evidence: In the late 1700s, the air contained about 280 parts per million (ppm) of CO2. We are now up to 420 ppm, an increase of about 50%; by the end of the century, we could reach 600 ppm or more. As a result, we are already somewhere along the uncertain warming curve, with a rise of about 1.2 degrees C (2.2 degrees F) since the late 19th century. [...] The most distant period, from about 66 million to 56 million years ago, has been something of an enigma, because the Earth was largely ice free, yet some studies had suggested CO2 concentrations were relatively low. This cast some doubt on the relationship between CO2 and temperature. However once the consortium excluded estimates they deemed the least dependable, they determined that CO2 was actually quite high—around 600 to 700 parts per million, comparable to what could be reached by the end of this century. The researchers confirmed the long-held belief that the hottest period was about 50 million years ago, when CO2 spiked to as much as 1,600 ppm, and temperatures were as much as 12 degrees C higher than today. But by around 34 million years ago, CO2 had dropped enough that the present-day Antarctic ice sheet began developing. With some ups and downs, this was followed by a further long-term CO2 decline, during which the ancestors of many modern-day plants and animals evolved. This suggests, the paper’s authors say, that variations in CO2 affect not only climate, but ecosystems.