Sea levels are rising, which will pose serious flood risks to large parts of the Gulf and Atlantic coasts over the next three decades.
A new report led by scientists from the National Oceanic and Atmospheric Administration warns that the United States should prepare for 10 to 12 inches relative sea level rise on average over the next 30 years. This increase is due to both land subsidence and global warming. And given the greenhouse gas emissions released so far, the country is unlikely to be able to avoid it.
This rise in sea level means cities like Miami experiencing damaging flooding during today’s high tides will experience more damaging flooding by mid-century. Nationally, the report predicts that moderate coastal flooding will occur 10 times more often by 2050. Without significant adaptations, high tides will spill onto streets more frequently and disrupt coastal infrastructure, including critical harbors to supply chains and the economy.
The upper ocean will also be bring sea water further inland. By the end of the century, an average sea level rise of 2 feet or more is likely, depending on how much the world reduces greenhouse gas emissions.
Thermal expansion of the oceans
As greenhouse gases from fossil fuel use and other human activities build up in the atmosphere, they trap energy that would otherwise escape into space. This energy causes an increase in average global surface temperatures, especially in the upper layers of the ocean.
Thermal expansion occurs when the ocean heats up. The heat pushes the seawater molecules away slightly, thus taking up more space. The result is that the ocean rises higher, flooding more land.
Over the past decades, about 40 percent of global sea level rise is due to the effect of thermal expansion. The ocean, which covers about two-thirds of the Earth’s surface, absorbs and stores more than 90 percent excess heat added to the climate system due to greenhouse gas emissions.
The melting of land ice
The other major contributor to sea level rise is the melting of land ice. Mountain glaciers and polar ice caps are shrinking at a faster rate than natural systems can replace them.
When land ice melts, this meltwater eventually flows into the ocean, adding new amounts of water to the ocean and increasing the total mass of the ocean. On 50% of global sea level rise has been induced by the melting of land ice in recent decades.
Currently, the Greenland and Antarctic ice sheets contain enough frozen water that, if they melted completely, it would raise global sea levels by up to 200 feetor 60-70 meters—about the height of the Statue of Liberty.
Climate change is also melting sea ice. However, since this ice is already floating on the surface of the ocean and displacing some liquid water below, this melting does not contribute to sea level rise.
Risk will continue to rise long after emissions stabilize
While the ocean surface height increases globally as the planet warms, the impact is not the same for all coastal regions. The rate of increase can be several times faster in some places due to unique local conditions, such as changes in ocean circulation or land subsidence.
The East Coast and the Gulf Coast of the United States, for example, face above average risksaccording to the new report, while the West Coast and Hawaii are expected to be below average.
Nearly 4 in 10 US residents live near a coastline, and much of the US economy is located there as well.
Even when greenhouse gas emissions eventually drop, sea levels will continue to rise for centuries as the huge ice caps of Greenland and Antarctica will continue to melt and take a very long time to reach a new equilibrium. A 2021 Report of the Intergovernmental Panel on Climate Change shows that the excess heat already present in the climate system has stalled the current rates of thermal expansion and melting of land ice for at least the next few decades.