Milankovitch became one of the many geniuses whose lives unknowingly paralleled each other’s during Vienna’s astonishing fin de siècle. He graduated at the top of his civil engineering class and, after being awarded his doctorate, began work at a Viennese engineering firm. In his spare time, he registered a series of successful patents that quickly made him wealthy, so much so that by 1912, at the age of 33, he was able to indulge his personal interests on a number of fronts. One of these was the orgin of ice ages.
In June 1914, just as war broke out, he married Kristina Topuzovich. They decided to spend their honeymoon in his home village of Dalj. As it happened, Dalj was in territory contested by Serbian nationalists, and Milankovitch, having been born there, was arrested by the Austro-Hungarian army and thrown in jail. He wrote in his diary, “The heavy iron door closed behind me . . . I sat on my bed, looked around the room and started to take in my new social circumstances.” Fortunately, the soldiers allowed him to keep his briefcase, in which he had hurriedly stuck his theoretical papers and some blank sheets. “I looked over my works, took my faithful ink pen and started to write and calculate . . . When after midnight I looked around the room, I needed some time to realize where I was. The small room seemed to me like an accomodation for one night during my voyage in the Universe.”
Milankovitch’s wife appealed to their highly placed social connections in Vienna to get him transferred to a military prison in Budapest, where his captivity was more lenient, a sort of house arrest with day privileges. He spent the balance of the war researching his climate theories at the Central Meteorological Institute in Budapest, and it was there that he began to investigate the extraterrestrial causes of ice ages. When the war ended, he rejoined his family in Belgrade and started work on his crowning theory: a mathematical description of Earth’s climatological history, published in 1924 as Climates of the Geological Past. His thesis is now known as the Milankovitch cycle, which, at its complex heart, is an interplay of three variables that coincide every 100,000 years to create an ice age.
The first variable is the Earth’s axial tilt, which is not fixed. It gradually alternates 2.4° (between 22.1° off the vertical and 24.5° off the vertical) over a period of 41,000 years. What produces the tilt are tidal forces exerted by the moon and nearby planets. Right now, the Earth’s tilt is 23.44°. The Tropics of Capricorn and Cancer are respectively 23.4° below and above the equator, while the Arctic and Antarctic circles lie the same distance away from the poles. If Earth was tilted any more obliquely, say at its maximum of 24.5°, then the Tropic of Cancer would also be shifted north significantly, to 24.5° latitude and instead of running through Havana, it would run through Miami. Likewise in the Middle East: instead of running through the United Arab Emirates, the Tropic of Cancer would run through southern Iran. The same expansion would apply to the southern hemisphere, so the overall area of the tropics would increase by more than 2° latitude. This expansion would come at the expense of the temperate zone, because the Arctic and Antarctic circles would also creep south and north, enlarging the polar regions. Any change in the Earth’s tilt affects the severity of the seasons. When the Earth is more vertical, summers get cooler in the northern hemisphere and this, more than cool summers in the southern hemisphere, favors the onset of an ice age. Why?
It’s a matter of landmass and water. Most of the Arctic is covered by ocean, which means it can’t get as cold as the Antarctic: the Arctic Ocean, despite its icy cover, acts as a reservoir of heat in the form of liquid water. The Antarctic, being solid land, has no such thermal check on how cold it can get. As a result, the northern hemisphere keeps our planet warm by counterbalancing and compensating for the southern hemisphere, which is cooler, on average, than the northern hemisphere. Just ask the someone living in the Falkland Islands. Although they’re on the same latitude in the southern hemisphere as London, England, is in the northern hemisphere, the Falklands have an average yearly temperature of 5.6°C whilst in London, the average is twice as warm, 10.4°C. Again, water drives the thermal imbalance here but in the opposite direction. Oceans may take longer to cool down, but they also take longer to warm up. The larger ocean surface of the southern hemisphere drives the overall temperature lower, even in the summer. Consequently, Earth has one foot in the South Pacific ice bucket, and the only thing keeping the planet warm is summer in the northern hemisphere.
So let’s fastforward Earth’s tilt 20,000 years to its most upright inclination, 22.1°. The summers will have been cooler in the northern hemisphere for thousands of years. Yet the glaciers haven’t started moving yet. Why? Well, there are two more things that need to happen at the same time to force Earth’s climate into an ice age. One of these, the second variable in Milankovitch’s trio, is the seasonal timing of our orbital perihelion, the point at which Earth’s orbit brings it closest to the sun. Right now that happens in January, when the southern hemisphere is tipped toward the sun. But over thousands of years, the point of perihelion moves. This means that in 13,000 years, the northern hemisphere will get the summer perihelion boost. This also means that one of the three requirements necessary to tip our climate into an ice age is currently in place. The northern hemisphere has lost the extra sunshine that the perihelion would give it, and the southern hemisphere simply squanders the extra heat — a 7 percent increase in solar energy — with its summer chilliness. Fortunately, the northern hemisphere can get along without the extra encouragement from the perihelion.) But when the perihelion cycle and Earth’s tilt coincide with the third and most important cycle — orbital eccentricity — then it’s time to get out the snow shovels and road salt.
Every 96,000 years or so, the Earth’s orbit switches from a nearly circular orbit to a more elliptical one. This changes everything. During a more elliptical orbit, the perihelion brings the Earth much closer to the sun and increases the solar energy received by a whopping 20 to 30 percent on the hemisphere facing the sun. You’d think that would warm things up considerably, but the good news is counterbalanced by the farthest point that an elliptical orbit would take Earth away from the sun, the aphelion. If the aphelion occurred while the northern hemisphere was tipped away from the sun (as it is now), it would cancel out the increase in solar radiation on the southern hemisphere. Every winter would add to the snow cover that remained over the summer in the northern hemisphere, because landmasses lose heat more quickly, particularily if the Arctic sea ice remained year round. Combine that with a less oblique axial tilt and, presto, you have an ice age underway.
Fortunately for us, that that won’t happen for another 60,000 years. Although there are still some scientific critics of Milankovitch’s cycle, the geological evidence from the Vostok ice cores and deep-sea deposition cores show that past ice ages are in lockstep with his axial precession index and orbital eccentricity charts. There seems to be little question that, at least over the past 2.6 million years, during the Pliocene-Quaternary glaciation, Milankovitch’s cycle has been driving Earth’s climate on the macro scale.
But even within these large, regular cycles, there are climatological events that are highly irregular and reveal the sensitivity and potential instability of the atmosphere. The advance and retreat of the glaciers and the beginning and ending of warm interglacial periods are not always orderly and gradual. A stressed atmosphere can be nudged out of balance by extraordinarily subtle influences.
Climate Instability
When climates are stable, whether in the deep freeze of a Snowball Earth or the tropical planetary conditions that prevailed for millions of years between ice ages, they have climatic equilibrium. When instability is introduced, some outside influence, say by the Milankovitch cycle or from a change in the percentages of atmospheric gases, then the climate goes into a period of wild instability. Unusual or extreme weather events sometimes accompany the state of transition between the prior set of conditions and the next. As we’ve seen, the evidence from the ice cores in Greenland suggest tipping points
that usher in apocalyptic climate change. Sometimes the early warning signs of change are registered by differences in the behavior of animals and plants. Like canaries in a coal mine or dogs barking before an earthquake hits.
Phenology . . .
I’m a naturalist. It might seem a contradiction, living as I do in urban Toronto, but you’d be surprised at the wildlife that calls downtown home. I keep a diary of the most interesting urban creatures that I see, as well as an annual “first sightings” chart, in which I record the day the maple leaves open — a robust indicator of the first warm weather — and when I first see my four favorite seasonal creatures: June bugs, nighthawks, swallowtail butterflies and cicadas. The study of these seasonal appearances (and disappearances in the fall) is called phenology and is, I suspect, the cardinal trait of the amateur naturalist. I’m in good company. I recently discovered that Thomas Jefferson was an obsessive phenologist. He kept a record of the difference in leaf-opening times between his estate in Virginia and his home in Washington. Over the course of the last 30 years, my charts have not only become a calendar of migratory and seasonal animals, they have also turned into a personal record of climate change. Contrary to what you’d expect, it seems that spring is arriving just a little later each year, at least it is in the North American Great Lakes region.
The most telling data comes from my leaf-opening records. During the 1980s, the maple leaves opened on April 23, but in the 1990s that shifted to April 29, a forward jump of six days. In the first decade of the new millennium, the average leaf-opening date remained at April 29, but in the first eight years of the second decade, from 2011 to 2018, the date moved a little farther, to May 1. Interestingly enough, that recent average includes a wildly early year, 2012, when the leaves opened on April 14.
So what’s happening? Every decade, spring is getting colder and winter is lasting just a little longer, at least in Toronto. Perhaps it’s just the maples. Certainly rising levels of carbon dioxide are influencing the atmosphere, but as it turns out there are other, human-originated factors that have equally potent effects on climate.
. . . and Contrails
Andrew Carleton, a geography professor at Pennsylvania State University, and David Travis, a climatologist at the University of Wisconsin, had always been curious about the influence of high-altitude air traffic on climate. What effect did jets have on the weather? Short of clearing the sky of all aircraft for several days — something they could never arrange — how would they be able to quantify any effect?
Then came 9/11 and the attack on Manhattan. For the three days that the U.S. grounded all commercial aircraft and almost all military flights, Carleton and Travis scrambled to amass the data that poured in from national sensors. When the numbers were gathered and crunched, the figure was surprising. After factoring out local weather and other thermal irregularities, they discovered that during the three-day, no-fly period after 9/11, the mean surface temperature of the United States climbed 1.2°C. Why? The simple answer: less cloud cover.
When jets fly at high altitude, their exhaust creates long streamers of ice crystals called contrails. Everyone is probably familiar with these twin white lines inscribed in the blue, and if the dew point is just right, they last well after the plane has disappeared over the horizon. Given how narrow contrails are, you’d think they’d be negligable in terms of cloud cover, but apparently, if there are enough of them, they contribute significantly to the continent’s shade. Carleton and Travis published their results in Nature, and a new, somewhat contradictory term, “global cooling,” was introduced into the climate-change lexicon. Perhaps that explains why spring seems to be arriving a little later every decade, at least here in eastern North America. But there are other ways that humans have been influencing the climate, and for much longer than you’d think.
Anthropogenic Warming
Presently we are in the Holocene epoch, which began 11,500 years ago at the beginning of our current interglacial period. By some estimates, the last interglacial period, the Sangamonian, lasted 11,000 years. Does that mean our own interglacial period is past its expiration date? Some climatologists think so — University of Virginia emeritus professor William Ruddiman for one. He believes that our current interglacial period should have ended thousands of years ago and we should be in the throes of a new glacial advance. But we’re not. Why? Ruddiman blames humans.
More than a decade ago, when Ruddiman was studying ice-core data from both Greenland and Antarctica representing thousands of years of atmospheric history, he noticed a spike in carbon dioxide levels that began about 8,000 years ago. Cross-referencing it with archeological records, he noticed a correlation. The carbon dioxide rise came at exactly the same time as slash-and-burn agriculture spread from the Middle East to Europe and western Asia. Then, about 5,000 years ago, there was another spike, this time in atmospheric methane levels. These Ruddiman linked to rice paddy agriculture, which was beginning in the Lower Yangtze region of China. The methane spike continued to rise as rice paddies expanded across the rest of China and into Asia. By 3,000 years ago, the combined effects of anthropogenic methane and carbon dioxide had elevated mean global temperature by 0.8°C at mid-latitudes and by 2°C at far northern latitudes. According to Ruddiman, this increase was large enough to have stopped a glaciation of northeastern Canada that, according to his calculations, should have started 2,000 or 3,000 years ago. “Whew!” you might say. “We dodged that bullet.” But things climatic aren’t straightforward. It’s quite possible that the Ruddiman effect is now shifting into high gear.
Since the onset of the Industrial Revolution in 1800, human contribution to atmospheric carbon dioxide has been increasing almost exponentially, primarily from burning fossil fuels. This process has released billions of tons of carbon dioxide (that had been previously sequestered over hundreds of millions of years by the carbon cycle) into the atmosphere.
Today carbon dioxide represents only 0.04 percent of our atmosphere, or approximately 400 parts per million. This is a tribute to the sequestering efficiency of the carbon cycle, considering that ocean plants, mostly plankton and algae, along with land vegetation, pump 771 gigatonnes of carbon dioxide into the atmosphere yearly. By comparison, our human contribution of 29 gigatonnes annually seems trivial, but the carbon cycle is finely tuned, and there might be little capacity for excess. In fact, most climatologists believe that the carbon cycle has already been overwhelmed, and they argue that is reason why measurements of atmospheric carbon dioxide are currently rising.
Clearly the world is no longer evolving as it would have without our intervention. Here, in the midst of a catastrophic man-made mass extinction where thousands of species are being lost due to human activities, we also appear to be altering the very atmosphere itself. In light of this, a number of scientists contend that we are entering a new geological era of our own making: the Anthropocene.
Tickling the Dragon’s Tail
When Robert Oppenheimer and General Leslie Groves were overseeing the construction of the first atomic bomb in Los Alamos, New Mexico, they set up an experiment to measure the critical mass threshold of the bomb: the point at which a runaway fission reaction would create an explosion. The experiment was rather primitive. A horseshoe-shaped stack of plutonium bricks, just slightly below critical mass, was arranged on a tabletop amid an array of radiation detectors. A little rail entered the opening of the horseshoe and stopped at the center of the pile. This rail was the track for a sliding rod, and at its tip the scientists placed a small chunk of plutonium, just enough that, when added to the pile, it would trigger a critical mass reaction. The idea was to slip the chunk in and out so quickly the pile wouldn’t get a chance to go completely critical and explode. That way they could measure the spike in radioactivity and fine-tune the mass necessary to detonate the first nuclear bomb. Rather aptly, they called this manoeuver “tickling the dragon’s tail.”
It seems that anthropogenic atmospheric change is a s
imilar experiment, though unsupervised, a kind of meddling with the dials on our global thermostat that might have disasterous and unpredictable consequences. Our atmosphere is a fluid system, prone to turbulence and the butterfly effect. In July 2012, for example, a warm air mass parked over Greenland for 12 days, and on one of those days, 11 inches of ice melted, representing 14 percent of the total annual melt. It was a weather anomoly, an unpredictable event. Another small imbalance, like the Greenland heatwave, seemingly trivial, might cascade up the chain of complexity and eventually destabilize the entire global system. It has happened before.
Climate and Weather
“Climate is what you expect, weather is what you get.”
Robert A. Heinlein
People often attribute extreme weather events to climate change, and although it’s true that one of the harbingers of abrupt climate change can be extreme weather, it’s best not to lose sight of the fact that it’s the averages, the mean temperatures and rainfalls and wind speeds that indicate real climate change.
But when, or where, does climate stop being climate and become weather? We can say that certain types of weather are endemic to particular climates. A hurricane is a major weathermaker but is indigenous to the tropics. Occasionally, the remnants of a hurricane will travel beyond the 40th parallel but never all the way to the Arctic Circle. Conversely, blizzards never strike the Amazon.
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