Read here. Scientists who base their livelihoods on global warming research funding claim that late 20th century warming was "unprecedented." This specific claim is embarrassingly false, and more peer-research science (study of fossil dinoflagellate cysts and pollen from sediment cores) from the Chesapeake Bay waters confirms that.
"The authors report that "several dry periods ranging from decades to centuries in duration are evident in Chesapeake Bay records." The first of these periods of lower-than-average precipitation, which spanned the period 200 BC-AD 300, occurred during the latter part of the Roman Warm Period,.....The next such period (~AD 800-1200), in the words of the authors, "corresponds to the 'Medieval Warm Period',..... First, it [Willard et al. research] demonstrates the reality of the millennial-scale hydrologic cycle that accompanies the millennial-scale temperature cycle that is responsible for producing alternating warm and cold intervals such as the Roman Warm Period, Dark Ages Cold Period, Medieval Warm Period, Little Ice Age and Modern Warm Period. Second, it [research] demonstrates that the global warming of the 20th century has not produced unusually strong wet and dry periods, contradicting climate-alarmist claims that warming will exacerbate extreme climate anomalies."