Thermal Expansion Has Oceans Rising Imperilling Coastal Communities

NASA’s Earth observation satellites have been monitoring ocean sea levels since 1993. The total rise since the first satellite measurements has been 10 centimetres (4 inches). In 2024, the rise amounted to 0.59 centimetres (nearly a quarter inch or 6% in this one year), a rate 35% faster than predicted by climate models. Some of that accelerated rise can be attributed to a strong El Niño, the periodic heating that occurs in the western equatorial Pacific Ocean. The El Niño lasted from July 2023 to May 2024. Its impact didn’t just contribute to sea level rise but also played a significant role in extreme weather events.

Josh Willis, at the Jet Propulsion Laboratory (JPL) in Pasadena, added another concern, noting “the rate of rise is getting faster and faster.” How do the scientists at JPL know the rate of rise? Earth observation satellites use an onboard radar altimeter that bounces radio and microwave pulses off the ocean’s surface and reads the returning results. The satellites precisely measure the time between sending and receiving the pulses to calculate distance to the ocean surface. If the timing is shorter than previously observed, it means the oceans are rising. If the timing is longer, the oceans are sinking.

New algorithms integrated into the satellite’s programming can now measure sea level changes to coastal areas to millimetre accuracy, even in the face of winter ice and extreme weather events like hurricanes.

NASA scientists also validate satellite readings by comparing them to the data coming from the Argo Program, 3,500 floating platforms deployed across the world’s oceans.

The majority of sea level rise in recent years has been attributed to the melting of glaciers, not thermal expansion, which is caused by an increase in temperature of seawater from heat-trapping greenhouse gases (GHGs) in the atmosphere.

The ocean is the largest global carbon sink. GHG molecules, as they warm, become more active and interact with the ocean surface before being absorbed. The molecular motion then gets transferred to the seawater.

GHG molecular motion decreases seawater density and increases ocean volume. The interaction begins in the upper layers but eventually is transferred throughout the water column down to the seabed.

The process of ocean thermal expansion is normally slow because seawater has a much greater heat capacity than the atmosphere. That should set off alarm bells because, right now, the rate of sea level rise is speeding up. Climatologists describe the impact of GHGs on the ocean as a challenge that will continue well after we begin to lower emissions.

In a June 3, 2025, article appearing in Inside Climate News, Emilie Lounsberry writes about sunny day flooding along New Jersey’s shores with sea levels rising almost double the global mean.

New Jersey has seen sea level rise more than 45 centimetres (18 inches) since the early 1900s. Future projections to 2050 add another 30 centimetres (a foot) to the rise. The implications for the Jersey shore are dire, especially from the mid-state to the south, beginning with Atlantic City and ending at Cape May and Delaware Bay.

What is sunny day flooding? It is the inundation of low-lying coastal areas caused by the ocean’s daily tidal movements, exacerbated by thermal expansion. In Lounsberry’s article, she writes that the flooding is becoming more frequent and reaching more locations. What she doesn’t write is the impact the flooding is having on the aquifer beneath the surface.

Nor does she mention how isostatic rebound from the last Ice Age is tipping the southern part of the state downward, causing subsidence. The combination of sinking land and rising sea levels makes New Jersey a good candidate for a planned retreat for many of its coastal communities.

In 2012, Hurricane Sandy devastated the New Jersey shore. No effort was made to move back en masse from the shoreline. Instead, communities in the state rebuilt homes in situ and added seawalls, built up beaches by adding dunes, installed drainage pumps, and did some wetland restoration. In the town of Wildwood, just north of Cape May, best known for its plastic palm trees lining the main street (I’ve been there and seen them), citizens in the western half of the town installed concrete ramps and elevated driveways. None of these measures has kept the water from rising. As one New Jersey citizen stated, ‘This is something that’s chronic and getting worse — and not going away.”