Boston Herald columnist Howie Carr explains why environmentalists have a legitimate cause for concern.

Try to restrain your excitement — Hillary Clinton, John Kerry and John Kasich are all writing books.

Think how many trees are going to die for … what? And then there’s the carbon footprint. All those trucks taking all those thousands of books to warehouses and stores, then picking them up after they don’t sell and taking them to some place to be pulped — at which point maybe they can at least be recycled into something more useful, like toilet paper.

Let’s start with Kerry’s proposed book. It’s going to be his “first memoir,” which you could take as a threat — there may be a sequel! Sadly, as someone pointed out yesterday, Kerry’s obvious title is already taken — “American Gigolo.”

You’ll be surprised to learn that he plans to mention his service in Vietnam, not to mention a “deep behind-the-scenes” take on his years as secretary of state. …

… Self-help books have always done very well. Kerry should think along the lines of “You Too Can Marry an Heiress — or Two!” Or he could offer more prosaic tips — “You Can Fight City Hall — How I Got that Pesky Fire Plug Moved.”

Then there’s Gov. John Kasich. His long-unawaited book is coming out in April. Another obvious title: “Son of a Mailman.” But no, his tome is going to be called “Two Paths.”

An Ohio newspaper claimed that this weighty opus enables the bust-out 2016 candidate to “leave his options open for a possible 2020 bid for president.”

They’re kidding, right? As the late Mumbles Menino used to say, hope burns eternal. But seriously …

The blockbuster new book, though, is by Hillary Clinton. It’s being published in September by Simon & Schuster, which is owned by CBS, or should I say, “See BS.” No word on what her advance was, but last September, even the worshipful New York Times couldn’t help noticing the downward trajectory of her first-week sales.

Her first book, in 2003, sold about 500,000 copies in its first week. Her 2014 “memoir” could only manage 85,000.

And then there was that final campaign book last fall, “Stronger Together.” It sold a whopping 2,912 copies.

In retrospect, those numbers should have been an early warning to the Democrats that perhaps the election wasn’t going to be the slam dunk they thought it would be.