The Frozen Thames, Looking Eastwards towards Old London Bridge, London, by Abraham Hondius in 1677. (Museum of London)
Something strange happened in Europe from the early 1600s. For months at a time, major rivers such as the Thames in London would freeze over. This had rarely happened, and never for more than a few hours at a time. Temperatures got so cold that the period to 1814 has been called the Little Ice Age.
This was before people started pumping huge amounts of carbon dioxide into the atmosphere, kick-starting the warming that is making the world’s climate change.
Scientists have been trying to work out why things got so cold back then. Now they think they have an answer. Research published in the journal Quaternary Science Reviews — Earth system impacts of the European arrival and Great Dying in the Americas after 1492— has linked the near-annihilation of indigenous people in the Americas to a decrease in carbon levels in the atmosphere.
In a few decades, the concentration of carbon in the atmosphere suddenly dropped by 10 parts per million. This kind of change usually takes thousands of years (except in recent years, when we have jumped levels by 150 parts per million in just two centuries). Today, with vast industries and nearly eight billion people, humanity is adding three parts per million of carbon into the atmosphere — a year.
In the 1600s temperatures dropped by an average of 0.15°C globally. In Europe, the thermometers dropped even further.
The team behind the research, from University College London (UCL), crunched through a range of data from the Americas to work out that this drop was because 56-million hectares of farmland had become fallow and trees and other plants moved in and took over an area half the size of South Africa. Really quickly.
All this happened at the same time that Europeans first started making contact with, and colonising, the people already living in North and South America. Starting in 1492, these expeditions brought with them a variety of diseases for which the indigenous people did not have any immunity.
At this point, people had been living in the Americas for about 20 000 years, and farming intensively for 10 000 years. Large cities were fed by vast tracts of farmland. In central Mexico — one of the most densely populated places on Earth — the population was about 25-million when Europeans arrived. Farmers cultivated maize, cacao and fruit, and homes had their own gardens. Further south, some nine million people lived in Peru and Bolivia, the heart of the Inca Empire. North America was far more sparsely populated, with estimates that just 5.7-million people inhabited the space between the Atlantic and Pacific oceans.
Getting to these numbers was hard because the Europeans either didn’t care about recording indigenous people when they did a census, or they did a census after the numbers of people had dramatically dropped. Instead, the researchers pieced together numbers from, among others, records of army sizes, the number of houses in cities, baptisms in newly colonised groups and how much tribute was paid to ruling elites in various empires.
Put together, more than 60-million people lived in the Americas. This was the perfect target for the diseases that Europeans and slave populations from Africa brought with them.
Waves of smallpox, measles, influenza, the bubonic plague, malaria, diphtheria, typhus and cholera hit civilizations, killing up to 30% of the population at any one time.
Multiple plagues killed 55-million people in just one century. With 92% of the population suddenly gone, there were not enough people to go into the fields and farm. This also collapsed the empires of that time. From a population of nine million in the Inca heartland of
Peru and Bolivia, 8.3-million people died.
A total of 1% of the Americas switched from agriculture to wilderness in just a few decades. Trees and other vegetation sucked carbon out of the atmosphere. Less carbon was also released because farmers were no longer cutting down forests and burning large areas of land to make space for fields.
This dropped global temperatures and created the mini ice age.
Besides solving the riddle of why a “random” ice age happened, the research raises two other key points.
The first deals with the epoch that we now live in, dubbed the Anthropocene because humans play a big role in how the climate works. Current logic says it started in the late 1700s, when industrial-scale burning of fossil fuels started pumping carbon into the atmosphere, kicking off global warming.
The researchers say their findings “show that human actions had global impacts on the Earth system in the centuries prior to the Industrial Revolution”. The Anthropocene’s starting date should now be taken as 1610, they say.
The second point is one that deals with how the mini ice age is used as a mainstay of the arguments of those who deny that climate change is happening, or that humans are responsible for it. Their argument points to this sudden freeze as an example of how the climate is always changing, so the present changes are just part of a natural cycle. But, with evidence showing that humans drove even that climactic event, this argument falls apart.
From the UCL research, we now know that humans have been fundamentally altering the climate for at least four centuries.