the sea. As the global sea level continues to rise whole human societies as well as coastal ecosystems are put under risk. These risks include coastal flooding and submergence through storm surges and stronger tides, coastal erosion as well as saltwater intrusion both on surface waters and groundwater.6 These impacts then have different socio-economic consequences depending on the vulnerability and the resilience of the area affected.
The future prospects are uncertain because of the sheer complexity of the ice-sheet dynamics of the huge Antarctic and Greenland ice sheets. It is certain though that because of the increasing temperature rise in the atmosphere and the positive feedback loop on the ice sheets and glaciers, the sea level will continue to rise.2 Even if we substantially manage to reduce our greenhouse gas emissions, the sea level will still continue to rise throughout the 21st century. Nevertheless, how much the sea level will rise is mainly determined by the amount of the emissions. According to the latest assessment report from the IPCC, the global sea level is projected to rise from everything between 0.26 to 0.98 meters for the period 2081-2100 fully depending on the amount of our greenhouse gas emissions.2 Many scientists believe that the rise could be a lot higher than estimated depending on how the big ice sheets will react, especially the melting West Antarctic ice sheet, which in case of a collapse could contribute to an additional global sea level rise of around 5 meters.7
About the author: Venni Arra is currently completing her bachelor’s degree in physical geography at Stockholm University and is doing her internship at CliMates in Paris for the Ocean and Climate change project 4sea.
This article has been written in the context of 4sea. 4sea, a project about the importance of the world oceans, addresses the interdependence between the oceans and climate change, entraining everyone to become ocean lovers – for now through articles and videos on this blog and in November on our own platform. 4sea is a joint project between the youth organisations CliMates, Youth for Ocean and Vitamin Sea. Love it? Stay tuned for our platform!
NASA, Understanding sea level – Causes, [website], https://sealevel.nasa.gov/understanding-sea-level/causes/overview (accessed 1 July 2017)
Church, J.A. et al., 2013: Sea Level Change. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
IPCC, Climate Change 2007: Working Group I: The Physical Science Basis. FAQ 5.1 Is Sea Level Rising? [website], 2007, https://www.ipcc.ch/publications_and_data/ar4/wg1/en/faq-5-1.html (accessed 6 July 2017)
National Ocean Service, Is sea level rising? [website], 2017, http://oceanservice.noaa.gov/facts/sealevel.html (accessed 6 July 2017)
UN, Percentage of Total Population Living in Coastal Areas, [website] http://www.un.org/esa/sustdev/natlinfo/indicators/methodology_sheets/oceans_seas_coasts/pop_coastal_areas.pdf (accessed 6 July 2017)
National Geographic, Sea Level Rise, [website], http://www.nationalgeographic.com/environment/global-warming/sea-level-rise/ (accessed 6 July 2017)
Modelling West Antarctic ice sheet growth and collapse through the past five million years David Pollard & Robert M. DeConto Nature 458, 329-332(19 March 2009) doi:10.1038/nature07809