"The best estimate of the centennial standard deviation of temperature during the Holocene is 0.98ºC± 0.27ºC. During the 20th century, thermometers recorded an increase of about 0.7ºC. It seems reasonably certain that there was some warming due to the increasing buildup of greenhouse gases in the atmosphere, but it seems difficult to estimate the magnitude of this warming in the face of a likely natural variation of the order of 1ºC. The signal of anthropogenic global warming may not yet have emerged from the natural background." (Lloyd PJ, 2015)
Whether you subscribe to the politicised UN definition of climate change™ or to the definition of the individual words provided by the Concise Oxford Dictionary, a recent study published in Energy and Environment (1) demonstrateed that the centennial variation of temperature within the last 11,700 years (The Holocene) since the last glacial epoch lies within natural variation. There is also a trendless temperature interval having no statistically significant warming since 1998 (2), possibly one reason why the falsifiable term 'global warming' was banished from the political vernacular in preference to the axiomatic, politically expedient and unfalsifiable term, climate change™ by the UN in 2000.
Abstract (1) Lloyd PJ. (2015) An estimate of the centennial variability of global temperatures. Energy & Environment · Vol. 26, No. 3.
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 up to 8000 years before present, from several ice cores 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 was 0.98ºC± 0.27ºC. 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.
Abstract (2) McKitrick RR. (2014) HAC-Robust Measurement of the Duration of a Trendless Subsample in a Global Climate Time Series. Open Journal of Statistics 4, 527 - 535.
The IPCC has drawn attention to an apparent leveling-off of globally-averaged temperatures over the past 15 years or so. Measuring the duration of the hiatus has implications for determining if the underlying trend has changed, and for evaluating climate models. Here, I propose a method for estimating the duration of the hiatus that is robust to unknown forms of heteroskedasticity and autocorrelation (HAC) in the temperature series and to cherry-picking of endpoints. For the specific case of global average temperatures I also add the requirement of spatial consistency between hemispheres. The method makes use of the Vogelsang-Franses (2005) HAC-robust trend variance estimator which is valid as long as the underlying series is trend stationary, which is the case for the data used herein. Application of the method shows that there is now a trendless interval of 19 years duration at the end of the HadCRUT4 surface temperature series, and of 16 - 26 years in the lower troposphere. Use of a simple AR1 trend model suggests a shorter hiatus of 14 - 20 years but is likely unreliable.