Read here. The Roman Warming was an extended period of atypical warmth associated with low levels of atmospheric CO2. Neither alarmists nor climate models have any explanation for this period of natural warming. Typical alarmist response is that Roman Warming was just a regional condition, despite a slew of peer-reviewed research finding it was of global nature.
"Hence, they cannot stomach the thought that the Medieval Warm Period of a thousand years ago could have been just as warm as, or even warmer than, it has been recently, especially since there was so much less CO2 in the air a thousand years ago than there is now. Likewise, they are equally loath to admit that temperatures of the Roman Warm Period of two thousand years ago may also have rivaled, or exceeded, those of the recent past, since atmospheric CO2 concentrations at that time were also much lower than they are today. As a result, climate alarmists rarely even mention the Roman Warm Period, as they are happy to let sleeping dogs lie."