Sea level rise has two main physical drivers. Getting these straight is one of the key goals of this lab — and one of your pre-lab questions. You may be surprised to learn that melting ice is not the whole story.

Driver 1 — Thermal Expansion

Water, like most substances, expands when it gets warmer. The ocean absorbs roughly 90% of the excess heat trapped by greenhouse gas emissions. As ocean water warms, individual water molecules move farther apart, and the entire ocean physically swells. You don't need to add a single drop of water for sea level to rise — warming alone does it.

This process, called thermal expansion, currently accounts for roughly 40–50% of observed global sea level rise. That's a bigger contribution than most people expect. Even if every glacier on Earth somehow stayed frozen, the ocean would still be rising because of warming.

Photo 1 — Ocean Heat Content

Ocean heat content has risen steadily since the 1970s — the energy stored in the ocean drives thermal expansion, one of the two main causes of sea level rise.

Driver 2 — Land Ice Melt

The second driver is the addition of water to the ocean from melting land-based ice. This is critical: only ice that sits on land contributes to sea level rise when it melts. Sea ice (like Arctic Ocean ice) is already floating — it displaces its own weight in water, so melting it doesn't raise sea level. It's the same reason a melting ice cube in your drink doesn't cause it to overflow.

Land ice comes from two main sources:

Glaciers and smaller ice caps — found worldwide, from Glacier National Park (Montana) to the Alps, Andes, and Himalayas. These are currently melting rapidly and contribute significantly to sea level rise.

Ice sheets — the massive continental glaciers covering Greenland and Antarctica. The Greenland Ice Sheet holds enough water to raise global sea levels by about 7 meters. The Antarctic Ice Sheet holds enough for roughly 57 meters. These are not going to melt overnight, but even a small percentage of loss has enormous consequences. The West Antarctic Ice Sheet is of particular concern because its base sits below sea level, making it especially vulnerable to warm ocean water intrusion from below.

📅 September 2009

📅 August 2017

Scientists don't give a single number for how much sea level will rise by 2100 — because it depends heavily on how much greenhouse gas humanity emits over the next 75 years. To handle this uncertainty, the IPCC (Intergovernmental Panel on Climate Change) uses standardized Representative Concentration Pathways (RCPs) — essentially different "futures" based on different emissions trajectories.

The Coastal Vulnerability Explorer uses four scenarios. Click each one to learn what it represents.

Here's a puzzle: sea level is rising equally everywhere around the globe (with minor regional variation), yet two coasts can respond very differently to the same amount of rise. Why? Because vulnerability is not the same as exposure. Vulnerability depends on several factors beyond simply "how much sea level rise is coming."

Click each region below to see how its geography shapes its vulnerability.

The Four Factors That Shape Vulnerability

1. Elevation. The most obvious factor. A coastal area that sits 0.5 m above the current high-tide mark is directly threatened by +1 m of sea level rise. A city averaging 10 m of elevation has more time. But elevation alone doesn't tell the whole story — read on.

2. Subsidence. Some coastal land is actually sinking at the same time seas are rising — a process called subsidence. In Louisiana, the land is sinking 1–2 cm per year due to sediment compaction, groundwater withdrawal, and loss of river sediment from levee construction. This means Louisiana's relative sea level rise is 2–3 times greater than the global average. Even without any climate change, Louisiana would be losing coastline.

3. Ecosystem buffers. Healthy salt marshes, mangrove forests, coral reefs, and barrier islands physically absorb wave energy and reduce storm surge. A coast with intact buffering ecosystems handles sea level rise very differently from one where those ecosystems have been degraded or removed. We'll explore this in depth in Part 4.

4. Adaptive capacity. A wealthy, technically advanced city like New York can build surge barriers, elevate infrastructure, and install tidal gates. Tuvalu — a small Pacific nation of ~11,000 people — cannot. This "adaptive capacity" means two places with identical physical exposure can have very different social and economic outcomes.

Photo 3 — Louisiana Coastal Land Loss

Louisiana's coast from above — the "Swiss cheese" pattern of open water replacing what was once continuous salt marsh. Louisiana loses approximately a football field of land every hour.

You've already learned about salt marshes, mangroves, estuaries, and wetlands in lecture. Here we're going to connect that knowledge to sea level rise — and to specific places in the Coastal Vulnerability Explorer. The key insight: coastal ecosystems are not just habitats, they are infrastructure. When they are lost, the consequences ripple far beyond wildlife.

Salt Marshes — New York / NJ: Jamaica Bay

Salt marshes are dominated by salt-tolerant grasses — especially Spartina alterniflora (smooth cordgrass) in North American Atlantic marshes. These dense grass beds are remarkably effective at dissipating wave energy, trapping sediment, and absorbing storm surge. Research has shown that every mile of healthy salt marsh can reduce storm surge by 6–9 inches.

Jamaica Bay, a large estuary within the Gateway National Recreation Area in New York Harbor, is a textbook example of what happens when salt marsh is lost. Jamaica Bay once contained approximately 4,100 acres of salt marsh. By the 2020s, roughly half of that had converted to open water — a loss rate of about 50 acres per year. The causes are complex: sea level rise, propeller wash from boat traffic, reduced sediment supply, and possibly excess nutrients. The result is that the marshes that once buffered Brooklyn and Queens from Atlantic storms are substantially diminished.

Jamaica Bay also supports a remarkable diversity of wildlife for an urban estuary: over 330 species of birds (including nesting American oystercatchers and laughing gulls), horseshoe crabs, diamondback terrapins, and serves as a critical migratory stopover on the Atlantic Flyway.

Smooth Cordgrass

Spartina alterniflora

Dominant grass of Atlantic salt marshes. Its dense rhizome mat stabilizes sediment and its above-ground stems dissipate wave energy. As sea level rises faster than marsh sediment can accrete, these marshes "drown."

Jamaica Bay / NY-NJ

Horseshoe Crab

Limulus polyphemus

Spawns on sandy beaches adjacent to salt marshes. Their eggs are the primary food source for migrating shorebirds, especially red knots traveling from South America to the Arctic. Loss of marsh shoreline directly reduces spawning habitat.

Jamaica Bay / NY-NJ

Piping Plover

Charadrius melodus

Federally threatened shorebird that nests directly on sandy beach and dune habitat on barrier islands including Fire Island, NY. As barrier islands erode and overwash events increase, nesting habitat is lost and nest success declines.

NY Barrier Islands

Photo 4 — Jamaica Bay, New York

Jamaica Bay, New York Harbor — the dark patches are surviving salt marsh; the open water was marsh within living memory. Over half of Jamaica Bay's original 4,100 acres of marsh has been lost since the mid-20th century.

Mangroves — Bangladesh: The Sundarbans

Mangroves are woody shrubs and trees that grow in intertidal zones throughout the tropics and subtropics. Their most distinctive feature is the prop root system — a tangle of aerial roots that creates a physical barrier against waves and traps sediment, actively building land elevation over time. Research from Bangladesh shows that mangrove forests can reduce cyclone wave height by 50–70% compared to unprotected shorelines. Every kilometer of mangrove is worth significant amounts of avoided storm damage — one study estimated mangroves provide over $1 billion annually in avoided flood damage globally.

The Sundarbans — shared between Bangladesh and India — is the world's largest mangrove forest (~10,000 km²) and a UNESCO World Heritage Site. It is the primary habitat of the Bengal tiger (Panthera tigris tigris), which has adapted to swimming between islands and hunting in the tidal channels. It also supports Irrawaddy dolphins, estuarine crocodiles, and serves as critical nursery habitat for fish and shrimp that underpin Bangladesh's fishing industry.

As sea level rises, the Sundarbans face a double threat: inundation of low-lying islands and salinization of freshwater areas at the forest margins. The Bengal tiger, which depends on freshwater for drinking, faces a shrinking habitat as fresh groundwater becomes saline.

Bengal Tiger

Panthera tigris tigris

The Sundarbans population (~100 individuals) is uniquely adapted to a tidal mangrove ecosystem. They swim between islands and drink brackish water — unusual among tigers. Rising seas are fragmenting their already limited habitat.

Sundarbans / Bangladesh

Red Mangrove

Rhizophora apiculata

Dominant tree in the Sundarbans. Prop roots create a physical buffer against wave action and provide nursery habitat for juvenile fish. The root system actively traps sediment, helping the forest "keep up" with modest sea level rise — but not the accelerating rates projected for 2100.

Sundarbans / Bangladesh

Irrawaddy Dolphin

Orcaella brevirostris

A critically endangered dolphin found in the tidal channels of the Sundarbans. Highly vulnerable to saltwater intrusion and habitat loss as the mangrove forest retreats with sea level rise. Also threatened by fishing entanglement and vessel traffic.

Sundarbans / Bangladesh

Prop roots in the intertidal zone

Mangrove forest canopy

Coral Reefs — Tuvalu: The Reef That Builds the Island

Coral reefs are both ecosystems and physical structures. A healthy coral reef does something remarkable for an atoll like Tuvalu: it produces the sand and sediment that the island itself is made of. Coral skeletons, broken down by wave action and bioerosion (parrotfish grinding coral with their beaks, for example), create carbonate sand that accumulates on the atoll's shoreline and windward reef flat.

If the reef bleaches and dies, this "sediment factory" shuts down. At the same time, the reef framework — the limestone structure that dissipates incoming ocean swell — weakens. Waves that would have been absorbed now strike the shoreline with full force. The result: overwash events increase, the freshwater lens (the only freshwater on the island, sitting like a bubble of fresh water below the sand) gets contaminated with seawater, and the island's physical foundation begins to erode.

Tuvalu's reefs are already experiencing significant bleaching stress from warming ocean temperatures. Combined with sea level rise, the physical existence of the islands themselves is at risk — not just their ecology.

Staghorn Coral

Acropora cervicornis

One of the fastest-growing and most structurally important reef-building corals in the Indo-Pacific. Its branching structure provides critical fish habitat. Also one of the most vulnerable to bleaching — a 1–2°C rise above average summer temperature triggers bleaching within weeks.

Tuvalu / Pacific

Bumphead Parrotfish

Bolbometopon muricatum

The largest parrotfish, capable of consuming massive amounts of coral skeleton, which passes through their gut and is excreted as fine white sand. A single large parrotfish can produce hundreds of kilograms of sand per year — the sand that builds and maintains atoll beaches. Overfishing of parrotfish reduces this sediment supply.

Tuvalu / Pacific reefs

Green Sea Turtle

Chelonia mydas

Nests on Pacific atoll beaches including Tuvalu. Sea level rise inundates nesting beaches during high tides, causing nest flooding and mortality of developing eggs. Warmer sand temperatures also increase the proportion of female hatchlings, disrupting the sex ratio over time.

Tuvalu / Pacific atolls

Funafuti Atoll from above

Tuvalu's nine atolls in the Pacific

Cypress-Tupelo Swamps — Louisiana Bayou

Louisiana's coastal wetlands — a mosaic of cypress-tupelo swamp, salt marsh, and freshwater marsh — function as the coastline's natural storm defense system. Research has documented that every 4 miles of healthy wetland reduces hurricane storm surge by approximately 1 foot. In 2005, Hurricane Katrina's devastation of New Orleans was made dramatically worse because decades of coastal land loss had removed much of that natural buffer.

Louisiana loses approximately a football field of coastal wetland every hour to erosion and subsidence. The primary drivers are: river channelization by the Army Corps of Engineers (which diverts sediment that formerly rebuilt the delta), oil and gas extraction (accelerating subsidence), and sea level rise. This is a compounding problem: less marsh means more storm damage; more storm damage accelerates marsh loss.

Roseate Spoonbill

Platalea ajaja

One of the most visually striking birds of Louisiana's coastal marshes and bayous. Nests in mangroves and willow thickets along marsh edges. Their feeding behavior — sweeping their distinctive spoon-shaped bill through shallow water — makes them highly dependent on shallow estuarine habitat that disappears as marshes are lost to open water.

Louisiana Bayou

American Alligator

Alligator mississippiensis

A keystone species of Louisiana's coastal wetlands. Alligator nest mounds — built above water level — provide nesting habitat for over 100 other species. As marshes are converted to open water and salinity increases, alligator habitat contracts. Saltwater intrusion also affects their prey base of fish, turtles, and wading birds.

Louisiana Bayou

Brown Shrimp

Farfantepenaeus aztecus

Juvenile brown shrimp depend entirely on Louisiana's coastal marsh estuaries as nursery habitat. Without marsh nursery habitat, shrimp populations collapse. Louisiana's $1.5 billion shrimp and seafood industry is built on the ecological productivity of coastal wetlands that are disappearing at an accelerating rate.

Louisiana Bayou

Photo 7 — "Ghost Forest," Louisiana

Dead cypress trees standing in open water — the marsh that once supported them has converted to open Gulf. These "ghost forests" are one of the most striking visual records of Louisiana's accelerating coastal land loss.

The impacts of sea level rise come in two flavors, and distinguishing them is one of your pre-lab questions.

A direct impact is a consequence that happens immediately and directly from sea level rise itself — flooding, erosion, saltwater intrusion into freshwater aquifers. If sea level rises 1 meter and a beach becomes permanently submerged, that is a direct impact.

A cascading impact (sometimes called an indirect or secondary impact) is triggered by a direct impact and flows downstream from it. Cascading impacts are often where the most serious long-term consequences emerge, because they ripple through ecological, economic, and social systems in ways that can be hard to predict or stop.

Tuvalu — Walk Through a Cascading Impact Chain

Here's how a single degree of warming and a meter of sea level rise can cascade into a full civilizational crisis for a Pacific atoll. Click "Next Impact" to reveal each step in the chain.

Louisiana — A Second Example Chain

The same principle applies to Louisiana's bayou country, where land loss creates cascading economic and ecological consequences. Click through the chain.

These five questions match exactly the pre-lab questions in your worksheet. Answer them here to check your understanding before class. Select an answer for each question.