Out of Season

Unless you've seen it year after year, the arrival of the autumn forest is one of those unimaginable events, as weird as the nice neighbor down the street who turns out to be a serial killer. How on earth could these leathery green leaves turn not only yellow but the absolute essence of screaming yellow, not tinted orange but so orange you should stare at them through a filter to spare your eyes?

It seems unimaginable, too, that some fall soon that blaze will not appear. Unimaginable, but almost certain. Perhaps in my lifetime, definitely in my daughter's, unless we do extraordinary things to stop it.

Most of us have accepted that the world is growing slowly warmer because of the carbon that pours from our tailpipes, our chimneys, our power plants. But very few realize how fast and how thoroughly the climate is changing — realize, for instance, that the latest computer models indicate that later this century the climate of Boston is likely to be somewhere between the current climates of Richmond, Virginia, and Atlanta. A thousand practical dangers arise from that — everything from marshland inundation to mosquito infestation. But if you want to know what it will feel like — it will feel like no more autumn.

Already it's begun. In 1998, the warmest year on record, the fall saw a drab, muted, late display. Why? As researchers from the University of New Hampshire pointed out in last year's federally sponsored regional assessment of climate change, hard frosts “hasten the loss of chlorophyll and enhance the colors of the other pigments.” Those frosts didn't come until November and December across much of the North Country, and so the glory was perceptibly dulled. In the years ahead, such damage will grow. Global warming drives up nighttime temperatures more than daytime highs. As a result, trees keep breathing all night, and that increased respiration robs the leaves of some of the sugars that keep them bright. And warming temperatures push pests into the region, robbing trees of the vigor that blasts through each fall.

Those shifts are minor compared to what will come during the next few decades. Since the late 19th century, we've seen temperature increases of about one degree, averaged globally. But the best guess is that the planet will warm 2 to 6 degrees more in the next century, and the models the UNH scientists used for New England suggest our totals may be slightly higher still. That's enough to make New England uninhabitable for beech and birch and maple — they'll want to be a couple of hundred miles farther north. In their place we can expect oak and hickory, the kind of forest that now dominates coastal Rhode Island. And you don't exactly see a flood of Vermont plates heading for Pawtucket come October: Oaks and hickories turn a nice greenish-brown.

Bundled children pulling sleds through snowy New England winters? By late this century, the computer models suggest warmer, shorter winters, especially in New England. Meanwhile, ponds and lakes will be less and less likely to freeze: In the past eight years, Lake Champlain froze solid only twice, the longest period of thaw in nearly two centuries of records. Pancakes on the griddle, with a big pitcher of this year's maple syrup? There's a reason no one sticks taps in oak trees come early March.

In other words, if you change the temperature, you change everything. Maybe you could genetically engineer oaks and hickories to turn yellow and red (and plant them along the Kancamagus Highway so they'd spell out the Monsanto logo) — but who would want to watch them on a muggy 80-degree October day? No, fall leaves go with apple snap and sweaters, and they lead into deer hunt and freeze-up, and it's all of a piece. Disconnected glory is only spectacle — might as well build an IMAX at the New Hampshire line.

What is to be done about all this? Well, nothing we seem willing yet to do. Turn in our gas-guzzlers for fuel-efficient hybrids (but the Senate this spring voted overwhelmingly against the idea, preferring a two-year delay in proposals to change the current standards). Stop generating our electricity from coal, and turn instead to windmills (like the ones Cape Codders are trying to block for fear they'll ruin the view from the chaise longue). Spend the money to shift from a fossil-fuel economy to a renewable-energy one (but a Bush administration report foresees that we will produce 43 percent more greenhouse gases by 2020).

So, in the meantime — like, this fall — what is to be done about all this? Watch the leaves with special care, with special urgency. Try to burn them onto your retina. And watch them from a bicycle or a bus. Not from your SUV — that would be ironic, and at least for now, the sheer enthusiasm of that blaze of color defies irony.