Objection: In the geological record, it is clear that CO2 does not trigger climate changes. Why should it be any different now?
Answer: Given the fact that human industrialization is unique in the history of planet earth, do we really need historical precedent for CO2-triggered climate change before we accept what we observe today? Surely it is not far-fetched that unprecedented consequences would follow from unprecedented events.
But putting this crucial point aside, history does indeed provide some relevant insights and dire warnings.
During the glacial/interglacial cycles, temperatures and CO2 concentrations showed remarkable correlation. Closer examination reveals that CO2 does not lead the temperature changes, but lags by many centuries. Even so, the full extent of the warming can not be explained without the effects of CO2. Though these cycles do not demonstrate that greenhouse gas initiated warming, they do lend credence to the importance of CO2 and CH4 in setting the planetary thermostat.
There are also events in geological history when sharp rises in temperature were initiated and driven by large spikes in greenhouse gases — not unlike the fossil-fuel-emissions spike today. The Paleocene Eocene Thermal Maximum is such a case. Roughly 55 million years ago, ocean pH levels dropped drastically and global temperatures rapidly rose over 5oC. The resolution of available proxy records indicates that this occurred in a period of time no longer than 5K years; it’s not possible to know if it happened even faster. The likely cause was massive releases of methane from the ocean floors, perhaps due to some smaller warming or changes in sea level. It took over 100K years for the ocean, atmosphere, and temperatures to return to their previous state. The result was a mass extinction event that took millions of years to recover from.
We can also look at the formation of the Deccan Traps. In this case, a massive and sustained volcanic action altered atmospheric chemistry and caused a drastic climate change, one that lead to the extinction of the dinosaurs. And Snowball Earth theories involve the build-up of greenhouse gases as the mechanism by which the earth eventually escaped its frozen state.
In short, it is simply untrue that history lacks precedent for greenhouse-gas-driven warming. The precedents are there, as are the dire warnings.