Although there is an overwhelming amount of evidence linking an increase in global temperatures to human activity, there is a tiny but vocal minority claiming that the warm years can be explained by the natural cycle that Earth and the Sun go through.
For this reason, an international group of researchers looked at the likelihood of having 13 of the last 15 years in the 21st century being hotter than average due to natural causes. They found that the odds are between 1 in 5,000 and 1 in 170,000. These odds don’t even include the data for 2015, which was recently declared the hottest year on record.
The scientists had to take into account a significant number of variables to produce this statistical analysis. They combined observational data and sophisticated computer simulations to separate natural irregularity in climate from the impact of human activities on the planet.
"Natural climate variability causes temperatures to wax and wane over a period of several years, rather than varying erratically from one year to the next," said lead author Michael Mann, a distinguished professor of meteorology at Penn State, in a statement. "That makes it more challenging to accurately assess the chance likelihood of temperature records. Given the recent press interest, it just seemed like it was important to do this right, and address, in a defensible way, the interesting and worthwhile question of how unlikely it is that the recent run of record temperatures might have arisen by chance alone."
The research, published in Nature, also characterized the probability of individual years being a record-breaking year. They found that the probability that 2014's temperature record could have arisen from natural fluctuations alone was no more than 1 in 1,000,000.
"2015 is again the warmest year on record, and this can hardly be by chance," added co-author Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research. "Natural climate variations just can't explain the observed recent global heat records, but man-made global warming can."
"It has led to unprecedented local heat waves across the world – sadly resulting in loss of life and aggravating droughts and wildfires," concluded Rahmstorf. "The risk of heat extremes has been multiplied due to our interference with the Earth system, as our data analysis shows."