|Changes in the solar and carbon dioxide forcings over the past 420 million years. Illustration: Foster et al. (2017); Nature Communications.|
In particular, as shown in the first chart above, Earth’s climate has been fairly stable over the past several million years. The wiggles in the blue line represent transitions in and out of ice ages, due to wobbles in the Earth’s orbit around the sun, amplified by changes in the amount of carbon dioxide in the atmosphere (these are known as Milankovich cycles).
In every case the line is already quite steep due to the hundreds of billions of tons of carbon pollution humans have dumped into the atmosphere thus far. The size of the global energy imbalance we’ve caused is already on par with those previous blue wiggles – Earth’s ice age transitions. If we keep burning lots of fossil fuels, we could soon cause higher carbon dioxide levels and faster climate change than the Earth has seen in 50 million years. If we burn all available fossil fuel reserves (the black “Wink12k” line), we’ll see faster climate change than in the entire 420 million year record.
It’s an alarming proposition. Climate deniers will often argue against taking action to curb carbon pollution because climate changed naturally in the past and carbon dioxide levels were higher in the past. One Republican congressman repeated these talking points in the latest House “Science” committee hearing. While both arguments are technically true, they miss several important points.
First, the rate at which we're currently causing climate change is alarmingly fast - much faster than in most natural climate change events. Second similar past rapid climate changes have caused mass extinction events. Third, the sun's cooler past helped keep temperatures lower.
As renowned climate scientist Wallace Broecker once said, "The climate system is an angry beast, and we are poking at it with sticks."