The science is pretty conclusive – our appetite for fossil fuels is having a devastating effect on the Earth's climate. We're already seeing the consequences, with 2019 having already set 33 world records for heat (and it's not even February). But it isn't the first time human actions have kicked the planet's temperature off kilter.  

According to research published in Quaternary Science Reviews, European colonization of the Americas may have been directly responsible for a shift in the Earth’s climate. Albeit, at a much, much smaller scale than what we're witnessing today. 

The "Little Ice Age" saw temperatures dip across many parts of the world. Although it covers a period from the late 13th century to the mid-19th century, it peaked, so to speak, in the 17th century when global temperatures were at their lowest points. Now, a team of researchers at University College London (UCL) in the UK are linking European frost fairs to events that took place on the other side of the world.

"The Great Dying of the Indigenous Peoples of the Americas resulted in a human-driven global impact on the Earth System in the two centuries prior to the Industrial Revolution," the study authors state. 

The European invasion of North and South America and subsequently, the "Great Dying", resulted in such substantial land changes that it upset the carbon cycle, they conclude. Specifically, the decimation of the indigenous people led to the abandonment of great swathes of previously cultivated land, which quickly turned to forests, savannas, and other wild environments that pull more carbon dioxide (CO2) from the atmosphere than farmland. The result: levels of atmospheric CO2 dipped and so did global temperatures. 

To back up their hypothesis, the researchers turned to population and land-use estimates pre- and post-colonization.

Before the Europeans arrived in 1492, it is thought that some 60 million people lived across the Americas. That would have been 10 percent of the world's population at that time. (Though, naturally, there are limitations to these records, which are based on indirect methods of data collection – army size, tribute records, number of buildings, etcetera – rather than thorough population censuses.) A century of war, disease, and famine and the population had collapsed by 90 percent to five or six million people by the turn of the 17th century. 

The team worked out that this would have meant 56 million hectares (138 million acres) of land previously cultivated for agriculture were deserted as a result of European colonization. The ensuing vegetation growth would have removed enough atmospheric carbon to reduce total concentration by 7 to 10 molecules of CO2 in every million molecules in the air (7-10ppm), they calculated. 

"To put that in the modern context – we basically burn (fossil fuels) and produce about 3ppm per year," Mark Maslin, Professor of Geography at UCL, told BBC News. "So, we're talking a large amount of carbon that's being sucked out of the atmosphere."

While the reliance on indirect historical records is a bit of a problem, the team points to charcoal and pollen deposits, which they say support their conclusion. Analysis of these deposits suggests a slow-down in the use of fire for land-management purposes and an overall increase in natural vegetation. Ice core records in Antarctica also suggest a fall in atmospheric carbon triggered by land processes occurred during that time.

Other theories have been put forward to explain the unusually frosty temps experienced during the "Little Ice Age" (including climate feedback, volcanic eruptions, and the Maunder minimum), but the researchers say that this particular cold spell that occurred between 1577 and 1694 CE was 20 to 87 percent, with a midpoint of 50 percent, driven by the lowering of atmospheric CO2 levels resulting from the "Great Dying".