What is Sea-Level Rise?

What is Sea-Level Rise?

Sea-level rise (SLR) is the sustained increase in mean ocean surface height relative to land, driven primarily by two mechanisms: thermal expansion (ocean water expands as it warms) and mass addition (melting glaciers and ice sheets add water volume). Global mean sea level has risen approximately 21–24 cm since 1880, with the rate accelerating from 1.4 mm/year in the 20th century to 3.6 mm/year from 2006–2015 and approximately 4.6 mm/year since 2020. The IPCC's AR6 projects 0.28–1.01 meters of rise by 2100, with higher outcomes possible if ice sheet instabilities are triggered.

Why It Matters

Sea-level rise threatens the physical and economic foundations of coastal civilization. Approximately 900 million people live in low-elevation coastal zones — roughly 11% of the global population. Eight of the world's ten largest cities are coastal. The OECD estimates that $14 trillion in global assets are exposed to coastal flooding, a figure that grows with continued development in vulnerable zones.

The financial system is recalibrating. Moody's incorporated sea-level rise into sovereign credit assessments starting in 2020, downgrading the outlook for nations with high coastal exposure and limited adaptation capacity. In the US, FEMA flood maps are undergoing the most significant revision in decades, expanding designated flood zones and increasing the number of properties requiring flood insurance. Coastal property values are diverging — high-exposure areas are experiencing "climate discounts" of 5–15% while elevated properties command premiums.

Insurance retreat is accelerating. Citizens Property Insurance, Florida's insurer of last resort, nearly doubled its policy count to 1.4 million between 2019 and 2023 as private insurers exited the market. Louisiana's FAIR Plan has similarly expanded. When private insurance withdraws, mortgage availability contracts, property values decline, and the fiscal burden shifts to taxpayers.

National security agencies treat sea-level rise as a threat multiplier. The US Department of Defense has assessed that 1,700 military installations worldwide face climate risks, with sea-level rise affecting major naval bases including Norfolk, San Diego, and Pearl Harbor. Rising seas compound saltwater intrusion into freshwater aquifers, contaminate agricultural soils, and erode the economic base of coastal communities — creating conditions for instability and migration.

How It Works / Key Components

Thermal expansion accounts for approximately 40% of observed sea-level rise. As the ocean absorbs over 90% of excess heat trapped by greenhouse gases, water molecules move faster and occupy more volume. This process is effectively irreversible on human timescales — even if greenhouse gas concentrations stabilized today, thermal expansion would continue for centuries as deep ocean layers equilibrate.

Ice sheet contributions are accelerating and represent the primary source of uncertainty in projections. Greenland lost an average of 270 billion tons of ice annually from 2006–2018, contributing roughly 0.7 mm/year to sea-level rise. Antarctica's contribution is smaller but accelerating, driven primarily by marine ice sheet instability in West Antarctica where warm Circumpolar Deep Water undercuts glaciers from below. The Thwaites Glacier alone could contribute 65 cm of sea-level rise if it collapses entirely.

Local sea-level rise often differs substantially from the global mean. Factors include land subsidence (Jakarta is sinking 10–25 cm/year due to groundwater extraction), gravitational effects (melting ice sheets redistribute gravitational pull on ocean water), ocean currents, and tectonic activity. The US East Coast experiences faster-than-average rise due to AMOC slowdown and post-glacial land subsidence. New York City's relative sea-level rise is roughly 50% faster than the global average.

Storm surge interaction magnifies SLR impacts. A 30 cm rise in baseline sea level can double or triple the frequency of flooding events in many coastal cities. What was once a 1-in-100-year flood becomes a 1-in-10-year event. This nonlinear relationship between sea-level rise and flood frequency means that small increments of rise produce large increases in damage.

Sea-Level Rise in Practice

Miami-Dade County invested $4 billion in its sea-level rise adaptation program, including stormwater system upgrades, raised roads, updated building codes, and saltwater intrusion barriers for the Biscayne Aquifer. The county uses a unified sea-level rise projection (developed with NOAA and the Army Corps of Engineers) for all capital planning: 10–17 inches by 2040, 21–54 inches by 2060.

The Netherlands' Delta Programme allocates €1.5 billion annually to flood protection, water management, and spatial planning, with plans extending to 2100. The strategy combines hard infrastructure (storm surge barriers, dikes), nature-based solutions (sand nourishment, restored wetlands), and adaptive spatial planning that limits development in the highest-risk zones. It's the most comprehensive national sea-level rise adaptation program globally.

Council Fire's Approach

Sea-level rise sits at the intersection of Council Fire's core competencies — ocean systems, coastal resilience, and stakeholder strategy. We help coastal organizations quantify their SLR exposure using the latest projections, develop adaptation pathways that phase investments over time as risks materialize, and communicate sea-level rise risk to boards, investors, and communities in terms that drive action. Our ocean expertise brings depth that generalist consultancies lack — understanding how SLR interacts with coastal ecosystems, marine economies, and blue carbon systems to create both risks and adaptation opportunities.

Frequently Asked Questions

How much sea-level rise is already locked in?

Even if emissions stopped today, committed warming from existing greenhouse gas concentrations would produce an estimated 0.3–0.6 meters of additional rise by 2100, with continued rise for centuries thereafter. The thermal inertia of the ocean and the slow response time of ice sheets mean that past emissions have locked in impacts that no mitigation can prevent. This makes adaptation investment essential regardless of how aggressively the world reduces emissions.

Which US cities face the greatest sea-level rise risk?

Miami, New Orleans, Houston, New York, Charleston, Norfolk, and Annapolis consistently rank highest based on elevation, subsidence rates, exposure, and population density. However, smaller communities often face greater relative risk — places like Isle de Jean Charles (Louisiana), Tangier Island (Virginia), and numerous Alaskan Native villages are facing relocation within years, not decades. Risk depends on local factors including subsidence, storm exposure, and infrastructure quality.

What is "managed retreat" from sea-level rise?

Managed retreat is the deliberate relocation of people, buildings, and infrastructure away from areas where sea-level rise makes continued habitation unsustainable. It contrasts with "hold the line" strategies that invest in ever-larger protective infrastructure. Examples include FEMA buyout programs that acquire flood-damaged homes, New Jersey's Blue Acres program, and community-scale relocations. Managed retreat is politically difficult but increasingly recognized as the only viable long-term response for the most exposed areas.