‘Climate history warns us to limit warming to 2°C –humans may not be able to adapt beyond that’

Dagomar Degroot is associate professor of history at Georgetown University. Speaking to Srijana Mitra Das at Times Evoke, he discusses archives of climate collapse — and resilience:

(Photo: Getty Images)

What is the core of your research?
I’m an environmental historian. I work on identifying how climate changed in the past, often by collabo-rating with scientists and scholars from related disciplines, and figure out how such climate change influenced diverse populations. Some collapsed while others managed to survive, even thrive. My work explores this and sees if these findings can inform climate policy now. LORDS OF THE RINGS: Trees have growth rings which record changing weather (Photo: Getty Images)

What sources are used in such history?
For identifying past climatic changes, I work with scientists in part. They use ‘the archives of nature’, aspects of the natural world that record the influence of earlier climatic variations. The rings of trees are an example — these correspond to how much a tree has grown. Bits of these rings could be thicker or thinner, depending on weather. You can use their width to figure out how the weather changed over a tree’s lifetime. Ice cores also accumulate gradually as layers, akin to tree rings. There’s some melting at the poles in summer, causing these layers. By measuring differ-ent isotopes in the layers, you gain a sense of the temperature of the whole planet because heavier isotopes tend to evaporate more when Earth is warmer and then precipitate into ice sheets.


Researchers like me also use ‘the archives of society’. These can be direct reports of weather. Educated people would often keep weather diaries while officers on ships would record changes in wind direction and velo-city. Many texts describe activities profoundly influenced by the weather — the date of harvests, sowing times, when cherry blossoms would reach full bloom or how battles were conducted, all these indicated weather. Archaeological remains show us mate-rial cultures while oral histories, through word of mouth, mirror the state of climates. You can combine all these different sources and synthesise them to see how the planet changed — and people responded.
When did it become clear humans were impacting climatic processes?
Humans have been altering the atmosphere for thousands of years, changing landscapes, wiping out megafauna and developing agriculture. Potentially bigger changes came with the 15th century genocide in the western hemisphere when European colonisers began killing millions of indigenous people, perhaps so many that previous agricultural and forest management regimes were no longer viable on a scale which could allow the regrowth of tropical forests that pull carbon dioxide (CO2) out of the atmosphere.

Large-scale pollution of the atmosphere came with industrialisation and the use of fossil fuels like coal, oil and natural gas releasing vast amounts of CO2, methane and other greenhouse gases — the weight of all the greenhouse gases pumped into the atmosphere over the last 150 years is estimatedly about 2 trillion tonnes. That outweighs the weight of all organisms on Earth plus everything we have built. In the mid-1890s, Svante Arrhenius, a Swedish chemist, wrote about how industrialisa-tion was releasing warming gases into the atmosphere which would cause a ‘benign’ heating of Earth. By the 1930s, Guy Callendar, an English engineer, figured out Earth was already warming due to CO2. The emergence of a climate consensus between a vast majority of scientists and experts, on the links between a warming world and human emissions, came in the 1970s-1980s.
ARCHIVES: Cherry blossoms tell tales (Photo: Getty Images)

Why do you posit the Dutch flourished under climate change?
Some of my work focuses on the Dutch but they were not an exception. I’ve been working with scholars all over the world to identify such resilience globally. Humans endured climatic changes even before industrialisation. Our species evolved in the Pleistocene epoch. This was distinguished by spectacular climate changes, a vast transition between glacial conditions, when Earth was about 7oC colder than now, and inter-glacial conditions when it was as warm as now. There was a huge oscillation with short-term instability but humans adapted.

Then, the Holocene enabled agriculture 11,700 years ago. The global climate became relatively stable but even here, there were profound regional changes, the most important being the drying-up of the Sahara, a grassland at the Holocene’s start, becoming the world’s largest desert very rapidly. Another example was the Little Ice Age, a period of dramatic cooling from the 13th to the 19th century. This was globally modest but locally powerful. In this cooling climate, the Dutch built up a trade empire with networks that gave them access to diverse resources which could be imported from several locations — if the prices of commodities like grains went up, they benefitted. Trade also meant breaking away from battles where you had to besiege cities for ages. Waiting around for very long was hard to do when it was so cold. Spanish soldiers, with a huge empire, had a terrible time waiting in sieges while the Dutch discovered sailing. They relied on wind power while other states were more dependent on farming, made harder by the Little Ice Age. But wind currents didn’t change then, which powered the Dutch.

FOSSIL FURY: Burning coal, oil and gas has released once-sequestered carbon (Photo: Getty Images)

Can climate history empower people today?
The world is currently headed for warming of about 3oC by the end of this century. This is extremely dangerous. Many collapses, from the Mayans to the Vikings, occurred when the climate wasn’t changing as much. Resilience might not be readily available because there are eight billion humans on the planet now. We must limit warming to 2oC because there is a possibility we can’t adapt beyond that. We’ve recently seen tragic losses of life due to heatwaves in India. Societies which survived climate change were very proactive — climate history can tell a hopeful story but only if we learn its lessons.