Science Matters – Human impact on climate change, then and now

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Human activity from the industrial revolution onwards is causing global warming which, if not halted, will have catastrophic consequences.

Now scientists suggest that human activity several centuries earlier also caused climate change*. But this had the opposite effect – cooling – resulting in the ‘Little Ice Age’ between the 16th to 19th centuries.

‘In 1492 Columbus sailed the ocean blue’, ‘discovering’ the Americas and heralding unprecedented prosperity for Europeans. But for indigenous Americans it caused rapid, massive loss of life. This was mainly caused by the spread of lethal, Eurasian microbes carried by the Conquistadors. Literally thousands of infectious diseases, including smallpox, measles, scarlet fever, diphtheria and scarlet fever, killed millions of Americans who had no immunity or genetic resistance against them.

‘The Great Dying’ saw the population plummet by 90 per cent, the final death toll being around 55 million. So began a chain of events which, scientists say, cooled the whole hemisphere.

The Great Dying caused abandonment of approximately 56 million hectares of cultivated land – an area over twice size of UK. Natural reforestation of this land increased carbon uptake by plants, leading to a  consequent decline in atmospheric CO2 and finally a decrease in temperature – the Little Ice Age.  This theory is supported by coincident changes in local pollen deposits and by Antarctic ice-core records revealing trapped air bubbles from this time period with low CO2 levels.

Any lessons for today?

Unfortunately, today’s problems are on a completely different scale. Reduction in CO2 by reforestation in the 16th century was comparable to just two years worth of fossil fuel emissions today.

*A Koch et al. Quaternary Science Reviews 207 (2019) 13-36.