Ian Musgrave has written an open letter to Opposition Leader Tony Abbott, correcting him on his claim that “at the time of Julius Caesar and Jesus of Nazareth the climate was considerably warmer than it is now”.
But where did Abbott get the notion that it was considerably warmer in Roman times? Most likely from Ian Plimer, who on page 59 of Heaven and Earth writes:
The Roman Warming Period (250 BC – 450 AD)
Warming started about 250 BC and was enjoyed by the Greeks and Romans. The Romans had it easy. Although the Empire started in cool period, grapes were grown in Rome in 150 BC. By the 1st Century BC, Roman scribes record little snow and ice and that vineyards and olive groves extended northwards in Italy. At he peak of the Roman warming, olive trees grew in the Rhine valley of Germany. The location of vineyards is a good climate proxy. Citrus trees and grapes were grown in England as far north as Hadrian’s Wall and most of Europe enjoyed a Mediterranean climate. This suggests a very rapid warming. It was also wetter. Temperatures in the Roman Warming were 2 to 6°C warmer than today. Sea level was slightly lower than today despite the fact that times were warmer suggesting that land movements associated with the collision of Africa with Europe influenced local sea level. Roman clothing also shows that it was warmer than today.
 Allen, H. W. 1961: The history of wine Faber & Faber, London.
 Lambeck, K., Anzidei, M., Antonioli, F., Benini, A. and Esposito, A. 2004: Sea level in Roman time in the Central Mediterranean and implications for recent change. Earth and Planetary Science 224: 563-575.
“2 to 6°C warmer” would certainly qualify as “considerably warmer”, but as he commonly does, Plimer provides no cite to support his claims. He only has two references in the entire paragraph. One, about vineyards, doesn’t support his claim that the Roman period was “2 to 6°C warmer” since there are plenty of vineyards in England today. The other, on sea levels contradicts it. If sea levels were lower in Roman times, then it was likely cooler. Plimer tries to wriggle out of this by suggesting that the difference is caused by land movement, but after adjusting for land movement, sea level was still lower. Look at the abstract:
These data provide a precise measure of local sea level of âˆ’1.35Â±0.07 m at 2000 years ago. Part of this change is the result of ongoing glacio-hydro isostatic adjustment of the crust subsequent to the last deglaciation. When corrected for this, using geologically constrained model predictions, the change in eustatic sea level since the Roman Period is âˆ’0.13Â±0.09 m.
In other words, sea level in Roman times was similar to that at the start of the 20th century, suggesting that temperatures were also similar to those at the start of the 20th century, and hence cooler than curent temperatures nad certainly not “considerably warmer”.