Natural climate variability: Interpretation of the post 2000 temperature standstill
The period from 2000 to 2016 shows a modest warming trend that the advocates of the anthropogenic global warming theory have labeled as the “pause” or “hiatus.” These labels were chosen to indicate that the observed temperature standstill period results from an unforced internal fluctuation of the climate (e.g. by heat uptake of the deep ocean) that the computer climate models are claimed to occasionally reproduce without contradicting the anthropogenic global warming theory (AGWT) paradigm. In part 1 of this work, it was shown that the statistical analysis rejects such labels with a 95% confidence because the standstill period has lasted more than the 15 year period limit provided by the AGWT advocates themselves. Anyhow, the strong warming peak observed in 2015-2016, the “hottest year on record,” gave the impression that the temperature standstill stopped in 2014. Herein, the authors show that such a temperature peak is unrelated to anthropogenic forcing: it simply emerged from the natural fast fluctuations of the climate associated to the El Niño–Southern Oscillation (ENSO) phenomenon. By removing the ENSO signature, the authors show that the temperature trend from 2000 to 2016 clearly diverges from the general circulation model (GCM) simulations. Thus, the GCMs models used to support the AGWT are very likely flawed. By contrast, the semi-empirical climate models proposed in 2011 and 2013 by Scafetta, which are based on a specific set of natural climatic oscillations believed to be astronomically induced plus a significantly reduced anthropogenic contribution, agree far better with the latest observations.
As explained in part 1 of this study , in the last decade future climate scenarios have been used to develop and politically enforce energy expensive policies to contrast catastrophic climate warming expectations for the 21st century. This has been done mostly by the United Nations Intergovernmental Panel on Climate Change [2, 3, 4]. Several studies based on general circulation model (GCM) simulations of the Earth’s climate concluded that the 20th century climate warming and its future development depend almost completely on anthropogenic activities. Humans have been responsible of emitting in the atmosphere large amount of greenhouse gases (GHG) such as CO2 throughout the combustion of fossil fuels. This paradigm is known as the Anthropogenic Global Warming Theory (AGWT).
However, before trusting GCM projections about future climatic changes, it is necessary to validate these models by testing whether they are able to properly reconstruct past climate changes. In Ref. , the authors have argued that since 2001 AGWT was actually supported by the belief that the “hockey stick” proxy temperature reconstructions, which claim that an unprecedented warming occurred since 1900 in the Northern Hemisphere, were reliable [2,5] and could be considered an indirect validation of the available climate models supporting the AGWT . However, since 2005 novel proxy temperature reconstructions questioned the reliability of such hockey stick trends by demonstrating the existence of a large millennial climatic oscillation [7-10]. This natural climatic variability is confirmed by historical inferences  and by climate proxy reconstructions spanning the entire Holocene [12, 13]. A millennial climatic oscillation would suggest that a significant percentage of the warming observed since 1850 could simply be a recovery from the Little Ice Age of the 14th – 18th centuries and that throughout the 20th century the climate naturally returned to a warm phase as it happened during the Roman and the Medieval warm periods [9, 11, 14- 16].
To test the reliability of the Coupled Model Intercomparison Project Phase 5 (CMIP5) GCMs, in Ref.  it was shown that for the period 1860-2016 they predict an excessive warming relative to four independent global surface temperature reconstructions. This was a first significant discrepancy between observations and models. Then, it was noted that AGWT advocates had claimed that discrepancies between observation and modeled predictions could occur because of an unforced internal variability of the climate system that the same GCMs are able to predict .
These people were very explicit by providing the following scientific criterion to validate the models: “The simulations rule out (at the 95% level) zero trends for intervals of 15 year or more, suggesting that an observed absence of warming of this duration is needed to create a discrepancy with the expected present-day warming rate” .
By using such a 15-year interval criterion, in Ref.  we tested the CMIP5 GCMs against the observations in the periods 1922-1941, 1980-1999 and 200-2016. The first two periods were selected because they are characterized by a strong and compatible warming rate but by very different rate of anthropogenic GHG emissions. On the contrary, the 2000- 2017 period is characterized by a very strong increase of anthropogenic GHG emissions while the temperature has been quasi stationary. Our statistical analysis  confirmed with a 95% confidence that the GCMs fail to properly reconstruct the temperature trends in 1922-1941 and in 2000-2017. Thus, according to the very criterion proposed by the AGWT advocates themselves, the GCMs used to support the AGWT are demonstrated to be flawed.
Herein, a detailed study of the natural climatic variability observed after 2000 in six available global temperature records versus the performance of the GCMs is carried out. We also critically analyze the year 2015-2016, which has been famed as the hottest year on record. We show that this anomaly is simply due to a strong El-Niño event that has induced a sudden increase of the global surface temperature by 0.6 oC. This event is unrelated to anthropogenic emissions. In fact, an even stronger El-Niño event occurred in 1878 when the sudden increase of the global surface temperature was 0.8 oC: see Figure 2 in Ref. . Finally, for the post 2000 period we compare the predictions of the CMIP5 GCMs used by the IPCC , against that of two semi-empirical models proposed a few years ago [15,19].
These models were based on a specific number of natural oscillations suggested by astronomical considerations plus an anthropogenic warming effect strongly reduced by 50% relative to the GCM predictions. We stress that the latter result is consistent with recent scientific literature findings  confirming that the real climate sensitivity to CO2 doubling is about half, that is between 1 oC and 2 oC, than what predicted by the GCMs supporting the AGWT, which is about 3 oC .