From the UN IPCC Fifth Assessment Report , Working Group I Contribution To The IPCC Fifth Assessment Report Climate Change 2013: The Physical Science Basis – Final Draft Underlying Scientific-Technical Basis, Chapter 9, Box 9.2: Climate Models and the Hiatus in Global-Mean Surface Warming of the Past 15 Years, page 31, :
(c) Model Response Error
The discrepancy between simulated and observed GMST trends during 1998–2012 could be explained in part by a tendency for some CMIP5 models to simulate stronger warming in response to increases in greenhouse-gas concentration than is consistent with observations… This finding provides evidence that some CMIP5 models show a larger response to greenhouse gases and other anthropogenic factors (dominated by the effects of aerosols) than the real world (medium confidence). As a consequence, it is argued in Chapter 11 that near-term model projections of GMST increase should be scaled down by about 10% (Section 11.3.6.3). This downward scaling is, however, not sufficient to explain the model-mean overestimate of GMST trend over the hiatus period.