Read here. A new peer-reviewed study uncovers that climate models, including those used by the IPCC, seriously overestimated the impact on ocean and atmospheric warming, and sea levels, from both major and minor volcanoes. When corrected for, the models produce non-realistic results.
In essence, the IPCC climate models pretend there were no volcanoes prior to 1880's, so that the very first eruption in 1883 (Krakatoa) has this huge, long-term impact on temperatures in the model. Obviously, there were large and small volcanic eruptions prior to 1883, and when these are accurately taken into account, the the models' predicted temperatures moving forward no longer resemble reality. Summary: the models' predictions fail.
"The net result of this is that when the first volcano erupts into this virgin, volcano-free environment, it has a huge and long-lasting impact as the modeled climate has to incorporate this new perturbation throughout the entire system. The impact of subsequent volcanoes are less than the first, but still produce nearly additive results as the modeled climate is still adjusting to this new form of (negative) climate forcing...In his new paper, Gregory details the above-described model behavior, and goes on to suggest that if climate models were to be equilibrated to conditions which include periodically occurring volcanic eruptions, the impact of new eruptions is much shorter-lived and doesn’t lead to a long-term trend in oceanic heat content (or, thus, sea level rise).....This is major blow to the climate models, for it means that they are improperly handling the exchange of heat through the earth’s atmosphere/ocean system, with the ultimate result being that they are over-responsive to rising levels of greenhouse gases. Getting these things fixed will mostly likely lead to much more modest projections of future temperature rise and accompanying impacts. It seems as if it is time for the modelers to head back to the drawing board."