In 2024, the global sea level rose faster than anticipated, primarily due to the phenomenon of thermal expansion as ocean water warms. A comprehensive analysis led by NASA revealed that the rate of sea level rise in the previous year was measured at 0.23 inches (0.59 centimeters) per year, significantly higher than the expected rate of 0.17 inches (0.43 centimeters) per year. Josh Willis, a prominent sea level researcher at NASA's Jet Propulsion Laboratory in Southern California, emphasized, "The rise we saw in 2024 was higher than we expected. Every year is a little bit different, but what's clear is that the ocean continues to rise, and the rate of rise is getting faster and faster."
Historically, approximately two-thirds of the sea level rise was attributed to the influx of water from melting ice sheets and glaciers, while about one-third stemmed from the thermal expansion of seawater. However, in 2024, this trend reversed, with two-thirds of the rise resulting from thermal expansion. This shift highlights the growing impact of warming oceans as the planet experiences unprecedented temperatures.
Nadya Vinogradova Shiffer, head of physical oceanography programs at NASA Headquarters, noted, "With 2024 as the warmest year on record, Earth's expanding oceans are following suit, reaching their highest levels in three decades." Since the onset of satellite measurements of ocean height in 1993, the annual rate of sea level rise has more than doubled, culminating in a total rise of 4 inches (10 centimeters) over this period.
This significant long-term record is made possible by an unbroken series of ocean-observing satellites, starting with the TOPEX/Poseidon mission in 1992. The current satellite in this lineage, Sentinel-6 Michael Freilich, was launched in 2020 and is paired with an identical twin, Sentinel-6B, which will continue to measure sea surface height with high precision across approximately 90% of the world's oceans for the next several decades.
Several factors contribute to the thermal expansion of water in the ocean. Generally, seawater organizes itself into distinct layers based on temperature and density, with warmer water being less dense and floating above cooler water. In typical circumstances, heat from the surface penetrates these layers slowly, moving down into the deeper ocean.
However, in regions with strong winds, the ocean layers can be agitated, leading to vertical mixing. Additionally, large ocean currents, such as those found in the Southern Ocean, can tilt these layers, allowing surface waters to descend more easily into the depths. The substantial water movement associated with phenomena like El Niño, where a significant pool of warm water shifts from the western to the central and eastern Pacific, can also facilitate vertical heat movement within the ocean.
As scientists continue to monitor these changes, the implications of rising sea levels and thermal expansion are becoming increasingly clear, underscoring the urgent need for global awareness and action regarding climate change.
