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Being the occasionally interesting ramblings of a major-league technophile.
First and foremost, keep in mind the difference between climate and weather. While it is natural for both to change with time, weather is short-term and local while climate is long term and global. What is being discussed here is climate. Weather - even seasonal or yearly weather - can have trends which are directly opposite to the current climate trend. So a freak cold snap does not disprove global warming, any more than a single heat wave proves it.
That the Earth is warming should surprise no-one. We're in the rising part of the 750/1500-year cycle, after all. During the peak two cycles ago, a time known as the Medieval Warm Period, Greenland (which was actually green then) was settled. However, within a few centuries the planet was well into in the Little Ice Age and anyone who didn't leave Greenland before the ice came back starved. Two cycles earlier saw the rise of Rome, during what is known as the Roman Warm Period.
But! Did the Roman Empire rise because the climate warmed... or did the climate warm because the Romans were clearing huge tracts of forest to build cites and farm the land? Or both? Professional opinion about the impact of human activity on climate ranges from that of people like W. S. Broecker, who said "Climate is an angry beast, and we are poking it with sticks." to folks who feel that climactic variation has so much inertia behind it from natural causes that believing humans can affect it is sheerest hubris. Most likely, the truth lies somewhere between.
We know for a fact that past warm periods were accompanied by increased levels of carbon dioxide in the air long before there were enough humans to cause the change. Periods of warmer climate also tend to be associated with higher atmospheric levels of methane, not easily produced by non-technical societies, but definitely produced in large quantities by natural means when the climate is warm enough. So which is cause and which effect in our current trend? Or is the situation far more complicated than that? Given the way the universe usually works, I'm putting my money on the latter.
We have some pretty good measurements of temperature over wide swaths of the Earth year-by-year going back over ten thousand years, with somewhat less detail known about the eras before this, and for other areas during the same ten millennia. There are multiple, overlapping climatic cycles ranging in length from about seven hundred fifty years to tens of millions of years. Picking what's "normal" or even "typical" requires specifying a time span, and perhaps even a region. Because complications in natural air and water currents can result in some regions experiencing the opposite of what the planet overall is going through.
There are also non-cyclic influences on climate, such as volcanic eruptions. Some significant short-term cold spells in history have been connected to known major eruptive events. Krakatau alone has been responsible for several. Besides the recent one, eruptions in 535 CE caused notable diminution of sunlight in much of the world, and associated low temperatures, unusual rains and crop failures. "The Year Without a Summer," 1816, was most likely caused by the eruption of Tambora. However, besides dust, which can cause cooling, volcanoes also emit carbon dioxide, methane and other greenhouse gasses. So a major volcanic eruption can cause a short-term cooling trend followed by a long-term warming trend.
While individual eruptions are apparently random, global volcanic activity may be cyclic, and some volcanoes do have very loose cycles of activity which may be real or due to accidents of record keeping. If such cycles are real they further complicate forecasting - or even tracking - climate changes.
Another complication in understanding global temperature change is the effect of certain major ocean currents, such as the Gulf Stream. These move staggering amounts of heat around. Parts of Europe and North America are far warmer now than they would be without the Gulf Stream carrying tropical heat north and east, then west. Bermuda has the same latitude as Charleston, South Carolina and Montgomery, Alabama, but a near-tropical climate because it is in the Gulf Stream. Also, Bermuda's climate has much less variation between seasons, thanks to the moderating effect of the Gulf Stream. Similarly, deep river valleys tend to have more moderate climates than the open areas around them.
All these factors and more need to be taken into account before we can start talking about trends. However, with some work such factors can be taken into account, and actually have been. So, what are the trends?
I worked with trendline analysis of traffic data for about a decade. Though I have no formal training in the mathematics behind the procedure, I have a good feel for what is involved in it. As an example, for the majority of purposes traffic count data from before about 1960 is best ignored for modern forecasts, because the construction of the Interstate system significantly changed the trend and data from before then often cannot be directly applied to current trends in most of the United States.
In discussing climate trends you likewise have to know when to ignore spikes in either direction, such as cold spells caused by volcanic eruptions. You also have to take much of the historical data with a generous grain of salt, because in most places through most of time symbolism and politics were considered more important than literal accuracy.
The shortest climate temperature cycle we know of comes from the 11/22 year Solar sunspot cycle. However, while the effect of sunspots on the weather can be significant, this cycle is so short that its effect on the climate can be largely ignored. There's simply too much inertia in the system for such short impulses to do much more than cause an occasional fluke storm or drought. (Though if the effect of this cycle reinforces one or more other cycles...) There is considerable evidence that the sunspot cycle is part of a longer cycle, lasting centuries, which influences the intensity of particular 11-year cycles in a longer trend. Combine that with such things as the slow precession of the Earth's axis, and the amount of sunlight falling on our globe can be seen to change dramatically in a long, slow cycle.
Note the "11/22" above, as well as the previous 750/1500. Many cycles are whole number multiples of shorter ones or fractions of longer ones. Some appear independent of other cycles. Occasionally, a short cycle can have a double or triple beat which makes a long cycle, often syncopated, just as the four chambers of the heart have a characteristic "lub-dub." What I'm going to focus on here is the 750/1500 year cycle. That's because this gives us about three complete fifteen hundred year cycles and a good part of another during recorded human history. Though for the earlier part of that period the historical data is mostly limited to just three or four regions. (Mostly Egypt, India and China.)
For non-historical data we have such sources as ice cores from glaciers and ice caps, tree rings and lakebed and seafloor sediments, plus a few other indicators. The sediment layers provide a pretty straightforward example of how non-historical temperature data can be determined. Plant debris such as bits of leaf and pollen can be used to identify the types of plants in the watershed of a lake which were washed into the deep water and incorporated into the sediments. These are often form in annual or seasonal layers. By identifying which plants grew in the region and how that mix changed with time, we can get a pretty good idea of the climate trend. The thickness of the sediment layers can also directly tell us how much rainfall fed the watershed in a particular year or season. The proportion of mineral to organic matter in a layer can also give an idea of the severity of rains and whether there was just one bad storm season or decades of torrents.
Likewise, tree rings give a good indication of rainfall. This is so consistent that tree ring timetables can be used to date when a log for a structure was cut. So with tree rings we not only get a direct climate indicator, but also a good calendar.
There are specific biological indicators of temperature which can give pretty precise and accurate breakpoints. For example, there's a type of snail which curls its shell in one direction above a certain temperature, and the other below that. Why it does this is known only to God (maybe to give us climate clues :-), but it happens consistently to this day. So if we find the shells of this snail curled one direction, we know for a fact that it grew at a time the temperature was above a certain value.
If the examination of lakebed sediments all around a continent show the hot curl moving north during a certain period, we can be pretty sure that meant the climate was warming. When the northernmost snails began curling the other way, and the change subsequently moved south, the climate was cooling.
Through these and other methods - checking one against the other - we can plot the change in temperature over much of the globe, focusing on relatively small areas or averaging for whole regions.
Because the thermometer is a relatively recent invention, most historical climate data comes from such things as the number of days of rain, total amount of rain, river levels, crop successes or failures, what day of the year the first frost was seen, when the Spring thaw came, subjective accounts of cloud cover, and so forth. The Ancient Egyptians kept records of the height of the Nile day by day at multiple locations using the same Nileometers (usually markings cut in rock the Nile flowed past) almost continuously for thousands of years. We can also make inferences from styles of architecture and clothing. For example, during the Medieval Warm Period many rural homes through much of Europe had no solid walls, just tarpaulins which were rolled down if the night became too cool in Winter. In the warm climate of the time more protection was simply not needed, and the open structures let cooling breezes blow through when the heat rose.
Art can also be a source of climate information. For example, there are certain alpine mountains which appear in paintings and drawings going back over a thousand years. By determining what season a particular piece was done, and what year, and noting where the foot of a particular glacier was then, and comparing with other pieces showing the same glacier through the ages for that same season, and compensating for precipitation changes and other factors, we can get a pretty good idea of temperature changes through time for that area. (See below for more on this.)
Now, here are some dates. Keep in mind that natural cycles can not only be irregular in length and magnitude, but asymmetrical. That is, the legs may not be even in length and magnitude. Note that during some cycles the extremes may be more extreme, while in others they are less so, even with the same averages. Also, one leg of a cycle may be more extreme than the other.
While turnaround points for temperature trends are pretty easy to agree on, and even peaks can usually be determined pretty well, there is still room for argument on when these occurred for a particular cycle. Short-term changes may be flukes or due to another trend or a unique event momentarily running counter to the overall trend. (See above for examples.) Different indicators may show a peak or turnaround at a different time, perhaps due to different lags in how they function, so a best-guess average must be taken. Finally, we are talking about cycles lasting multiple centuries. Even rounding to the nearest half-century may not be general enough. I'm also skipping some benchmark events just to save space. Finally, note that warmer climates tend to have smoother trends (that is, fewer and slower changes in climate) while colder ones can have numbers all over the chart while remaining cold on average.
150,000 BCE The world's climate is cold and dry, and much of sea and land are covered by ice.
140,000 BCE Ice age peaks, temperatures start back up.
130,000 BCE A rapid warming begins the Eemian interglacial
During this interglacial period Earth's climate on average is much like it is today but a bit warmer and moister in many regions. There may have been at least one major cold and dry event during this period, at around 120,000 BCE. Even then, temperatures were little or no lower than today.
110,000 BCE End of the Eemian Interglacial period. The climate cools and mostly stays cold for 82,000 years. The cooling at the start was a rather sudden event, the initial drop to full ice age conditions perhaps taking less than four centuries. Some recent Atlantic floor sediment analysis suggests the change could have been even more rapid.
Following this initial cooling, temperatures changed both ways in often dramatic leaps, but there were also periods of thousands of years of stable climate. Some of those periods were quite warm compared to the average for this long span, and may even count as interglacial periods. However, for tens of thousands of years the trend was towards cooler temperatures. Northern forests were replaced by dryer grasslands. The coldest, driest period was the Pleniglacial, about seventy thousand years ago.
Between sixty and fifty-five thousand years ago the climate warmed slightly. This was followed by a long phase of climate oscillation. Temperature changes in this period often occurred in sudden and dramatic jumps.
About thirty thousand years ago the climate began cooling again, in what is known as the Late Glacial Cold Stage, or Upper Pleniglacial. This reached its coldest between twenty-one thousand and seventeen thousand years ago. The first part of this period is known as the Last Glacial Maximum. (The reduced precipitation which accompanies deep cold spells means the glaciers grow very slowly, perhaps not even keeping up with the slow creep of ice to the sea. However, the ice is still seasonally layered; the layers are just thinner.)
Note that even the worst ice ages we know of have had periods of relief, known as interstadials. During these the climate will warm for a few centuries or even as much as two millennia, sometimes approaching modern temperatures. Again, peak temperatures were sometimes reached in only a few decades, with the subsequent coolings being as rapid. Interstadials tend to occur during the warm part of the Atlantic temperature cycle, which has a length of about 1500 years.
Similarly, Heinrich events are cold snaps which also tend to occur in 1500 year cycles. (Note that the period is uncertain, since the term tends to be reserved for only the most extreme of the rapid cold snaps, and there is disagreement over which of the temperature dips we know of qualify. Several firmly established Heinrich events are listed below.)
Most of the information for this period tends to be for the northern hemisphere, and focus on regions bordering the Atlantic, due to extensive work in obtaining and analyzing sediment cores there. However, the less extensive work done in other areas of the Earth tends to show similar trends for the rest of the world.
103,000 BCE The climate warms slightly but is still colder and dryer than at present for the next ten thousand years
93,000 BCE Cooling phase begins
91,000 BCE Climate warms slightly
73,000 BCE Ice returns, the world becomes cold and dry in the Lower Pleniglacial period
60,000 BCE A thirty-five thousand year period begins of intermediate glacial climate, on average cooler and dryer than today but with much variation
50,000 BCE Peak of warming period
39,000 BCE Heinrich event
33,000 BCE Heinrich event
23,000 BCE For ten thousand years the world is cold and dry, during a period which includes the Last Glacial Maximum
21,000 BCE Heinrich event (extreme)
19,000 BCE End of extreme Heinrich event
15,000 BCE Heinrich event (extreme)
13,500 BCE Nearly all areas of the world had climates at least as warm and moist as today
12,500 BCE End of extreme Heinrich event
12,000 BCE Rapid global warming (most likely an interstadial) which lasted a few thousand years and was followed by a return of cold known as the Younger Dryas
Temperatures dropped rapidly at the beginning of this period, reaching ice-age extremes in less than a century. After about 1300 years there was an equally or even more rapid rise. As much as half the warming may have occurred in as little as fifteen years. The Younger Dryas was apparently not global, but it did affect large portions of the world.
10,800 BCE Onset of the Younger Dryas
10,500 BCE Warm, wet period
Broad areas of land now occupied by Egypt, Chad, Libya and Sudan experienced increased temperatures and precipitation, resulting in the desert blooming. Most of the people in the eastern Sahara region followed the rains and migrating animals.
9500 BCE End of the Younger Dyras; beginning of the Holocene phase
The cold ends abruptly, but continued warming is a gradual, long-term temperature increase. During much of the next several thousand years large parts of the Earth were warmer and moister than now, but there were also periods of cold and dry. The Saharan and Arabian Deserts almost completely disappeared under vegetation. Large forests covered much of the land and even swamps were present
The period between nine thousand and five thousand years ago (seven and three thousand BCE) is known as the Holocene Optimum.
9100 BCE Cold phase
8500 BCE Cold phase
During this time the Sahara region had greatly increased rainfall, including monsoon-like conditions.
8000 BCE Severe cold and dry phase lasting one or two centuries
7400 BCE Cold phase
7000 BCE Climates were warmer and often moister than today
6200 BCE 200-year cooling period
This reduced precipitation levels in the Fertile Crescent by thirty percent, according to marine and geological records, and may have sparked a mass migration away from dry-land farming to the creation of irrigated fields along the Tigris and Euphrates Rivers.
This is one of the two most extreme cold periods in the last ten thousand years, the other beginning in 600 BCE.
6000 BCE Climate somewhat warmer and moister than today
5300BCE Rains in eastern Sahara begin to decline, starting a return to desert conditions
3900 BCE Cold phase begins, forcing more migrations
The Sahara region becomes very dry. Early settlers enter Nile Valley, which previously had few human occupants.
3100BCE Pre-Dynastic Egypt rises as a power at the start of a warm spell
During this period in what would become Egypt the hieroglyphic script was developed and pottery invented (apparently independently of its invention at about the same time in the Middle East). Towards the end Narmer unified Upper and Lower Egypt.
2700 BCE Old Kingdom begins early in a cold period
The first stone pyramids are built.
2400 BCE Egypt continues to grow and consolidate in a warm period
2500 BCE End of warm period
2200 BCE Cold phase
Many early Bronze Age societies collapse as their climate cools; the Old Kingdom in Egypt begins to fall apart.
2000 BCE Rise of Middle Kingdom during cold period
1650 BCE Hyksos kings seize power in northern Egypt as warming period starts
1300 BCE Long period of political turmoil and upset
Perhaps as many as 300 conquests and collapses occur in Egypt and the immediate surrounding areas during a persistent warm period with relatively stable climate.
800 BCE Cold phase
600 BCE Cold phase (unusual low temperatures) 150 BCE Roman Warming begins
Rome grows greatly in power, expanding and consolidating.
200 CE Roman Warming peaks
550 CE Roman Warming ends
642 CE Egypt conquered by Arabs
800 CE Medieval Warming begins
1100 CE Medieval Warming peaks
1150 CE Medieval Warming peaks again
1200 CE Medieval Warming ends
1250 CE Observers in northern Europe note Arctic pack ice is growing
1300 CE Little Ice Age begins
1315 CE Europe suffers three years of torrential rain prefacing decades of unpredictable weather
During this span there is one three year period known as the Great Famine.
1500 CE Little Ice Age becomes milder, but global temperatures still low
1550 CE Evidence of glaciers expanding worldwide
Before this time in post-Medieval Europe depictions of Winter in art were rare, and the effects portrayed mild. They now became more and more common and the scenes more harsh, for about a century.
The pattern repeats from roughly Seventeen-Eighty to Eighteen-Ten, another cold period. The famous Emanuel Leutze painting of Washington crossing the Delaware was made during this time. Today the Delaware River rarely freezes at all, but the painting shows what looks very much like an Arctic ice pack, and is apparently true to actual conditions the General and his troops encountered.
1650 CE Climatic minimum
Glaciers in the Swiss Alps advance, gradually engulfing farms and crushing entire villages. Some today theorize that part of the secret to the violins, cellos and so forth created by Antonio Stradivari and other master instrument makers of this time was the colder climate resulting in denser wood. Most authorities on the subject say there is no one secret.
1770 CE Climatic minimum
In the winter of 1780 New York Harbor froze, allowing people to walk from Manhattan to Staten Island. The winter of 1794/95 was particularly harsh. During this time the French invasion army under Pichegru marched on the frozen rivers of the Netherlands; meanwhile the Dutch fleet was stuck in the ice in Den Helder harbor.
1783 CE Iceland's Laki volcano erupts, possibly the most damaging volcanic activity to humans in 500 years. Temperatures around the Northern Hemisphere drop. Seasonal rains in Africa and India are disrupted. Much of Egypt is also affected, with perhaps a sixth of the population dying.
By the early 19th Century the extent of mountain glaciers and snow lines had been mapped around the world. Comparing those with positions in the same areas today shows them to have risen on their respective mountainsides by over a hundred meters. (A 1 degree Centigrade increase in temperature applied over the entire surface of a glacier will reduce it's depth by about one to three meters per year, depending on compaction.) 1850 CE Climatic minimum; end of Little Ice Age
The climate begins to warm worldwide (note that both events are usually dated as "mid-19th Century," which probably means something like ten to thirty years between the cold peak and temperatures becoming distinctly warmer).
1900 CE Modern Warming begins
1940 CE Temperature peak
Average annual temperatures exceed 1961 - 1990 average by half a degree for a roughly three year period, not to be topped until 1980. This is the peak of a long, steady climb beginning in 1910. Temperatures drop for a while after this, but soon begin climbing again.
Naturally, many of the apparent correlations shown above between climate change and social change could easily be coincidence... but in a number of cases records from the time blame changes in the social and political situation on changes in the climate. Of course these things are pretty complicated. A warming period can stimulate growth in a region... but it can also stimulate more growth in a neighboring region, which then becomes ambitious. Mild cold periods can cause already established kingdoms to consolidate their power, and annex other kingdoms which don't handle the downturn as well. Extreme cold periods can cause even long-established hierarchies to collapse, from the inside or outside.
Whatever you do, don't underestimate the effect of changing climate on human history. The Black Death came during a cold time, when people were living closer together for warmth and washing less. It killed so many people that some believe the climate was actually made worse afterwards because forests were reclaiming cleared land and there was less combustible material being burned, both because of the smaller population. These and other factors consequently reduced the atmospheric levels of carbon dioxide, which could in turn have made the climate colder than it would have been without the Black Death. The world is a complicated place and cause and effect are rarely clear.
I focused on Classical history, here, but there were similar correspondences with Chinese, Indian and New World events. When Columbus made his journeys the population of North America had been reduced greatly by climate change. Prolonged drought brought by a cooling of the climate caused many Pueblos to be abandoned over a period of several centuries during this event, as typified by the Anasazi.
So what causes the 750/1500 year cycle? One prime candidate is sunspots. The worst part of the Little Ice Age occurred during what is known as the Maunder Minimum, when observed sunspots were exceedingly rare. There was also a lot of volcanic activity worldwide. Periodic variations in the flow of the Gulf Stream and other major ocean currents may have contributed. Some of these factors have also been connected with other cold periods.
Conversely, the earlier Medieval Warm Period was during the Medieval Maximum sunspot cycle. The thermohaline circulation of the Gulf Stream was likewise in a part of its cycle when it would bring warmth to much of the northern hemisphere. And there appear to have been fewer major volcanic eruptions in this time. The 750/1500 cycle may, itself, merely be an average of the effects of several shorter-term natural cycles, moderated or exacerbated by random events.
Keep in mind that through much of the Holocene (roughly the last ten thousand years) a change in average annual temperature over a few decades of two degrees Centigrade would be extreme... but such changes do seem to have happened. Also, changes in average temperature tend to be much milder than changes in peak high and low temperatures in the same period.
So, is the Earth getting warmer? Almost certainly, at least for the short (that is, the next few decades to couple of centuries) term. Are humans causing this? Probably not, but we may be accelerating the process.
Nuclear power, anyone?
This document is Copyright 2007 Rodford Edmiston Smith. Anyone wishing to repost it must
have permission from the author, who can be reached at: stickmaker@usa.net
Go to my Joy of High Tech page.
The Joy of High Tech
by
Rodford Edmiston