The large scale syntheses of global mean temperature in the IPCC fourth report suggest that the Northern Hemisphere temperature in the second half of the 20th century was likely the highest in at least the past 1300 years and the 1990s was likely the warmest decade. However, this remains debated and the controversy is centered on whether temperatures during the recent half century were higher than those during the Medieval Climate Anomaly (MCA, AD 800-1300) and the Roman warm period (RWP, BC 20-A.D. 400) the most recent two natural warm periods of the late Holocene. Here the high resolution sea surface temperatures (SSTs) of two time windows around AD 900 (±40) and AD 50 (±40) which located in the MCA and RW P respectively.
CO2 Science reports this:
In an eye-opening study published in the Chinese Science Bulletin, Yan et al. (2014) recount how they derived high-resolution sea surface temperature (SST) histories of two 80-year time windows centered at approximately AD 990 and AD 50 within the Medieval Climate Anomaly (MCA) and the Roman Warm Period (RWP), respectively, by analyzing the Sr/Ca ratios and δ18O values of Tradacna gigas (giant clam) shells collected from the northern South China Sea.
As indicated in the figure below, this undertaking revealed that the mean annual SSTs of the 80-year periods centered on AD 990 (MCA) and AD 50 (RWP) were 0.8°C and 1.4°C higher than the mean SST during the AD 1994-2005 portion of the Current Warm Period (CWP). Likewise, they also report that the mean summer SSTs of the MCA and RWP were, respectively, 0.2 and 1.0°C higher than that of the CWP, while the mean winter SSTs of the MCA and RWP were, respectively, 1.3 and 1.8°C higher than that of the CWP.and later:
Last of all, the Chinese group of five write that a "recent study in Qaidam Basin of northwest China also indicated a warmer MCA," noting that "quantitative reconstructions from Sugan Lake and Gahai Lake both suggested a much higher temperature in the MCA than in the recent warm period," citing He et al. (2013). And, we might add, the second NIPCC report of 2013 also provides a wealth of data that contradict the claim of the IPCC that "recent decades were the warmest in at least the past 1,300 years," clearly demonstrating that there has been nothing unusual or unprecedented about the mean level of global warmth experienced over the past 18 years of no significant upward or downward trend.
Read More at CO2 Science.