The Roman Warm Period or the Roman climatic optimum has been proposed as a period of unusually warm weather in Europe and the North Atlantic that ran from approximately 250 BC to AD 400.
Roman Warm Period Wikipedia
The phrase "Roman Warm Period" appears in a 1995 doctoral thesis. It was popularized by an article published in Nature in 1999.
Theophrastus (371 – c. 287 BC) wrote that date trees could grow in Greece if planted, but could not set fruit there. This is the same situation as today, and suggests that southern Aegean mean summer temperatures in the fourth and fifth centuries BC were within a degree of modern temperatures. This and other literary fragments from the time confirm that the Greek climate during that period 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 resembling the modern pattern of variation. Tree rings from Italy in the late 3rd century BC indicate a period of mild conditions in the area at the time that Hannibal crossed the Alps with elephants.
Cooling at the end of this period in south west Florida may have been due to a reduction in solar radiation reaching the Earth, which may have triggered a change in atmospheric circulation patterns.Pollen: A high resolution pollen analysis of a core from Galicia concluded in 2003 that the Roman Warm Period lasted from 250 BC–AD 450 in northwestern Iberia.
Glaciers: A 1986 analysis of Alpine glaciers concluded that the AD 100 to 400 period was significantly warmer than the immediately preceding and following periods.
Deep ocean sediment: A 1999 reconstruction of ocean current patterns based on the granularity of deep ocean sediment concluded there was a Roman Warm Period that 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.