Scenarios for the Fourth NCA: Sea Level Rise
The Fourth National Climate Assessment (NCA4) in the United States, which assesses the science of climate change and its impacts across the country, includes scenarios for sea-level rise that are critical for planning and policy-making. These scenarios are based on a combination of climate models, historical data, and projections of future greenhouse gas emissions. They take into account factors such as the melting of ice sheets, glacial retreat, and the thermal expansion of seawater.
Some potential scenarios that could be considered for sea-level rise in the context of the Fourth NCA:
1. Low Emissions, Slow Ice Melt Scenario:
This scenario assumes that global efforts to reduce greenhouse gas emissions are successful, leading to lower atmospheric concentrations of CO2.
Ice melt occurs at a slower rate than currently observed, possibly due to successful mitigation efforts or lower climate sensitivity than expected.
Sea-level rise is still ongoing but at a slower pace, allowing more time for adaptation and reducing the immediate risk of inundation and erosion.
2. Intermediate Scenario:
Emissions peak mid-century and then decline, in line with some of the more moderate Representative Concentration Pathways (RCPs) used in climate modeling.
Ice sheet melt continues at a pace consistent with current observations, contributing to an intermediate level of sea-level rise.
Coastal areas experience significant impacts, including increased flooding and erosion, but there is still potential for effective adaptation strategies to be implemented.
3. High Emissions, Rapid Ice Melt Scenario:
This scenario assumes that emissions continue to rise throughout the 21st century, leading to high concentrations of greenhouse gases in the atmosphere.
Ice sheets, particularly in Greenland and Antarctica, respond more sensitively to warming, leading to accelerated melting.
Sea levels rise rapidly, overwhelming many existing coastal defenses, leading to widespread inundation, loss of coastal ecosystems, and significant socio-economic disruption.
4. Extreme Scenario:
An extreme scenario might consider the potential for abrupt and deep ice sheet instabilities or the disintegration of marine-based sectors of the Antarctic ice sheet, leading to much higher rates of sea-level rise than currently projected.
Such a scenario would have profound implications for coastal cities, infrastructure, and ecosystems, potentially displacing millions of people and requiring massive investment in adaptation and protective measures.
5. Stabilization Scenario:
In a more optimistic scenario, global temperatures stabilize due to aggressive mitigation efforts, and the rate of sea-level rise begins to slow down.
While sea levels continue to rise due to the lagged response of the ocean and ice sheets to warming, the rate is slow enough that adaptation measures can keep pace with the changes.
6. Regional Variability Scenario:
This scenario acknowledges that sea-level rise is not uniform across the globe due to factors like ocean currents, land subsidence, and regional temperature differences.
It would provide projections for different regions of the U.S. coastline, recognizing that some areas may experience significantly higher or lower rates of sea-level rise.
In each scenario, the NCA would likely discuss the implications for various sectors, including water resources, infrastructure, human health, and ecosystems. It would also consider the effectiveness of different adaptation strategies, such as managed retreat, hard and soft engineering solutions, and policy measures like zoning and building codes.
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Scenarios for the Fourth NCA: Sea Level Rise
Description
The Fourth National Climate Assessment (NCA4) presents four scenarios for sea level rise (SLR) along the U.S. coastline by the year 2100. Scenarios would be used not only to inform policymakers and the public about the risks of sea-level rise but also to guide investment in research, monitoring, and infrastructure to prepare for and mitigate these risks.
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