The first scientist to measure carbon dioxide’s potential to absorb heat was an American woman, Eunice Foote, in 1856. She filled one glass cylinder with carbon dioxide, and another with air, and set them out in the sunshine. Her scientific brief, published in the American Journal of Science and Arts, recorded the temperature in the carbon dioxide-filled cylinder as rising much higher than in the one with air.
Ever since, scientists have been steadily building confidence in the idea that humans are changing the climate by dumping billions of tons of greenhouse gases into the atmosphere. The advent of supercomputers has given researchers massive computer simulations of the climate system, recreating the interactions between oceans, land, and the atmosphere to detect humanity’s role in global warming. The science has been right most of the time. After analyzing 17 climate models designed between 1970 and 2007, researchers found the majority of the predictions were “indistinguishable from what actually occurred.”
But scientists are a famously cautious bunch. When asked how much we know about the climate, and our role in altering it, they almost always talk in probabilities. The UN Intergovernmental Panel on Climate Change (IPCC) has developed an entire nomenclature to describe the scientific certainty of their findings. The terms describing statistical confidence range from very high confidence (a 9 out of 10 chance) to very low confidence (a 1 in 10 chance). You can see the full range here.
In the IPCC’s latest assessment report (AR6), the first section of which was published on Aug. 9, scientists sent a clear signal of growing confidence in their findings. Comparing the text with that of the first piece of the group’s previous assessment report (AR5), from 2013, “low confidence” statements fell from about 20% to 6% of the total number of confidence statements, while the portion of statements expressing “high confidence” rose from 36% to 56%.
The assessment reports are designed to give other scientists and policymakers a comprehensive understanding of the state of climate science.
Not all confidence statements are created equal, of course, and scientists always review data to revise their conclusions. Yet one of the most important statements in the latest assessment report came with no caveat whatsoever: “It is unequivocal that human influence has warmed the atmosphere, ocean and land.”