Losing higher ground: hurricanes and sea level rise

New Jersey was hit hard by the storm surge. Michael Reynolds/AAP

As I write this, the worst coastal flooding effects from Hurricane Sandy’s attack on the densely populated regions of the US northeast are almost over. Even so, the effects have been significant: record coastal inundation in lower Manhattan and flooding of many subway lines, leading to the greatest disaster in the history of the subway system.

The impact of Hurricane Sandy has been great. The bad news is that in the future, the impact of a similar storm would be even greater, due to the projected sea level rise in this region caused by global warming.

How Sandy hit so hard

Unusual meteorological conditions: Sandy is occurring quite late in the hurricane season. It is certainly not unheard of for hurricanes (severe tropical cyclones as they known in the Australian region) to occur in the Atlantic in late October, but they are much less frequent at this time of year than during the peak of the season in August and September.

Sandy’s track: Normally at this time of year, storms head out to sea after approaching the US east coast, but instead Sandy made a left turn and struck the coast of southern New Jersey. This was likely caused by its interactions with other meteorological systems already present over the northeast of North America. Although this track was unusual, it had been predicted by US meteorologists as at least possible since last Friday, when the storm was still well south of the US in the Bahamas.

The location: Sandy affected some of the most densely populated and heavily built-up areas of the United States. As a result, the potential for damage was great, simply because there is a lot of expensive infrastructure there that is located in low-lying areas close to the coast.

The storm itself: As it approached the New Jersey shore, Sandy was no longer an intense hurricane, and indeed started to lose its tropical shape as it was affected more and more by the intense temperature gradients and associated wind fields typical of mid-autumn in this region. Compared to Cyclone Yasi that struck Australia’s northeast coast last year, Sandy had much weaker winds.

Sandy was a very large hurricane, though, much larger than normal, with hurricane-force winds extending a couple of hundred kilometres on either side of its centre. Thus it was able to push elevated water levels onshore over a very large region, included New York City and its vulnerable and densely built-up lower Manhattan region.

Sandy was a rare event and indeed the media are calling it a once-in-a-generation storm, which is an accurate description. Damaging hurricanes, though, have occurred before along the US east coast.

Storms and rising sea levels

In 1938, a powerful storm smashed into Long Island, east of New York City, with winds more powerful than Sandy’s. On Long Island and in New England, it killed hundreds and caused hundreds of millions of dollars damage. Other well-documented, powerful hurricanes affected New York City in the nineteenth century. So storms like Sandy will happen again in the future. When they do, their effects will be increased by sea level rise.

Sea level rise is a highly confident prediction of climate change research. It is clear from what we know about the climate system that the sea is now rising and this will continue and indeed accelerate into the future.

What is not certain is how fast this acceleration will occur. This uncertainty is largely due to our lack of precise knowledge regarding how quickly land ice is melting in Antarctica. Without going into details, the upshot is that New York City could be facing a one metre sea level rise by 2100.

It is reasonably accurate to assume that this sea level rise can just be added on top of any coastal flooding caused by a tropical cyclone. Tropical cyclone flooding is caused by strong winds pushing extra water onto the coastline, known as storm surge. During the height of Hurricane Sandy, in lower Manhattan a storm surge of 4.2 metres above the typical low tide mark was recorded.

In 2100, that would become 5.2 metres due to sea level rise. More importantly, quite modest hurricane surges in today’s climate would become considerably more significant in a world with higher sea levels.

Luckily, New York City has considerable adaptive capacity to deal with these possible future risks. The same cannot be said about many poorer, vulnerable locations elsewhere in the world.