2010 Diary week 37
Global warming, climate change and weather extremes
Book Review
Below you will find the review of Part V of With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change by Fred Pearce. These are some snippets: “Advances in geology revealed that not one ice age but a whole series of glaciations had occurred, separated by warm periods like our own. The onset of the last ice age must have been rather fast, with temperatures crashing in a couple of hundred years at most, and very probably much less.” “We now know that two main ice sheets formed. One stretched from the British Isles across the North Sea and Scandinavia, and then west through Russia and western Siberia, and north across the Barents Sea as far as Svalbard.” “A second, even larger sheet covered the whole of Canada and southern Alaska, with a spur extending over Greenland. A smaller sheet sat over Iceland, and the seas around were full of thick floating ice.” “Combined with the older ice covering Antarctica, these ice sheets contained three times as much ice as is present on Earth today – enough to keep sea levels worldwide some 400 feet lower than they are now – and 30% of Earth’s land surface.” “Temperatures fell by 9°F as a global average; the world beyond the ice sheets became dry and cold; deserts covered the American Midwest, France, and the wide lands of Europe and Asia between Germany and the modern-day Gobi Desert, in Mongolia; the Sahara expanded; the Asian monsoon was largely extinguished; and the tropical rainforests of Africa and South America contracted to a few refuges surrounded by grasslands.”
WITH SPEED AND VIOLENCE
WHY SCIENTISTS FEAR TIPPING POINTS IN CLIMATE CHANGE
FRED PEARCE
BEACON PRESS 2007
PART V
PART V. ICE AGES AND SOLAR PULSES
Chapter 21: Goldilocks and the Three Planets
• Some planetary scientists have dubbed Earth the “Goldilocks planet” because of its rich and chemically active atmosphere and a sufficiency of greenhouse gases to maintain equable temperatures and lots of liquid water.
• Although other planets were similar at birth 5 billion years ago, Venus is too hot and Mars is too cold and only Earth is just right for the development of myriad life forms.
• For the past half-billion years Earth has maintained a surprisingly constant temperature, while the sun has changed a great deal over the lifetime of Earth. For the first billion years, the sun emitted a third less energy that it does today.
• Did this happy goldilocks outcome occur entirely by chance? Or could the planet have developed some kind of crude thermostat? The surprising answer is that it seems to have done just that.
• Back in the 1980s the charismatic British chemist and maverick inventor Jim Lovelock wondered if life itself might be controlling this process. Two of his acolytes Tyler Volk and David Schwartzman suggested he was right by demonstrating that basalt rocks erode a thousand times faster in the presence of organisms such as bacteria, introducing a highly dynamic negative feedback as most bacteria will keep the planet cool.
• If the air gets too cool, the planet becomes covered by ice, the bacteria die, the erosion slows, and the atmosphere warms again.
• For many years, Lovelock was virtually cast out of the scientific community, and Gaia was often seen as quasi-religious mumbo-jumbo. Major journals like Nature and Science would not publish his work but his idea has made him the spiritual father of a whole generation of Earth system scientists.
• The thermostat, whether run by life or by geology, is pretty crude. For 400 million years planet Earth has been getting cooler. Greg Retallack of the University of Oregon argues that the cooling happened because large parts of life wanted it that way.
• Plants have proved extremely efficient at capturing carbon dioxide and burying it permanently where it cannot return to the atmosphere. Some 7 trillion tons of old vegetable carbon has been stored for tens of millions of years in the form of fossil fuels beneath Earth’s surface.
• In addition, probably as much methane is captured in frozen clathrate beneath the ocean bed.
• Around 55 million years ago, earth experienced the “biggest fart in history,” a vast surge of methane into the atmosphere from the undersea clathrate store, which pushed the temperature up by around 9°F.
• The methane eventually decayed to carbon dioxide, which was absorbed back into the oceans. Levels of carbon dioxide in the air were still about 5 times as high as they are today – at around 2,000 ppm. Within a million years these levels began to fall sharply and by 40 million years ago they had subsided to 700 ppm.
• Around 24 million years ago they were below 500 ppm, ice sheets spread across Antarctica and about 3 million years ago another surge of cooling had begun, resulting in ice sheets forming in the Northern Hemisphere, too.
• Unravelling the causes of the ice ages may, many climate scientists believe, provide vital clues to our fate in the coming decades.
Chapter 22:The Big Freeze. How a wobble in our orbit triggered the ice ages
• While Louis Agassiz was summering in the alps in 1836, his host pointed out giant scratch marks on the mountainsides that showed, he said, how the glaciers must once have extended much farther down their valleys.
• There were similar marks distant from present-day glaciers in Europe and North America.
• He had read of perfectly preserved mammoths being dug from the snow in Siberia. He concluded that much of the Northern Hemisphere was covered by ice and that the event happened very suddenly, in a vast, icy apocalypse.
• Advances in geology revealed that not one ice age but a whole series of glaciations had occurred, separated by warm periods like our own. The onset of the last ice age must have been rather fast, with temperatures crashing in a couple of hundred years at most, and very probably much less.
• We now know that two main ice sheets formed. One stretched from the British Isles across the North Sea and Scandinavia, and then west through Russia and western Siberia, and north across the Barents Sea as far as Svalbard.
• A second, even larger sheet covered the whole of Canada and southern Alaska, with a spur extending over Greenland. A smaller sheet sat over Iceland, and the seas around were full of thick floating ice.
• Combined with the older ice covering Antarctica, these ice sheets contained three times as much ice as is present on Earth today – enough to keep sea levels worldwide some 400 feet lower than they are now – and 30% of Earth’s land surface.
• Temperatures fell by 9°F as a global average; the world beyond the ice sheets became dry and cold; deserts covered the American Midwest, France, and the wide lands of Europe and Asia between Germany and the modern-day Gobi Desert, in Mongolia; the Sahara expanded; the Asian monsoon was largely extinguished; and the tropical rainforests of Africa and South America contracted to a few refuges surrounded by grasslands.
• Humans lived by hunting on the plains and bunkering down in the small areas where lush vegetation persisted despite the cold and arid conditions.
• Something drastic must have triggered all this. The first man to subject it to detailed analysis was James Croll with virtually no formal learning, but a passion for self-education.
• Astronomical forces, he discovered, have three principal effects on earth, all of which slightly alter the distribution of the solar radiation that reaches it. The effects are greatest in the polar regions, where they can alter the amount of sun by as much as 10%.
• First, they change the shape of earth’s annual orbit around the sun. This “eccentricity” has a cycle that repeats itself about every 100,000 years.
• Second, looked at from space, Earth appears to be spinning on a slight tilt. The combination of the orbit around the sun and the tilt of Earth’s axis is what gives us our seasons. Astronomical forces alter the tilt of the axis, causing a difference in the intensity of the season. It has a 41,000-yaear cycle.
• Third, there is a further wobble in the axis around which Earth rotates, called the precession, that repeats on a cycle of 19,000 to 23,000 years. Currently the Northern Hemisphere has its summer, and the Southern Hemisphere has its winter, when Earth is furthest from the sun; 10,000 years ago, it was the other way round.
• The eccentricity of Earth’s orbit around the sun drives the 100,000-year cycles into and out of ice ages. The other two effects, especially the precession, seem to trigger the short warm episodes that punctuate each ice age.
• Croll realized that these changes made little difference to the amount of radiation reaching Earth, but the changes did alter where and when the sun hit and they coincided with what geologists were discovering about the timing of Earth’s progress into and out of ice ages.
• Ice sheets would grow when northern winters were coldest. Once ice sheets started to grow, they would reflect ever more sunlight back into space, intensifying the cooling.
• Today the idea that astronomical forces influence the formation of ice sheets is back in vogue. Proof of its worth came in the 1970s. The orbital changes are known as the Milankovitch wobbles after Milutin Milankovitch revived and elaborated Croll’s ideas in the early 20th century.
• Earth’s system contains powerful embedded amplifiers that can make it highly sensitive to relatively small forcings. Identifying those amplifiers should help answer how Earth’s climate might respond to our interference in its actions today.
• Jim Hansen calculates that at the height of the last glaciation, it reduced the amount of heat absorbed by the planet’s surface by 4 watts per 10.8 square feet. But why, after reaching their greatest extent 21,000 years ago, did the ice sheets begin to retreat rather than continue to grow until they covered the entire planet?
• There are today about 44 trillion tons of carbon dissolved in the oceans – 50 times as much as in the atmosphere. A minor uptake of carbon by the oceans could have a huge effect on the atmosphere.
• Colder water dissolves carbon dioxide better than warmer water. Plankton like colder temperatures and the Southern Ocean around Antarctica is today one of the most productive. As the plankton grew, they drew more carbon dioxide out of the atmosphere.
• Methane may have been important as its atmospheric concentration is in lockstep with temperature apparently as fixedly as that of carbon dioxide.
• A final amplifier may have been the ocean circulation system, with its huge ability to move heat around the planet. There is good evidence that the circulation system slows down during ice ages, and may have shut down entirely at the coldest point in the last glaciation.
• Over the past couple of million years at least, the natural climate system has constantly returned to one of two conditions. One is glaciated; the other is interglacial. The former has an atmosphere containing around 440 billion tons of carbon dioxide; the latter has an atmosphere containing about 660 billion tons.
• The planet oscillates between the two states regularly, repeatedly, and rapidly. But it doesn’t hang around in any in-between states. Each time the guiding feedback seems to have rapidly moved about 220 billion tons of carbon between the atmosphere and the ocean.
• In the past century or so, human activity has moved another 220 billion tons of carbon into the atmosphere, in addition to the high concentrations of the interglacial states.
• The atmosphere now contains twice as much carbon as it did during the last ice age, and a third more than in recent interglacial eras, including the most recent. And we are adding several billion tons more each year that comes mainly from fossilized carbon.
• We are in uncharted territory and the big question is: How will the system respond to this vast injection? There seem to be three possibilities.
• The system may deploy negative feedbacks to suppress change; the system will carry on operating normally, gradually accumulating the carbon and gradually raising temperatures; the feedbacks may flip the system into a new, as-yet-unknown state.
• We don’t know. But hold on to your hat: we could be in for a bumpy ride.
Chapter 23: The Ocean Conveyor. The real day after tomorrow
• Broecker is a geochemist with an unimpeachable track record in pioneering the use of isotopic analysis to plot ocean circulation.
• The conveyor begins with the strong northward flow of the Gulf Stream pouring warm, salty water from the South Atlantic across the tropics and into the far North Atlantic.
• In the North Atlantic, the water is cooled by the bitter winds blowing off Canada and Greenland, increasing the density of the water, a process amplified by the formation of ice, which takes only the freshwater and leaves behind increasingly saline and dense water.
• The dense water sinks to the bottom of the ocean, in two spots: one to the west of Greenland, in the Labrador Sea, and the other to the east, down Wadhams’s vertical chimney’s.
• The circulation has many roles: distributing warm water from the tropics to the polar regions, mixing the oceans, and aiding the exchange of carbon dioxide between the atmosphere and the oceans.
• It keeps Europe warm, allowing Europeans to grow roses farther north than Canadians meet polar bears.
• On the face of it, the circulation is self-sustaining. The operation of the chimneys draws Gulf Stream water north, which provides more water for the chimneys. But it is also temperamental, prone to switching on and off abruptly.
• Broecker uses chemical tracers to identify movements of water in the oceans. By 1995, he felt confident enough to title a lecture on the conveyor to a big science conference “Abrupt Climate Change: Is One Hiding in the Greenhouse?”
• In it he outlines how evidence from sea-floor and lake sediments, ice cores, coral, and glacial records “demonstrates unequivocally” that an on-off switch on the global conveyor operated at the beginning and the end of the last ice age.
• The suggestion was that the conveyor had shut down and single-handedly started the ice ages, lowering temperatures by “4 degrees C (7°F) or more, often within the lifespan of a generation” – a claim he inflated soon afterward, in the pages of Scientific American, to “10 degrees C (18°F) over the course of as little as a decade.”
• When a threshold is crossed and sudden climate change occurs, it is the conveyor that throws the switch. These claims remain extremely controversial.
• Two years after making his claims for the ocean conveyor and the ice ages – and just a week before the world met in Japan to agree to the Kyoto protocol – he was warning that climate change could trigger a future shutdown of the conveyor.
• “There is surely a possibility that the ongoing buildup of greenhouse gases might trigger yet another of those ocean reorganizations.” If it did, “Dublin would acquire the climate of Spitzbergen in ten years or less. The consequences would be devastating.” He called the conveyor the “Achilles heel of the climate system.”
• Climate systems work, he suggests, not gradually, through constant incremental change, but in sudden bursts.
• I met Michael Schlesinger of the University of Illinois at Urbana-Champaign at a conference on “dangerous” climate change held at the Hadley Centre for Climate Prediction, in Exeter in 2005.
• For more than a decade, Schlesinger has been making Broecker’s case that a shutdown of the ocean conveyor could be closer than mainstream modelers think, criticizing the IPCC and its models for systematically eliminating a range of quite possible doomsday scenarios from consideration.
• “The trouble with trying to reach a consensus is that all interesting ideas get eliminated,” he said at the conference. Science by committee ends up throwing away the good stuff – like the idea of the conveyor’s shutting down.
• In Exeter Schlesinger had been invited to present his model findings that a global warming of just 3.6°F would melt the Greenland ice sheet fast enough to swamp the ocean with freshwater and shut down the conveyor. The risk, he said, was “unacceptably large.”
• Later in the day, Peter Challenor, of the British National Oceanography Centre, in Southampton, said he had shortened his own odds about the likelihood of a conveyor shutdown from 1 in 30 to 1 in 3. He guessed that a 3-degree warming of Greenland would do it. Given how fast Greenland is currently warming, that seems a near certainty.
• In the 1960s some 8 billion acre-feet of freshwater gushed out of the Arctic through the Fram Strait. Studies by Ruth Curry of the Woods Hole Oceanographic Institute published in Science in June 2005, concluded that 7 billion acre-feet would have been enough to “substantially reduce” the conveyor, and double that “could essentially shut it down.” So it was a close call.
• With the region’s water still substantially fresher than it was at the start of the 1960s, the conveyor remains on the critical list.
• “A shift in the ocean conveyor, once initiated, is essentially irreversible over a time period of many decades to centuries. It would permanently alter the climatic norms for some of the most densely populated and highly developed regions of the world.”
• Harry Bryden, of the National Oceanography Centre, had strung measuring buoys in a line across the Atlantic, from the Canary Islands to the Bahamas, and found that the flow of water north from the Gulf Stream into the North Atlantic had faltered by 30% since the mid-1990s.
• Less warm water was going north at the surface, and less cold water was coming back south along the ocean floor. This weakening of two critical features of the conveyor was, so far as anyone knew, an unprecedented event.
• The volume of deep water coming from the Greenland Sea, the site of Wadhams’s chimneys, had collapsed to half its former level.
• None of this demonstrated that Broecker’s bleaker predictions of what would happen if the conveyor shut down were about to come true – that “London would experience the winter cold that now grips Irkutsk in Siberia.”
• Something more like the little ice ages was the worst that most climate modelers feared. But there did seem to be a real possibility that many of Broecker’s ideas were about to be put rather dramatically to the test.