Global warming lengthens the growing season and increases net rainfall worldwide. The enrichment of the atmospheric carbon dioxide accelerates the rate of plant growth. These are all very positive developments, both for humanity and for wild nature. As a result of climate change, the Earth is becoming a more fertile planet.

Nevertheless, say the representatives of the green movement, economically destructive -- and highly regressive -- carbon taxes must be imposed, because otherwise global warming will result in catastrophic floods of coastal areas.

This assertion is quite problematic because global warming has been going on for four hundred years. We can know this with certainty, not from the doubtful claims of researchers who assert that they can measure average global temperatures to within a tenth of a degree, but from readily available historical accounts. Civil War buffs are familiar with the massive snowball fights engaged in by Confederate armies stationed as far south as Georgia, and everyone who has read Dickens encounters tales describing much more severe winter weather in mid 19th-century London than anything we see today. If we read back further in time, we hear of a world that is much colder still, with the Thames freezing up regularly, sometimes for months on end, during the 1600s.

The profound global warming of the past four centuries cannot be plausibly ascribed to anthropogenic causes, but it certainly has happened, and the greens cannot deny it. That being the case, the catastrophic effects predicted for global warming should now be apparent. In particular, many proud cities and towns that were thriving in the seventeenth, eighteenth, and nineteenth centuries should now be sleeping beneath the waves. As Earth Day approaches, it is time that the reality, or lack thereof, of such losses be assessed.

So let’s have a look. Several major American cities were founded during the 1600s. One of the first was Boston. As the seas have grown over the past four centuries, how much of the Puritans’ fabled “City on a Hill,” has been lost to the rising tides?

This map, comparing Boston’s coastline in 1630 with that of modern times shows the inconvenient truth. None of colonial Boston has been lost. Far from it; the city’s coastline has expanded considerably.

The greens might argue that Boston is an anomaly So let’s consider another city of colonial vintage which is very much in the news these days: New York. We have maps of Manhattan going back to Dutch times; do they report a different result?

No, they do not. Rather, what a series of maps starting with the Castello map of 1660 show is that, for the past four centuries, the Big Apple has been growing, not shrinking.

It is also apparent to the casual observer that other celebrated colonial cities, including Philadelphia, Baltimore, and Charleston, are not underwater either.

But these are only the cities that we know about today. Maybe there were others, now lost to human memory -- their records of existence having been erased by global warming deniers -- whose ruins might still be found sunk in the gloomy darkness of the ocean’s bottom, far out from the ever retreating shore. This is, after all, what allegedly happened to the lost civilizations of Atlantis and Mu. Perhaps the greens might wish to make the case this happened to colonial America as well.

Personally, I’m skeptical. But this is a question of science. Either the drowned cities exist, or they don’t. If they do, then clearly everyone will have to take the warnings of those sounding the climate alarm seriously. But if they are not found, the green’s flood story will need to be rejected as a myth.

This being so, it is apparent that there can be no more important area of environmental research than the hunt for America’s vanished subsea civilization. Unless evidence for its prior existence can be produced, the central case for global warming catastrophe will lack proof. All environmentalist organizations need to stop whatever else they are doing, and refocus all their efforts and resources on the search.

Show us the lost cities, dear greens. Find the colonial American Atlantis. Don’t wait for summer. Start diving today. The fate of the planet could be at stake.

Dr. Robert Zubrin is president of Pioneer Energy of Lakewood, Colo., and the author of The Case for Mars. The paperback edition of his latest book, Merchants of Despair: Radical Environmentalists, Criminal Pseudo-Scientists, and the Fatal Cult of Antihumanism, was recently published by Encounter Books.