Judith Curry: climate models can't be trusted

She concludes they are "not fit" for the purpose and have failed to predict even the past.

Climate models infer that all of the warming since 1950 can be attributed to humans. However, there have been large magnitude variations in global/hemispheric climate on timescales of 30 years, which are the same duration as the late 20th century warming. The IPCC does not have convincing explanations for previous 30 year periods in the 20th century, notably the warming 1910-1945 and the grand hiatus 1945-1975. Further, there is a secular warming trend at least since 1800 (and possibly as long as 400 years) that cannot be explained by CO2, and is only partly explained by volcanic eruptions.

Given the uncertainties in equilibrium climate sensitivity and the magnitude and phasing of natural internal variability on decadal to century timescales, combined with the failure of climate models to explain the early 20th century warming and the mid-century cooling, I conclude that the climate models are not fit for the purpose of identifying with high confidence the proportional amount of natural versus human causes to the 20th century warming.

As for predicting the future, Curry said the models are "warming too much":

The IPCC’s projections of 21st century climate change explicitly assume that CO2 is the control knob on global climate. Climate model projections of the 21st century climate are not convincing because of:

• Failure to predict the warming slowdown in the early 21st century
• Inability to simulate the patterns and timing of multidecadal ocean oscillations
• Lack of account for future solar variations and solar indirect effects on climate
• Neglect of the possibility of volcanic eruptions that are more active than the relatively quiet 20th century
• Apparent oversensitivity to increases in greenhouse gases

There is growing evidence that climate models are warming too much and that climate sensitivity to CO2 is on the lower end of the range provided by the IPCC...

The 21st century climate model projections do not include:

• a range of scenarios for volcanic eruptions (the models assume that the volcanic activity will be comparable to the 20th century, which had much lower volcanic activity than the 19th century
• a possible scenario of solar cooling, analogous to the solar minimum being predicted by Russian scientists
• the possibility that climate sensitivity is a factor of two lower than that simulated by most climate models
• realistic simulations of the phasing and amplitude of decadal to century scale natural internal variability.

... Hence we don’t have a good understanding of the relative climate impacts of the above or their potential impacts on the evolution of the 21st century climate.

(Thanks to reader Margaret.)