AP Syllabus focus:
‘Excess greenhouse gases drive global climate change, including rising sea levels from melting ice sheets and from the expansion of warming ocean water.’
Sea level rise is a major, measurable outcome of global climate change.
This time-series plot shows global mean sea level rise since 1900 alongside the major drivers identified by climate scientists. It visually reinforces that modern sea level rise is explained primarily by added ocean water from land-ice melt (glaciers and ice sheets) plus increasing ocean volume from thermal expansion. The figure also contextualizes why attribution matters by comparing the observed record with the contributions that make up the “sea level budget.” Source
In AP Environmental Science, focus on two main mechanisms: melting land ice adding water to oceans and thermal expansion as seawater warms.
What “sea level rise” means in this context
Sea level rise: An increase in the average height of the ocean surface relative to land, driven mainly by added water from melting land ice and by the expansion of warming ocean water.
Sea level is commonly discussed as a global average, even though local sea level can differ due to currents, winds, and vertical land movement.
Mechanism 1: Rising seas from melting ice sheets (and other land ice)
Why land ice melt raises sea level
Only ice on land (especially ice sheets and glaciers) raises sea level when it melts, because the meltwater flows into the ocean and increases ocean volume. The largest potential contributions come from Greenland and Antarctica, where ice is stored in massive, long-lived reservoirs.
This IPCC AR6 figure connects ice-sheet behavior to changes in global mean sea level, highlighting that once large ice sheets become destabilized, regrowth can take extremely long times. The lower panels provide conceptual cross-sections showing how melting can be driven by ocean temperature (ice–ocean interaction) and air temperature (surface melt and elevation feedback). Together, the diagrams reinforce why Greenland and Antarctica are considered high-impact, long-lived contributors to sea level rise. Source
Key ideas for APES:
Ice sheets store freshwater on land; melting transfers that water to the ocean.
Warming increases surface melt and can also speed the movement of ice toward the sea (e.g., faster glacier flow).
Because ice sheets are so large, even small percentage losses can contribute substantially to sea level rise over time.
Distinguishing land ice from sea ice
Sea ice (floating ice) melting has little direct effect on sea level because it already displaces seawater while floating.
Land ice melting directly adds water to the ocean, raising sea level.
Mechanism 2: Rising seas from thermal expansion of warming ocean water
Why warming water expands
As the ocean absorbs heat trapped by excess greenhouse gases, seawater molecules move slightly farther apart, increasing the water’s volume. This thermal expansion can raise sea level even without any additional water entering the ocean.
Important APES points:
The ocean has a very large heat capacity, so it can store vast amounts of heat.
Warming can be distributed through the upper ocean and, over longer timescales, deeper layers—supporting sustained expansion.
Thermal expansion is a volume change, not a mass change: the same water takes up more space.
A simple way to represent this process uses the volumetric expansion relationship.
= change in seawater volume (m)
= volumetric thermal expansion coefficient (per °C)
= initial seawater volume (m)
= temperature change (°C)
In real oceans, varies with temperature, salinity, and pressure, but the core APES concept is that warming oceans expand, contributing to sea level rise.
How the two mechanisms work together
Combined contribution to observed rise
Sea level rise is best understood as the sum of:
Added water from melting land ice (especially ice sheets)
Increased volume from thermal expansion of warming seawater
Both are driven by excess greenhouse gases warming the Earth system, but they differ in how they respond over time:
Thermal expansion can respond as ocean heat content increases.
Ice melt can accelerate as warming intensifies and as ice dynamics change, increasing the rate at which land ice is delivered to the sea.
Why this matters environmentally
Rising sea level increases the frequency and severity of coastal flooding, enhances storm surge reach, and can inundate low-lying areas. It also pushes saltwater farther inland, threatening freshwater resources and coastal ecosystems.
FAQ
Sea ice already displaces seawater while floating. Land ice adds new water to the ocean when it melts, increasing total ocean volume.
No. Expansion varies because warming is uneven with depth and region, and the expansion coefficient depends on temperature, salinity, and pressure.
Ocean heat uptake and mixing can continue, and land ice can keep losing mass due to lagged responses, sustaining thermal expansion and meltwater input.
Both are land ice, but ice sheets (Greenland/Antarctica) are vastly larger, so their potential contribution to sea level rise is much greater.
They combine satellite and ocean observations, including ocean temperature profiles (for expansion) and measurements of land-ice mass change (for meltwater input).
Practice Questions
State two processes that cause global sea level to rise as greenhouse gases increase. (2 marks)
Melting of land ice (ice sheets/glaciers) adds water to the ocean. (1)
Thermal expansion of warming ocean water increases ocean volume. (1)
Explain how (i) melting ice sheets and (ii) thermal expansion of seawater each contribute to rising sea level. (6 marks)
(i) Ice sheets are land-based stores of ice. (1)
When land ice melts, meltwater flows to the ocean, increasing ocean volume/mass. (1)
This raises average sea level. (1)
(ii) Oceans absorb heat as greenhouse gases trap more energy. (1)
Warmer seawater expands (greater volume for the same mass). (1)
Increased seawater volume raises sea level even without added water. (1)
