Where we are headed: sea level

Back in late 2017, I wrote an item about sea level rise and how climate change deniers lie about it¹. At the time, the mean sea level rise for the interval between 1993 and 2015 was 3.3 millimetres (mm) per annum². The data used, as noted, only went up to 2015. However, since then, the rate of sea level rise has increased, and it seems that rate is itself increasing. The average sea level rise in the interval between 2013 and 2022 was 4.62 mm per annum. This dramatic increase has been attributed to extreme glacier melts and record ocean heat levels³. In the years from 1993-2018, it was estimated that 42% of sea level rise was from ocean thermal expansion, 21% from melting of glaciers, 15% from melting of the Greenland ice sheet and 8% from melting of the Antarctic ice sheet⁴.

Complete melting of the Greenland and Antarctic ice sheets would cause sea level to rise about 65 metres. While that will not happen in the foreseeable future, the rate of melting is increasing. Between 1990 and 2020, the rate of ice loss from ice sheets increased from about 105 billion tonnes to about 372 billion tonnes per annum⁵.

Sea level rise is not uniform across the ocean. It is uneven because of ocean dynamics and the Earth’s uneven gravity field. Ocean dynamics refers to ocean currents driven by wind, uneven heating of the water column, as well as uneven evaporation and rainfall⁶. The Earth’s gravity field varies because the mass and density of the planet varies from place to place. One reason that variations in the Earth’s gravity is measured is because gravity plays a major role in determining mean sea level. In general, in areas of the planet where gravitational forces are stronger, the mean sea level will be higher. In areas where the Earth’s gravitational forces are weaker, the mean sea level will be lower⁷. For instance, as noted above, between 1993 and 2018 average sea level rose about 3.3 mm per annum. However, in some regions it rose almost 5 times that (i.e. up to 15 mm), while it actually went down in other areas. Most of this variation is cause by ocean dynamics⁷.

A 2010 study linking ice sheet loss and sea level rise forecast that the hardest hit areas for sea level rise would be coastal regions between 20°N and 40°S across the western Pacific and eastern Indian oceans. This includes coastal populations in places such as Thailand, Cambodia, Malaysia, Indonesia, the Philippines, Australia, New Zealand, Kiribati, Papua New Guinea, Tuvalu, and Fiji, among others⁸.

While it would be comforting to suggest that if we decrease greenhouse gas emissions the ice sheets might stop melting, this is not the case. A recent study has found that irreversible loss of the West Antarctic and Greenland ice sheets, and a corresponding rapid acceleration of sea-level rise, may be imminent if global temperature change cannot be stabilized below 1.8°C, compared to preindustrial levels⁹.

The melting of floating ice-shelves in the Amundsen Sea is currently the main process controlling Antarctica’s contribution to sea-level rise. A recent study has found that rapid ocean warming, at approximately triple the historical rate, is probably inevitable over the twenty-first century, with widespread increases in ice-shelf melting, including in regions crucial for the stability of the ice-sheet located behind the ice shelf and buttressed by it. These results suggest that mitigation of greenhouse gases now has limited power to prevent ocean warming that could lead to the collapse of the West Antarctic Ice Sheet¹⁰.

The implications of this inevitable ice-shelf melting are dire. The rate at which this will occur is unclear, but all that means is that many coastal towns and cities may have to be abandoned, perhaps starting in the middle of this century. Nearly a billion people live in coastal cities¹¹. Melting of the west Antarctica ice shelf would push up sea level by 5 metres if it is lost completely. It seems it is doomed to collapse completely over the course of centuries. However, many cities will be in deep trouble well before that mark is reached, and will likely feel the effects in the next 30 years.

Sources

1. https://blotreport.com/2017/10/30/deniers-sea-level/
2. https://climatedataguide.ucar.edu/climate-data/global-mean-sea-level-topex-jason-altimetry
3. https://www.reuters.com/business/environment/pace-rise-global-sea-level-has-doubled-un-climate-report-2023-04-21/
4. https://essd.copernicus.org/articles/10/1551/2018/
5. https://nsidc.org/learn/ask-scientist/where-will-sea-level-rise-most-ice-sheet-melt#
6. https://sealevel.nasa.gov/faq/9/are-sea-levels-rising-the-same-all-over-the-world-as-if-were-filling-a-giant-bathtub/
7. https://oceanservice.noaa.gov/education/tutorial_geodesy/geo07_gravity.html#
8. https://sealevel.nasa.gov/faq/17/which-areas-of-the-world-will-be-most-affected-by-sea-level-rise-over-the-next-century-and-after-that/
9. https://www.hawaii.edu/news/2023/02/14/irreversible-loss-of-ice-sheets/
10. https://www.nature.com/articles/s41558-023-01818-x
11. https://www.theguardian.com/environment/2023/oct/23/rapid-ice-melt-in-west-antarctica-now-inevitable-research-shows