The Global Economic Case for Immediate and Continuous Coastal Adaptation

Technical Summary: The Global Economic Case for Immediate and Continuous Coastal Adaptation

Problem Statement
Despite accelerating sea-level rise (SLR) and a growing global coastal adaptation gap, there is a scarcity of global economic evidence regarding the optimal timing and sequencing of adaptation measures. Previous global economic assessments often assume immediate, static implementation of adaptation strategies, overlooking the inherent flexibility in timing decisions and switching between options over time. Conversely, non-economic studies highlight the need for timely action but lack the economic cost-benefit framework to justify specific pathways. Furthermore, existing economic models often fail to account for "adaptation tipping points" (ATPs)—conditions where a current strategy becomes economically insufficient—limiting their ability to guide dynamic, long-term planning.

Methodology
This study extends a dynamic cost-benefit optimization framework previously applied at the European scale to a global level. The analysis covers 61,828 coastal floodplains (derived from a high-resolution grid of 144,361 segments) containing exposed populations and assets, spanning the time horizon from 2020 to 2150.

• Adaptation State Space: The model evaluates four distinct adaptation options:

  1. Protection: Hard infrastructure (e.g., dikes) at the coastline.
  2. Accommodation: Flood-proofing buildings up to one meter.
  3. Retreat: Relocating people and assets from the floodplain.
  4. Retreat & Inland Protection: A combination of retreat and building protection inland.
     The framework allows for flexible timing, simultaneous activation of options (e.g., retreat combined with inland protection), and switching between options once ATPs are reached.

• Cost and Damage Functions:

  • Investment Costs: Modeled using linear cost functions with fixed components for protection (based on Dutch dike costs adjusted by country-specific factors) and retreat (estimated at six times local GDP per capita per migrant). Accommodation costs are derived from U.S. case studies and adjusted globally.
  • Flood Damages: Computed using the DIVACoast framework with a "bathtub" inundation model and a logarithmic depth-damage function. Damages occur when extreme water levels exceed the maximum of protection or accommodation heights.
  • Optimization: A dynamic programming approach minimizes the expected total discounted sum of adaptation investments and residual flood damages using a 3% discount rate.

• Scenarios: The study utilizes three Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, SSP5-8.5) with median SLR projections from the IPCC Sixth Assessment Report. A counterfactual scenario with zero SLR is also analyzed to isolate the impact of sea-level rise.

Key Results

• Immediate Action Required: Economically optimal pathways indicate that immediate adaptation is necessary for 28% of the global coastline, irrespective of the climate scenario. This reflects a current state of under-adaptation where existing protection levels are insufficient from an economic perspective.
• Long-term Pathways: By 2150, adaptation is economically warranted along 31% of the global coastline. The optimal mix of strategies includes:
  • 20% managed through retreat.
  • 7% through coastal protection.
  • <1% through accommodation.
  • 4% through a combination of retreat and inland protection.
  • 69% of the coastline requires no adaptation over the entire time horizon.
• Timing and Switching: For 94% of the coastline requiring a switch from "doing nothing" to an adaptation option, the transition occurs in the earliest time step (2030). Adaptation tipping points emerge as early as 2040, with the majority occurring over the next century under the medium emissions scenario (SSP2-4.5).
• Regional Disparities: The relative cost burden (adaptation investments plus flood damages relative to GDP) is highest in the Global South. Countries such as Suriname, Myanmar, and Guyana face the highest relative costs (up to 929% of GDP for Suriname), while the Netherlands, USA, and China face the largest absolute costs.
• Investment Magnitude: Cumulative global costs are estimated at 12.6–13.4 trillion USD (2024) by 2150 under medium and high emissions scenarios. These estimates are slightly higher than some previous studies, driven by higher assumed retreat costs and the exclusive use of median SLR projections.

Significance and Claims
The paper claims to provide the first global economic evidence supporting immediate and continuous coastal adaptation, independent of the specific climate change scenario. Its primary significance lies in demonstrating that:

1. Current Under-adaptation: A significant portion of the global coastline is currently under-adapted from an economic standpoint, necessitating immediate upgrades or strategic retreat even without considering future SLR acceleration.
2. Dynamic Necessity: Adaptation cannot be a one-time investment; continuous action is required to maintain protection levels and switch strategies as ATPs are reached.
3. Equity Implications: The study highlights that the Global South will bear the highest relative future cost burdens, exacerbating existing inequities, as these regions currently have lower levels of public adaptation implementation compared to Europe and North America.
4. Retreat Viability: The results suggest that retreat is the economically optimal strategy for the majority of the coastline requiring adaptation, challenging the prevailing global reliance on hard coastal protection.

The authors acknowledge limitations, including the exclusion of nature-based solutions, land reclamation, and accommodation options exceeding one meter, as well as the focus on coastal flooding alone without accounting for compound hazards like riverine flooding. Despite these constraints, the study concludes that the economic case for early, flexible, and continuous adaptation is robust.