Illustration 75 The great ocean conveyor belt redistributes temperatures around the world. Source IPCC.
The return flow for the Gulf Stream comes from dense water flowing out of the Arctic basin, which flows south along the deep sea floor. The water in the Arctic basin is dense because it is both colder and saltier than average. The increased salinity is caused when water crystallizes to form sea ice—salt ions are excluded raising the concentration of salt in the remaining sea water. The salinity of water in the tropics can also be increased through surface evaporation. The interaction of surface and deep ocean currents, influenced by solar radiation, ice formation, fresh water runoff and evaporation, is highly complex and poorly understood. Observation of the MOC has been ongoing for years, but still, little is known about its fluctuations and their effect on global climate.249
We do know that this mechanism is not directly tied to the El Niño mechanism. The El Niño-Southern Oscillation (ENSO) creates surface water temperature anomalies in the equatorial Pacific because of changes in trade winds. It temporarily affects atmospheric circulation patterns over a sizable portion of the globe. The effects of the MOC are longer term and more substantial than an El Niño or the corresponding North Atlantic Oscillation (NOC).
It is known that temperature fluctuations in the Northern Pacific have a significant effect on marine ecosystems and the climate of North America. Observations over the past 50 years show that this overturning circulation has been slowing down since the 1970s, causing a rise in equatorial sea surface temperatures of about 1.5°F (0.8°C). Like many aspects of Earth's climate system, the physical mechanisms responsible for these fluctuations are also poorly understood.250
Scientists suspect that the conveyor belt has been disrupted in the past. Toward the end of the last glacial period, when the Wisconsin ice sheet was retreating northward into Canada, a large freshwater lake formed. The lake water's path to the sea was blocked by glacial ice. As the climate warmed, the ice continued to melt, adding to the water trapped in the lake.
Illustration 76: Seed head of Dryas octopetala on the Burren, County Clare.
This lake, named after glaciologist Louis Agassiz, was twice the size of the Caspian Sea, the largest body of freshwater on Earth today. At its greatest extent, Lake Agassiz stretched from Saskatchewan, east to Quebec, and from Minnesota, north to the Hudson Bay.251 After collecting water from melting glaciers for 4,000 years, the ice dam broke and the lake suddenly emptied into the Arctic Ocean through Hudson Bay. The water flowed from the Arctic basin into the North Atlantic, and the great ocean conveyor belt stopped.
This incident, which occurred 12,700 years ago, is believed to have been the cause of a period of sharply colder temperatures called the Younger Dryas.252 This period is named after Dryas octopetala, a flowering plant found in Arctic tundra regions, because great quantities of dryas pollen were found in ice core samples from that time. The Younger Dryas was the most significant rapid climate change event to occur during the onset of the Holocene warming. Recent studies show this interval began with a 12°F (7°C) drop in temperature, in the span of only 20 years.253
Work done by Wallace Broecker provided the link between the emptying of Lake Agassiz, the disruption of the ocean conveyor, and the sudden shift back to glacial conditions.254 The Younger Dryas lasted about 1,300 years, then the Holocene warming trend reasserted itself. Theory suggests that this event is an example of Earth's climate shifting between different stable modes, and has led to warnings that current melting of glacial ice might trigger another sudden cold snap.255 ,256
Interestingly, the end of the Younger Dryas was marked by a period of rapid temperature increase—an estimated 27°F (15°C) rise over half a century. A visual comparison of these changes and more recent temperature variation can be seen in Illustration 77.
Illustration 77: Temperatures during the Younger Dryas and the present day. Source data from Richard B. Alley.
While some species may have gone extinct during this period, it was not a noticeable biotic calamity. This is a direct contradiction of claims that a temperature rise of a few degrees will lead to widespread extinctions. Climate stress or habitat destruction might have helped kill off the mastodon and the mammoth, but nature did not collapse, nor did our stone-age ancestors. As usual, Earth's resilient life adapted and thrived.
No wonder the IPCC's claim of “unprecedented” global warming is always qualified by “in the last 10,000 years.” Go back a bit farther in time and there is a temperature drop of 12°F (7°C) and a subsequent rise of 27°F (15°C). The rate of temperature rise at the end of the Younger Dryas is 20 times the most widely quoted IPCC prediction of 1.5°C per century.
The Only Constant is Change
As we have seen, the ever shifting arrangement of Earth's tectonic plates is a source of slow but inexorable climate change. This change is due to modification of ocean currents and atmospheric currents that redistribute heat energy around the world. Though these changes assert themselves over very long time periods, they are a reminder that there are much more powerful influences on Earth's climate than human activity. Other effects of ocean currents are visible on a more human time scale.
The global nature of ocean circulation is shown by a recent story about rubber bath toys, reported in England's Daily Mail. It seems that 15 years ago a shipment of 29,000 plastic yellow ducks, blue turtles and green frogs, traveling on a cargo vessel from China to America, were washed overboard. Most of the toys circled the northern Pacific once before being washed up on the shores of Alaska and the West coast of Canada and the US. Since then, some of the toys have traveled 17,000 miles, landing in Hawaii, spending years frozen in the Arctic pack ice, and floating over the site where the Titanic sank.257
At some point, the toy armada is expected to wash up on beaches in South West England. This slow journey from the Pacific to the Atlantic, by way of the Arctic, was predicted by British oceanographer, Curtis Ebbesmeyer. Having heard about the toys' escape at sea, Ebbesmeyer theorized that some of the floating objects would eventually end up in the Arctic Ocean and become embedded in the ice. Hitching a ride on the Arctic ice cap, which moves as much as a mile a day, the rubber ducks found their own Northwest Passage to the Atlantic.
This event illustrates how interconnected all Earth's oceans are, and that ocean circulation includes both water and ice. Scientists are now using the lost toys' journey to help understand ocean currents. In the future, as the continents continue to drift and ocean currents change, scientists may dump more rubber duckies overboard on purpose.
The circulation of the deep Atlantic Ocean during the height of the last glacial period appears to have been quite different from today. The conveyor belt didn't halt, as it did during the Younger Dryas, but changes in temperature and flow occurred.258 The British Oceanographic Data Center, as part of the Natural Environment Research Council (NERC) Rapid Climate Change (RAPID) program, and the University of Miami's Rosenstiel School of Marine and Atmospheric Science (RSMAS) have moored buoys in the Atlantic to monitor the MOC in real time.259 Perhaps they will be able to provide a warning if the MOC shuts down again. As Earth's climate continues to change, variation in ocean circulation should be expected. Whether these changes will cause sudden shifts in temperature, in some regions, cannot be foretold.
Even so, we need to be prepared. Man can no more control Earth's climate than he can stop the drifting continents. As we have seen from studying the causes of extinction, successful species are those that can adapt to Earth's ever changing environment. To quote physicist Stephen Hawking, “We've got to start adapting, we've got to start understanding how severe the changes are and that there's very little than we can do to stop them.”
Variations In Earth's Orbit
“At present the globe goes with a shattered constitution in its orbit.... No doubt the simple powers of nature, properly directed by man, would make it healthy and a paradise”
— Henry David Thoreau
The chang
ing seasons are among the most familiar short term climate cycles. The change of summer into fall, or winter into spring illustrates the variability of temperature and the adaptability of Earth's living creatures and plants. The parade of seasons is caused by the tilt in Earth's axis of rotation, an imaginary line drawn through the north and south poles.
Illustration 78: Earth's seasons due to axial tilt. Source NOAA.
Earth spins like a top, completing a revolution every 24 hours, but it is slightly tilted from the plane of its orbit around the Sun. Today, this tilt is 23.5° but, as we shall see, this amount changes over time—referred to as variation in axial obliquity. Why Earth tilts like this is a mystery. Some think it might be a result of the collision that formed the moon.
As Earth orbits the Sun, the direction of the tilt remains constant, just like a spinning top. So, during the summer, the tilt in the northern hemisphere is toward the Sun and, during the winter, it points away. The longer summer days allow more sunlight to warm Earth, while shorter, winter days mean less warmth and colder weather. When the northern hemisphere is tilted toward the Sun, the southern hemisphere is tilted away. This causes the seasons to be reversed between the north and south. At the South Pole, winter starts in June.
People have long understood what causes the seasons and ancient observers of the night sky noticed other changes as well. Early astronomers noticed that the canopy of stars was slowly moving around Earth, causing the stars of the zodiac to slowly move through the seasons. This is due to a phenomenon known as precession. The discovery of precession is usually attributed to the Greek astronomer Hipparchus of Nicaea,260 in the 2nd century BC. He was the first to develop accurate models of solar and lunar motion.
Reportedly, Hipparchus made use of the observations recorded by the Chaldeans for several centuries preceding his own observations. Virtually all Hipparchus' writings were lost, including his work on precession. We know of it today because Ptolemy mentioned the works of Hipparchus in his Almagest. Ptolemy explained precession as the rotation of the celestial sphere around a motionless Earth. It is reasonable to assume that Hipparchus, like Ptolemy, thought of precession in geocentric terms as a motion of the heavens (see page 223).
There has been speculation that other cultures discovered precession prior to Hipparchus. The Babylonians may have known about precession as early as 330 BC. According to al-Battani,261 Chaldean astronomers had measured the difference between the solar and sidereal year. The solar year is the length of time that the Sun takes to return to the same position along its path among the stars relative to one of the equinoxes. The sidereal year is the time it takes for the Sun to return to the same position with respect to the stars of the celestial sphere. This is the same length of time as the orbital period of Earth. Because precession causes the equinoxes to move backward along the ecliptic, a solar year is shorter than a sidereal year. The value of precession is the difference between the solar and sidereal years.
Claims have been made that precession was known in Ancient Egypt. Some buildings in the Karnak temple complex were allegedly oriented toward the point on the horizon where certain stars rose or set at key times of the year. When precession made the orientations inaccurate, the temples would be torn down and rebuilt. Of course, observing precession's effects doesn't mean the Egyptians understood what caused the changes.
The first known reference to precession resulting from the motion of the Earth's axis appears in Copernicus's De Revolutionibus Orbium Coelestium, where he called precession the third motion of the Earth. Over a century later it was explained in Newton's Philosophiae Naturalis Principia Mathematica to be a consequence of gravitation. Newton's original precession equations contained errors and were later revised by d'Alembert.
Scientists came to suspect that these variations, linked to Earth's travels around the Sun, might have an impact on climate. Establishing the connection between variations in Earth's orbit, attitude and long term climate change took more than 150 years and was an on again, off again affair.
Cycles of Earth
Scientists in the mid 19th century had come to accept Louis Agassiz's Ice Age theory and were casting about for explanations as to why they would occur. In the ninth edition of his Principles of Geology, Charles Lyell speculated that variations in Earth's orbit might be the cause, but provided no proof for the conjecture. The first scientist to attempt to prove this link between Earth's orbit and ice ages was a self-educated Scot named James Croll.
Illustration 79: James Croll, 1821-1890.
James Croll (1821-1890) was one of those people who seem to have been prevalent in the Victorian British Empire, the self-taught amateur scientist who ended up a major contributor to a number of fields of study. Born to a poor rural family, Croll only attended school until 13 years of age, when he had to quit to help work the family farm. He had a varied career outside of science, working as a hotel manager, an insurance salesman, an itinerant industrial equipment repairman, and other odd jobs. Eventually, he found a position as caretaker at Anderson College in Glasgow, which gave him time to pursue more scientific matters.
Though his first publication was on philosophy, after reading a book by the French scientist Joseph Adhémar on the influence of wobble in Earth's axial tilt and its eccentric orbit on ice ages, Croll became obsessed with climate change. Adhémar claimed that the combined effects of these two factors would result in alternating glacial periods in the northern and southern hemispheres. Some of Adhémar's other claims were a bit wacky, which diminished the impact of the good ideas contained in his book. But, Croll and several others took note and pushed the investigation further. For 25 years, Croll was to be captivated by what he termed “The Fundamental Problem of Geology.”
Croll was a very methodical man, with a deep desire to understand the fundamental nature of the problems he studied. His approach to understanding the impact of Earth's orbital eccentricities involved arduous, intricate calculations. Though the elliptical nature of Earth's orbit had first been calculated by Laplace, in 1773, Croll's calculations were much more painstaking.
He published his first paper on the causes of climate change in 1864, and it rocketed him to prominence in geological circles. Ice ages and their causes were a topic of great interest at the time. No one had developed an accurate way to tell when the last glacial period had ended or how long it had lasted. Croll corresponded with Charles Lyell, sending his ideas of links between ice ages and variations in Earth's orbit.
In 1875, Croll published his major work on the linkage between Earth's orbital variations and climate, Climate and Time in their geological Relations: A Theory of Secular Changes of the Earth's Climate. In it, he estimated that the last glacial period had ended 80,000 years ago. He also reiterated Adhémar's conjecture that the glaciers would alternate between the poles in a 11,500 year cycle. Acceptance of Croll's theory was at its peak.
Geologists working in the field began to use glacial drift deposits and erosion to try and date the glacial episodes experimentally. As their data came in, discrepancies with Croll's carefully calculated, but theoretical dates began to arise. The alternation between northern and southern hemispheres was also shown to be erroneous. By the end of the 19th century, Croll's astronomical theory of climate change was discredited and discarded.
The rise and fall of Croll's theory is a good example of how scientific progress is made. Croll built on the works of others, including Agassiz and Adhémar, both of whom he readily acknowledged. His work corrected and refined their theories while making new predictions of its own. Other scientists, trying to confirm or disprove Croll's predictions, showed the astronomical theory to be incorrect in several ways, leading to its fall from favor. But sometimes, the basic idea behind a theory is correct, even if its expression is flawed. In this case, the fundamental ideas were correct—Croll's theory would rise again.
The scientist who revived Croll's theory was Milutin Milankovitch, a Serb mathematician and engineer whose initial fame was due t
o concrete. Born in 1879, in the town of Dalj on the Danube, he was well-educated during his youth. He attended university in Vienna, eventually earning his PhD in engineering. His dissertation was on the uses of reinforced concrete in buildings, and provided a wealth of data about strengths and shapes used for construction.
Illustration 80: Milutin Milankovitch
With his detailed knowledge of concrete, Milankovitch quickly found work with a large engineering firm in Vienna, eventually becoming head engineer. He earned notoriety designing buildings all across Europe, and his career seemed on a steady course. But the political storm clouds, that would eventually lead to World War I, were starting to gather.
Though he loved his work and life in Vienna, in 1908, political tensions led him back to Serbia. He became a professor at the University of Belgrade, giving up his much higher paying job as an engineer. Milankovitch was a man driven to succeed at whatever he worked on, and he was soon looking around for problems where he could apply his considerable mathematical skills. After reading Croll's work on orbital variation and climate, Milankovitch decided to investigate the causes of climate change.
A great deal of scientific progress had been made since Croll's work. Better measurements of planetary movements were available, along with better historical climate data. Milankovitch was not trying to explain ice ages, but all climate changes. He was seeking a mathematical theory that would give the temperature for any point on Earth's surface at any given time. In fact, he would eventually extend his calculations to the Moon and Mars.
Though his work was interrupted by WWI, during which he was briefly imprisoned, by 1920 he published A Mathematical Theory of the Thermal Phenomenon Produced by Solar Radiation. His book generated interest among meteorologists, but gained little notice from geologists of the time.
The Resilient Earth: Science, Global Warming and the Fate of Humanity Page 16