Discrepancy found between historical temperature data and models

Ars Technica : A history of Earth’s average temperature over the last 12 000 years was recently created by using a wide range of local direct records and proxies for temperature. Zhengyu Liu of the University of Wisconsin and his colleagues compared that historical data with three climate-model simulations spanning the last 21 000 years. They found that, where the historical data showed a peak and then a brief drop in temperatures before continued increases, the models showed a continual temperature increase, albeit at a slower rate. The models based their simulations on variations in sunlight due to Earth’s orbital changes and the known growth in greenhouse gases. A closer examination of the models and data revealed that some of the proxies likely indicated summer temperatures, not yearly averages. Adjusting the models to favor summer temperatures produced a closer fit to the historical data, but significant differences remained. It is likely that adjustments need to be made to both the model and the interpretation of the historical data.