They treat horrifying statistical conditions such as unobserved heterogeneity, selection bias, heteroscedasticity, and autocorrelation.

FORBES: Publication Bias (or, Why You Can't Trust Any of the Research You Read)

The paper didn't consider spatial autocorrelation at all.

WSJ: Climate Emails: Science and Candor

On the autocorrelation issue: The models have a much larger range of lag-1 autocorrelation coefficients (0.66 to 0.95 for T2LT, and 0.69 to 0.95 for T2) than the UAH or RSS data (which range from 0.87 to 0.89).

WSJ: Climate Emails: Science and Candor