This study from Yale, the University of Hong Kong and U of C Santa Cruz looked at climate between 3 and 5 million years ago, the point in the past when temperatures were as close to today as they have been since. Their goal was to find a stable warm climate state and evaluate the equilibrium conditions for climate sensitivity. You see, our climate today is far from equilibrium after 200 years of fossil fuel carbon emissions. One of the reasons is the great heat content of our oceans. For our planet to be in equilibrium, the oceans have to absorb the excess CO2 in the atmosphere. Surface ocean waters can only absorb so much CO2, but ocean currents allow for the surface waters to circulate to the deep ocean. Deep waters have lost much of their CO2 load by the time they are circulated back to the surface. This process takes about a thousand years and it is just one of the slow feedback processes that helps our climate return to equilibrium after some external ‘force" pushes it "out" of equilibrium.
Normally the external forcing is one of the three solar cycles associated with Earths orbit around the sun. But today, mankind’s civilization is forcing our climate with all of these extra greenhouse gases that we are putting into the atmosphere. So on a ‘normal" planet, when climate is in equilibrium, or when CO2 is about 40% less than it is today with the same average planetary temperature, these scientists found a sensitivity to climate change that is somewhere between 40% again as much and twice what the current models show. That is, the same forcing produces twice the warming.
Pagini et. al., High Earth-system climate system sensitivity from Pliocene carbon dioxide concentrations, Nature Geoscience, January 2010.