By Dr. Robert Thorson

`This is our tsunami.” That was the haunting phrase of Biloxi’s mayor, which I read beneath the televised images of destruction from Hurricane Katrina.

But the comparison between the late August Mississippi Gulf Coast storm and last December’s Indonesian tsunami is terribly inadequate – – even misleading — with respect to earth science.

The Sumatran tsunami of December 2004 was a surprise attack that struck a largely underdeveloped coast. Its waves traveled sight unseen at speeds up to several hundred miles per hour, destroying coastal communities in minutes.

Katrina, on the other hand, was spotted nearly a week before it ground its way into a wealthy, educated nation that prides itself on homeland security and civil preparedness.

The flood surge that swept through New Orleans was created largely by human folly. We let the city sink below sea level (5 to 15 feet). We built dikes higher and higher on foundations resembling brown butter. We developed an evacuation plan that failed miserably, especially for the poor.

“This was our Atlantis.” That’s what I want my great- grandchildren to hear when they ask what happened to New Orleans in 2005.

I want them to learn about the fatal mix of rising seas, sinking lands, elevating rivers, shoreline erosion and increased storminess that finally caught up with public ignorance and government arrogance.

I want them to hear that Hurricane Katrina gave this country the “shock and awe” it needed to force an unconditional surrender to nature’s power; to realize that rebuilding New Orleans was as pointless as it was prohibitively expensive. I want them to understand that life in the Big Easy had gotten so impossible that the only answer remaining was permanent evacuation.

There are five geological factors that lead me to this sad conclusion about the lower Mississippi Delta. They are the steady rise in sea level, the sinking of the delta plain, the increased height of the riverbed, the loss of wetlands due to coastal erosion, and the increased storm strength. Rebuilding New Orleans under such circumstances is not the kind of risk that a prudent insurance company or an overspent federal government should take.

Rising seas. Sea level has been rising since the last Ice Age on low-latitude coasts throughout the world. The pace slowed about 9,000 years ago, creating the coastal deltas and marshes on which early civilizations arose. In the past few thousand years, the pace had slowed even more, to less than a tenth of an inch per year. But the pace has nearly doubled in the last century because of global warming, which melts glaciers and causes the ocean to expand.

Sinking land. Muddy deltas sink as they compact over time. This isn’t a problem as long as every layer that settles out of sight is replaced by another, courtesy of the Mississippi’s seasonal flooding. This balancing act held sway over New Orleans for thousands of years until it was stopped by the dikes in the late 18th century. The precious sediment needed to keep a great American city alive is being wasted in a place where nobody lives.

Elevating riverbed. The sediment being forced to flow past the city is dumped where the dikes finally end. This creates miles of new land, meaning that the river has to flow a longer distance before reaching its ultimate low, the sea. Rivers, however, can’t flow when there is no slope. Hence, any added length to the channel requires that its height be raised. The bed of the river at New Orleans must rise to make this happen.

Receding shoreline. With the sea coming up and the land going down, the marshy shoreline of Louisiana’s south coast has no alternative but to migrate inland via normal coastal erosion. This process has been accelerated by human activity: canals dug for oil exploration, the starvation of the delta vegetation of fertile mud, and the introduction of plant-eating water rats (nutria). As the shoreline moves closer to New Orleans, there is less and less land to buffer storms before they hit densely occupied areas. Shoreline loss causes incoming storms to have faster wind speeds, stronger waves and a higher flood surge.

Stronger storms. This variable is still being debated by climate- change scientists. The idea is that a warmer greenhouse Earth will offer up stronger, more frequent hurricanes. This seems to be taking place, but it’s too early to tell for sure.

“Let bayous be bayous,” I wrote almost a year ago when Hurricane Ivan sloshed a wall of mud soup against the tottering dikes of New Orleans, which managed to hold. “Let Old Man River do its delta thing. Let the City of Jazz become the wonderful swamp that nature intended … New Orleans will someday go under. It’s a modern-day Atlantis in the making.”

Please dig deep in your pockets to help with medical and humanitarian needs and to help the poor relocate their families to higher ground. But I advise you not to spend a dime to subsidize federal flood insurance for the Gulf Coast, or to rebuild America’s Atlantis.

By the way, my 13-year-old daughter is named Katrina. She’s another force of nature.