A key difference from FEMA is First Street incorporates historical climate and insurance data.

property, whether it comes from high tides, rainfall, rivers or hurricane storm surge. Future risk calculations are based on the set of scenarios used by the Intergovernmental Panel on Climate Change.

Past efforts have focused on separate models for each risk.

“The thing that we did that’s unique is we not only pulled that together and created this model at the property level that hadn’t been done before, then we also adjusted it into the future,” said Matthew Eby, First Street’s founder and executive director.

A key difference from FEMA’s approach is that First Street incorporates historical climate and insurance claim information, as well as future climate projections from the climate change panel. FEMA develops regulatory flood maps based on historical data alone.

First Street also included floodmitigating infrastructure in its model, such as levees, beach nourishment projects and wetland restoration projects.

The estimates of numbers of properties at risk under First Street’s model versus FEMA’s maps differ dramatically in many areas of the country, with FEMA underestimating risk in the majority of the U.S. and overestimating it in some pockets.

For example, in Cook County, Illinois, which encompasses the city of Chicago, about 25,000 properties fall within FEMA’s flood risk area. First Street’s model shows roughly 150,000 properties – 11% of properties in the county – are at risk. By 2050, an additional 11,000 properties will be at risk.

In some Gulf communities, First Street’s estimates are in line with or lower than FEMA. In Port Arthur, Texas, the group estimates 15% of properties are at risk, compared with 17% under FEMA’s maps. But by 2050, First Street estimates as many as 96% of properties will be at risk for flooding.

The foundation’s methodology is still undergoing peer review. Three independent experts reviewed and approved of the model before its release.