Many of the recently observed climate changes are unprecedented over decades to millennia.
A stable climate enabled the development of modern civilization, global agriculture, and a world that could sustain a vast population, now exceeding 7 billion people. We already have unprecedented levels of CO2 in the atmosphere, so it would not be surprising to learn that some of the CO2-driven climate changes are unprecedented.
Until the last century, global temperatures over the past 11,000 years varied quite slowly, generally not more than a degree Fahrenheit (under a degree Celsius) over a period of several thousand years. In its final 2014 synthesis of more than 30,000 scientific studies, the Intergovernmental Panel on Climate Change concluded, “Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia.” How unprecedented those changes were became clear in an earlier 2012 study, the most comprehensive scientific reconstruction of global temperatures over the past 11,000 years ever made. The study’s funder, the National Science Foundation, explained in a news release: “During the last 5,000 years, the Earth on average cooled about 1.3 degrees Fahrenheit–until the last 100 years, when it warmed about 1.3 degrees F.” In short, primarily because of human-caused greenhouse gases, the global temperature is changing 50 times faster than it did during the time when modern civilization and agriculture developed, a time when humans figured out where the climate conditions—and rivers and sea levels—were most suited for living and farming.
In 2013, scientists from the International Programme on the State of the Ocean reported that the rate we are acidifying the oceans is also “unprecedented.” Approximately one quarter of the CO2 humans emit into the air gets absorbed in the oceans. The CO2 that dissolves in seawater forms carbonic acid, which in turn acidifies the ocean. As a result, the oceans are more acidic today than they have been over the last 300 million years. A 2010 study concluded that the oceans are acidifying 10 times faster today than 55 million years ago when a mass extinction of marine species occurred.