The climate on Earth has not always been the same; the average temperature has varied significantly throughout Earths history. These temperature variations are strongly related to the amount of CO₂ in the air, as revealed by a recent reconstruction of the climate over the past 485 million years.
The Past as a Key to the Present
Much is now known about climate change in the distant past. When we compare the current rapid warming of the Earth with that, we see both similarities and differences. Two important differences are that the climate has never changed this quickly before, and that the current climate change is caused by human activity. One similarity is that CO₂ has played a leading role both in the past and now.
The problem of climate change is not so much about how warm or cold it is, but how quickly the climate is changing. We are simply adapted to the current climate. This applies to all life, and certainly to humans with all their infrastructure. For example, sea level is highly dependent on the amount of ice on Earth and thus on temperature.
Temperature Reconstruction of the Last 485 Million Years
A 2024 study provides a detailed picture of changes in global climate over the last 485 million years. The authors have rigorously worked: they have used many indirect data about temperature (so-called proxies) and a climate model, taking into account, for example, the position of the continents and the steadily increasing solar radiation over that time period. Earlier reconstructions were often based either on proxy measurements or a climate model. The combination of methods is a strong point of this study.
Their analysis shows that the average temperature over the last half billion years has varied between 11° and 36°C, while CO₂ concentrations varied between just over 200 and 2000 particles per million (see image). These are significant variations, but they occurred over long timescales. For most of the time, it was much warmer on Earth than it is now, with a global average surface temperature of about 15°C.
However, the Earth has also experienced very cold periods, so cold that even the oceans were frozen. The most recent of such “snowball Earth” periods was 640 million years ago, which goes even further back in time than this study examines.