Theclimate of ancient Rome varied throughout the existence of that civilization. In the first half of the 1st millennium BC, the climate ofItaly was more humid and cool than now and the presently arid south saw moreprecipitation.[1] The northern regions were situated in thetemperate climate zone, while the rest of Italy was in thesubtropics, having a warm and mild climate.[1] During the annual melt of the mountain snow, even small rivers would overflow, swamping the terrain (Tuscany and thePontine Marshes were deemed impassable in antiquity).[1] The existence of Roman civilization (including theEastern Roman Empire) spanned three climatological periods: EarlySubatlantic (900 BC–175 AD), Mid-Subatlantic (175–750) and Late Subatlantic (since 750).[2]
The written, archaeological and natural-scientificproxy evidence independently but consistently shows that during the period of theRoman Empire's maximum expansion and final crisis, the climate underwent changes.[3] The Empire's greatest extent underTrajan coincided with theRoman climatic optimum.[4] Theclimate change occurred at different rates, from apparent near stasis during the early Empire to rapid fluctuations during the late Empire.[3] Still, there is some controversy in the notion of a generally moister period in theEastern Mediterranean inc. 1 AD–600 AD due to conflicting publications.[5]
Throughout the entireRoman Kingdom and theRepublic there was the so-calledSubatlantic period, in which theGreek andEtruscan city-states also developed.[7] It was characterized by cool summers and mild, rainy winters.[7]
At the same time there were a number of drastic winters, including the complete freezing of theTiber in 398 BC, 396 BC, 271 BC and 177 BC.[8] In subsequent centuries the reports of occasional harsh winters became associated with flooding rather than ice on the Tiber.[8] Evidence for a coolerMediterranean climate in 600 BC–100 BC comes from remains of ancient harbors atNaples and in theAdriatic which are located about one meter below current water level.Edward Gibbon, citing ancient sources, thought that theRhine and theDanube were frequently frozen, facilitating the invasion of barbarian armies into the Empire "over a vast and solid bridge of ice".[9] Suggesting colder climate, Gibbon also contended that duringCaesar's timereindeer were commonly found in the forests of modern Poland and Germany, whereas in his time reindeer were not observed south of theBaltic.[9]
During the reign ofAugustus the climate became warmer and the aridity in North Africa persisted.[10] Thebiotopes ofHeterogaster urticae, which in Roman times occurred farther north than in the 1950s, suggest that in the early Empire mean July temperatures were at least 1 °C above those of the mid-20th-century.[3]Pliny the Younger wrote that wine and olives were cultivated in more northerly parts of Italy than in the previous centuries,[4] as didSaserna in the last century BC (both father and son).[8]
A comparison of modernwind roses with the situation in the 1st century AD shows some differences: in that time northern inflows in winter were quite rare.[11] The typical northwest winds which regularly blew in July are presently non-existent.[11] Thesea breeze began a month earlier, in April.[11]Vitruvius mentioned moisture-carrying winds blowing from the south or west which could damage books.[11] There is also evidence indicating that in the Roman period the Mediterranean climate was influenced by low frequency fluctuations insea level pressure over theNorth Atlantic, called the Centennial North Atlantic Oscillation (CNAO).[12]
During theFirst Punic War, the Mediterranean coast saw such powerful storms that the Roman fleet was destroyed twice (in 255 BC and 249 BC).[13] This was followed by drought in Italy in 226 BC, which lasted six months.[13] In December of 170 BC there was ablood rain in Rome.[13] Written sources from about 75 BC toc. 175 AD also emphasize moisture, mainly in the form of the Tiber floods inRome.[3] Large Tiber floods occurred in 5 (lasted seven days), 15, 36, 51, 69, 79 and 97 AD.[13] Starting from theRoman annexation of Egypt in 30 BC and until 155 AD favorable floods occurred more frequently in theNile.[3]
The winter of 69/70 AD was the driest known toTacitus when he wrote hisHistories around 100 AD; exactly at the same time the dry season persisted in the Americas.[14] Dry conditions returned during the reign ofHadrian.[14] InTimgad – on Hadrian's visit to that city in 133 rain fell for the first time in five years.[6] Some parts of the empire, however, saw better precipitation. A weather diary, compiled byPtolemy inAlexandria in around 120, mentioned rain in every month except August and thunder throughout the summer. It helps to explain the agricultural prosperity ofRoman Africa (the granary of Rome), and the prosperity of southern Spain in the Roman era.[15] According toRhoads Murphey, the total yearly grain supply from North Africa to Rome, "estimated as enough to feed about 350,000 people, is by no means impossible to produce for export under present conditions".[6]Columella's weather calendar suggests that summer precipitation in southern Italy, particularly in Rome andCampania, occurred more often than now. Unusually high precipitation levels were inRoman Spain during the so-calledIberian–Roman Humid Period.
Roman Spain experienced three major phases: the most humid interval in 550–190 BC, an arid interval in 190 BC–150 AD and another humid period in 150–350.[16] In 134 BC the army ofScipio Aemilianus in Spain had to march at night due to extreme heat, when some of its horses and mules died of thirst[17] (even though earlier, in 181 BC, heavy spring rains prevented theCeltiberians from relieving the Romansiege of Contrebia).[17] Through the 2nd century AD warm temperatures dominated particularly in theAustrian Alps, punctuated by further cool spells fromc. 155 to 180.[3] After about 200 the temperatures fluctuated, trending toward cool.[3]
According toSheldon Judson, in the 2nd century BC the rate ofsoil erosion inLatium increased ten times, which is associated with the increased number of settlements in southEtruria.[15] Additionally, from thefoundation of Rome until possibly 165 AD,the Romans deforested huge areas forarable land.[18] In 61 ADSeneca the Younger described the high level ofair pollution in Rome, which was associated with the extensive wood burning for fuel.[15]
Fromc. 200 toc. 290 there was a period of cooling, which affected the northwesternprovinces of the Empire.[3]Dendrochronology indicates that severe drought which began in 338 and persisted until 377 forced the nomadic pastoral federation ofHuns to seek pastures and predation farther to the west and south.[3] Their attacks north of theBlack Sea drove theGoths to flee into the Roman Empire and ultimately to attack it (particularly in theBattle of Adrianople).[3] Increased climate variability fromc. 250 to 600 coincided with the decline of theWestern Roman Empire.[19] For theEastern Roman Empire there is evidence for a regional prolonged drought in modern central Turkey inc. 400–540 AD.[5]
