Are Increasing Flood Losses Due to Climate Change?

Federal aid in response to the damage caused Hurricane Sandy currently amounts to over $60 billion dollars, while total economic damage will likely be over $80 billion dollars. This follows just a year after 2011’s Hurricane Irene and tropical storm Lee, which caused damages of over $16 billion. Much of the damage caused by these storms was due to extensive flooding from the large amounts of rain delivered by the storms. Figures from the National Weather Service show an increasing trend in flood damages in recent decades.

The consensus within the scientific community, and increasingly in the public opinion, is that climate change is causing more frequent and intense storm events. But climate change is not the whole story behind the increase in economic costs from storms. A significant factor behind increased losses from floods is that more people live in high-risk areas such as floodplains and coastal areas. A related cause is that the value of our assets has increased in the past several decades. To illustrate this, compare the value of everything you own in your house to what your grandmother would have owned at your age.

Why have we allowed so much development in high-risk areas? Ironically, much of the reason is because of federal policy originally designed to reduce the cost of flood damages. The National Flood Insurance Program was established in 1968 in response to rising concern over the amount of federal aid given to victims of floods. The program mandated flood insurance for homeowners in designated high-risk areas. The standard chosen for delineating these high-risk areas was the “100-year” flood.

The “100-year” flood is a term that is often misunderstood, because it does not refer to the flood that only occurs once every hundred years. Rather, it is the magnitude of flood that has a 1% chance of occurring in any given year. Stated another way, over a very long period of time, this is the magnitude of flood that would occur on average every 100 years.

Hydrologists determine the magnitude of the “100-year” flood at a given location by using available streamflow data from the U.S. Geological Survey. Hydrologists assume that the chance of a future flood of a certain size will be the same as the chance that size flood occurred in the past, which can be estimated using the streamflow data. This method assumes the conditions causing floods in the pasts will be the same as future conditions. There are two major problems with this assumption. The first is that the watershed and stream channel may not have stayed the same over time. In many places we have modified rivers by building dams (decreasing flood magnitude), and levees (decreasing flood risk locally but increasing flood magnitude downstream). Also, urbanization creates more impermeable surfaces, which increases the amount of runoff after storms and leads to larger floods.

As mentioned before, the frequency and intensity of storms is likely changing, such that the climatic conditions that led to flood events in the past will be different from those in the future. This means that the “100-year” estimates underlying the National Flood Insurance Program are not accurate reflections of true flood risk in many areas, and has led to inappropriate development in high-risk areas. Scientists such as myself are interested in better quantifying the relationships between climate and large floods in order to support improved risk management policies.