
By Anthony C. Didlake Jr.
Of all the hazards that hurricanes bring, storm surge is the greatest threat to life and property along the coast. It can sweep homes off their foundations, flood riverside communities miles inland, and break up dunes and levees that normally protect coastal areas against storms.
But what exactly is storm surge?
What Storm Surge Looks Like From Shore
As a hurricane reaches the coast, it pushes a huge volume of ocean water ashore. This is what we call storm surge.
This surge appears as a gradual rise in the water level as the storm approaches. Depending on the size and track of the hurricane, storm surge flooding can last for several hours. It then recedes after the storm passes.
Water level heights during a hurricane can reach 20 feet or more above normal sea level. With powerful waves on top of it, a hurricane's storm surge can cause catastrophic damage.
What Determines How High a Storm Surge Gets?
Storm surge begins over the open ocean. The strong winds of a hurricane push the ocean waters around and cause water to pile up under the storm. The low air pressure of the storm also plays a small role in lifting the water level. The height and extent of this pile of water depend on the strength and size of the hurricane.
As this pile of water moves toward the coast, other factors can change its height and extent.
The depth of the sea floor is one factor.
If a coastal area has a sea floor that gently slopes away from the coastline, it's more likely to see a higher storm surge than an area with a steeper drop-off. Gentle slopes along the Louisiana and Texas coasts have contributed to some devastating storm surges. Hurricane Katrina's surge in 2005 broke levees and flooded New Orleans. Hurricane Ike's 15- to 17-foot storm surge and waves swept hundreds of homes off Texas' Bolivar Peninsula in 2008. Both were large, powerful storms that hit in vulnerable locations.
The shape of the coastline can also shape the surge. When a storm surge enters a bay or river, the geography of the land can act as a funnel, sending the water even higher.
Other Factors That Shape Storm Surge
Ocean tides – caused by the gravity of the moon and sun – can also strengthen or weaken the impact of a storm surge. So, it's important to know the timing of the local tides compared to the hurricane landfall.
At high tide, the water is already at an elevated height. If landfall happens at high tide, the storm surge will cause even higher water levels and bring more water further inland. The Carolinas saw those effects when Hurricane Isaias hit at close to high tide on Aug. 3. Isaias brought a storm surge of about 4 feet at Myrtle Beach, South Carolina, but the water level was more than 10 feet above normal.
How a storm surge and high tide add up to coastal flooding. The COMET Program/UCAR and National Weather Service
Sea level rise is another growing concern that influences storm surge.
As water warms, it expands, and that has slowly raised sea level over the past century as global temperatures have risen. Freshwater from melting of ice sheets and glaciers also adds to sea level rise. Together, they elevate the background ocean height. When a hurricane arrives, the higher ocean means storm surge can bring water further inland, to a more dangerous and widespread effect.
Anthony C. Didlake Jr. is an Assistant Professor of Meteorology, Pennsylvania State University.
Disclosure statement: Anthony C. Didlake Jr. receives funding from the National Science Foundation.
Reposted with permission from The Conversation.
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The speed and scale of the response to COVID-19 by governments, businesses and individuals seems to provide hope that we can react to the climate change crisis in a similarly decisive manner - but history tells us that humans do not react to slow-moving and distant threats.
A Game of Jenga
<p>Think of it as a game of Jenga and the planet's climate system as the tower. For generations, we have been slowly removing blocks. But at some point, we will remove a pivotal block, such as the collapse of one of the major global ocean circulation systems, for example the Atlantic Meridional Overturning Circulation (AMOC), that will cause all or part of the global climate system to fall into a planetary emergency.</p><p>But worse still, it could cause runaway damage: Where the tipping points form a domino-like cascade, where breaching one triggers breaches of others, creating an unstoppable shift to a radically and swiftly changing climate.</p><p>One of the most concerning tipping points is mass methane release. Methane can be found in deep freeze storage within permafrost and at the bottom of the deepest oceans in the form of methane hydrates. But rising sea and air temperatures are beginning to thaw these stores of methane.</p><p>This would release a powerful greenhouse gas into the atmosphere, 30-times more potent than carbon dioxide as a global warming agent. This would drastically increase temperatures and rush us towards the breach of other tipping points.</p><p>This could include the acceleration of ice thaw on all three of the globe's large, land-based ice sheets – Greenland, West Antarctica and the Wilkes Basin in East Antarctica. The potential collapse of the West Antarctic ice sheet is seen as a key tipping point, as its loss could eventually <a href="https://science.sciencemag.org/content/324/5929/901" target="_blank">raise global sea levels by 3.3 meters</a> with important regional variations.</p><p>More than that, we would be on the irreversible path to full land-ice melt, causing sea levels to rise by up to 30 meters, roughly at the rate of two meters per century, or maybe faster. Just look at the raised beaches around the world, at the last high stand of global sea level, at the end of the Pleistocene period around 120,0000 years ago, to see the evidence of such a warm world, which was just 2°C warmer than the present day.</p>Cutting Off Circulation
<p>As well as devastating low-lying and coastal areas around the world, melting polar ice could set off another tipping point: a disablement to the AMOC.</p><p>This circulation system drives a northward flow of warm, salty water on the upper layers of the ocean from the tropics to the northeast Atlantic region, and a southward flow of cold water deep in the ocean.</p><p>The ocean conveyor belt has a major effect on the climate, seasonal cycles and temperature in western and northern Europe. It means the region is warmer than other areas of similar latitude.</p><p>But melting ice from the Greenland ice sheet could threaten the AMOC system. It would dilute the salty sea water in the north Atlantic, making the water lighter and less able or unable to sink. This would slow the engine that drives this ocean circulation.</p><p><a href="https://www.carbonbrief.org/atlantic-conveyor-belt-has-slowed-15-per-cent-since-mid-twentieth-century" target="_blank">Recent research</a> suggests the AMOC has already weakened by around 15% since the middle of the 20th century. If this continues, it could have a major impact on the climate of the northern hemisphere, but particularly Europe. It may even lead to the <a href="https://ore.exeter.ac.uk/repository/handle/10871/39731?show=full" target="_blank" rel="noopener noreferrer">cessation of arable farming</a> in the UK, for instance.</p><p>It may also reduce rainfall over the Amazon basin, impact the monsoon systems in Asia and, by bringing warm waters into the Southern Ocean, further destabilize ice in Antarctica and accelerate global sea level rise.</p>The Atlantic Meridional Overturning Circulation has a major effect on the climate. Praetorius (2018)
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<p>At what stage, and at what rise in global temperatures, will these tipping points be reached? No one is entirely sure. It may take centuries, millennia or it could be imminent.</p><p>But as COVID-19 taught us, we need to prepare for the expected. We were aware of the risk of a pandemic. We also knew that we were not sufficiently prepared. But we didn't act in a meaningful manner. Thankfully, we have been able to fast-track the production of vaccines to combat COVID-19. But there is no vaccine for climate change once we have passed these tipping points.</p><p><a href="https://www.weforum.org/reports/the-global-risks-report-2021" target="_blank">We need to act now on our climate</a>. Act like these tipping points are imminent. And stop thinking of climate change as a slow-moving, long-term threat that enables us to kick the problem down the road and let future generations deal with it. We must take immediate action to reduce global warming and fulfill our commitments to the <a href="https://www.ipcc.ch/sr15/" target="_blank" rel="noopener noreferrer">Paris Agreement</a>, and build resilience with these tipping points in mind.</p><p>We need to plan now to mitigate greenhouse gas emissions, but we also need to plan for the impacts, such as the ability to feed everyone on the planet, develop plans to manage flood risk, as well as manage the social and geopolitical impacts of human migrations that will be a consequence of fight or flight decisions.</p><p>Breaching these tipping points would be cataclysmic and potentially far more devastating than COVID-19. Some may not enjoy hearing these messages, or consider them to be in the realm of science fiction. But if it injects a sense of urgency to make us respond to climate change like we have done to the pandemic, then we must talk more about what has happened before and will happen again.</p><p>Otherwise we will continue playing Jenga with our planet. And ultimately, there will only be one loser – us.</p>By John R. Platt
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