This is yet another oft-repeated but problematic assertion based in this case on the mis-characterization of the so-called Mid-Holocene Climatic Optimum” or “Mid-Holocene Warm Period”. Paleoclimate experts now know that the mid-Holocene warmth centered roughly 8000 to 6000 years ago was probably restricted to high latitudes and certain seasons (summer in the Northern Hemisphere and winter in the southern hemisphere). Because much of the early paleoclimate evidence that was available (for example, fossil pollen assemblages) came from the Northern Hemisphere extratropics, and is largely reflective of summer conditions, decades ago some scientists believed that this was a time of globally warmer conditions. More abundant evidence now demonstrates, for example, that the tropical regions were cooler over much of the year.
All of these changes are consistent with the expected response of surface temperatures to the known changes in the Earth’s orbital geometry relative to the Sun during that time period and associated climate feedbacks, as detailed in the peer-reviewed literature (see Ganopolski et al, 1998; Hewitt and Mitchell, 1998). The best available evidence suggests that annual, global mean warmth was probably similar to pre-20th century warmth, but less than late 20th century warmth, at this time (see Kitoh and Murakami, 2002).
More information about the Mid-Holocene warm period can be found here.
Ganopolski, A., C. Kubatzki, M. Claussen, V. Brovkin, and V. Petoukhov, The Influence of Vegetation-Atmosphere-Ocean Interaction on Climate During the Mid-Holocene, Science, 280, 1916-1919, 1998.
Hewitt, C.D. and J.F.B. Mitchell, A Fully Coupled GCM Simulation of the Climate of the Mid-Holocene, Geophys. Res. Lett., 25, 361-364, 1998.
Kitoh, A., and S. Murakami, Tropical Pacific Climate at the mid-Holocene and the Last Glacial Maximum simulated by a coupled ocean-atmosphere general circulation model, Paleoceanography, 17, 1-13, 2002.