The major climate changes of the past all occurred because the climate was driven to change by some external change, which is typically called a climate forcing. These forcings include changes in the intensity of the sun’s radiation, volcanic eruptions (which generally cause a short-term cooling), rapid releases of greenhouse gases, and changes in Earth’s orbit.
In particular, the biggest climate changes in the past 800,000 years have been the ice-age cycle, “slow changes in Earth’s orbit which alter the way the Sun’s energy is distributed with latitude and by season on Earth,” as the U.S. National Academy of Sciences and British Royal Society put it in 2014. A key point about the global climate is that it does not appear to be inherently stable. As Wallace Broecker, a leading climatologist, wrote in the journal Nature in 1995, “The paleoclimate record shouts out to us that, far from being self-stabilizing, the Earth’s climate system is an ornery beast which overreacts even to small nudges.”
Here, for instance, is the paleoclimate record of recent ice ages: an overlay of CO2 levels in parts per million by volume (ppmv; Figure 1.2, top curve) over the past 800,000 years with the temperature in Antarctica during the same period (in °C, Figure 1.2, bottom curve) derived from various ice core samples. The trace gases that are found in deep ice layers reveal both temperatures and CO2 levels.
The data reveal that when an initial warming is triggered by an external forcing (such as an orbital change), the planet can warm up at a fast rate. That in turn implies the climate system has strong amplifying feedbacks, which turn a small initial warming into a large heating fairly quickly.