2010 Diary week 38
Global warming, climate change and weather extremes
Book Review
Below you will find the review of Part VI of With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change by Fred Pearce. These are some snippets: “Today we can see this extraordinary climatic history recorded in ice cores extracted from the ice of Greenland and Antarctica. They show a climate system in a protracted series of spasms.” “The last great cold snap of the ice age, 12,800 years ago, is known today as the Younger Dryas era. Within about a generation, temperatures fell worldwide – perhaps by as little as 3 to 5°F in the tropics, but by an average of as much as 28 degrees farther north, and, according to ice cores analysed by George Denton, of the University of Maine, by 54 degrees in winter at Scoresby Sound, in eastern Greenland.” “Richard Alley, who was there handling the ice cores, says: “Most of that change looks like it happened in a single year. It could have been less, perhaps even a single season. It was a weird time indeed.”
WITH SPEED AND VIOLENCE
WHY SCIENTISTS FEAR TIPPING POINTS IN CLIMATE CHANGE
FRED PEARCE
BEACON PRESS 2007
PART VI
Chapter 24: An Arctic Flower. Clues to a climate switchback
• Some 18,000 years before the present, there was still a full-on ice age. By 16,000 years ago, the world was warming strongly. But by 15,000 years ago, it was cold again, with ice sheets reforming.
• At 14,500 years ago, it became so warm that within 400 years the ice caps melted sufficiently to raise sea levels worldwide by 65 feet. The cold gained the upper hand once more, only to give way to the pronounced warming of 13,000 years ago, which crashed again 12,800 years ago.
• Today we can see this extraordinary climatic history recorded in ice cores extracted from the ice of Greenland and Antarctica. They show a climate system in a protracted series of spasms.
• The last great cold snap of the ice age, 12,800 years ago, is known today as the Younger Dryas era. Within about a generation, temperatures fell worldwide – perhaps by as little as 3 to 5°F in the tropics, but by an average of as much as 28 degrees farther north, and, according to ice cores analysed by George Denton, of the University of Maine, by 54 degrees in winter at Scoresby Sound, in eastern Greenland.
• Analysts of the Greenland ice-core chronology say publicly that the warming must have happened within a decade. But that is the minimum time frame for the change of which they can be certain, given the resolution of the ice cores.
• Richard Alley, who was there handling the ice cores, says: “Most of that change looks like it happened in a single year. It could have been less, perhaps even a single season. It was a weird time indeed.”
• Chaos theory may help here. Alley says that it is just when conditions are changing fastest that the chances for seemingly random, unexpected, and abrupt change are greatest.
• In the final millennia of the ice age, as melting made its fitful but sometimes dramatic progress, a very large amount of liquid water was produced, forming giant lakes on the ice, the largest of which was Lake Agassiz draining down the Mississippi River into the Gulf of Mexico.
• 12,800 years ago something stopped this and forced the lake to drain east and there was a huge breakout of freshwater into the basin now occupied by the Great Lakes, and on into the North Atlantic.
• A fresher ocean shut down the conveyor once more. The warm Gulf Stream was no longer drawn north. The entire global system would have been shaken, and may have lurched back from its interglacial to its glacial mode.
• The evidence is patchy, but new evidence is emerging all the time. Modeling studies by John Chiang, of the University of California at Berkeley, suggest that an invasion that diluted the flow of warm water from the Gulf Stream would have rapidly frozen the ocean surface, flipping a climate switch and through the ice-albedo feedback dramatically chilling the entire region.
• Alley says: “It looks like this is the real switch in the North Atlantic. In the winter, does the water sink before it freezes, of freeze before it sinks? Sink or freeze. There are only two possible answers. That’s the switch.”
• If the water sinks, the conveyor remains in place and the Northern Hemisphere stays warm. If it freezes, the circulation halts and the westerly winds crossing the ocean toward Europe and Asia stop being warmed by the Gulf Stream and instead are chilled by thousands of miles of sea ice.
• And that switch flipped, Alley argues, at the start and the finish of the Younger Dryas. At the start, freshwater invaded the North Atlantic; the ocean froze, and within a decade “there were ice floes in the North Sea and permafrost in the Netherlands.”
• The westerly winds would have picked up the cold of the Atlantic ice, cooled the heart of the Eurasian landmass, preventing it from warming enough to generate the onshore winds that bring the monsoon rains to Asia. This revised narrative also explains the concurrent warming in the Southern Hemisphere.
• It took about 1,300 years before the North Atlantic water switched back to sinking rather than freezing in winter. When it happened it was at least as fast as the original freeze. The ocean warmed; the winds warmed; temperatures were restored in a year; nature returned to reclaim the tundra; and deglaciation got back on track.
• 8,200 years ago there was one last intrusion of cold freshwater into the North Atlantic, a lesser event than the younger Dryas, probably only regional in its impact, and lasting 350 years.
• In recent decades large slugs of freshwater have poured into the far North Atlantic. They may have come close to triggering a shutdown of the ocean conveyor. This trend is unlikely to end.
• As the climate warms and the permafrost melts in Siberia, river flows from there into the Arctic Ocean are rising strongly. And there is always the prospect of future catastrophic melting of the Greenland ice sheet, where glaciers are accelerating and lakes are forming.
Chapter 25: The Pulse. How the sun makes climate change
• In the 1690s temperatures in Scotland were more than 3°F below normal. Those who stayed behind were reduced to eating nettles and making bread from tree bark. After widespread famine brought despair about the future for the Scots as a nation, the clan chiefs forged a union with England.
• This was the little ice age: a climatic affair that began early in the 14th century and flickered on and off before peaking in the late 17th century and finally releasing its grip some 150 years ago.
• It spread its icy fingers from the north across Europe, creating spectacular skating scenes for the Dutch painters Breugel and Van der Neer, and allowing Londoners to enjoy the frolics of regular frost fairs on the frozen River Thames.
• There were some warm periods amid the cold. Between 1440 and 1540 England was mild enough for cherries to be cultivated in the northeastern Durham hills. Much of Europe was exceptionally warm in the 1730s.
• At the height of the little ice age, the Baltic Sea froze over, and there was widespread famine across northern Europe. Some suggest that half the populations of Norway and Sweden perished. Some say the cold was the hidden hand behind the famine, raising grain prices, and bread riots that triggered the French Revolution in 1789.
• If the Viking settlers had followed the ways of their Eskimo neighbors and turned to hunting seal and polar bears, they might have survived. Instead, they stuck to their hens and sheep and grain crops, and built ever-bigger churches in the hope that God would save them. He did not.
• Creeping starvation had cut the average height of a Greenland Viking from a sturdy 5 feet 9 inches to a stunted 5 feet. The last women were so deformed that they were probably incapable of bearing a new generation. We know all this because their buried corpses were preserved in the spreading permafrost.
• It is increasingly clear that what Europe termed the little ice age was close to a global climatic convulsion, which took different forms in different places. Reasonable cases have been made that it blanketed parts of Ethiopia with snow, destroyed crops and precipitated the collapse of the Ming dynasty in 17th century China, and spread ice across Lake Superior in North America.
• Rainfall patterns altered. In the Amazon basin fires ravaged the tinderbox rainforests. In the Sahara repeated floods in the early 17th century washed away the great desert city of Timbuktu.
• The medieval warm period ran from 800 to 1300, ending just as the little ice age began. Grains grew further north in Norway than they do today, and vineyards flourished on the Pennines, in England. Warmth brought Europe wealth. There was an orgy of construction of magnificent Gothic cathedrals.
• Reconstructions of past temperatures come mainly from looking at the growth rings of old trees. It looks likely that much of Europe was between 1.8 and 3.6°F warmer in the medieval warm period than it was in the early 20th century, while the little ice age was a similar amount cooler in Europe.
• Away from the North Atlantic, those centuries were characterized by long superdroughts that caused the collapse of several major civilizations. Best documented are the Anasazi people, ancestors of the modern Pueblo Indians who were forced to flee into the wilderness after a long drought that peaked in the 1280s.
• The little ice age and the medieval warm period appear to have been natural examples of climate change. The pendulum moves too fast for any orbital cycles and volcanic dust clouds cool temperatures for only a few years. Most climatologists believe that the sun should get the blame.
• The popularising of the telescope by Galileo a few decades before the Maunder Minimum meant that astronomers of the day were able to note the virtual disappearance between 1645 and 1715 of the by-then-familiar spots on the surface of the sun, now recognized as a good indicator of a reduced output of solar energy.
• The best guess is that solar radiation reaching Earth’s surface during the Maunder Minimum fell by 0.2%. But climatologists find it perplexing that such a widespread effect could result from such a modest change.
• Gerald Bond working at the Lamont-Doherty Earth Observatory argued forcefully until his death in 2005, that the little ice age and the medieval warm period were the most recent signs of a pervasive pulse in the world’s climatic system. This pulse had a cycle that occurred once every 1,500 years or so and was unaffected by bigger influences on global climate like the Milankovitch orbital cycles that triggered the major glaciations.
• In the early 1980s Hartmut Heinrich examined cores of sediment drilled from the bed of the North Atlantic, leading to researchers using the term Heinrich events. In the early 1990s Willi Dansgaard of the University of Copenhagen discovered in the Greenland ice-core record a series of large and sudden temperature changes that punctuated the last glaciation.
• Several times, temperatures leaped by up to 3.6 to 18°F within a decade or so, before recovering after a few hundred years. So far, more than 20 of these warm phases have been identified.
• Temperatures moved from cold to warm with a cycle ranging between 1,300 and 1,800 years.
• Bond had a hunch that the two events were connected with other climate changes such as the advances and retreat of glaciers in Europe and North America. It became clear that the iceberg armadas of the Heinrich events occurred during unusually cold phases of the Dansgaard-Oeschger cycle.
• Bond’s colleague Peter deMenocal examined seabed sediments off Africa’s west coast and found that every 1,500 years or so there were huge increases in dust particles in the sediments, suggesting big dust storms on land.
• The sediments also revealed dramatic increases in the remains of temperature-sensitive marine plankton, suggesting a temperature switchback in tropical Africa of as much as 9°F. “Climate changes that we thought should take thousands of years to happen occurred within a generation or two.”
• Researchers had discovered that isotopic traces of cosmic rays bombarding the atmosphere were left in the ice cores – and that when solar radiation is at its most intense, cosmic rays are literally blown away from the solar system. Fewer “cosmogenic” isotopes, like carbon-14 and beryllium-10, are left in the ice cores during periods of strong solar radiation.
• The evidence tallied. Over the past 12,000 years, fluctuations in detritus from the iceberg armadas in the Atlantic coincided with changes in cosmogenic isotopes in the ice cores. Thus there was a solar pulse that translated into a pulse in icebergs, global temperatures, and recurrent climatic events found through both the glacial and the postglacial eras.
• Bond told me in an interview shortly before his death that the most important lesson from his research is what it shows about the sensitivity of the system itself. “Earth’s climate system is highly sensitive to extremely weak perturbations in the sun’s energy output.” And if it is sensitive to weak changes in solar forcing, it is likely to be sensitive also “to other forcings, such as those caused by human additions of greenhouse gases to the atmosphere.”
PART VI. TROPICAL HEAT
Chapter 26: The Fall. The end of Africa’s golden age
• If there was a golden age for humans on Earth – a Garden of Eden that flowed with milk and honey – then it was the high point of the Holocene, the era that followed the end of the last ice age.
• The Holocene era, and its abrupt end, may offer important lessons about our future climate in the 21st century. No place on Earth exemplifies the fall from this climatically blessed state better than the Sahara.
• The world’s largest desert was once vast lakes, swamps, and rivers the size of France, Spain, Germany, the UK put together, draining south via Nigeria into the Atlantic ocean, or east down a vast wadi to the Nile.
• The whole of North Africa was watered by a monsoon system rather like the one that keeps much of Asia wet today. Archeologists digging in the sands of northern Chad, currently the dustiest place on Earth, have found human settlements around the shores of the ancient Lake Megachad. Paintings in caves deep in the desert depict the lives of the inhabitants of the verdant Sahara of the Holocene.
• The wet Sahara and the era known more generally as the African Humid Period began around 13,000 years ago, as the ice age abated; and, except for the Younger Dryas hiatus, it lasted right through to the end of the golden age.
• The monsoon rains were recycled by the rich vegetation across North Africa. Rather as in the Amazon today, the rain nurtured lush vegetation that ensured that much of it evaporated back into the air. The continually moistened winds took rain to the heart of the Sahara.
• But the African Humid Period came to an end very suddenly. In the space of perhaps a century, the rivers of the Sahara emptied, the swamps dried up, the bush died, and the monsoon rain clouds were replaced by clouds of wind-blown sand.
• The first answer is that the sun moved. Or, rather, the precession continued its stately progress and gradually took away the extremely favourable conditions for Saharan rains.
• Martin Claussen, of the Potsdam Institute for Climate Impact Research, in Germany, has played out this tragedy in detail in his model. He turns time forward and backward, recreates the subtle orbital changes, and fine-tunes the vegetation feedbacks. More or less whatever he does to mimic the conditions of 5,500 years ago, the result is the same. The system flips abruptly turning bush to desert, and seas of water to seas of sand.
• Other researchers have replicated his findings. Peter deMenocal, of Lamont-Doherty, calculates that the system flipped when solar radiation in the Sahara crossed a threshold of 470 watts per 10.8 square feet.
• Jon Foley, of the University of Wisconsin, found that a reduction in Holocene summer sun sufficient to reduce temperatures by just 0.72°F would have cut rainfall across the Sahara by a quarter, and by much more in the farthest interior of the continent.
• The evidence is as yet sketchy, but the dramatic drying of the Sahara and Arabia appears to coincide with other climate changes around the world. In the Pacific Ocean, El Niňo appeared to switch into a more active mode at around this time.
• The signs are that worsening droughts are becoming the norm in regions that have suffered megadroughts in the past. In the American West, the biggest river, the Colorado, is a shadow of its former self. Early in the 20th century, the average flow was 13 million acre-feet a year. From 1999 to 2003, the average sank to 7 million acre-feet. In 2002, it fell to just 3 million acre-feet.
• Kevin Trenberth, of the National Center for Atmospheric Research, reports that the percentage of Earth’s land area stricken by serious drought has more than doubled in 30 years. “The climate models predict increased drying over most land areas. Our analyses suggest that this may already have begun.”
• Mark Cane, a specialist in pacific weather at Lamont-Doherty, say scarily: “The medieval warm period a thousand years ago was a very small forcing compared to what is going on with global warming now. But it was still strong enough to cause a 300- to 400-year drought in the western U.S. That could be an analogue for what will happen under anthropogenic warming. If the mechanisms we think hold true, then we’ll get big droughts in the West again.”