Some ingenious writers have suspected that Europe was much colder formerly than it is at present; and the most ancient descriptions of the climate of Germany tend exceedingly to confirm their theory. The general complaints of intense frost, and eternal winter, are perhaps little to be regarded, since we have no method of reducing to the accurate standard of the thermometer the feelings or the expressions of an orator, born in the happier regions of Greece or Asia. But I shall select two remarkable circumstances of a less equivocal nature. 1/. The great rivers which covered the Roman provinces, the Rhine and the Danube, were frequently frozen over, and capable of supporting the most enormous weights. The barbarians, who often chose that severe season for their inroads, transported, without apprehension or danger, their numerous armies, their cavalry, and their heavy wagons, over a vast and solid bridge of ice. Modern ages have not presented an instance of a like phenomenon. 2/. The reindeer, that useful animal, from whom the savage of the North derives the best comforts of his dreary life, is of a constitution that supports, and even requires, the most intense cold. He is found on the rock of Spitzberg, within ten degrees of the Pole; he seems to delight in the snows of Lapland and Siberia; but at present he cannot subsist, much less multiply, in any country to the south of the Baltic. In the time of Caesar, the reindeer, as well as the elk and the wild bull, was a native of the Hercynian forest, which then overshadowed a great part of Germany and Poland. The modern improvements sufficiently explain the causes of the diminution of the cold. These immense woods have been gradually cleared, which intercepted from the earth the rays of the sun. The morasses have been drained, and, in proportion as the soil has been cultivated, the air has become more temperate. Canada, at this day, is an exact picture of ancient Germany. Although situated in the same parallel with the finest provinces of France and England, that country experiences the most rigorous cold. The reindeer are very numerous, the ground is covered with deep and lasting snow, and the great river of St. Laurence is regularly frozen, in a season when the waters of the Seine and the Thames are usually free from ice.
My first reaction is, I wish I could write like that; and my second is to note the usual bigotry against the "savage of the North" with "his [not her] dreary life". But apart from that, it is a passage with interesting resonances.
Of course, Gibbon is probably unaware of the Gulf Stream warming northwestern Europe (Benjamin Franklin described it a few years later, in 1786). It's also fairly clear to the modern reader that human destruction of their habitat alone is enough to drive the reindeer and elk beyond the Baltic, rather than the trees providing some sort of continental cooling effect as Gibbon seems to believe. Like Gibbon, I do wonder a bit if the cold was exaggerated by Roman writers - he footnotes Ovid describing frozen lumps of wine being served at dinner, but I would observe that the Danube was a cold place for Ovid in more ways than one, and he was also a master of figures of speech.
Data are few and dubious, but the "Little Ice Age" generally described as having lasted from about 1500 to 1850 seems to have let up - and warmed up - specifically at the time that Gibbon was writing. It's not at all clear if this took Europe back up to the temperature levels of the first or second century, though. Yet his fundamental conclusion, that the major cause of climate change was anthropogenic and related to environmental exploitation, is well ahead of his time, even if the details are mostly wrong.