The vast majority of paleo-climate studies have been unequivocal - there are multiple past periods of warmer than current temperatures when low atmospheric CO2 levels existed. Empirical evidence, such as the Greenland ice cores, point to three specific Holocene periods of ancient historical warming: the Minoan, the Roman, and the Medieval eras.
A new research study focusing on the U.S. Western area provides unimpeachable scientific data confirming warmer the temperatures that the previous studies have found.
"In a recent treeline study from central Colorado, Carrara and McGeehin (2015) employed a combination of 23 radiocarbon ages and annual ring counts from 18 Rocky Mountain bristlecone pine (Pinus aristata) remnants found above the local present-day limits of such trees near the Continental Divide in central Colorado, which work revealed that the majority of the tree remnants "were established above the present-day limit of bristlecone pine from prior to 2700 cal. years BP to no later than about 1200 cal. years BP." ... "...has also been found and described in a number of other studies cited by Carrara and McGeehin, namely, those of LaMarche and Mooney (1967), LaMarche and Mooney (1972), LaMarche (1973), Petersen and Mehringer (1976), Scuderi (1987), Carrara et al. (1992), Fall (1997), Lloyd and Graumlich (1997), Doerner (2007), Benedict et al. (2008), Carrara (2011), Madole (2012), Lee and Benedict (2012), Lee et al. (2014) and Morgan et al. (2014)." ..."Clearly, therefore, there exists a wealth of real-world data from the western United States that testifies of the fact that there is nothing unusual, unnatural or unprecedented about that region's current mean yearly maximum and minimum temperatures...".
This should come as no surprise since both empirical and anecdotal evidence establish that natural climate change, with wide variation and extremes, has been a constant for humanity since the beginning.
Take home global warming (and cooling) message: the climate is not stable and never will be - natural climate change rules.
Additional climate-history articles.