Michael Dobie Michael Dobie is a member of the Newsday

Michael Dobie is a member of the Newsday editorial board.

Fifty or sixty or eighty years ago, Louisiana’s loggers and oil drillers and flood-control experts weren’t thinking about the tiny community of Isle de Jean Charles. They had jobs to do, and did them.

The logging and oil companies cut channels through the Mississippi River delta to move their goods. The flood experts built levees and other structures to control the mighty Mississippi and stop it from overflowing its banks.

And the channels eroded the spit of land that is Isle de Jean Charles. And the efforts to stop flooding also stopped the river from carrying mud and sediment downstream that for generations built up and replenished delta lands like Isle de Jean Charles.

And now, exacerbated by climate change, Isle de Jean Charles, like much of Louisiana’s coast, is sinking into the Gulf of Mexico. More than 90 percent of it has disappeared in the last 60 years.

The federal government last year allocated $48 million to resettle the remaining 60 people, most of them part of the Biloxi-Chitimacha-Choctaw tribe. And a community and culture will be gone.

No one planned for that to happen. But it did. It’s an environmental version of the law of unintended consequences. And it happens all the time. We ought always to think about the repercussions of what we’re about to do. Sometimes they’re hard to see. Sometimes we don’t even try to look.

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Take DDT. The chemical was invented to kill mosquitoes, moths, beetles and other pests. It also thinned the eggshells of birds like bald eagles, ospreys and peregrine falcons, contributing to their rapid decline. All have rebounded since DDT was banned in 1972.

In California’s Central Valley, one of the country’s most productive agricultural regions, farmers fighting drought have been furiously pumping groundwater for irrigation. Now the Central Valley is sinking, as much as a foot a year. Besides the potential damage to all sorts of infrastructure, the drawdown of water means drier soil, which means harsher growing conditions, a real problem since almost half of America’s produce comes from California.

Here on Long Island, the law of unintended consequences plays out on our coastline. Groins built from the shore out into the ocean protect communities from losing sand. But they also disrupt the natural flow of that sand along the coast and cause erosion further down the line. It’s happened in places like Westhampton Dunes and in Montauk, where a group of homeowners is suing over erosion caused by groins on Lake Montauk Harbor.

Similarly, hard structures like bulkheads and stone revetments are springing up on Long Island Sound and Peconic Bay, from East Hampton and Orient to Sound Beach and Belle Terre. Erected by homeowners, they’re intended to protect the property behind them, but they destroy the beach in front as waves smack into the barriers and reverse direction, carrying sand out to deeper depths.

Then there are sewers. The region needs more of them, to reduce the nitrogen pollution killing our bays and rivers and to help revitalize moribund downtowns. But we need to be aware of a potential unintended consequence: When we reduce the number of septic systems and expand the sewer system, more water is dumped into the ocean and less water goes into the ground to recharge the aquifer.

That increases the risk that saltwater will intrude on the aquifer, already an issue in vulnerable areas like parts of Nassau County and the South Fork. It also draws down our tributaries and lakes. We won’t know how much of a problem this is until the U.S. Geological Survey completes its ongoing groundwater study, but we won’t be able to ignore the findings.

Unintended consequences are bad. Unlearned lessons are far worse.

Michael Dobie is a member of Newsday’s editorial board.