Illustration 8 Temperature variation over the last million years.
The previous interglacial, called the Eemian, ended around 120,000 years ago. It had lasted about 20,000 years, which is the average length for interglacials during the last million years. The temperatures during the Eemian were as high or higher than they are today.49 CO2 levels were also about what they are today, though they fell considerably when the last glacial period began. The most recent glacial period is called the Wisconsin in the US. In Europe, it is often referred to as the Weicksel glaciation.
How cold did it get during the last icy period? Actual low temperatures are hard to determine, but ice core data indicates that the average temperature in Antarctica was about 12°F (7°C) colder than today. That may not sound like a lot, but the impact it had on the ice caps was huge. In North America, ice covered all of Canada and the northernmost parts of the United States. The extent of northern hemisphere ice coverage is shown in Illustration 9.
Illustration 9 Ice sheets during last glacial period.
At its peak, some 20,000 years ago, the last glacial advance locked up so much water in ice sheets that sea levels were 400 ft (120 m) below today's levels. Proof of this can be found in the channel of the Hudson river in New York.
Most river valleys end at the ocean's edge, where the lighter, fresh river water disperses on top of the denser, salty ocean water. The power of a river to cut a channel ends there as well. Oceanographers have discovered that the channel of the Hudson extends far beyond the New York-New Jersey coast.
Some parts of the Hudson Canyon rival in size the Grand Canyon of the Colorado. Wide and deep, the Hudson Canyon continues over 450 miles across the continental shelf.50 This magnificent and mysterious canyon lies next to one of the largest metropolitan areas in the world, yet remains unseen under hundreds of feet of ocean. This hidden valley, one of the largest submarine canyons in the world, bears testimony to a time when the oceans were much shallower than today.
The fact that alternating cold and warm periods have been occurring for a long time indicates that Earth can experience significant climate change without man's interference. The latest warming period, the Holocene, started before human civilization took root, and well before the industrial revolution. This tells us that humans were not the cause of the Holocene warming.
Since the Ice Started Melting
Even though we are in a warm period, that doesn't mean there haven't been recent swings in temperature. Just as temperatures vary from day to day, month to month, and year to year, there are longer term cycles that occur over centuries. Sometimes these changes are significant enough and happen fast enough for people to notice. When this occurs, the episodes are often given names: the Roman Warm Period, the Dark Ages Cold Period, the Medieval Warm Period, the Little Ice Age, and so on.
Illustration 10 shows variation in average temperature since the beginning of the Holocene. Notice the long period starting about 9,000 years ago, when the temperatures were at or above today's level. This period is called the Holocene Climate Maximum (HCM), or sometimes the Holocene Climate Optimum, and it lasted for 4,500 years. The Iceman's ill-fated journey through the Alps took place near the end of the HCM.
Illustration 10 Temperature variation during the Holocene.
During this time, temperatures at the North Pole rose 7°F (4°C). Some studies suggest that winter temperatures rose more than summer temperatures, and that the warming was limited to the northern hemisphere.51 These studies claim that temperatures in the southern hemisphere remained close to modern levels or a bit cooler. Some authorities claim this lack of worldwide change proves that warming during the HCM was fundamentally different than global warming today.
Reliable temperature records have existed for only the last 150 years, worldwide records for 50 years, and satellite data for 35 years. We observe greater variation today, in part, because there are more sources of data, providing details about more sources of change. For example, possible sources of temperature change are variation in insolation (the energy from the Sun), increase in greenhouse gases due to rock weathering or volcanic activity, the extent of sea ice, and variation in cloud formation rates. There are also a number of temperature exchange mechanisms that affect Earth's climate; ocean currents, deep water up-welling, air currents, floating sea ice, and ice shelf base melting, to name a few. Today all these sources and mechanisms are probed and measured worldwide. We have no way to recreate a similar wealth of data for days long past.
Scientists have discovered that “the Holocene climate developed differently in different regions of the North Atlantic due to different dominant forcing factors.”52 It also appears that different parts of the world experienced the warming at different times, with the southern hemisphere having warming periods before and after the northern hemisphere.53 According to data taken from Antarctic ice cores:
“All the records confirm the widespread Antarctic early Holocene optimum between 11,500 and 9000 yr; in the Ross Sea sector, a secondary optimum is identified between 7000 and 5000 yr, whereas all eastern Antarctic sites show a late optimum between 6000 and 3000 yr.”54
Accordingly, ice core data indicates that not all parts of the world were “hot or cold” at the same time, or by the same amount. Today, not all parts of the world experience the same temperature trends at the same time. The same was true during the HCM.
Recent studies have revealed that, despite the general worldwide warming trend, Antarctica is actually growing colder. From 1986 to 2000, central Antarctic valleys cooled 1.2°F (0.7°C) per decade with serious ecosystem damage from cold.55 Most of Antarctica now experiences a longer sea-ice season, lasting 21 days longer than it did in 1979.56 Satellite radar altimetry measurements from 1992 to 2003 indicate that, on average, the elevation of about 8.5 million square kilometers of the Antarctic interior has been increasing. The rising elevation has been linked to increases in snowfall, which translates into a mass gain of 45 billion tons per year, tying up enough moisture to lower sea level by 0.005 inches per year.57
Criticisms that claim no global warming took place during the HCM are not substantiated by current available data. Both hemispheres experienced warming during the early Holocene, but not simultaneously. Today, similar conditions exist. The northern hemisphere is warming, while the Antarctic regions are growing colder. Claiming the HCM warming was not global because both hemispheres did not warm simultaneously implies that the current spate of warming isn't global either.
Historic Climate Changes
So far, we have addressed climate change in the pre-historic past, long before there were written records or thermometers. All of the data we have discussed come from what scientists call proxies. Proxies are measurements like oxygen isotope ratios extracted from the tiny skeletons of dead sea creatures, pollen counts from lake bed sediment layers, or tree ring widths. These proxies stand in for actual measurements of other parameters, like temperature or CO2 levels. Proxies are second-hand data. There are many different proxies and we will explain how they are used in Chapter 13, Experimental Data and Error.
The task of compiling climate data becomes easier during recorded human history. Even if our ancestors did not have thermometers, we can collect anecdotal evidence from written documents. Several warm periods and cold periods, that lasted for centuries, were noted by commentators in the past: The Roman Warm Period, 400 BC - 100 AD, the Dark Ages Cold Period, 100-800 AD, and the famous Medieval Warm Period, 800-1300 AD.
Though these historic periods were named by European scientists, their effects were global. From a study of climate in China during the same times comes evidence that China experienced the same climate variations. K. F. Yu, et al. state, “historic records show that it was relatively warm and wet in China during 800-300 BC (Eastern Zhou Dynasty), but was significantly colder and drier in east China during the period of 386-589 AD.”58
The Chinese researchers continue: “it was so warm during the early Eastern Zhou Dynasty (770-256
BC) that rivers in today's Shangdong province (35-38°N) never froze for the whole winter season in 698, 590, and 545 BC,” but that “the period of Southern-Northern Dynasties (42-550 AD) was so dry and cold ... that Beiwei Dynasty (386-534 AD) was forced to move its capital from Pingcheng (40.10°N) to Luoyang (34.67°N) in 493 AD after a series of severe droughts.”
Research into the collapse of ancient civilizations in the Americas have revealed links to these shifts in climate. A paper in Science observed, “the collapse of Maya civilization in the Terminal Classic Period occurred during an extended regional dry period, punctuated by more intense multi-year droughts centered at approximately 810, 860, and 910 AD.”59 The paper concludes, “new data suggest that a century-scale decline in rainfall put a general strain on resources in the region, which was then exacerbated by abrupt drought events, contributing to the social stresses that led to the Maya demise.”
Sometimes, the change in climate was viewed as beneficial. In Europe and the rest of the northern hemisphere, the Medieval Warm Period was considered a period of plenty, with warm summers and mild winters. Crops were good and living was easy, unlike the Dark Ages Cold Period that preceded it, and the Little Ice Age that followed.
The Little Ice Age
One of the most notorious periods of climate variation during the Holocene Epoch is the Little Ice Age. This worldwide cooling trend began at different times in different parts of the planet. There was a great deal of local temperature variation, but it was not just a cold snap—its depths lasted for nearly four centuries. This frigid time coincided with a reduction in the Sun's activity, as indicated by sunspots, known as the Maunder Minimum. This observation has lead many scientists to investigate the link between solar activity cycles and climate.60 ,61 ,62 ,63 The swing in temperature from the Medieval Warm Period to the Little Ice Age is shown in Illustration 11.
Illustration 11 Temperatures over the last 1,000 years.
Europe experienced a general cooling between the years 1150 and 1460 and very cold conditions between 1560 and 1850. This climate shift, which brought late harvests and wet summers, is blamed for poor health, crop failures, and bad years for wine making. Prior to this period, England had a significant wine industry but, after the 1400s, temperatures became too cold for widespread grape production. Poor grape harvests in continental Europe, particularly in Switzerland and Austria, resulted in a change in drinking preference from wine to beer.
The Little Ice Age caused many disasters: the disappearance of the orange groves from Jiang-Xi province in China, the failure of the Viking colonies in Greenland and North America, and the Black Death in Europe. The disappearance of cliff-dwelling Anasazi culture in the American Southwest, the bitter cold at Valley Forge during the American Revolution, and the Irish Potato Famine have all been attributed to the Little Ice Age.64 ,65 There are even claims that the storm that destroyed much of the Spanish Armada, during its disastrous attempt to circumnavigate the British Isles in 1588, was due to shifting climatic conditions.66
The harsh climate played an unmistakable role in the course of human events. During the late 13th century, Kublai Khan turned his attention to the Islands of Japan. Mongolian invaders from the Asian mainland were pushed back twice in the span of only a few years. Not by Japanese soldiers, but rather by a devastating natural phenomenon—the Japanese typhoon season. The grateful Japanese named the storms “kamikaze,” meaning “divine wind,” for protecting their homeland.
In the early 1300s, the old sea route from Iceland, directly west to Greenland, became impassable due to southward expansion of sea ice. Floating ice in the North Atlantic adversely impacted fisheries in Iceland and Scandinavia while Eskimos were seen paddling kayaks off the coast of Scotland. By 1350, this had caused a serious decline in the Viking Greenland settlements. The last reliable account of Norsemen still living in Greenland was around 1410. When German merchants visited Greenland in 1510, they found Inuits living among ruins of Norse settlements.67
Mountain glaciers advanced in Scandinavia, New Zealand and the Alps during the 1600s and early 1700s, threatening villages and pasture lands. Today, the Chamonix valley of France is a popular resort area and tourist destination, nestled in the Alps in the shadow of Mont Blanc. But starting in the late 1500s, advancing alpine glaciers, accompanied by massive floods and sudden landslides, began a reign of terror. The rivers of ice descended to the lowest levels in recorded history. In the Chamonix valley, the glacial onslaught entirely destroyed three villages; Bonnenuit, Le Chatelard and La Bonneville. A fourth village, La Rosiere, was severely damaged. Le Chatelard, the oldest village, had been continuously inhabited since the 13th century.68
Around 1700, the Mer de Glace (sea of ice) glacier, the largest of the seven Chamonix glaciers, threatened to obstruct the Arve river valley. Local inhabitants of the village of Les Bois resorted to an “exorcism” of the evil glacier spirits. The Bishop was summoned, a procession made its way to the base of the glacier, singing hymns and praying all the way. The Bishop blessed the glacier, sprinkling it with holy water, in the hope of halting the great wall of ice. A statue of Saint Ignatius was erected barring the glacier's path. These religious exhortations only provided temporary relief; as late as 1825, when Les Bois village had to be evacuated, the ice was still advancing.69
There were a few positive aspects; in London, the Thames River froze during the winters, leading to big winter carnivals on the ice. In New York City, the harbor froze solid, allowing people to walk to Manhattan and Staten Island. Some have suggested that the cold temperatures increased the density of tree wood, helping to create the distinctive sound of Stradivarius violins.70 The variable climate forced changes in agricultural methods that ushered in land reform in several parts of Europe.
In 1812, the French Emperor Napoleon Bonaparte, flush from victories across Europe, fixed his gaze on Russia. Napoleon invaded Russia with more than 600,000 men and, though he looted and burned Moscow, the invasion turned into one of the worst military disasters of all time. The coldest temperature recorded on the army's return from Russia was -36°F (-38°C), freezing men and horses to death as they marched. Fewer than 20,000 of Napoleon's men survived the Russian campaign. Many were lost in battle and typhus killed even more, but the enemy that destroyed Napoleon's Grand Armée, the largest army Europe had ever seen, was the cold.
In 1816, New England experienced its “year without summer,” when cold, rainy weather during the growing season caused many crops to fail. This low point has been linked to several volcanic eruptions around the world, their volcanic ash further chilling the already cold climate. The art and literature of the time reflected the menacing climate. A survey of historical paintings has revealed that images of the sky painted during the Little Ice Age show significant cloud cover. Depiction of low clouds increased sharply after 1550 and did not decline until after 1850.71 When Dickens wrote “A Christmas Carol” in 1843, it reflected the cold, snowy winters Britain experienced at that time. By the 1850s, the climate relented and average yearly temperatures slowly began to rise—a trend that continues to this day.
These well-documented historical episodes have been presented here to underline the fact that these periods of climate change actually happened. The evidence for these periods is clear and can be found in the written histories of every nation on Earth. Yet, some modern scientists have cast doubt on their existence.
The IPCC Changes Climate History
Early IPCC reports reflect these well-established historical climate periods,72 but since the publication of two papers by Michael E. Mann, et al.73 ,74 that has all changed. These papers presented a new climate history for the past thousand years based on a reinterpretation of selected proxy temperature records, primarily tree ring data. They are the source of the famed “hockey stick” temperature graph (page 19) that has caused so much controversy.
This revised climate history totally ignores a century of scholarship and scientific study. Both the Medieval Warm Period and Little Ice
Age are eliminated—replaced with a slight, almost linear temperature decrease from 1000 A.D. to the early 1990s. All that’s left is an abrupt rise in temperature over the last decade, as shown in Illustration 3, on page 19. Bob Carter, Research Professor of Geology at James Cook University, in an appearance on Radio National, had this to say about the IPCC's revised history:
“That’s where the big argument is centered over the ‘hockey stick’ because that purports to summarize accurately the temperatures, particularly for the northern hemisphere, over that time period. And the answer is that there is good evidence that in the Medieval warm period and also prior to that in the Roman warm period before the birth of Christ, temperature was at least as warm as it was today, if not indeed a little bit warmer still. Then, in between those warm periods, of course, we’ve had cold periods such as the famous Little Ice Age experienced in Europe in the 17th, 18th centuries.”75
There are any number of studies that contradict the IPCC findings by reporting temperatures during the historical warm periods at least as warm as today. For example, scientists have reported that the mean temperature of the Medieval Warm Period in northwest Spain was 1.5°C warmer than it was over the preceding 300 years. During the Roman Warm Period the average temperature was 2°C warmer than today, several decades were more than 2.5°C warmer than today, and one interval longer than 80 years was more than 3°C warmer than today.76 Many other examples have been reported.77 ,78 ,79 ,80 ,81
As a result of these contradictions, the IPCC's new climate history has caused a furor in scientific circles.82 ,83 ,84 Refusal to release source data and analysis programs prevented other researchers from reproducing Mann's results. This recalcitrance led to charges that Mann and company had “manufactured” the hockey stick by selectively “censoring” datasets that did not agree with their desired outcome.85 ,86 In science it is acceptable, even encouraged, to introduce new theories that re-interpret the meaning of data and observations from the past. Changing the data itself, in effect rewriting history in order to better match a theory, is not science. It is deception.
The Resilient Earth: Science, Global Warming and the Fate of Humanity Page 5