Boys prepare to walk across a makeshift bridge to reach their neighbourhood that lies below sea level in Jakarta, Indonesia. Picture: GETTY IMAGES/ED WRAY
Boys prepare to walk across a makeshift bridge to reach their neighbourhood that lies below sea level in Jakarta, Indonesia. Picture: GETTY IMAGES/ED WRAY

Climate change poses risks to real estate that home buyers may not be able to predict. As sea level rises, coastal properties, for example, may be subject to increased flooding and intensifying storm surges. First-time home buyers often lack the expertise to evaluate these new risks, and thus tend to under-estimate them and overpay for increasingly exposed properties.

Unfortunately, the risks they accept are not borne by themselves alone. Rather, our research has found, it is shared by mortgage lenders and, through the operations of Freddie Mac and Fannie Mae, American taxpayers.

Consider how most home purchases are arranged with long-term mortgages. While the conditions vary, a buyer of a $400,000 home may arrange a 5% fixed-rate, 30-year mortgage on $320,000, and agree to pay it back by making 360 monthly payments of about $1,700. If the lender holds this loan on its balance sheet, and climate change creates new expenses — from flooding, storms or wildfires — the borrower becomes more likely to default on the loan.

Consider that lenders originate an estimated $60bn to $100bn in mortgages on coastal properties, and it’s clear the potential aggregate impact of default due to climate change is significant.

Lenders can lower their risk, however, by exercising their option to sell loans to Fannie Mae and Freddie Mac, the government-sponsored enterprises that were created by US Congress to improve access to mortgage lending. Fannie and Freddie have an important public mission, but lenders’ ability to sell them mortgages means that the risk of climate-related real estate expenses is easily relayed from home buyers to taxpayers.

The fee structure should be designed to shift along with changes in risk and improvements in forecasting, as new climate-change scenarios materialise

To gain some insight into the scope of this problem, we examined what happened in mortgage markets after 15 “billion-dollar” disasters, including Hurricanes Katrina ($119bn) and Sandy ($73bn). We found that natural disasters significantly raise the number of delinquencies, defaults and foreclosures. This is probably a consequence of the decline in flood insurance: when fewer homes are insured, less of the damage from storm surges and the like is repaired. Fewer homes are rebuilt. And many more home owners default on their loans.

These mortgage defaults and payment delinquencies affect both lenders and Fannie Mae and Freddie Mac. In areas where natural disasters hit, we found, bank lenders transfer substantially more mortgages to the government-supported enterprises. And this increase is largest in neighborhoods where floods are “new news” — that is, where flooding has only recently become a regular occurrence. Indeed, lenders quickly learn where not to hold loans in their portfolios.

The existing rules of the mortgage-lending game actually maximise this risk to taxpayers, as Fannie and Freddie’s guarantee becomes a substitute for flood insurance. Simple reforms could discourage lenders from leaning so hard on the government-supported enterprises to absorb climate risks. The securitisation fees (also called guarantee fees) that Fannie Mae and Freddie Mac charge lenders to take on their loans should be higher for properties at relatively high risk of flooding.

And the fee structure should be designed to shift along with changes in risk and improvements in forecasting, as new climate-change scenarios materialise. At the same time, when home buyers neglect to buy the insurance that’s federally mandated in flood hazard areas, Fannie Mae and Freddie Mac should exercise their existing authority to transfer losses back to the lenders.

To make sure federal mortgage-market regulators have an accurate picture of flood risks, they should encourage the private-sector data-science industry to compete to provide the best possible forecasting algorithms. If mortgage lenders can steadily improve their understanding of climate risks, they can increasingly work those risks into their loan calculations, by asking for larger down payments and charging higher interest rates to borrowers buying vulnerable houses.

The risks of climate change keep growing, and home buyers may never develop the expertise required to recognise them. But mortgage lenders have a responsibility to see what’s ahead. They should ensure that their customers know what they’re getting into, and that taxpayers are not unwittingly exposed.

• Kahn is a Bloomberg Distinguished Professor of economics and business and the director of the 21st Century Cities Initiative at Johns Hopkins University. Ouazad is an associate professor in the department of Aapplied economics at HEC Montreal. This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

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