The word climate was originally a geographical term. Long before we changed the climate, the Greek philosopher, Aristotle divided the known world into five zones called climes (κλίμα). There were two “frigid” climes, corresponding to the polar regions and an uninhabitable “torrid” clime corresponding to the equatorial region. Between them, were two temperate climes, which were neither too hot nor too cold. These two climes, which constituted the habitable world, were further divided by the astronomer Ptolemy into seven latitudinal zones that were also called climes. He defined them mathematically, based on the length of the longest day such that it increased in half-hour steps from 13 to 16 hours. This forgotten definition of climate is worth reflecting on because it reminds us that climate defines the bounds of habitability. Climate defines the limits of life.

It was not until the seventeenth century that the word climate started to be associated with weather conditions. In this presently accepted meaning, climate is defined as the average of weather, expressed by factors such as temperature and rainfall, in a certain place over a “certain period of time”. According to the World Meteorological Organization, this certain period of time is 30 years.

I find this meteorological definition of climate concerning. This is because it is anthropocentric. It centres on humans. We can relate to a time frame of 30 years. Three decades is somewhere between a half and a third of an average human lifetime depending on where one lives and what one’s socioeconomic circumstances are. However, with this definition of climate, we lose touch with a longevity that was an implicit part of Aristotle’s frigid, torrid and temperate climes. This longevity matters. It gives us a sense of scale. Climate is big. We are small.

As a geologist, I prefer to think of climate as a descriptor of what a part of the world is like, or what it was like, for the full duration a specific chapter of geological time. These chapters are long. Geologists call them epochs, periods, eras and eons. They range in length from thousands of years to millions of years. By thinking about climate in this way, we acknowledge its true longevity. We show it respect. And with this definition, we are forced to accept that by changing the climate, we have become geological actors.