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 (371 – c. 287 BC) 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, implying that South Aegean mean summer temperatures in the 4th and 5th centuries BC were within a degree of modern ones. That and other literary fragments from the time confirm that the Greek climate then was basically the same as it was around 2000. Tree rings from the Italian Peninsula in the late 3rd century BC indicate a time of mild conditions there at the time of Hannibal's crossing of the Alps with imported elephants (218 BC).
Dendrochronological evidence from wood found at the Parthenon shows variability of climate in the 5th century BC, which resembles the modern pattern of variation.
Cooling at the end of the period is noted in Southwest Florida. This 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 this putative period, along with other warmer or colder pre-industrial periods such as the "Little Ice Age" and "Medieval Warm Period" were regional phenomena, not globally coherent episodes. This analysis uses the temperature record of the last 2,000 years dataset compiled by the PAGES 2k Consortium 2017.
A 1986 analysis of Alpine glaciers concluded that the period AD 100–400 period was significantly warmer than centuries before and after. Artifacts recovered from the retreating Schnidejoch glacier have been taken as evidence for the Bronze Age, Roman, and Medieval Warm Periods.