It is almost certain that global mean temperatures were warmer during certain past geological periods (e.g., the Cretaceous, when we suspect that Co2 levels were higher than today, and that the globe, w/ Dinosaurs wandering around near the poles, was almost certainly warmer). These changes occurred over many millions of years, due to the influence of plate motion on the production of co2 by geological sources (e.g. volcanic outgassing). Of course, that warming occurred over many millions of years. The present warming is occurring on a century time scale, so it is the *rate* of recent warming that may be particularly anomalous in the long-term history of the climate.
...There is still some debate about this. We now have excellent CO2 records from ice cores dating back to more than 400,000 years. The present Co2 concentration appears higher than at any time during that record. The longer-term evidence is more tenuous (based on trace gases trapped in ambers, evidence from fossil leaf stomata, etc.), but it is quite likely that co2 levels were higher as one gets back towards the Cretaceous period (e.g. more than 50 million years ago), precisely how much higher is still a subject of dispute. The present thinking is that current co2 levels are probably the highest in about 20 million years.
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