One of the recurrent themes of the climate change denialists is that we can’t blame humans, since “climate is always changing!” My general response to this is a shrug and “aren’t we the ones who told you that?” The idea that climates of the past have been different than that of today is obvious to any paleontologist or other student of Earth’s past. Data from the fossil record is what first gave scientists an appreciation of changing global climates and remains essential for unraveling the causes, patterns, and consequences of these shifts. It is our appreciation of these changes that makes us so worried; we know the historical context of our modern climate and that it is increasingly anomalous.
The really relevant time is the last two million years, the period during which humans evolved and developed civilization. It is in this context that that truly anomalous features of today’s climate stand out. Multiple lines of evidence, including many based on fossils, tell us that carbon dioxide levels in the atmosphere have been between 200 and 300 parts per million (ppm) for this entire interval, with carbon dioxide levels and temperature rising and falling in step. Last year, the value rose to above 400 ppm. We have to go back at least 4 million years, well before our species evolved, to find similar levels. It is predicted that these values might rise as high as 600–800 ppm by the end of the century, levels not seen for 25 million years, when apes first appeared, and the world was much warmer than today.