The Roman Warm Period, or Roman Climatic Optimum, was a period of unusually warm weather in Europe and the North Atlantic that ran from approximately 250 BC to AD 400. Theophrastus wrote that date trees could grow in Greece if they were planted, but that they could not set fruit there. That is the case today, which suggests that South Aegean mean summer temperatures in the 4th and 5th centuries BC were within a degree of modern temperatures. That and other literary fragments from the time confirm that the Greek climate then was basically the same as it was around AD 2000. Dendrochronological evidence from wood found at the Parthenon shows variability of climate in the 5th century BC, which resembles the modern pattern of variation. Tree rings from the Italian Peninsula in the late 3rd century BC indicate a period of mild conditions in the area at the time of Hannibal's crossing of the Alps with elephants. Cooling at the end of the period in Southwest Florida may have been due to a reduction in solar radiation reaching the Earth, which may have triggered a change in atmospheric circulation patterns. The phrase "Roman Warm Period" appears in a 1995 doctoral thesis. It was popularized by an article published in Nature in 1999. More recent research, including a 2019 analysis based on a much larger dataset of climate proxies, has found that the putative "Roman Warm Period", along with other warmer or colder pre-industrial periods such as the "Little Ice Age" or the "Medieval Warm Period" were regional phenomena and not globally coherent episodes.
Proxies
Pollen
A high-resolution pollen analysis of a core from Galicia concluded in 2003 that the Roman Warm Period lasted from 250 BC to AD 450 in northwestern Iberia.
Glaciers
A 1986 analysis of Alpine glaciers concluded that the period AD 100–400 period was significantly warmer than the periods that immediately preceded and followed. Artifacts recovered from the retreating Schnidejoch glacier have been taken as evidence for the Bronze Age, Roman and Medieval Warm Periods.
A 1999 reconstruction of ocean current patterns, based on the granularity of deep ocean sediment, concluded that there was a Roman Warm Period, which peaked around AD 150.
Mollusk shells
An analysis of oxygen isotopes found in mollusk shells in an Icelandic inlet concluded in 2010 that Iceland experienced an exceptionally warm period from 230 BC to AD 40.
