Natural Variation

The earth has experienced roughly cyclical warming and cooling periods over at least the past 800,000 years, accompanied by roughly cyclical increases and decreases in atmospheric CO2. The positive temperature anomalies over this period have ranged from approximately +0.3 to +2.7°C, while the negative temperature anomalies have ranged from approximately -3 to -5°C. The lowest negative temperature anomaly in recent times was approximately -1.2°C in 1579, during the Little Ice Age. The current temperature anomaly is approximately 0.92°C, calculated in July 2021. Atmospheric CO2 concentrations have ranged from approximately 177 ppmv to the recent approximately 420 ppmv. Graphical presentations of this information can be viewed at the linked site by clicking on the icons at the upper left-hand corner of the graph. The darker lines on the 1000-year graphs represent the instrumental data period.

The shorter-term graphs do not show the Roman Warm Period, the Medieval Warm Period or the Little Ice Age as clearly as some other sources. While the graphs in the longer term reference above show a total temperature range of approximately 4°C over the past 800,000 years, the graph in the second reference shows a total temperature range of approximately 2°C over the past 4,500 years. This graph provides a clearer comparison of recent warm and cool periods including the modern warming period.

There was extensive discussion within the consensed climate science community approximately 10 years ago regarding the inconvenience of the Roman and Medieval warm periods, which could not be explained as being the result of increased atmospheric CO2 concentrations resulting from anthropogenic emissions, There was discussion of the advantages of “disappearing” these warm periods. There has recently been discussion in the consensed climate science community about the end of natural variation, suggesting that all recent changes in climate are the result of anthropogenic CO2 emissions.

However, there is no scientific basis for suggesting that the roughly cyclical pattern of global temperature change over the past 800,000 years has ceased, and that all temperature change over the past 70, or even 140, years has been the result of anthropogenic GHG emissions. There is no question that anthropogenic GHG emissions have occurred and that they have had some effect on global temperatures. However, there is also no scientific basis on which to assert that anthropogenic emissions are totally, or even primarily, responsible for increasing global temperatures. There is also no scientific basis upon which to allocate the anthropogenic influence on increased global temperatures among GHG emissions, land use changes and the Urban Heat Island effect on temperature measurements.

There is also little or no evidence that anthropogenic influence on climate has had any adverse impact on the frequency or intensity of weather events such as tropical cyclones, tornadoes, droughts, floods, heat and cold waves, wildfires, etc. However, there is evidence that increased atmospheric CO2 has had a dramatic positive impact on global greening, as well as a positive impact on the efficiency with which many common plant species use available water resources.