1.7 What’s happened in the past few thousand years?
Around 17,000 years ago the Earth began to come out of its most recent ice age. As carbon dioxide levels naturally rose, the planet warmed. Then, about 12,000 years ago, global temperatures reached a plateau and we entered a period of relative climatic stability known as the Holocene. It is this stability that allowed humans to settle and farm. Then, around 5,000 years ago, greenhouse gas levels started to naturally fall again and temperatures began a slow decline, which would eventually have sent us towards our next ice age.
However, a few thousand years ago, human actions began to disrupt the Earth’s natural cycles. We started cutting down trees to clear land for farming or to burn them to keep warm - reducing the amount of carbon dioxide that could be removed from the atmosphere by photosynthesis. At the same time, we planted paddy fields to grow rice, releasing methane into the atmosphere from microbes growing in waterlogged soils. For a long period of time the levels of greenhouse gases in our atmosphere rose slowly and steadily. It has been proposed that this increasing greenhouse warming effect counteracted what should have been a period of natural cooling due to the Earth’s orbital cycles, leaving our global temperatures to remain pretty constant.
That is, until the last couple of centuries. With the start of the Industrial Revolution came mass burning of fossil fuels, large-scale deforestation and intensive farming. As a result, the levels of greenhouse gases began to shoot up astronomically. And with them, our temperatures.