Welcome, dear readers of FreeAstroScience. Here’s the question that keeps city planners and coastal families awake: how fast are the oceans rising, and which neighborhoods will go under first? In this article—written by FreeAstroScience only for you—we’ll stitch together the latest satellite evidence with an unprecedented building-by-building risk map of the Global South. Stick with us to the end and you’ll know the pace, the physics, and the places—plus what “good decisions today” can still save tomorrow.
Why does the speed of sea-level rise matter more than the average?
Over the past 30+ years, satellites have tracked global mean sea level (GMSL) with centimeter precision. The headline is stark: the rate has more than doubled, from about 2.1 mm/yr in 1993 to ~4.5 mm/yr in 2023, adding ~111 mm to the global ocean height since 1993. If the current trajectory holds, we’re looking at another ~169 mm by 2050. That shift isn’t gradual—it’s accelerating, a climate-system “pedal to the metal” moment that compresses adaptation timelines for ports, roads, and homes.
What’s driving the rise?
Two big levers do the lifting:
- Thermal expansion as the ocean warms.
- Land-ice loss from glaciers and ice sheets, which adds water to the ocean.
Both are tightly tied to greenhouse gas increases and are now tracked by modern observing systems (e.g., Argo, GRACE/GRACE-FO). The longer the satellite record gets, the clearer the signal becomes relative to year-to-year swings like El Niño and La Niña.
Where will the water actually go on land?
Here’s the aha: what drowns first depends on local elevation, tides, subsidence, and gravity/rotation fingerprints of ice melt. So we need local exposure metrics, not just global averages. A 2025 study mapped 840 million buildings across the Global South—Africa, Southeast Asia, South and Central America—then asked, building by building, at what local sea-level rise (LSLR) will the ground floor touch water at high tide? The result is the first broad, high-resolution risk picture for the built environment in these regions.
How did researchers decide if a building is “inundated”?
They combined:
- High-resolution, bias-corrected topography that removes trees and roof heights (FABDEM).
- Google Open Buildings footprints and centroids for billions of structures.
- Tidal modeling to reference elevations to local high tide.
A building is flagged “inundated” if its centroid elevation falls below the high-tide water level plus local sea-level rise—a conservative metric that doesn’t yet count storm surges, erosion, or protective infrastructure. Think of it as a clear, minimum estimate.
The inundation rule (as a compact formula)
Where:
- E(building) is the centroid elevation (meters above mean sea level, adjusted to high tide).
- Htide is the local high-tide offset.
- LSLR is the Local Sea-Level Rise (meters).
How many buildings are at risk—and how quickly does risk spike?
The Global South analysis shows a nonlinear surge in exposure: low at first, then escalating steeply between ~2 and 4 meters of local rise. Early on, Africa sees the largest counts; with higher LSLR, Southeast Asia dominates. Deltas and estuaries (think Nile, Amazon, Río de la Plata) pull the ocean inland along gentle slopes, amplifying building losses.
Key exposure counts (buildings) by LSLR
| LSLR scenario | Inundated buildings (approx.) | Notes |
|---|---|---|
| 0.5 m | ~3.0 million | Lower bound for global mean SLR by 2100 under low emissions |
| 5 m | ~45 million | Some countries lose >80% of building stock |
| 20 m | ~136 million | Long-term, multi-century potential under high warming and ice loss |
| Source: Willard-Stepan et al., 2025; npj Urban Sustainability. :contentReference[oaicite:5]{index=5} | ||
News coverage from ScienceAlert underscores the same message: even 0.5 m of rise threatens millions of buildings; at 5 m, risk expands to ~45 million buildings; at 20 m, ~130+ million are in peril.
How does “acceleration” translate into planning realities?
City budgets and construction timelines live in decades, not centuries. That’s why the trajectory of rise (not just total centimeters) matters. A quadratic fit to the satellite record captures both the current rate and its acceleration:
- Rate at time t:
- Observed acceleration: ~0.08 ± 0.06 mm/yr² (1993–2023).
- Rate increased from ~2.1 mm/yr (1993) to ~4.5 mm/yr (2023).
- Projected 2020–2050 rise (observation-driven): ~169 mm (90% CI).
A quick rate snapshot
| Year | Rate (mm/yr) | What it means for planning |
|---|---|---|
| 1993 | ~2.1 | Slow climb; long lead times felt “safe” |
| 2023 | ~4.5 | Rate has doubled; adaptation windows shrink |
| 2030 (est.) | ~5.0 ± 1.4 | Design to higher baselines, earlier |
| 2040 (est.) | ~5.8 ± 2.0 | Protection/retreat trade-offs get tougher |
| 2050 (est.) | ~6.5 ± 2.6 | Compounding extremes test defenses |
| Source: Hamlington et al., 2024; Communications Earth & Environment. :contentReference[oaicite:8]{index=8} | ||
Which regions face the steepest building losses?
- Africa (early stages): Highest number of impacted buildings at low LSLR.
- Southeast Asia (higher LSLR): Rapidly dominates exposure as rise intensifies.
- Deltas & estuaries: Gentle slopes and dense settlement magnify inland reach.
- Megacities on coasts: 20 of the world’s 26 largest megacities sit by the sea—a legacy now becoming a liability.
Want to explore your coastline? The authors host an interactive risk explorer with building footprints and inundation masks for different scenarios.
How uncertain is all this?
We should be honest about complexity:
- Local sea level ≠ global mean. Gravity/rotation effects of ice loss push more rise toward mid–low latitudes; many Global South coastlines may see more than the global average.
- Subsidence from groundwater pumping (e.g., Jakarta Basin) can bring water to doorsteps sooner than elevation maps suggest.
- Tides and surges can intensify with rising mean levels; the building metric here is high tide only—a minimum that excludes storm surges and erosion.
Still, the acceleration measured by satellites is robust, and the building-by-building mapping sets a spatial baseline you can plug into any emissions pathway or local projection.
What should cities and communities do now?
Because infrastructure lifespans are measured in decades, decisions we take in the 2020s and 2030s lock in outcomes far beyond 2100.
Practical moves:
- Stop placing new assets in zones flagged for inundation in the 0.5–2 m range.
- Elevate and flood-proof critical services (power, water, hospitals), test them under combined tide + surge.
- Monitor subsidence and cap groundwater pumping where feasible.
- Stage adaptation using trigger-based plans: build when observed thresholds are hit (rates, frequencies, damage).
- Budget for maintenance and retreat, not just walls—especially where estuarine geometry multiplies risk.
Remember, low-emissions pathways dramatically reduce long-term losses—those millions vs. tens of millions of buildings are the gulf between decisive mitigation and business-as-usual.
A quick reference: global pace vs. local places
- Global pace: Acceleration documented by satellite altimetry; another ~169 mm by 2050 if current trajectory continues.
- Local places: Nonlinear building losses, especially 2–4 m LSLR; clustered exposure in deltas; country-level exposure can exceed 80% of building stock at 5 m LSLR.
- Minimum estimate: Storms, erosion, and some land motions are not in the baseline inundation metric—real-world damage can arrive earlier.
- Public summary: ScienceAlert offers a concise overview aligned with the peer-reviewed findings.
Conclusion: What’s at stake—and what’s still possible?
We’re living through a measurable quickening of the oceans. Satellites say so. Neighborhood maps now say where the water goes next. Between a 0.5 m and 5 m future lies the difference between millions and tens of millions of buildings at risk across the Global South, with cascading effects on trade, food, and culture. The path we choose—emissions, planning, and the humility to work with water—can still keep more homes, schools, and ports on dry ground.
Come back to FreeAstroScience.com as we keep translating complex Earth science into choices you can use. This post was written for you by FreeAstroScience.com, which exists to explain complex science simply—because the sleep of reason breeds monsters.
Sources
- Hamlington, B. D., et al. (2024). The rate of global sea level rise doubled during the past three decades. Communications Earth & Environment.
- Willard-Stepan, M., et al. (2025). Assessing the exposure of buildings to long-term sea level rise across the Global South. npj Urban Sustainability.
- McLendon, R. (2025). Millions of Buildings Threatened by Rising Seas This Century, Study Warns. ScienceAlert.
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