In the winter of 1780, New York harbour froze over: people walked across the ice from Manhattan to Staten Island. Around the same time, the French invaded the Netherlands over frozen rivers, while - on a lighter note - Londoners held Frost Fairs on the iced-up River Thames (pictured).
This was the period of the Little Ice Age. The world was then about one degree cooler than the average for the twentieth century.
The Solar Connection
Meteorologists aren’t the only people to pick out this period as unusual: they’re joined by astronomers who study sunspots (pictured, below). Today, these dark patches – marking magnetic storms on the Sun - come and go in an 11-year cycle. But there was a complete dearth of dark spots on the Sun from 1645 to 1715, the time that the Earth plunged into the Little Ice Age.
This apparent link between the Sun’s magnetic activity and the Earth’s climate could be coincidence. But German researcher Sami Solanski has confirmed the effect. Based at the Max Planck Institute for Solar System Research, in Katlenburg-Lindau, Solanski has pushed the Sun’s magnetic record back to 9000 BC. No-one was observing sunspots then, of course, but the Sun’s activity shows up in certain isotopes - carbon-14 and beryllium-10 - that are preserved in ancient pieces of wood and deep in the ice of Greenland and Antarctica.
Solanski has found that the Sun’s magnetism and the Earth’s temperature march hand-in-hand over the millennia. When the Sun is violent, our temperatures go up. When the Sun is dormant (as in the Little Ice Age), our temperatures drop. He finds, however, that this correlation stops around 1970, when manmade global warming starts to kick in. Solanski reckons that solar activity can account for only 30%, at best, of the Earth’s warming in the past 40 years.
Volcanic Cool
In 1783, the inhabitants of Iceland were treated to the spectacle of a wall of fire spouting from the ground. It was accompanied by the greatest lava outpouring in history, along with lethal gases, such as acidic sulphur dioxide. A quarter of Iceland’s population died. And the following winter, Benjamin Franklin – then in Paris – noted the exceptionally cold conditions, and was astute enough to blame the eruption in Iceland.
The great explosions of the Indonesian volcanoes Tambora in 1815 and Krakatoa in 1883 each lowered Earth’s temperature by more than a degree. But in the geological past, eruptions had an even more dire effect. At the Siberian Traps in Russia, some 250 million years ago, the Earth witnessed its largest ever outpouring of lava – enough to cover the entire planet with three metres of molten rock. The ash and dust first cooled the planet to freezing point; then greenhouse gases heated it up. The toxic gases from the eruption, followed by the freeze/fry cycle, killed 95% of species on Earth.
The Ocean Conveyor
Great Salpetre Cave, Kentucky, is a treasure trove for fossil hunters. Oddly enough, near the surface, fossils are few. But when you dig to a level corresponding to 13,500 years ago, you suddenly come across a plethora of large mammal bones, including the giant beaver and the long-nosed peccary.
Some 30 species became extinct in a very short space of time. And the culprit was a sudden drop in temperature.
It occurred as the world began to warm up from the last Ice Age. For a period of 1000 years, the northern hemisphere was suddenly thrown back into an even deeper freeze. This period is known as the Younger Dryas (after a wild flower whose remains are found then) and – incredibly – it both started and ended in just ten years.
The clue to the Younger Dryas event lies deep in the oceans. All the oceans of the world are connected with great loops of flowing water: the Ocean Conveyor (pictured, above). Warm water from the Equator travels north up the Atlantic, until it reaches Greenland; here it cools and sinks, and the cold water travels back to the Equator along the ocean bed.
The conveyor is uniquely sensitive to the water’s salinity. If too much fresh water flows in, it sits on top of the salty water and caps off the conveyor. Instead of warm water moving up as far as Greenland, it will dive down before it’s travelled that far.
At the end of the last Ice Age, a huge ice-impounded lake in the middle of North America suddenly gushed out into the North Atlantic. The freshwater from Lake Agassiz stopped the Ocean Conveyor in its tracks.
In the space of less than ten years, the ocean circulation stopped. Deprived of warm water, Europe froze solid. After a thousand years, the freshwater dispersed, and the Ocean Conveyor started up again.
Orbital Mechanics
The Ocean Conveyor can modify the beginning and end of period of glaciation, but what drives the fundamental advance and retreat of the ice-sheets? The answer was put forward in 1941, by a Serbian engineer, Milutin Milankovic. He suggested that the Earth’s Ice Ages are governed by changes in the Earth’s tilt and its orbit around the Sun.
There are three parts to the theory. Our planet’s orbit changes from more circular to more oval, over a period of 100,000 years. The Earth nods up and down, changing its tilt, every 41,000 years. And its axis swings around in 23,000 years, altering its angle to the Sun when it is at its closest point.
Although the Earth receives the same amount of sunlight over a year, the Milankovic cycles can make the seasons more extreme. Ironically, an Ice Age begins when seasonal changes are small. A mild winter is more humid and produces more snow; during the ensuing cool summer, the snow that’s built up on land doesn’t melt. As a result of the land never warming up, more snow accumulates the next winter, driving temperatures ever downwards.
Since Milankovic’s day, more and more evidence has accumulated to show how prescient he was. Ice cores drilled in Greenland and Antarctica contain a thermometer that reveals the changing temperature, going back hundreds of thousands of years. It shows up in the different isotopes of oxygen preserved in the ice. The temperature pattern is a sawtooth through time (pictured, with data from different ice cores), with the ice coming and going with a periodicity that fits Milankovic’s theory.
With this theory so well extablished, what can we say about the future? We currently live in a warm “interglacial”, which climatologists call Stage 1. Until recently, many researchers thought that a new Ice Age is imminent at any time. But David Hodell from the University of Florida disagrees. He has investigated sediments on the floor of the Atlantic Ocean, as well as lake sediments and ancient stalactites. And he has found that our present interglacial most closely resembles Stage 11 of the Ice Age (the fifty or so “stages” number backwards in time). Stage 11 occurred over 400,000 years ago – and lasted for 30,000 years.
That’s bad news for anyone who had hoped that our current phase of global warming will soon be offset by the next glaciation. The glaciers won’t be marching to help us for another 20,000 years.
The most deadly extremes in our planet’s history involve Snowball Earth and extraterrestrial impacts…
Snowball Earth
According to Paul Hoffman from Harvard, some 700 million years ago all the planet’s landmasses formed a single continent – Rodinia - straddling the Equator. The polar seas began to freeze. Their brilliant white surface reflected sunlight, cooling the planet until the entire Earth froze over. Rather like Jupiter’s moon Europa today (pictured), our planet was a cosmic snowball. Life could only survive at hot volcanic vents on the ocean floor.
Hoffman believes that volcanoes eventually pumped enough carbon dioxide and other greenhouse gases into the atmosphere, to heat the Earth until the ice began to thaw. As more of the dark ocean surface was exposed, it absorbed more of the Sun heat: the seas became warmer and the ice rapidly melted.
Hammer from the Heavens
Mike Baillie, of Queen’s University, Belfast, has been checking out wood from ancient forts in Northern Ireland. Within the tree-rings of the ancient oak logs, he’s found one band where the rings are very narrow – a few years when the tree struggled hard to survive. Baillie dates it to AD 536.
That had to be a period when the climate was extremely bad. Indeed, chroniclers from around the world – from Italy to China – complained about the bad weather and consequent famine. Baillie links it to the appalling conditions that were supposed to follow the death at that time of Britain’s warrior hero, King Arthur.
Baillie blames the appalling conditions on a comet (like Hale-Bopp, pictured below) that impacted the Earth, and spread cooling dust around our planet – a bigger cousin of a comet or asteroid that was seen to explode over Siberia in 1908.
That’s not the only danger we face from the sky. Jan Veizer of the University of Ottawa has been looking inside meteorites, to see how the Solar System – including the Earth – is affected by cosmic radiation, as the Sun moves around the Milky Way Galaxy. He has found the radiation peaks every 143 million years – matching the changing temperature of the Earth. Veizer proposes that the cosmic rays would seed clouds on Earth, so reducing the planet’s temperature. He claims that – through the geological past – cosmic rays have been twice as effective as carbon dioxide in controlling Earth’s temperature.
Whatever effects the Cosmos has on us, it’s something we have no hope of controlling – and not even of predicting. As the dinosaurs found out 65 million years ago, if you’re unlucky enough to be in the path of a runaway asteroid, then the climate is going to be unimaginably extreme. When that asteroid hit the Earth, it created a fireball at over a thousand degrees that literally roasted the dinosaurs alive.
That was the ultimate “fry” in the Earth’s long history, just as Snowball Earth was the ultimate “freeze”. These geological perspectives prove that our current global warming is a mere blip as compared to the changes that nature can inflict on our planet.
But that’s no reason for us to be complacent. Though our planet has survived these extreme climate swings, and life has somehow pulled through, the Earth’s cycles of freeze and fry have always destroyed the dominant species of their time…
For more information
US Environmental Protection Agency on causes of past climate change
http://www.epa.gov/climatechange/science/pastcc.html
Christopher Scotese on changing climate throughout Earth’s history
http://www.scotese.com/climate.htm
Source: http://www.firstscience.com
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