Ice core data shows CO2 levels changed less than 10 parts per million from 1600-1800 during the MWP.
From the Hockey Schtick: A new paper from Schurer et al (with Mann as co-author) finds that climate “models cannot explain the warm conditions around 1000 [years before the present, during the Medieval Warming Period] seen in some [temperature] reconstructions.”
According to Schurer et al, “We find variations in solar output and explosive volcanism to be the main drivers of climate change from 1400-1900.” They also claim, “but for the first time we are also able to detect a significant contribution from greenhouse gas variations to the cold conditions during 1600-1800.” This claim is highly unlikely given that ice cores show CO2 levels only changed by less than 10 ppm from 1600-1800, and the effect of 10 ppm CO2 on the climate today remains undetectable even with modern instrumentation.
Separating forced from chaotic climate variability over the past millennium
Andrew Schurer,1 Gabriele Hegerl,1 Michael E. Mann,2 Simon F. B. Tett,1 and Steven J. Phipps3
Journal of Climate 2013 doi: http://dx.doi.org/10.1175/JCLI-D-12-00826.1
Reconstructions of past climate show notable temperature variability over the past millennium, with relatively warm conditions during the ‘Medieval Climate Anomaly’ (MCA) and a relatively cold ‘Little Ice Age’ (LIA). We use multi-model simulations of the past millennium together with a wide range of reconstructions of Northern Hemispheric mean annual temperature to separate climate variability from 850 to 1950CE into components attributable to external forcing and internal climate variability. We find that external forcing contributed significantly to long-term temperature variations irrespective of the proxy reconstruction, particularly from 1400 onwards. Over the MCA alone, however, the effect of forcing is only detectable in about half of the reconstructions considered, and the response to forcing in the models cannot explain the warm conditions around 1000 [years before the present] seen in some reconstructions. We use the residual from the detection analysis to estimate internal variability independent from climate modelling and find that the recent observed 50-year and 100-year hemispheric temperature trends are substantially larger than any of the internally-generated trends even using the large residuals over the MCA. We find variations in solar output and explosive volcanism to be the main drivers of climate change from 1400-1900, but for the first time we are also able to detect a significant contribution from greenhouse gas variations to the cold conditions during 1600-1800. The proxy reconstructions tend to show a smaller forced response than is simulated by the models. We show that this discrepancy is likely to be, at least partly, associated with the difference in the response to large volcanic eruptions between reconstructions and model simulations.