This implies that climate models are unable to fully simulate natural global warming, and the error of the underestimation is about the same as the 0.7C global warming since the end of the Little Ice Age in ~1850. Thus, the possibility that present-day temperatures could be entirely the result of natural processes cannot be ruled out in comparison to the last interglacial period.
Further, during the last interglacial, Greenland temperatures were naturally up to 8C higher and sea levels up to 29 feet higher than today. And, during another interglacial, all of Greenland and West Antarctica melted & sea levels were 79 feet higher. Since this low-CO2 global warming occurred entirely naturally, there is no evidence that global warming during the present interglacial is unnatural or man-made.
Clim. Past, 10, 1633-1644, 2014
P. Bakker1,2 and H. Renssen1
1Earth and Climate Cluster, Department of Earth Sciences, VU University Amsterdam, 1081HV Amsterdam, the Netherlands
2now at: College of Earth, Ocean and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA
Abstract. The timing of the last interglacial (LIG) thermal maximum across the globe remains to be precisely assessed. Because of difficulties in establishing a common temporal framework between records from different palaeoclimatic archives retrieved from various places around the globe, it has not yet been possible to reconstruct spatio-temporal variations in the occurrence of the maximum warmth across the globe. Instead, snapshot reconstructions of warmest LIG conditions have been presented, which have an underlying assumption that maximum warmth occurred synchronously everywhere. Although known to be an oversimplification, the impact of this assumption on temperature estimates has yet to be assessed. We use the LIG temperature evolutions simulated by nine different climate models to investigate whether the assumption of synchronicity results in a sizeable overestimation of the LIG thermal maximum. We find that for annual temperatures, the overestimation is small, strongly model-dependent (global mean 0.4 ± 0.3 °C) and cannot explain the recently published 0.67 °C difference between simulated and reconstructed annual mean temperatures during the LIG thermal maximum. However, if one takes into consideration that temperature proxies are possibly biased towards summer, the overestimation of the LIG thermal maximum based on warmest month temperatures is non-negligible with a global mean of 1.1 ± 0.4 °C.