When did we start warming the planet?

Ask most people when humans started warming the planet and they are likely to mention the industrial revolution, and pick a date of around 1850 or perhaps later. However, a paper published in 2003 suggested a radically different answer, with very profound implications for us today. Known as the ‘Ruddiman Hypothesis’ (after the guy who wrote the paper) it is still not universally accepted by scientists, but unlike a lot of radical theories, this one is gaining ground as the evidence supporting it slowly grows.

The science behind the hypothesis gets pretty technical but you can grasp it by looking at the graph below. (I know graphs aren’t everyone’s cup of tea, but stick with it – it’s worth it).

The graph shows Earth’s temperature over the last half a million years or so, as recorded in ice cores. There are only two things you need to check out. The first is that there are five ‘peaks’. In between the peaks are long periods of low temperatures – these are ice ages. The peaks are known as interglacials – warm periods between the ice ages. You can see we are in one now – at the right hand edge of the graph. It covers a period of about 12,000 years and is a period known as the Holocene. All human history happens here.

The second thing to note is that our current interglacial looks very different from all the others. The temperature for the previous four drops quite quickly from a peak whereas the Holocene temperature doesn’t. It’s basically more stable. The ice core used to create the graph was extracted in 1998 and there’s not enough detail to see the last 100 years, but the long term picture is clear. Previous ‘interglacials’ showed a significant cooling period before plunging into a new ice age, but the Holocene is different: Earth’s temperature has been broadly steady for about ten thousand years (until recently). And here’s the key point. It shouldn’t have been.

So here’s the last technical bit in this story. The peaks in the graph show temperature but are also associated with high levels of greenhouse gases. The levels of these gases start to tail off after the peak and with less greenhouse effect, the temperature drops accordingly. This is what happened in the first four interglacials. What William Ruddiman noticed when analysing the ice core for the Holecene was that about 8,000 years ago, the downward trend in carbon dioxide reversed. Atmospheric CO2 started rising. And then about 5,000 years ago the downward trend in methane also reversed. There were no volcanoes, no solar cycles or sunspot patterns, no wobbles in the earth’s orbit that could explain these reversals. None of the standard physics of natural events plugged the gap between what science would predict, and what actually happened.

But there was a possible explanation. Eight to ten thousand years ago, agriculture resulted in massive forest clearances in Europe and northern China releasing large amounts of CO2. And five to six thousand years ago, the rapid spread of rice cultivation across Asia started generating methane in huge quantities. Could these early human activities have reversed the natural trend in greenhouse gases and kept the earth warm? Ruddiman argued that although the human population was much smaller than now, the passage of thousands of years could achieve a climate impact similar to that of a hundred years of industrialisation, and published his paper in Climate Change in 2003.

It was an interesting idea from a respected scientist, but did it stand up to scrutiny? Climate scientists ran Ruddiman’s data through their models and found… that it didn’t. It seemed early human populations would never have cleared enough land to make enough of a difference. It was only later, after archaeologists and anthropologists studied the findings that attitudes changed. They pointed out that the models had been based on current land usage, but early farmers would have used far more land than is needed today to grow the same amount of food, and when these findings were fed into the models, the numbers started to add up. The latest models suggest that by 1850, the globe would have been 1.2 C cooler if there had been no warming from human activity since the dawn of agriculture. Put another way, that’s more warming effect before 1850 than since 1850.

Just as the Arctic is currently warming more than the rest of the planet, so a pre-industrial global cooling of 1.2 C would have resulted in 5 to 6 C of cooling in the Arctic. By 1850 an additional area the size of Greenland would have developed permanent snow cover. We can only guess what impact this amount of cooling would have had on human history and civilisation. It’s entirely possible that there would never have been an industrial revolution.

While the details of Ruddiman’s hypothesis are still being debated, it has changed (and continues to change) current thinking. Most scientists now accept that agriculture has been affecting the climate for thousands of years, and though the amount of pre-industrial warming is a moot point, there is a consensus that it has made enough difference to stop the Holocene dipping towards a new ice age.

So we’re bound to ask, ‘has human’s global warming activity saved civilisation from icy destruction?’ The answer seems to be that very modest amounts of greenhouse gas emissions have contributed to a stable climate that has allowed human development to flourish. But this observation can’t be used to suggest that the rapid acceleration in emissions unleashed by industrialisation is a good thing. Quite the opposite. If a relatively small population with basic tools was capable of warming the planet, there is all the more reason to be concerned about the effect we are having now. The pace of current climate change means we live in dangerously volatile times. 

It’s absolutely right that we should be striving for carbon-neutrality to deal with the current climate emergency. But it’s equally true that in the very long term, if a policy of zero carbon and massive reforestation allows the usual pattern of interglacial periods to resume, Swaledale will once again be covered with ice and devoid of life. We’ve been unwittingly steering the climate away from its natural course for thousands of years. That we now know we are doing so and have a reasonable understanding of how should be seen as progress. The challenge is to use this new knowledge well.

Further Reading

Blaustein, Richard (2015)  William Ruddiman and the Ruddiman Hypothesis. Minding Nature: Winter 2015, Volume 8, Number 1 at https://www.humansandnature.org/william-ruddiman-and-the-ruddiman-hypothesis

Headrick, Daniel (2015) Global Warming, the Ruddiman Thesis, and the Little Ice Age. Journal of World History, Volume 26, Number 1, March 2015, pp. 157-160 at https://edisciplinas.usp.br/pluginfile.php/4633011/mod_folder/content/0/Global-warming-the-Ruddiman-thesis-and-the-little-ice-ageJournal-of-World-History.pdf?forcedownload=1

Ruddiman, William F (2005) How Did Humans First Alter Global Climate? Scientific American, vol. 292, issue 3, pp. 46-53  at http://www.atmosedu.com/Geol390/articles/SciAmerRuddimanMarch2005.pdf

Tyrrell, Kelly April (2018) Ancient farmers spared us from glaciers but profoundly changed Earth’s climate. https://phys.org/news/2018-09-ancient-farmers-glaciers-profoundly-earth.html

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