My students believe that climate science is the study of how human-created greenhouse gases will impact the planet. And not much else.
As their curmudgeonly historian of science, I point out: ’Twas not always so.
There are plenty of researchers doing diligent, non-headline-seeking work in fields like paleoclimatology. They have my respect and gratitude.
But the popular confection my students think of as ‘climate science’ was the outcome of a hijacking, which took place in the last decades of the 20th century.
Policy-makers wanted anthropogenic CO₂ studied with the urgency of a military mission. The ‘carbon dioxide problem’ became the only legitimate problem in climate science.
Paradigms, as my students know from being forced to read Thomas Kuhn, define the legitimate problems of a research field.
So was the 1990s shift a scientific revolution, or a palace coup?
My students don’t get my humor. But they do like a good detective story, with hints and foreshadows along the way. The ‘usual suspects’ in climate change were all well known to 19th century science.
What was this big 19th century mystery?
The Ice Age.
Eyeballs roll. “That old cold case?”
The 19th century detectives turned up a lot of suspects. We, in our superior wisdom, have given them a pass. They are no longer, as the detectives say, persons of interest.
I casually suggest my students read the most recent IPPC science report (AR6) closely. With the numerical models in trouble, the usual suspects in climate change are starting to show up again…
A quick way-back to make sure we know what are we talking about
• The Cambridge dictionary defines “climate” as “the general weather conditions usually found in a particular place.”
• If we unpack this, there are two elements: a time span (“general conditions”) and a geographic region (“particular place”).
• Of the two, geography came first. The Greek klima literally means ‘lean’ or ‘incline’. But it was also their word for what we now call latitude. Based on the inclination of the Pole Star, the Greeks divided what they knew of the northern hemisphere into seven horizontal bands or climates.
• In the 1500s, after the start of European exploration, the word started to be used to describe the overall weather in a particular place. For example, in 1587 a location in the New World was reported as having a ‘warme climat’.
• Regional climates were described, as in geography. There was little science about it, except in relation to agriculture.
• The fused etymology raises questions still with us. If climate is something a geographic region has, what exactly is ‘global climate’? People may be interested in what’s going on where they live, but can they relate to Global Mean Surface Temperature? Where exactly is that, anyway? Halfway to the South Pole?
• The time span used to discuss climate is critical. In fact, it may be everything. Is it decades, centuries, millennia, epochs, or eons?
But first, the Earth must get older
• To give them their due, some ancient Greek, Roman, Chinese, and medieval Islamic scholars correctly believed that geologic changes take place over long periods of time.
• In Europe, religious doctrine held that the world’s geographic regions and their climates had not changed since the Creation. This was calculated by Bishop Ussher to have taken place at 6 pm on 22 October 4004 BC.
• The only geologic event with Biblical approval was Noah’s Flood.
• Let’s treat ‘Biblical catastrophism’ as a scientific paradigm. It struggled to explain two pesky anomalies: geologic strata and fossils.
Some Enlightenment
• The religious grip on geology only began to loosen in the late 1600s.
• Nicolas Steno, a Dane, in 1669 was among the first to suggest that fossils came from once-living organisms. Steno also outlined the common-sense rules for interpreting stratigraphic layers: the lowest layer is the oldest, and so on. Steno believed the age of the earth just had to be longer than 6,000 years.
• In the 1690s, Robert Hooke of the Royal Society in London also took issue with the Biblical view of the earth’s age. Hook argued that marine fossils found on top of hills and mountains must have gotten there by some geologic process. Hooke also suggested they came from extinct species.
• Stratigraphy became a valuable applied science in the early years of the industrial revolution, especially in coal mining.
• It was not until the mid-1700s that Enlightenment naturalists dared venture in public that the earth was older than 6,000 years.
• Even then, they did so at their peril. Buffon (Georges-Louis Leclerc, Comte de Buffon), who published the Histoire Naturelle in 1749, was “cancelled” for his views by his university in Paris. He was obliged to take them back.
Uniformitarianism and catastrophism
• It took until the early 1800s for Biblical catastrophism to be supplanted by a new paradigm in geology, uniformitarianism.
• Uniformitarianism emphasized gradual changes over the vastness of geological time.
• Precisely how vast was woefully underestimated until the development of radioactive dating techniques in the 20th century.
• Catastrophism may have stopped quoting the Bible, but never went away. It continued (and continues) in secular form.
• Spoiler: Uniformitarianism and catastrophism can be seen as two lenses for viewing a long record that contains unpredictable, chaotic events.
• Uniformitarianists see the long stretches of relative stability. Swings and variations may occur, but some mechanism is at work to restore balance. Oscillations and cycles are acceptable. In fact, cycles are good.
• Catastrophists, like The Joker in Batman, love chaos. Their attention is drawn to sudden discontinuous changes in the record. They like feedback effects, in which tiny changes amplify to have outsize influence. These can spin out of control. There is no ‘hidden hand’ trying to push everything back into balance. A new ‘equilibrium’ may not be a return, but a state change.
• The DNA of both camps runs through subsequent debates in geology, evolution, and climate, to the present day.
• Like Montagues and Capulets, the two can intermingle. For example, evolutionary biologist Stephen Jay Gould’s theory of “punctuated equilibrium” holds that the evolution of life on earth is characterized by long periods of stability infrequently punctuated by swift periods of branching speciation.
The Ice Age gets off to a rocky start
• That there had even been an Ice Age was highly controversial in the early 19th century.
• The fundamental tenets that had separated the Uniformitarianists from the Biblical catastrophists was: (a) that the earth in the past was shaped by the same natural processes we see at work in the present and (b) a lot of time has gone by.
• A layer of sediment deposited on a sea floor could have been slowly uplifted. That would explain why marine fossils are found in hillside strata.
• The Swiss Alps stick out in this story. They were the hot travel destination for Romantic poets and naturalists in the early 1800s. The poets include Goethe, Wordsworth, Schiller, Coleridge, Byron and of course the Shelleys, Percy Bysshe and Mary.
• Naturalists who tramped the Swiss Alps could see giant boulders in alpine valleys. The big rocks or ‘erratics’ looked a lot like the ones found at the base of glaciers. But there were no glaciers around. Those erratics needed explaining.
• A few independent thinkers got it right early on. In 1815, Jean-Pierre Perraudin hypothesized that glaciers had been responsible for the moving the giant boulders found in alpine valleys. In 1833, Ignaz Venetz, a Swiss naturalist, went further, proposing that much of Europe had been covered by glaciers at some point in the past.
• That concept was highly distasteful. People didn’t want to think about their beloved Europe being covered by a gigantic ice sheet.
• They can be excused for not knowing much about ice sheets. The first scientific expedition to Greenland was in 1852. Antarctica was a big white question mark.
• To Uniformitarianists, promoters of the ‘Glacial Theory’ were drama queens. It had a strong whiff of catastrophism about it.
• The Uniformitarianists had other explanations for the big rocks. These were sort-of plausible.
• The boulders could have gotten there in some ancient flood. Not Noah’s flood, of course, but some other flood. This was the ‘diluvial’ theory.
• The ‘drift’ theory hypothesized that the big rocks may have been carried on top of ice floes. This does actually happen. Alternatively, the big rocks may have been pushed along the bottom of a shallow ancient sea by an iceberg.
• Some Big Names just didn’t like the Glacial Theory: Georges Cuvier, Alexander von Humboldt, Charles Lyell, Charles Darwin, and Hermann von Helmholtz.
• In the late 1830s, the Glacial Theory found a charismatic evangelist.
• Louis Agassiz was elected president of the Swiss Natural History Society at age 30 . He was a expert on fossil fishes.
• Agassiz then proceeded to scandalize the Swiss naturalists at a meeting in Neuchâtel on July 24, 1837. Instead of giving a lecture on fossil fishes, Agassiz delivered a TED talk about Glacial Theory.
• There were facts on the ground to support the Glacial Theory, if anybody wanted to see them. Terminal moraines are ridges of rubble left behind when a glacier retreats. If you plot them on a map of Europe, they form nice curves suggesting the southern extents of ice advances.
• The evidence and Agassiz’s persuasiveness eventually brought people around. In 1840, even Charles Lyell, who was the The Godfather of geology, grudgingly admitted there might be something to the Glacial Theory.
• Aside: Agassiz later moved to America and became a well-known and popular professor of geology and zoology at Harvard.
The Mystery of the Ice
• By 1840, (an) Ice Age (wait, wait, there’s more!) was accepted as something that had happened in earth’s history.
• Not that anyone had a clue what could have brought it about.
• The Grand Prize of 19th century natural science would go to whoever could explain The Origin of the Ice Age.
First suspect: sunspots
• In 1838, Claude Pouillet was the first to measure of amount of the solar energy that strikes the earth.
• The units are: energy per unit of time per square area. An understandable value is 1.951 calories per minute per square centimeter (≅ 1376 W/m²).
• Sunspots had been observed by astronomers for centuries.
• Sunspots suggested solar variation. Sunspots also appeared to have a cycle. That was a good thing.
• In 1775, astronomer Christian Horrwbrow of Denmark concluded a decade-plus of sunspot observations. He found a cycle.
• In the 1830s, German astronomer Heinrich Schwabe spent 17 years searching for the planet Vulcan against the disk of the sun. Bored, he kept a record of sunspots while he was at it. In 1843, Schwabe proposed a 10-year sunspot cycle.
• The sunspot cycle could not only be run forward, but backward into the past. Old astronomical records were collected, collated, and re-examined.
• A surprise emerged: from around 1640 to 1715, sunspots were exceedingly rare. This coincided with a century of extremely cold weather, well documented by those who lived through it.
• This low sunspot period is called the Maunder Minimum. The cold spell is called the Little Ice Age. Start and end years vary.
• To be fair, the Maunder Minimum was first noticed by German astronomer Gustav Spörer. But the definitive documentation was by a husband-and-wife team, Annie Russell Maunder and E. Walter Maunder, at the Royal Observatory in Greenwich.
• Annie Russell Maunder deserves be the subject of one of those “unsung women in science” documentaries.
• Temperatures in the Little Ice Age dropped by as much as 2° C. The Bosporous froze over; people could walk between Europe and Asia. 1641 was the coldest winter ever recorded in Scandinavia. And so on.
• For those afraid of Eurocentrism, the Little Ice Age affected the entire planet.
• It also coincided with an era of social and political upheaval known to historians as the General Crisis. Crop failure and famine are two obvious reasons for political upheaval and social unrest.
• More subtle might be changes in pathogen behavior. Daniel Defoe’s Plague Year is 1665. The plague of Justinian (541–549 AD) took place in a notably cold century.
Second suspect: volcanoes
• In the summer of 1783, a strange “dry fog” appeared in the air over Europe. It appeared first over northern Scotland, then western Norway.
• French naturalist Mourgue de Montredon suspected the fog came from a volcano. Somewhere.
• Benjamin Franklin, then in France on behalf of the rebel American colonies, agreed with de Montredon. Franklin wrote about the exceptional cold and how the fog dimmed the sunlight. “We could view the sun as if through smoked glass,” one of his contemporaries wrote.
• After communication delays, the source of the fog was tied to activity of the Laki fissure in Iceland.
• The Laki eruption started 8 June 1783 and continued to February 1784. It maintained a sulfuric aerosol veil over the Northern Hemisphere for 5 months.
• Aside: Europe got off easy compared to Iceland. Closer to the eruption, poisonous hydrofluoric acid killed crops and cattle. The famine that followed resulted in the death of about one-quarter of Iceland’s population.
• 1816, the eruption of Tambora in the Dutch East Indies (now Indonesia), produced yet another “year without a summer.”
• Tambora lifted an estimated 60 gigatons of sulfate aerosol into the air. The aerosol traveled the globe via the jet stream and produced remarkable bright red-orange sunsets in Europe for three years.
• Volcanic “fog” is fine particulate matter. It’s nasty stuff, but at least it’s visible. Hydrogen sulfide, likewise, has that rotten-egg smell. What wasn’t obvious then was that volcanic magma puts a lot of carbon dioxide into the atmosphere.
Third suspect: carbon dioxide
• French polymath Jean-Baptiste Joseph Fourier wrote the book on heat flow, Théorie analytique de la chaleur (The Analytical Theory of Heat) in 1821.
• Fourier then attempted to calculate how warm Earth (and Venus, and other planets) should be given their distance from the sun.
• There was a whopping residual error in the number Fourier came up with for Earth. By his calculation, it should be ~36°C cooler than it actually is.
• Looking for an explanation, Fourier read about a 1760s experiment by de Saussure.
• Horace Bénédict de Saussure of Geneva was a great character, a sort of late 1700s gadget freak.
• De Saussure loved his thermometer, a relatively new toy. The thermometer was invented many times, but only given a standard scale by Daniel Fahrenheit in 1724.
• In 1767 de Saussure built a set of glass-topped boxes, one inside the other, like Russian dolls. He exposed the glass on the outer box to sunlight and recorded a progressive increase in temperature in the inner boxes. The innermost box got hot enough to boil water.
• De Saussure concluded that the layers of glass were one-way with respect to heat. They prevented it from radiating back out.
• De Saussure was yet another avid Alpinist and mountain climber.
• He built a well-insulated and portable version of his box, effectively inventing a solar oven. De Saussure humped his box up to the top of Mt. Cramont in the Swiss Alps.
• De Saussure’s hypothesis was that in terms of physics the same sunlight struck the mountain top as down below.
• Therefore, the temperature inside his insulated solar box should max out the same in both places. This despite the air being colder on the mountain. It did.
There was clearly something in the air…
• …but it was not all clear what it was.
• An American woman, Eunice Foote, discovered the absorption of thermal radiation by carbon dioxide and water vapor in 1856. Add her to the list of unsung women in science.
• Irish physicist John Tyndall carefully studied the different thermal absorption rates of various gases. He got the credit.
• Tyndall was another avid Alpinist and glaciologist. He led one of the early teams to the top of the Matterhorn in 1868.
• Tyndall’s 1859 description of the greenhouse effect is admirably clear: “Thus the atmosphere admits of the entrance of solar heat; but checks its exit, and the result is a tendency to accumulate heat at the surface of the planet.”
• Believe It Or Not Dept.: CO₂ levels in the atmosphere interested 19th century scientists as a cause for the Ice Age.
• CO₂ is put into the atmosphere by volcanic eruptions. Chemically, it disappears (temporarily) via photosynthesis and (permanently) by combining with rocks, a process known as ‘weathering.’
• While individual volcanic eruptions had obvious appeal to Catastrophists, the long history of volcanism on Earth gave the Uniformitarianists an equilibrium mechanism they liked.
• If a period of volcanic inactivity resulted in too little CO₂, that would bring on an Ice Age.
• On the other hand, if volcanos produced so much CO₂ it warmed up the earth, plants would grow faster and the weathering reaction would speed up.
• Spoiler: Modern studies show atmospheric CO₂ did reach a very low level about 18,000 years ago at the peak of the last glaciation. It was almost low enough to stop plant photosynthesis.
Sidebar: Winter is coming!
• By the late 19th century, it was clear from the geologic evidence that there had been not one Great Ice Age, but a succession of glaciations. By 1909, the accepted number was four.
• Worse, it was clear we are living in an interglacial period, and towards its end. Someday, for sure, the ice would be back.
• In popular media, ‘New Ice Age?’ scares recur periodically. The most recent was in 1974:
A reassuring(?) calculation
• In the 1890s Swedish physicist Svante Arrhenius was the first to put numbers on the greenhouse effect. Arrhenius’ laborious calculations, done by hand, took into account feedback effects. His numbers weren’t all that bad, considering.
• Arrhenius, like most scientists in the 19th century, talked about carbon dioxide (CO₂) and water vapor (H₂O) in the atmosphere in their combined form, carbonic acid.
• Arrhenius was foremost interested in how CO₂ cooling might explain the ice: “If the quantity of of carbonic acid in the air should sink to one-half its present percentage, the temperature would fall by about 4° [C]; a diminution to one-quarter would reduce the temperature by 8° [C].”
• As an aside, he went on: “On the other hand, any doubling of the percentage of carbon dioxide in the air would raise the temperature of the earth’s surface by 4° [C].”
• All this allowed Arrhenius to reassure the public. The dreaded return of the ice was unlikely. Anthropogenic emissions from “the enormous combustion of coal by our industrial establishments” would save us.
• In 1901, Nils Gustaf Ekholm, a Swedish meteorologist and associate of Arrhenius (and another mountaineer) suggested humans could use CO₂ as a global thermostat, cranking it up if necessary: “[I]t seems possible that Man will be able efficaciously to regulate the future climate of the earth and consequently prevent the arrival of a new Ice Age.”
• As an issue, anthropogenic carbon dioxide lay dormant until 1938. It was then revived, with new numbers, by British engineer Guy Callendar. Callendar thought greenhouse warming was essentially beneficial and that it might hold off another ice age.
Fourth suspect: Earth’s orbit
• The four recent glaciations seemed to have a regular cycle. Volcanoes were too unreliable to explain that.
• Regular cycles on a time scale scientists wanted could, however, be produced by an astronomical theory.
• In 1864, James Croll, a Scottish scientist, proposed a link between between the ice ages and subtle variations in the earth’s orbit.
• Since the effect in terms of physics would be tiny, Croll elaborated his theory by proposing the small changes might trigger a feedback loop.
• Earth’s albedo is the proportion of the incident light or radiation that is reflected by a surface. Additional ice at a pole could reflect more sunlight, leading to further cooling, leading to more ice, etc.
• Others in the 19th century had had the same idea, but Croll was ahead of his time.
• Croll was also very interested in a possible effect on ocean currents. The Gulf Stream, a large warm surface current, was well-known to be responsible for the warmer climate of northwest Europe.
Which introduces another suspect: the ocean
• Croll’s interest in the Gulf Stream was prescient, if Eurocentric. The Gulf Stream was first noticed by a European, Juan Ponce de León, in 1512.
• Benjamin Franklin became fascinated in the Gulf Stream on hist first voyage from America to England. He created this chart for mariners printed in London in 1769:
• El Niño, now officially the El Niño–Southern Oscillation (ENSO), got its name from Peruvian fishermen. They were aware that warm water appearing around Christmas could affect their catch (El Niño is ‘the Christ child’). The oscillation has a cycle between two and seven years.
• El Niño is bad for anchovies, good for sardines.
• Spoiler: Large-scale reorganizations of the earth’s atmospheric and ocean circulation, triggered by astronomical obliquity, is a prime suspect.
Back to Croll
• Croll’s hypothesis appealed to many, including Charles Darwin. The long time periods felt right.
• One problem with Croll’s hypothesis was that ice advances would occur in only one hemisphere at a time.
• In a very long view, the two-hemisphere problem brings up plate tectonics. There is now a big frozen continent, Antartica, at the South Pole.
• In the 1920s Serbian engineer Milutin Milankovitch redid the astronomical calculations. Milankovitch calculated more orbital variables, such as eccentricity, axial tilt, and precession, that produced a lot of cycles:
• The major cycles had promising numbers with respect to glacial epochs. The obliquity cycle had a period of 41,000 years. The eccentricity cycle had a period of 100,000 years.
• But all these superimposed cycles were like analyzing the spectrum of a noisy complex music chord. There weren’t very many data points in the geological record.
• Scientists picked their favorites. Cycles, like trends, are easy to see when you want to.
• For my money, Croll picked the wrong horse, orbital eccentricity. Obliquity is the one. But it’s very complicated.
• Like Croll’s, Milankovitch’s variations in the distribution of solar radiation were small in terms of the physics. To trigger a big event like a glaciation, there had to be an amplifying process.
A line in the sand
• So will the mystery of the Ice Age ever be solved?
• Not in this installment. Tune in next week. Teaser: Milankovitch gets tossed out of court! Temporarily, anyway. Volcanoes reappear shrouded in controversy!
• 1950 is a good year for the end of climate science prehistory.
• Radioactive dating techniques perfected in the 1950s greatly expanded the evidence. The detectives had more to work with, as if fingerprints and DNA had suddenly been invented. The trail went much farther back in time. It was filled in with much more detail.
• Another sea change was taking place.
• The 1970s were the ‘Me’ decade.
• Climate science began to be all about us. Human influence, that is.
• We even named a geological time unit, the Anthropocene, after ourselves.
• The Ice Age? Ancient history! This ain’t your grandfather’s Holocene any more!
Until then…