Sea Level Rise in Earth’s Temperature Regime: Causes and Regional Impacts

Sea levels are rising faster today than they have in thousands of years. This acceleration poses an urgent threat to coastal communities worldwide, with some regions experiencing disproportionate impacts. Understanding the causes and regional patterns of sea level rise is essential for preparing communities to adapt to these changes.

Historical trends reveal accelerating rise 🔗

According to recent IPCC findings, global mean sea level increased by 0.20 meters (20 centimeters) between 1901 and 2018. The pace has accelerated significantly since the late 1960s. From 1971 to 2018, sea levels rose at an average rate of 2.3 millimeters per year, but this increased to 3.7 millimeters per year during 2006 to 2018.

What makes this trend particularly concerning is the historical context. NOAA reports that global mean sea level has risen approximately 8-9 inches (21-24 centimeters) since 1880, with more than one-third of that rise occurring in just the past 25 years. This rate of increase is much higher than the relatively stable levels observed during the previous two millennia.

Two primary drivers of sea level rise 🔗

Sea level rise results from two main physical processes that respond to increasing global temperatures.

Thermal expansion of seawater 🔗

When ocean water warms, it expands and occupies more volume. This thermal expansion is a major contributor to rising seas. The IPCC assessment indicates that ocean thermal expansion accounted for 38% of the total sea level rise between 1901 and 2018. During the period from 1961 to 2003, thermal expansion contributed approximately 0.42 millimeters per year to sea level rise, which increased to 1.6 millimeters per year during 1993 to 2003.

Under different emissions scenarios, thermal expansion alone could raise sea levels significantly by 2100. Projections suggest increases ranging from 18 to 38 centimeters under low emission scenarios and 26 to 59 centimeters under high emission scenarios.

Melting land ice 🔗

The second major contributor is water added to the oceans from melting glaciers and ice sheets. Between 1901 and 2018, melting glaciers contributed 41% of total sea level rise. The contribution from the Greenland and Antarctic ice sheets has grown dramatically in recent decades.

The IPCC reports that ice sheet contributions to sea level rise were four times larger during 2010-2019 compared to 1992-1999. By the 2006-2018 period, total land ice loss (from both glaciers and ice sheets) became the largest contributor to global mean sea level rise.

Greenland ice sheet changes 🔗

The Greenland ice sheet is experiencing rapid transformation. NASA research shows that the Greenland ice sheet lost approximately one-fifth more ice mass in the past four decades than previously estimated. The majority of the 207 glaciers studied showed significant retreat since 1985.

Between 1992 and 2020, the Greenland ice sheet lost 4,890 gigatons of mass, equivalent to 13.5 millimeters of global mean sea level rise. The mass loss rate increased dramatically from an average of 39 gigatons per year during 1992-1999 to 243 gigatons per year during 2010-2019.

Recent studies reveal that the summer melting area has expanded by 16% over the last 30 years. University of California researchers found that warm ocean water intrusion beneath glaciers is accelerating ice loss at rates faster than previously projected, with some glaciers experiencing melt rate increases from 3 meters per year in the 1990s to 10 meters per year in the 2020s.

Antarctic ice sheet dynamics 🔗

The Antarctic ice sheet, Earth’s largest ice mass, has also lost substantial mass in recent decades. According to IPCC data, Antarctica lost 2,670 gigatons of mass between 1992 and 2020, contributing 7.4 millimeters to global sea level rise. The mass loss rate increased from 49 gigatons per year during 1992-1999 to 148 gigatons per year during 2010-2019.

West Antarctic outlet glaciers have experienced the most dramatic changes, driven primarily by ice shelf basal melt caused by warming ocean waters. Over the past 50 years, the Antarctic ice sheet lost approximately 13,500 square kilometers of outlying ice shelves, though warming in the Southern Hemisphere has been less pronounced than in the Arctic.

Future projections for the 21st century 🔗

Under a relatively low greenhouse gas emissions scenario (corresponding to approximately 2°C warming), sea levels will likely rise by more than one meter by 2100. The latest IPCC projections indicate that seas may rise anywhere between 75 centimeters and 1.9 meters by century’s end when considering all emissions scenarios and uncertainties.

These projections have been revised upward compared to earlier assessments because ice sheets have shown a quicker response to temperature increases than previously anticipated. The accelerating pace of ice sheet melting represents one of the largest uncertainties in sea level projections.

Regional impacts: Bangladesh under threat 🔗

Bangladesh exemplifies the severe regional impacts of sea level rise. With its 710-kilometer coastline along the Bay of Bengal and location at the tail end of the Ganges-Brahmaputra-Meghna river system, the country is extremely vulnerable.

Approximately 80% of Bangladesh consists of floodplain, and nearly 70% of the country’s land area is less than 1 meter above sea level. With a 1.0-meter sea level rise, about 10% of the total land area would be lost, displacing an estimated 14.8 million people.

Even a modest 0.3-meter rise would affect 5% of Bangladesh’s land and cause a reduction of 0.5 million metric tonnes in rice production. Climate Central research projects that over 1,000 square kilometers of land could be permanently below the high tide line by 2100 under current emission trends, with more than 800,000 people currently living in these areas.

The densely populated coastal regions and the Ganges-Brahmaputra-Meghna delta system are particularly vulnerable to both permanent inundation and more frequent coastal flooding. Current estimates suggest that 3.5 million people are at risk of river flooding annually due to sea level rise and increasingly intense monsoon seasons.

Island nations facing existential threats 🔗

Small island developing states face even more acute threats to their existence. The Maldives, where 80% of land is less than 1 meter above sea level, could see 80% of the country become uninhabitable by 2050 according to projections. The World Bank estimates that with future sea levels projected to increase between 10 and 100 centimeters by 2100, the entire nation could be submerged.

Pacific island nations like Kiribati and Tuvalu are experiencing similarly dire circumstances. NASA assessments indicate that Kiribati has already seen sea levels rise 2 to 4 inches (5 to 11 centimeters) over the past 30 years. By 2050, the islands will likely experience an additional 6 to nearly 12 inches (15 to 30 centimeters) of rise.

Areas that currently experience fewer than five flood days per year could face 65 flood days annually by the 2050s in Kiribati. The threat is so severe that in 2012, Kiribati purchased land in Fiji as a potential relocation site for its citizens. Some uninhabited islands have already disappeared, with Tebua Tarawa and Abanuea vanishing underwater in 1999.

Tuvalu, with an average elevation of just 2 meters, faces similar challenges. The nation has explored creating digital replicas of itself to preserve its sovereignty and culture even if its physical territory becomes uninhabitable. These unprecedented measures highlight the existential nature of the threat facing low-lying island nations.

Adaptation challenges ahead 🔗

The accelerating rate of sea level rise presents profound challenges for coastal adaptation worldwide. Beyond the direct loss of land, rising seas bring saltwater intrusion into freshwater aquifers, increased coastal erosion, loss of agricultural land, and more frequent and severe coastal flooding during storms.

For densely populated coastal regions like Bangladesh and small island states, the scale of required adaptation measures-from building protective infrastructure to potentially relocating millions of people-is unprecedented. The impacts will continue for centuries even if greenhouse gas emissions are reduced today, as the ocean’s thermal inertia means warming and sea level rise will persist long after atmospheric temperatures stabilize.

What do you think? How should the international community support vulnerable nations facing displacement from sea level rise? What role should historical greenhouse gas emitters play in helping countries adapt to rising seas?

References

1. https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-9/
2. https://www.climate.gov/news-features/understanding-climate/climate-change-global-sea-level
3. https://www.jpl.nasa.gov/news/nasa-study-more-greenland-ice-lost-than-previously-estimated/
4. https://news.uci.edu/2024/03/13/grounding-zone-discovery-explains-accelerated-melting-under-greenlands-glaciers/
5. https://www.ebsco.com/research-starters/environmental-sciences/bangladesh-and-sea-level-rise
6. https://sealevel.climatecentral.org/uploads/ssrf/Report-Bangladesh.pdf
7. https://earth.org/sea-level-rise-in-bangladesh/
8. https://en.wikipedia.org/wiki/Climate_change_in_the_Maldives
9. https://sealevel.nasa.gov/news/276/nasa-sea-level-team-examines-an-island-nation-at-risk/