“The medieval warm period” was the time when English kings grew their own wine, and Vikings settled on Greenland. It lasted three centuries from around 950 to 1250, when climate in the North Atlantic and the surrounding continents was warmer than before.

“The little ice age” was the time when the Thames froze over each winter, when the Dutch painting masters portrayed skating on the channels, and the Vikings on Greenland vanished in the snow and frost. It lasted the five centuries from the early 1300s to around 1850, with climate colder than today, especially in Europe.

A Scene on the Ice by the Dutch painter Hendrik Avercamp; one of his many paintings from the cold winters during the Little Ice Age.

These are examples of natural climate changes, on a time scale which we humans can experience. Both names are somewhat exaggerated, but they are handy labels on two periods of distinct climate, which may tell us something about the effects of climate change on human civilization.

Much scientific debate has circled around how global these periods were. Probably, the periods were most pronounced in the North hemisphere.

But in terms of temperature trends, the differences were not that big: The warm period was only around 0,4 C warmer than the average, and the little ice age was around 0,4 C colder. The numbers are somewhat uncertain; the Vikings did not use thermometers or write temperature records in decimals of centigrade. The temperature record is cobbled together from measurements of oxygen isotopes in fossil microorganisms, shapes of tree leaves and more qualitative descriptions of weather and crops (like wineyards in England), records of sowing and harvest times (the earlier harvest, the warmer weather), tree rings– and images of skating Dutchmen. Nevertheless, they show how even small changes in temperature can have big impact.

The little ice age probably happened because of two events that coincided: Firstly, there was a period of very low solar activity, called the Maunder Minimum, from around 1645 to 1715, when the sun emitted less energy than normal. The sun’s activity goes in cycles, but this minimum was particularly low. Why is a topic of discussion among astrophysicists, and we will not dig further into it here (avoiding yours truly from embarrassing myself too much), but it is important to note that the sun’s activity does impact temperature on Earth.

In addition, there were several large volcano eruptions in the 13^th century, which probably gave an extra important push. We know these eruptions happened because they left tiny bands of volcanic on the ice in Antarctica and Greenland, which we later found in ice cores drilled in the glaciers. Unfortunately, only one of these eruptions is pinpointed to a known volcano; the 1257 eruption of Samalas on Lombok island in Indonesia. Other eruptions may have prolonged the little ice age, like the 1452 eruption of Kuwae in Vanuatu, in the Western Pacific, and the 1600 eruption of a volcano in the Peruvian Andes, with the tongue-bending name of Huaynaputina.

Volcanoes can have quite an impact on climate in the time scale of human civilization. In the long term, millions of years, they heat the globe by emitting CO₂, as we saw with the big thaw after the snowballs. But on the short time scale of humans, years to centuries, volcanoes instead cool the climate by emitting sulfur aerosols.

It is a common misconception that volcanic ash cools the Earth. But ash particles cannot stay up long enough to cool the Earth over time.

Aerosols do that job. Volcanoes spew out sulfur dioxide, SO₂, which rises in the atmosphere, where it reacts with water to form tiny droplets, a.k.a. aerosols, of sulfuric acid, H₂SO₂. These aerosols form a thin veil that blocks out a fraction of the sun light from reaching Earth’s surface, cooling the Earth. Aerosols can stay aloft in the atmosphere for several years.

Aerosols do the cooling job so well – or bad – that they can cause global humanitarian crises, and possibly change the direction of history. It may not be the final verdict on the cause of the little ice age, but it seems that it happened because of the combined effect of the volcanoes and the solar maunder minimum.

The little ice age came gradually, but its “official” start is often set at the great famine that ravaged Europe from 1315 to at least 1317. In the spring of 1315, rain started to pour down across Europe, and it did so more or less until the fall of 1317. Crop fields drowned, Europe starved and epidemics followed the hunger. The death toll is uncertain, but up to 25% of the population died in some towns.

This great famine was neither the first nor last time volcanoes have wreaked havoc on human civilization.

The year 536 has been called the worst year in history. It was a year when Europe was covered in cold fog, crops failed, people starved. Chinese chronicles report snow in the mid summer. Tree rings show that trees almost stopper to grow. Average temperatures plunged up to 2.5^o, and it took a decade to reach back to the normal. And in 541, a bubonic plague, the same as the Black Death, struck the eastern Mediterranean, killing between one third and one half of the population there.

Ice cores show that there was an enormous volcanic eruption in 535 or 536, which probably caused the cooling. At least one more eruption followed around 540. Geochemistry of the ash generally points to Iceland for the first eruption, but otherwise the jury is still out on where those eruptions were.

Let’s jump forward some centuries: 1816 was the year without summer. It was the year when rain poured down across Europe. Potatoes rotted in the fields and the grain never ripened. Europe already struggled to recover from the carnage of the Napoleon wars, with infrastructure and agriculture in the shambles, and grain stores running low. Hunger stroke, farmers fled from their soaked fields and flocks of beggars besieged city gates. The next two years were not much better.

These three years were almost one long, muddy, wet fall. The years 1816 to 1818 was the last big subsistence crisis in Europe, and while the wake of the Napoleon wars worsened it, the cause was on the other side of the planet:

The year before, the volcano Tambora had exploded.

Tambora was the landmark of Sumbawa island in Indonesia. A classic volcano cone, rising elegantly towards a peak in the sky above a peninsula north on the island.

In the evening of April 10^th 1815, Tambora unleashed Hell. Fountains of light shot from the summit, rivers of lava poured down its side and hailstorms of pumice drowned or incinerated whole villages on the volcano’s flanks. At least ten thousand people perished within the first hour. Tsunamis washed onshore and drowned rice fields. Finally, when the magma chamber was empty, Tambora collapsed into itself. The sky stayed dark for three days.

Europe was not the only part of the world to suffer the cold climate. In North America, the eastern coast areas suffered just as badly. New England had blizzards in June 1816. But, the emerging United States were sort of lucky: Western parts of the continent were not so hard affected, and produced enough crops to at least avoid famine.

Out in the world, Tambora altered the monsoons in India, creating draughts through the summer season of 1815. Then, when the rain came, it was a torrent with devastating floods.

The southwestern Yunnan province in China was called the “rice chamber” of the Empire, due to its stable, warm, humid climate. Tambora’s veil against the sun made it bitterly cold and dry. Famine gripped the Chinese empire. Millions perished of hunger.

Tambora’s caldera today; a wide cauldron with lush green mountain sides. (Image By Jialiang Gao (peace-on-earth.org) [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons).

Volcanoes can be really, really mean. But these volcanoes have one thing in common: They cool the Earth in the short timescale of human lives.

Climate sceptics sometimes claim that volcanoes spew out much more CO₂ than humans. This is simply not true. We have good control on how much oil and gas we burn, the amount from volcanoes is somewhat more uncertain, but the estimates are that volcanoes emit only between around 1/100^th to 1/40^th the amount of fossil fuels.

“Fossil fuels” are interesting words. Fuels made of fossils. Coal and oil are simply the remains of dead plants and algae, respectively. Plants and algae are made by photosynthesis. All life on Earth is thus ultimately made by CO₂ out of the air. The same goes for all limestone, most of which was originally the tiny shells of microorganisms.

Through the long time, burial of fossil fuels, coal and oil, means that CO₂ gradually is taken out of the atmosphere and stored in the ground, only to possibly be returned by volcanic activity.

It is therefore no surprise that the CO₂ content of the atmosphere has had bumpy ride up- and downward through the thousands of millennia. Notably, the trend has been steadily downward for around the last 40 million years.

Through most of the Earth’s time, the CO₂ content in the atmosphere has been higher than today. The last time CO₂ was almost as low as today was during the last great ice age 360 to 260 million years ago. The graph is compiled by Wikipedia; the individual sources are plotted individually and highlight the uncertainties ins the estimates; however, the overall trend is clear. Note that the time line goes from old at the left to recent at the right.

Some climate sceptics even argue that by burning fossil fuels and returning the CO₂ to where it came from, we make life on Earth a favor by halting this depletion of atmospheric CO₂, and returning some of the fuel for photosynthesis to the atmosphere.

What will happen with the temperature on Earth, then? And what will then happen to life on Earth, including us hairless apes?

Climate models from the 1970’s and onwards have predicted the temperature development the last decades pretty well. Models are always approximations and thus always somewhat wrong; some hit above the actual temperature, some hit below, but on average they have fared quite well. But the spread shows that predicting the future is an uncertain business.

Contrary to climate skeptics’ claims, the climate models have fared quite well in reproducing the temperature development.

The Paris agreement aims to limit global warming to 2^o C – hopefully 1.5^o. Even then, the outlook, according to the IPCC is grim, although not catastrophic:

IPCC predicts that the sea level will rise between 0.3 and 0.8 meter by 2100 for a 1.5^o warming. This may not sound much, but because so much of the world’s population lives by the sea, this will impact between around 100 and 150 million people by 2100, in the form of flooding and drinking ground water becoming salty from the rising salt water.

With increasing temperatures, IPCC expects that dry areas will be drier and wet areas will be wetter – more people will be victims of draughts and famine, more people will be displaced by floods and have their crops drowned.

Uncertain is the key word when discussing climate change. IPCC estimates that that to be almost certain to reach the 1.5^o target, emissions must be cut so that the actual outcome may even be less than one degree temperature rise. The path with 1.5^o increase as most likely outcome may actually end between 1.1^o and 2^o. And each of these temperature outcomes may end up in more or less severe sea level rise, droughts and floods. Best case, the impact on nature and humanity is fairly limited, worst case, entire ecosystems collapse, include important fishing stocks.

It is not the purpose of this article to give the definitive answer to these questions, because I am far from an expert on those issues, and the real experts emphasize the uncertainty.

Rather, we go back to geology and look for examples from the past, what have happened when temperature has risen fast, 1.5^o – or much more. Because, finding a historical analogue to the 1.5^o goal – a natural change of 1.5^o – isn’t that easy. Nevertheless, we may draw some inferences from history. The difference between the medieval warm age and the little ice age was around 0.8^o, and, due to the recent warming, today is around as warm as the medieval warm period. If that difference is to be used as a standard, we may even say that today is an improvement from the little ice age: Better crops, less fuel needed in the winter – and no catastrophic global sea rise. But that was from 0.8^o colder than today – which does not mean that adding the double of that from today will make the world triple as good. What we can learn is that also Mother Nature can create major climate swings: those 0.8^o clearly had impact on our civilization, which we had to adapt to – although that impact was mainly good.

The big volcanic eruptions also tell us that small changes may have big impact: Tambora lowered global temperature by no more than 0.7^o, and still the results were devastating. But that was a cooling, not a warming, of the Earth.

Looking further back in time, the warming from the glacials and interglacials happened rapidly in geological terms, within a few thousand years. But those rapid temperature ascents were not 1.5 but up to ten degrees – a clear reminder that also natural climate changes can be large can create large swings, but far from what we expect human-made warming to do.

Finally, rounding of this series about geology and climate change, we will take a look at another time when temperatures rose between 5^o and 8^o – with scary results.

The Paleocene-Eocene Thermal Maximum, PETM, was a, geologically speaking, brief warming period 55 million years ago. Through a period of around 20 000 years, the global temperature increased by between 5^o and 8^o, and the whole event lasted for around 200 000 years before temperature was back to normal. The major reason was probably a small version of the end-Permian extinction; volcanoes during the opening of what is now the North Atlantic Ocean.

PETM happened at a time when both global temperature and atmospheric CO₂ was higher than today, before the onset of the current ice age. This was a time with coal swamps on Spitsbergen. But it is relevant because the speed of the increase in temperature was comparable to today, and because it may point to the consequences if we do not act: With a temperature increase of 5^o-8^o. it could mean a mass extinction of life.

The PETM was far from as bad as the extinctions at the end of the Permian or the Cretaceous. In the ocean, 35-50% of bottom-dwelling foraminifera died out. Foraminifera are a group of tiny one-celled organisms that build shells, usually less than 1 mm large, and investigation of sediments suggest that they died because the ocean bottom became anoxic. But mussels and other molluscs took little hit, and higher life; plants, fish and land-dwelling mammals, do not show an extinction of large groups of animals, with a void following. Instead, the warming led to flora and fauna of the warm regions on Earth spreading out and replacing those adapted to colder climate.

What, then, if we copy-paste the PETM temperature increase onto our current Earth? The sea level would rise several meters, because of the melting of the polar ice caps. In fact, the sea level rise would be much worse than during the PETM, because there were no ice caps to melt back then.

Climate zones would spread from the Equator and towards higher latitudes, with tropical fauna following, squeezing temperate fauna northwards and the arctic fauna towards…well, there is no north of the North Pole, so the result could be extinction.

Overall, he planet may even benefit from higher temperatures and better growing seasons, but as climate zones shift, wet areas become wetter and dry areas become drier, there would be a wholesale moving of the agricultural infrastructure of mankind. And with the moving climate zones would follow millions of climate refugees.

The fossil record of the PETM may seem comforting on a large scale; no big catastrophe. But those are snapshots of life back then; for billions of animals, the climate changes meant displacement or starving from droughts or soaked gracing lands. If the same happens now, those animal victims will be…us.

Will our burning of fossil fuels make it so bad as the PETM? Probably not. But you do still pay the fire insurance on your house, even if the risk that it actually burns down is low, don’t you?

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