Holocene century-on-century changes have a standard deviation close to 1deg C, so if there is a signal due to carbon dioxide, it still has not emerged from the background noise
A paper of mine has just appeared in Energy and Environment:
(Lloyd, Philip J. An estimate of the centennial variability of global temperatures. Energy & Environment, 26(3), pp. 417–424 2015. DOI: 10.1260/0958-305X.26.3.417
with an abstract:
“There has been widespread investigation of the drivers of changes in global temperatures. However, there has been remarkably little consideration of the magnitude of the changes to be expected over a period of a few decades or even a century. To address this question, the Holocene records from several ice cores up to 8000 years before present were examined. The differences in temperatures between all records which are approximately a century apart were determined, after any trends in the data had been removed. The differences were close to normally distributed. The average standard deviation of temperature over a century was 0.98 ± 0.27 oC.
This suggests that while some portion of the temperature change observed in the 20th century was probably caused by greenhouse gases, there is a strong likelihood that the major portion was due to natural variations. “
I was led to this by a statement from Trenberth et al in AR4 “The standard deviation of the HadCRUT3 annual average temperatures for the globe for 1850 to 2005 shown in Figure 3.6 is 0.24°C. The greatest difference between two consecutive years in the global average since 1901 is 0.29°C between 1976 and 1977, demonstrating the importance of the 0.75°C and 0.74°C temperature increases (the HadCRUT3 linear trend estimates for 1901 to 2005 and 1906 to 2005, respectively) in a centennial time-scale context.”
This can only be regarded as naïve – the standard deviation of annual temperatures cannot indicate much about the standard deviation
over a century.