On a careful reading of the historical sources, we cannot fail to notice some important facts. The first is that we cannot state with certainty which pass in the western Alps Hannibal went through.
The preferred hypothesis seems to be the Col Clapier. The Col Clapier (2,491 m) is an Alpine pass with a relatively easy crossing, located in the Cottian Alps along the border between Italy and France. It lies south of the Mont Cenis pass and north of the Rocca d'Ambin, and connects the municipality of Giaglione (near the town of Susa) with the French municipality of Bramans.
The descriptions of Hannibal’s march given by Polybius and Livy fit the geographical and morphological features of this pass rather well.
According to Polybius, once Hannibal reached the pass, he delivered his famous speech to the troops to encourage them after the immense hardships they had overcome. In the speech he pointed out the Po plain, which would have been visible from the pass: and this is the case for the Col Clapier.
Livy, for his part, recounts that, during the descent on the Italian side, it became necessary to cut through rock to overcome an obstacle. The work was carried out by lighting great fires to heat the rock and then cooling it abruptly, before attacking it with pickaxes. A study by Canadian physicists analysed rocks from the Italian side of the Col Clapier that show signs of burning: it concluded that these were caused by firewood fires.
Having said this, namely that crossing by the Col Clapier is a more than plausible hypothesis, there are at least two good reasons to refute the claim that Hannibal’s crossing of the Alps shows that the climate at the time was warmer.
Indeed, if we look carefully at the historical sources of Livy and Polybius, we can suppose that the situation may even have been the opposite.
The moment of the Carthaginian troops’ crossing is to be placed between September and the end of October 218 BC. We know this from an astronomical detail: the sources mention the setting of the Pleiades.
Historians tell us that Hannibal and his men had to walk on recent fresh snow covering old snow from the previous winter: this made the crossing very slippery and dangerous.
Now, the fact is that today, according to mountain experts, Alpine passes at around 2,500–3,000 metres with snow surviving from the previous winter can no longer be found anywhere.
So this detail, reported by both Livy and Polybius, suggests that, if anything, 218 BC was an autumn colder than ours.
The greenhouse effect in Roman times. It seems impossible, but it already existed.
Back in 2012, a study conducted by a group of scientists from several northern European universities, coordinated by Célia Sapart (a Swiss climatologist, at the time a lecturer at Utrecht University and today director of the non-profit organisation “CO2 Value Europe”), made a remarkable discovery, which was then picked up by news agencies and newspapers across half the world. Already two thousand years ago, the ancient Romans and their Chinese contemporaries were releasing large quantities of methane into the atmosphere.
The study was published in “Nature” and, as mentioned, caused quite a stir (the question of the “greenhouse effect” was already well present in the debate on the environment, although not as all-encompassing as today). The scientists hypothesised that at least 20–30% of methane, which together with carbon dioxide and the other three greenhouse gases is chiefly responsible for the “greenhouse effect”, had been produced long before the industrial revolution.
The study was carried out for the most part among the glaciers of Greenland, measuring the amount of methane that had remained trapped for millennia inside the great blocks of ice.
Thanks to the methods of palaeoclimatology, it was possible to establish that a strong surplus of the gas could be dated to around the 1st and 2nd centuries AD: exactly the period of greatest development, on the one hand, of imperial Rome and, on the other, of the Han dynasty in China.
How, though, was all this methane produced?
The scientists pointed to two sources: one natural, the other resulting from human action.
For the first, bacteria (properly, methanobacteria) were called into question, which release methane in wetlands. It goes without saying that this production decreases in periods of drought, as the wetlands shrink.
Human action, for its part, contributed to methane emissions, for example through rice paddies (hence the reference to China), or through the burning of forest areas for cultivation, or finally through the use of wood in kilns (activities that developed greatly with the expansion of the Roman empire).
But which of the two sources can be considered predominant in this singular phenomenon of pre-industrial “greenhouse effect”?
One of the scientists who signed the research, Prof. Thomas Blunier (a climate physicist still at the prestigious Niels Bohr Institute in Copenhagen), explained that “each source has a different composition. Methane produced by burning wood contains more heavy isotopes (carbon-13) than light ones (carbon-12).”
And so, by classifying the quantities found according to these differences, it was possible to establish that the abundance of gas was due to the hand of man, and not to natural factors.
The counter-proof of this thesis, according to the scientists, lies in the fact that, in ice samples dated to the time of the decline of the Roman Empire, the amount of methane found turned out to be much lower.
