Zachary Brown of Stanford University sips freshwater from a melt pond on sea ice during a NASA investigation to study how changing conditions in the Arctic are affecting the ocean’s chemistry and ecosystems. Image: Kathryn Hansen/NASA/Climate Visuals

The warnings are stark. “It is virtually certain that global mean sea level will continue to rise over the 21st century,” wrote scientists in the August 2021 report of the United Nations Intergovernmental Panel on Climate Change (IPCC), the key UN scientific body focusing on this crisis.

The fundamental reason for rising sea levels is global warming caused primarily by human use of fossil fuels. Warming causes ice sheets and glaciers to melt, increasing the volume of water in the oceans. Plus, warming causes the water to expand.

Scientists measure overall change in the oceans by calculating what they call global mean sea level (GMSL) – the average height of the entire ocean surface. GMSL “has accelerated since the late 1960s,” according to the IPCC, but has been on the rise for at least a century. “Human influence was very likely the main driver of these increases since at least 1971,” the IPCC reported.

The sea level increase in the 20th century was greater than in any prior century over the last 3,000 years, rising 20 centimeters (7.9 inches) between 1901–2018, according to a 2021 IPCC report (p. 1216). The report also said the average rate of increase was 2.3 mm (0.09 inches) per year from 1971–2018, while the rate increased to 3.7 mm per year (0.15 inches) from 2006–2018.

The magnitude varies significantly by location. This happens because of variations in water temperatures, wind, ocean currents, and the movement of the coastlines (some areas are sinking, others rising). Sea levels are rising fastest in the western Pacific and slowest in the eastern Pacific, according to the IPCC (p. 1216).

The IPCC describes five future scenarios – since outcomes will vary depending on what is done to limit greenhouse gas concentrations in the atmosphere. The scenarios are based on processes from which projections can be made with at least “medium confidence.”

Even under a “very low emissions scenario,” the IPCC projects that by the year 2100 global mean sea levels will likely rise by 28 to 55 centimeters (11 to 22 inches). Under a “very high emissions scenario,” the increase over that same time period will be between 63 centimeters and 1.01 meters (25 to 40 inches), according to the IPCC’s 2021 Summary for Policymakers.

Although unlikely, worse outcomes are possible. “Global mean sea level rise above the likely range – approaching 2 m by 2100 and 5 m by 2150 under a very high greenhouse gas (GHG) emissions scenario (SSP5-8.5) (low confidence) – cannot be ruled out due to deep uncertainty in ice-sheet processes,” according to the IPCC summary.

Changes in global surface temperature, which are assessed based on multiple lines of evidence, for selected 20-year time periods and the five illustrative emissions scenarios considered. Source: IPCC’s 2021 Summary for Policymakers

About 230 million people live on land less than one meter above current high tide lines, according to one study. Asian countries make up nine of the top 10 most at-risk large nations, according to another study, which also says many small island nations are at risk of “near-total loss” of land.

In addition to rising sea levels, flooding and storm surges will increase, caused by related phenomena. “In coastal cities,” the IPCC wrote in its 2021 report summary, “the combination of more frequent extreme sea level events (due to sea level rise and storm surge) and extreme rainfall/riverflow events will make flooding more probable.”

Further complicating the picture, land is subsiding in many locations for a variety of reasons, including extraction of groundwater. The exacerbating effect of coastal land subsidence is discussed in IPCC reports and measurements are provided in in academic research such as this 2022 article about 99 coastal cities and another one about subsidence in 48 coastal cities.

Extensive research is being conducted on sea level rise, sometimes abbreviated as SLR, offering journalists many sources.

In addition to major international reports, governments at all levels are looking at the issue. Reports are emerging not only from environmental agencies, but from those bodies responsible for infrastructure, health, and the economy.

Corporations, industry groups, and a burgeoning world of consultants are anticipating the future, especially regarding real estate. Environmental groups are active, but so too are civil society organizations and shareholder activists. Finally, many scientific articles are emerging. The authors are often willing to explain their findings.

But first, there are a number of key reports that journalists should read related to sea level rise to gain a basic understanding of the problem. They include:

• Climate Change 2022: Impacts, Adaptation, and Vulnerability: This report from IPCC Working Group II issued in February 2022 assesses the impacts of climate change on ecosystems, biodiversity, and human communities at global and regional levels. See the executive summary and press release (explaining the process). Sea level rise is covered in chapter three: Oceans and Coastal Ecosystems and their Services.
• The IPCC Sixth Assessment Report from Working Group I, on The Physical Science Basis, an August 2021 report. See especially chapter nine, Ocean, Cryosphere and Sea Level Change. We refer to this report as AR6 in the text below.
• Sea Level Rise and Implications for Low-Lying Islands, Coasts and Communities, chapter 4 of the 2019 IPCC Special Report on the Ocean and Cryosphere in a Changing Climate.
• A 2022 report by the UN World Meteorological Organization documents that sea levels and carbon emissions broke records in 2021.
• Resources from NASA, the US National Aeronautics and Space Administration.
• Climate Change: Global Sea Level, an explainer from the US National Oceanic and Atmospheric Administration (NOAA).
• Global and European sea level rise, by the European Environment Agency.

Higher Risk of Adverse Consequences

The negative consequences of rising sea levels are wide-ranging. Setting the stage, the IPCC February 2022 report (Chapter 3, p. 3) states:

“Ocean and coastal ecosystems support life on Earth and many aspects of human well-being. Covering two-thirds of the planet, the ocean hosts vast biodiversity and modulates the global climate system by regulating cycles of heat, water, and elements including carbon. Marine systems are central to many cultures, and they also provide food, minerals, energy, and employment to people.”

The pressures of migration may well intensify regional and inter-state conflicts. Damage will occur to nature, agriculture, and infrastructure. Besides higher water, warming seas will cause multiple changes in marine ecosystems.

Much detail on the predicted consequences is provided in chapter three of the February 2022 IPCC report, including emission scenarios that do not limit warming to 1.5°C, the global target set in the 2015 Paris Agreement. The IPCC report was a collaboration among 270 scientists from 67 countries. They looked at 34,000 scientific papers to assess how climate change could impact human civilization and the planet, as well as offer possible solutions to the impending crisis. The authors state how likely each prediction is to come true. So there are categories, such as “medium confidence” and “high confidence.”

Summarizing the chapter doesn’t do justice to the many foreseeable consequences of global warming. Consider the IPCC report as a sourcebook of ideas. Here are some examples of what the IPCC foresees:

• Increased risk of coastal erosion and submergence of coastal land
• Loss of sandy beaches, salt marshes, and mangrove forests
• Loss of coral reefs, kelp fields, sea grasses, and mangroves
• Salinization of groundwater
• Damage to coastal ecosystems
• Collapse of regional fisheries and aquaculture
• Migration of marine species to new areas
• Coastal eutrophication (algae blooms) and associated oxygen loss (hypoxia), causing ocean dead zones
• Increased risk of exposure to toxins, pathogens, and contaminants for seafood-dependent people.

All of these predictions have far-reaching consequences. One major concern is the likelihood of migration away from coastlines. The World Bank’s Groundswell report released in 2021 estimates climate change could force 216 million people across six world regions to move within their countries by 2050. As many as 250 million people, spanning all continents, could be “directly affected” by 2100, according to a 2019 study in the journal Nature Communications.

Island nations like the Republic of the Maldives — and its capital city, Malé (pictured) — are particularly vulnerable to small increases in sea levels. Image: Shutterstock

Many islands are severely threatened by sea level rise, including nations such as Fiji, Tuvalu, and the Maldives. In the atoll nation of the Marshall Islands, for example, rising seas could endanger 40% of the buildings in the capital, Majuro, according to a 2021 World Bank study. A coalition representing 39 island states and coastal countries in international negotiations was created in 1990, called the Alliance of Small Island States (AOSIS).

For journalists, the vast range of possible effects from sea level rise presents opportunities and challenges. While some of the science may seem arcane, developing stories is possible using conventional reporting techniques and available data.