Learn how sea levels are measured, why they are changing, and how they are incorporated into Flood Factor scores.
Measure sea level rise
To monitor local sea levels, scientists use different equipment such as satellites, floating buoys off the coast, and tidal gauges to accurately measure the local sea level as it accelerates and changes.
Satellites are able to measure sea level from outer space as they orbit the Earth and can accurately calculate the depth of the ocean within an inch. Buoys off the coast float on top of the water and measure the depth. Tidal gauges, which are large tubes in the ocean that look similar to an upside down measuring cup, measure how high the water moves up the tube. They have been measuring the height of the ocean every 6 minutes for over 20 years and are the main source of the sea level trend data that is used in the First Street Foundation Flood Model.
Sea level, on average, is increasing across the country, however, the increase varies by region. That is why the Flood Model uses around 130 tide gauges in the methodology to ensure that each area is accurately accounting for the regional trends in sea level.
Global causes of sea level rise include ice melt and thermal expansion, warming waters that expand, which raise the sea levels everywhere.
Ice melt as a contribution to sea level rise, refers to ice that was once on land, melting into the sea. This does not include icebergs, which are large chunks of ice already floating in the ocean. Ice melt can occur from different types of glaciers, including ice sheets, ice caps, or glaciers in mountains and valleys. Currently, ice melt is causing about two-thirds of global sea level rise. Scientists worry about Antarctica the most as it is nearly twice the size of the United States and contains 90% of the earth’s ice. Scientists are aware that Antarctica is melting, however, they do not know how much, and how quickly, it will melt. Because of this, scientists have created different forecasts using their best estimates to determine when the ice will melt.
Thermal expansion is the scientific term for expanding water, and it is responsible for about one-third of global sea level rise. As the ocean gets warmer each year, it expands, which then raises the sea level. Think of it like a thermometer: the hotter it gets, it expands and shoots upward. A warming ocean rises the same way, rising higher as it gets hotter.
Regional causes of sea level rise include the slowing Gulf Stream and land sinkage.
The Gulf Stream is part of a vast oceanic conveyor belt system around the world. It circulates ocean water in a loop, from the south to the north and back again. This causes different amounts of sea level rise along the East Coast of the United States. Since the stream is slowing, less water is taken. As a result, more water stays on our shores which causes sea level rise in parts of Florida.
Land Sinkage (subsidence) is generally a small contributor to sea level rise, however, in some places it is responsible for more than half of the sea level rise. Sea levels are measured relative to land, which means that when the ground sinks lower, sea levels reach higher. There are two main causes to land sinkage. Groundwater withdrawal and tectonic plates. Groundwater withdrawal results in gaps forming where the water used to be. As people pump water out of the ground, the land sinks in order to fill the empty space. Tectonic plates can also cause the earth to sink as they slide under each other.
Sea level rise and flooding
Sea level rise exacerbates the risk and damage of tidal and storm surge flooding. Homes that were once safe from this type of flooding may be at risk now or in the near future due to the sea level rising.
As the sea levels rise, there is more water available to contribute to hurricane storm surge. This means that water can travel further inland, or flood in greater depths. During high tide events, pressure from rising sea levels and higher tides can push seawater into drainage pipes and spill water out into the streets, causing tidal floods even on sunny days.
The inclusion of environmental changes that impact flood risks, such as sea level rise, is an essential trademark of the Flood Model. Using 1980-2010 as a baseline period, the Flood Model analyzes multiple environmental possibilities from the local tidal gauge data. This includes what the sea level projections would look like in each area, under the Representative Concentration Pathways (RCP) 4.5 carbon emissions scenario with high and low uncertainty bounds. The resulting high, median, and low projection scenarios are then used as inputs when calculating current and future flood risks to show how flood risks will change in the future.