While it's not too easy to explain why the temperatures in ancient Rome would be a good degree warmer than they are today, at least the Roman cold snaps are open to some exploration. For anyone who remembers the winters of 1991 and 1992, you might have noticed they were a bit colder than usual, and the sunsets at the time were a bloody red. These of course coincided with the Mt Pinatubo Eruption in the Philippines. It was the biggest bang in recent history, ejecting 16 cubic kilometres of material - not a lot on the scale of things, but enough to effect the global climate for several years once it had finished pumping millions of tonnes of ash and sulphur dioxide into the atmosphere.
So, going back to the big freeze around 110BC - the one that sowed the seeds of destruction for the Roman Republic - mean temperatures fell by nearly two degrees Celsius in just a couple of years. If a volcanic eruption caused this, it must have been a big one, and it appears Mt Churchill in Alaska is a strong contender. Its eruption was considerably larger than Mt Pinatubo, producing 25 cubic kilometres of material (the famous 1883 Krakatoa eruption was 21 cubic km). Another smaller eruption (6 cubic km) from the Okmok Caldera in the Aleutian Islands may have occurred within a year or so too.
Then there's the long cooling period beginning after 56AD and bottoming out around 300AD. A lot can happen in 250-years (think what's happened since 1762). But it does coincide with a number of big volcanic eruptions. The first was Ambrym in Vanuatu, it occurred sometime around 56AD but was only half the size of the Pinatubo. It was followed in 79AD by the much smaller eruption of Mt Vesuvius (just 3.8 cubic km). And then in 186AD the Roman historian Herodian describes several weeks when the day turned to night and the evil portents - not to mention the cold and failed crops - would mire the reign of the Emperor Commodus (the one who got snuffed out a few years too early in the movie Gladiator) for the remainder of his life - he was strangled in his bath on December 31st, 192AD. This darkness most likely marks the Taupo Eruption in New Zealand - the biggest eruption during the Roman period, producing 120 cubic km atmospheric pollution - and no doubt cooling down the globe for decades. Then to cap off the 3rd-Century AD, the cold snap got colder with the eruption of Ksudatch in Eastern Russia, ejecting 26 cubic kilometres of material.
Anyone notice that apart from Mt Vesuvius all of those eruptions, which may or may not have changed Roman history, happened on the other side of the world, sometimes in places where no humans had even reached? Goes to show, that no matter how big an empire you have, nature can throw it all back at you. For more on how the Romans coped with the cold you can read 'A Body of Doubt' - available from Amazon, just follow the links