A veil of debris from two massive volcanic eruptions in the early 19th century plunged the planet into a frozen decade by blocking the Sun's rays.
Blasted into an Ice Age
Something very bad has happened to the climate. Across the world, people are struggling to cope with extremes of weather that threaten their survival. Deaths from famine and disease are soaring, and it is clear that disaster looms unless the current climatic upheaval ends. No, it is not another grim communique from the climate change conference in Copenhagen: it is a summary of what was starting to happen to our planet exactly 200 years ago. And back then it was not global warming that threatened mankind, but the precise opposite. From 1809 onward, our entire planet was plunged into a decade-long Ice Age which devastated communities from Canada through Europe to China. The exact toll in human lives will never be known, but an estimated 200,000 died in Europe alone during the depths of the crisis in 1816 - the "year without a summer".
At least there has been little doubt about the cause. The finger of blame is usually pointed at Mt Tambora, a still-active volcano in Indonesia that exploded in 1815 in the most powerful eruption of modern times. The blast injected billions of tonnes of debris into the atmosphere, the resulting veil of dust cutting the level of the Sun's heat reaching the Earth's surface, and triggering global cooling.
All very neat and tidy. But new research has now revealed a twist to the tale - and a new mystery. According to an international team of scientists, analysis of climate records shows that the global cooling actually began around five years before Mt Tambora blew up. It thus seems something else must have been involved; but what? The obvious suspect is another volcanic eruption. Certainly, the early 19th century did seem to have been afflicted by a plague of the events: between 1812 and 1814 no fewer than four volcanoes erupted at sites as far afield as the Caribbean and Japan. Yet they were all relatively minor events - and the timing is still wrong in any case.
A search of the historical record for eyewitness reports of an eruption or weird atmospheric effects around 1810 has also come up empty. The absence of such evidence does not imply evidence of absence: the explosion may just have happened in some remote part of the world. What is certain is that any eruption capable of disrupting the climate must have injected a huge amount of dust high into the atmosphere - and what goes up must come down. This prompted Professor Jihong Cole-Dai of South Dakota State University and his colleagues to look for a telltale sign of an eruption around 1810 in samples of polar ice.
Now they have found it, in the form of sulphur trapped in ice-cores extracted from the ice-caps of Greenland and Antarctica. The exact characteristics of their discovery are highly significant. First, analysis of the sulphur revealed anomalous amounts of a certain isotope, implying that it had interacted with air in the stratosphere - which only truly colossal eruptions can reach. Second, the discovery of the anomaly at ice-core levels corresponding to the same date point to a single event. Using ice-core data for the Mt Tambora event, the team estimates that the earlier eruption took place in 1809, possibly in February of that year.
But where did it happen? Reporting their findings in the journal Geophysical Research Letters, Prof Cole-Dai and his colleagues argue that the discovery of the sulphur anomaly at both poles points to a location somewhere in the tropics - the only part of the world from which dust can travel to both Greenland and Antarctica. Yet this serves only to deepen the mystery of the eruption of 1809, as it seems incredible that a colossal explosion in so populous a part of the world could have gone unreported.
In the case of Mt Tambora, the explosion killed tens of thousands locally, generated a tsunami that killed thousands more across Indonesia, and was heard more than 2,000km away. An eruption as violent as that which seems to have taken place in 1809 would surely have done more than merely generate some dust at the poles. Clearly, some more detective work is needed to solve the mystery of the 1809 eruption. Whatever is found, the disastrous events of the subsequent decade underline the swift, global and ineluctable climatic influence of volcanoes.
It is a phenomenon attracting increasing interest among researchers trying to fathom the evolution of modern humans. Most palaeo-anthropologists believe that Homo sapiens spread across the globe after leaving Africa around 70,000 years ago. What is not clear, however, is precisely why these early humans left their homeland. One possibility is that they were driven out by climate change, which made parts of Africa inhospitable. This version of the so-called Out of Africa Hypothesis has been bolstered by studies of Lake Toba in Sumatra - the site of the largest eruption on Earth in the last 2.5 million years.
Like Mt Tambora, this Indonesian volcano blasted colossal amounts of dust into the air, triggering a plunge in global temperatures of as much as 5°C. And according to geologists, what has been called the Toba Catastrophe took place around 74,000 years ago - right in line with the Out of Africa date. According to some researchers, the resulting climatic upheaval was so terrible that the human race may have been pushed to the brink of extinction. Others insist that Homo sapiens was smart enough by then to adapt to the dire conditions.
It is a debate with echoes that can be heard this week in Copenhagen. Some believe global warming threatens the very survival of Homo sapiens; others argue we can adapt. At Copenhagen, most are pushing for measures to tackle the root cause, by restricting greenhouse gases. With volcanic eruptions, that is not an option. No amount of technology or legislation can prevent a future Mt Tambora event. When one strikes, we will have no choice but to do as we did exactly 200 years ago, and just get through it.
Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England