Simple statement: 13 of the 15 warmest years ever recorded have occurred within the first 15 years of this century. Complicated question: what is the probability that this happened purely by chance? The latest calculated answer: it could be one in 5000, or it could be as unlikely as one in 170,000, or it could be somewhere in between.
Conclusion: whatever the odds, the probability that this run of warmest years is an accident is vanishingly small. The odds instead that it has all happened because humans have been clearing forests and burning fossil fuels − increasing the carbon dioxide content of the atmosphere, and thus warming the planet inexorably − are overwhelming.
Michael Mann is a distinguished climate scientist at Penn State University in the US who has been frequently and furiously attacked by climate denialists, and who has been a consistent critic of government complacency on climate change.
He and colleagues state in Scientific Reports journal that they thought it important to take another and more careful look at the probabilities involved in global warming as a possible symptom of some natural cycle, rather than a consequence of human profligacy.
Natural climate variations just can’t explain the observed recent global heat records, but man-made global warming can.
Stefan Rahmstorf, professor of the physics of the oceans, Potsdam Institute for Climate Impact Research
Looked at simply as a run of heads-or-tails tosses of the coin, the odds that the consistent and consecutive rises in annual average temperatures are a consequence of some random chance are astronomically low: figures of one in 27 million and one in 650 million have been cited.
But climate fluctuations are not random, and there are natural cycles to be considered, some of which would in any case be involved in year-on-year increases in annual average global temperatures.
“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,” Professor Mann says.
“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 to 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 scientists used computer simulations of climate plus observational data to look at temperatures from both the Northern Hemisphere and the entire globe for specific groups of years. They examined scenarios for record warm years of 1998, 2005, 2010 and 2014; for nine of the 10 warmest years occurring since 2000; and for 13 of the warmest 15 years occurring since 2000.
They played with a variety of data sources and statistical approaches. But, in all cases, the odds of the patterns of warming occurring with no human intervention were similarly low.
The study was completed before the close of 2015, which was even warmer. Co-author Stefan Rahmstorf, professor of the physics of the oceans at the Potsdam Institute for Climate Impact Research in Germany, says: “2015 is again the warmest year on record, and this can hardly be by chance.”
Professor Rahmstorf has warned that climate change increases the probability of dangerous extremes of heat and threatens to affect the turnover of circulation in the North Atlantic. And this latest study reinforces the link between climate change and human activity.
“Natural climate variations just can’t explain the observed recent global heat records, but man-made global warming can,” he says. “It has led to unprecedented local heat waves across the world – sadly, resulting in loss of life and aggravating droughts and wildfires.
“The risk of heat extremes has been multiplied due to our interference with the Earth system, as our data analysis shows.”