The Roman Warm Period, or Roman Climatic Optimum, is a proposed 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, which suggests that southern 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 the variability of climate in the 5th century BC that resembles 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 the period in southwestern 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 Naturein 1999.
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.
Deep ocean sediment
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.