The Earth is always in one of two climatological states; an icehouse (or an ice age) or a greenhouse climate. A so called "greenhouse climate" is simply one where there isn't any place on Earth which is permenantly frozen or "glaciated". An icehouse is the opposite - so long as at least one region is home to permenant glaciation year round, the Earth is in an icehouse. ^[ⅰ] The last time Earth was in a greenhouse climate was around 34 MYE (million years before present). ^[ⅱ] Since then, Earth has been in what's termed the "Late Cenozoic Ice Age".

A glacier is a snow deposit that manages to stay frozen year round. In nearly every case, they begin to form on mountaintops. Glaciers are subject to what's commonly referred to as "positive feedback". ^[ⅲ] If permanent snow manages to get a foothold it quickly becomes compacted into ice. The surrounding enviroment is cooled at a steady rate. Glaciers expand in time. Over very long periods, glaciers are best conceptualized not as a single region or object, but as the sum of all areas frozen within a complex system of glaciers. In addition to the semi-permenantly frozen areas, many sub-systems are subject to temporary freezing as a result of the glacier which may otherwise remain above freezing outside their influence.

As glaciation gathers momentum in an ice age, whole continents or oceans may freeze over entirely. In an ice age, cooling of the planet will generally continue to grow and spread until the Earth's climate reaches a tipping due to the greenhouse effect. The point of reversal may occur at an arbitrary stage within a glacial period or may only occur after the entire surface of Earth is glaciated as it has in the past. After an extended period, historically over the span over hundreds of millions of years, freezing produces an Earth climate so dry and dusty that the atmospheric greenhouse effect outweighs the cooling of glaciated areas. Incoming sunlight is reflected very strongly by ice. Reflected sunlight gets trapped by the dust in the atmosphere and bounced back to Earth, producing a kind of "oven". ^[ⅳ] At the tipping point, more heat is generated due to the greenhouse effect than by the direct sunlight itself - at this point an ice age come to an end. The planet's climate is always teetering back and forth as a result of these basic mechanisms of glaciation and the greenhouse effect - tipping too far in either direction is cataclysmic. Just as runaway glaciation can result in global glaciation, the greenhouse effect is capable of heating the planet far beyond the habitable range. A prime example of the potential outcome of the runaway greenhouse effect on a planet can be observed on Venus, whose surface temperature is 872 degrees farenheit, regardless of season. ^[ⅴ]

Within an ice age Earth climate is subject to recurring cycles of glaciation (stadials) and de-glaciation (interstadials). ^[ⅵ] De-glaciation within an ice age is simply a period where many or all of the peripheral glaciers and ice are melted. What constitutes "de-glaciation" is subject to interpretation. Recalling that many subsystems of glaciers may exist simultaneously across the Earth, it's often the case that a small geographical area experiences de-glaciation while the rest of the glacier system stays intact. De-glaciation on Earth generally occurs as a consequence of periodic changes in the orbital motions of the Earth about the Sun.

The Earth revolves about its axis producing day and night. While it spins Earth orbits the sun annually, giving rise to the progression of the seasons. These familiar patterns are subject to a series of periodic changes. These motions, termed the "Milankovitch Cycles", alter the climate of Earth in a dramatic and decisive fashion. ^[ⅶ] Together they effectively define the greater context which night, day, and the progression of the seasons throughout the year occur in.

34 million years ago, Earth entered our current ice age - called the "Cenozoic". Glaciation gained a foothold in Antarctica, freezing the entire continent. Prior to the Cenozoic, Australia and South America were joined to Antarctica. ^[ⅹⅴ] An immediate consequence of glaciation was that the two adjoining continents were split and dispersed northward. Between Antarctica and South America, the "Drake Passage" opened up. Australia's northward drifting formed the "Tasmanian Passage" The new passages allowed oceanic currents to surround the Antarctic continent, acting as a connecting current between previously isolated Atlantic, Pacific, and Indian oceans. The new encircling current isolated the newly glaciated Antarctia - ensuring that its freezing temperatures would be a long-term characteristic. ^[ⅹⅵ]

As South America drifts north, volcanic activity is triggered along the fault line connecting it with North America. An increase in seismic activity in time produces a series of new islands which eventually bridges together the two continents. The final formation of the Isthmus of Panama takes place around 3 million years before present. While the Istmus plays a pivotal role in the ecology of the Americas, even more significant were the resulting changes in oceanic current due to narrowing the once open ocean. ^[ⅹⅷ]

The formation of the Isthmus of Panama radically transformed Earth's climate. It introduced the "Gulf Stream" to the Atlantic Ocean. Warm water originating from Panama is directed east, to the eastern coast of North America, then north. The warm water current flows north around Greenland, before branching off into the coast of north-western Europe. Warm water intoduced into the water cycle near the north pole has a reverse effect on the overall climate. The end result of the warm current in the northern hemisphere is that it cooled the area significantly. ^[ⅹⅹ]

With some foundational information laid out, the bigger picture becomes easier to understand. All planets, by necessity are either in a greenhouse or ice age. Earth entered the current ice age 34 million years ago. Glaciation began in Antarctica, which fragmented the super continent (a remnant of Pangea) into South America and Australia. Antarctica became permenantly affixed to the south pole. Changes in continental distribution altered the oceanic current. Around 4 million years ago, the northern hemisphere began experiencing glaciation during stadial periods. Glaciation in the north first appeared in Greenland.

As stadial periods began producing more glaciers throughout the northern hemisphere, we entered a period called the Quaternary, which we are in today. The Quaternary began 2.58 million years ago. The Quaternary period is further sub-divided into the Pleistocene and the Holocene. The Pleistocene stretches the vast majority of the timeline (2.58 MYE - 11.7k MYE), while the Holocene only corresponds to the last 11.7k MYE - the end of the last stadial period. ^[ⅹⅻ]

The Pleistocene period, like any other ice age, is divided up into stadials (active glaciation) and interstadials (de-glaciation). The stadial-interstadial cycles within the Pleistocene have shown remarkable regularity in their periodicity. In nearly every instance, the stadial period occurs on a time scale of around 100,000 years - with short stadials at around 40,000 years occuring every so often. The interstadial period, in every instance, lasts around 10,000 - 15,000 years. ^[ⅹⅹⅲ] While there is some scholarly debate about causality in specific instances, academia seems to look to the Milankovitch cycles for an explanation. There does seem to be a rather suspicious correlation: Earth's orbital incline has a periodicity of 100k years. Earth's eccentricity in orbit around the Sun occurs at time intervals of 95k and 125k years. Others suspect precession to be a lone explanation.

The Pleistocene begins with the glaciation of the northern hemisphere but around the same time (2.58 MYE) Earth simultaneously experienced another change of equal importance. The beginnning of the Pleistocene marks the first emergence of human ancestors on Earth. ^[ⅹⅹⅳ] It seems that throughout our development as a species, we endured these conditions. It's a bit of a misnomer - popular media depicts "the ice age" as a single event. Similarly, we often describe "humanity" as beginning at the end of the ice age. As it turns out, humans have seen dozens of these cycles and it's very likely that some cultural memory remains.

For modern researchers to begin forming a hypothesis about climate change and the transition from greenhouse to ice periods, the past offers the best model for comparison. In considering our current climate and what may lie in Earth's immediate future, it may be useful to understand how we entered the current ice age. From the start, we can be assured that our current climate is a temporary phenomenon. At present, Earth is in a period of unusually moderate temperature and uncharacteristic stability - the result of our residing in an inter-stadial period in the Cenozoic ice age. Earth has been transitioning between active glaciation and inter-stadial periods of warmth for around 34 million years. ^[ⅹⅹⅴ] Prior to this, Earth was in a greenhouse climate, free of long-term freezing across the planet (including the poles). In exploring what may have caused the shift in the distant past, we may hope to gain some insight into our future.

The likely cause of our current climate can be found long ago, during the Cretaceous period 66 million years ago. Researchers disagree about a single cause while some dismiss the notion of a sole explanation. What we can be certain of is that around 66 million years ago, Earth suffered cataclysmic loss of life. The Shiva crater event is dated at 66 million years. If it truly is an impact crater, it would be the largest crater on Earth. ^[ⅹⅹⅶ] Some scientists argue that the Shiva crater isn't a crater at all - asserting instead that the submerged trenches are naturally occurring. Others hypothesize that the Shiva crater impact gave rise to the nearby Deccan Traps. The Deccan Traps are home to one of the largest volcanic structures on Earth: An enormous shield volcano. During the end of the Cretaceous period, massive volcanic eruptions released sulfur dioxide into the atmosphere. These eruptions caused the Earth's temperature to drop 3.6 degrees fahrenheit. According to some researchers, this event was soley responsible for the extinction of the dinosaurs and our transition into the ice age of the present. Scientific consensus points in a slightly different direction. ^[ⅹⅹⅷ]

The Chicxulub crater is what remains beneath the Yucatan Peninsula in Mexico of a large asteroidal impact; remnants of an event that took place 66 millions years ago. The crater is the second largest confirmed impact of its type on Earth. Many believe that following the event, dust and debris were thrown high into the atmosphere at an unfathomable scale. Some hypothesize that the result of the collision was an "impact winter" on Earth. An impact winter occurs when dust strewn into the atmosphere to such an extent that sunlight is unable to reach Earth's surface and warm the planet. Such an event would inevitably result in rapid climate shift - many believe this to be the cause of our last global climate transition. ^[ⅹⅹⅹ]

The atmosphere filters light moving in both directions - as it move towards the Earth, and in as it travels back into space after being reflected off the surface. Different particles filter light differently. In general, particles in Earth's atmosphere reflect or scatter high frequency light (radiation) and bounce it back into space before it reaches Earth - this is a part of what makes life on Earth possible. The higher the frequency of light moving through the atmosphere, the more heavily strongly it's reflected. Once light reaches the Earth's surface it can either be reflected back into the atmosphere or absorbed as heat. Infrared (IR) light oscillates at a slightly lower frequency than visible light. IR on its way back out into space is subject to a second pass filtering by the atmosphere. Not all gasses in the atmosphere filter infrared to the same degree. Many gasses allow IR to pass back into space, untouched. Some gasses (Greenhouse gasses) filter infrared heat very heavily, trapping it within the atmosphere of Earth where it is dispersed. As a general rule, gasses which are the by-product of burning (soot, smoke, carbon) are extremely insular - blocking the movement of infrared through the atmosphere. Water vapor is also very good at trapping heat within the atmosphere.

An important aspect to consider is that all matter on Earth exists in a "state". The "land" or planet's surface is in "solid state". Under the right (or wrong) set of circumstances, solid state matter can transition into a different state: liquid or gas. As a rule, heating solid matter causes it to become liquid (like when a glacier melts), or to become a gas (like when water evaporates). When the planet experiences volcanic activity, it's helpful to conceptualize the state change that is occurring. Solid matter through conflagration is being converted to gas (smoke, soot, and dust) and liquid (magma or lava) - solid state carbon which had no effect on Earth's climate is converted into carbon in the atmosphere, a greenhouse gas. When large quantities of greenhouse gasses are released into the atmosphere at once, the planet may undergo rapid cooling. The effects of a global-scale event such as am asteroidal impact on climate are not entirely understood. Fallout from an asteroid impact will occur across the planet for (possible millions) of years after the initial collision.

Prior to the onset of our current ice age, Earth was in a greenhouse climate. Greenhouse systems are distinct from our familiar icehouse era in the effects of seasons upon the environment. In our icehouse age, seasonal progression across most of the planet transitions from hot/wet to cold/dry. The hot/wet summer-spring months are the most temperate and life supporting. Selective pressure is exerted on life during the cold/dry fall-winter months. The relationship of the seasons to life is inverted in a greenhouse climate. During the hottest months, moisture within tropical biomes begins to evaporate. Hot months are often absent of rainfall producing long droughts during the summer. Hot conditions place extreme selective pressure upon life systems. The return of moisture is associated with the cool period - Earth's winter. The characteristic yearly change of the seasons isn't associated with the shift from woodlands to tundra that takes place throughout most of the northern hemisphere in the modern era, but instead periodically caused a shift from rainforest to desert climate.

Plant and animal species flourishing across the planet during the greenhouse period were primarily adapted to the tropical and desert climate. These species are distinguished by their lack of genetic adaptation to cold weather. Selective factors in evolution within a greenhouse environment produce a different ecological system as its outcome. Generally, reptiles and insects have an evolutionary advantage in hot and dry environments - their bodies make them nearly indifferent to temperatures which would kill warm-blooded mammals. Much like rainforest biomes today, insects and reptiles excel within jungle ecosystems while mammals must be highly specialized and adaptive. In a greenhouse climate, the yearly change in seasons from tropical to desert would have little effect on reptile or insect species, who could easily endure hot periods without much loss of life. Recall however, that many insects are subject to seasonal displacement in the present climate.

Plant species of the prevoius greenhouse period possessed anatomical adaptations which made them uniquely suited to the inversion of seasons. Evergreens are a class of plant which don't lose their foliage in response to seasonal changes. There are a variety of evergreen species which have adapted to cold and warm climate alike. Prior to the onset of the Cenozoic, these species dominated the landscape. Deciduous plants are their counterpart - weilding the ability to shed foliage in response to seasonal and environmental cues. The trigger for shedding to occur in deciduous planet species isn't a change in temperature or season, as is often mistakenly assumed, but is instead correlated to a lack of rainfall. Plants abandon their leaves to preserve resources, opting instead to pull nutrition exclusively from the soil. The deciduous mechanism can successfully operate in hot and cold climate alike. Deciduous species in hot climates (greenhouse) lose their leaves with hot months in response to drought, instead of cold months. Prior to icehouse conditions there wasn't much evolutionary advantage in reactive shedding. Across most of the globe seasonal change was moderate during the greenhouse period, offering little benefit over evergreen species competing in the same biosphere.

When Earth transitioned into the Cenozoic selective pressure upon all species was reversed. Life systems across the planet whose characteristics had for millenia evolved within a moderate-warm climate were upended. Cold-weather pressure became the new standard. Instead of the hottest regions being inhospitable, many became an oasis from icehouse cooling. Most importantly the seasons began to exert real selective pressure on life. One of the first consequences of the transition was the spread of deciduous plant species. According to some researchers, this was a pivotal point in the history of life on Earth.

In the previous era, the dynamics governing evolution across the globe followed the same basic pattern. Most life supporting regions were tropical. The most inhospitable areas were deserts. Seasonal change occurred from tropical (wet) to desert (dry). Thus, the most adaptive life forms were insects/reptiles. When environmental pressure increased, evolution continuously favored these species. Their populations boomed throughout the greenhouse period. Mammal species didn't have much of a "niche". With the onset of the ice age, life ecosystems became cooler everywhere. The most inhospitable areas became dry tundras instead of dry desert. Seasonal change began to resemble the modern period. During the early cenozoic, deciduous plant species began outcompeting evergreen species. Seasonal adaptation became more practical. Tropical plant species that had evolved during the greenhouse period had adapted to excessive heat, not cold. Consequently, most evergreen species were unable to endure cold periods. The success of the deciduous species meant that cold regions could sustain animal life, instead of becoming barren tundra under pressure. Deciduous species facilitated the spread of mammals who could endure cold temperature when warm weather animals could not.

In the new climate, mammals fared better than warm weather species of the past. Warm weather species became restricted to a narrow territory in the most temperate zones. Active stadial periods of glaciation were cataclysmic for warm weather species. Mammals had an advantage - they were capable of occupying territory inaccessible to their competitors. With the rise of deciduous plants, woodlands and tundra which experienced regular freezing and thawing due to the seasons could sustain mammal life. Groups of mammals who were driven from highly populated tropical regions due to extreme competition could now find refuge in cooler climates. These isolated cold weather regions allowed mammals to evolve within a "buffer zone" free from selective pressure exerted by other species. Novel mammal species began appear - horses are one noteworthy example. In time, the Cenozoic produced most of the mammals familiar to us today, including humans.

Throughout the Cenozoic, the cyclical transition from stadial (glaciation) to interstadial (deglaciation) has occured at varying intervals. The pace of the stadial cycles seems to directly mediate the nature of life on Earth for all species. Stadial acceleration means mass extinction, limited resources, genetic isolation, and climate shifts occuring at very short intervals. Acceleration makes it difficult for complex environments to manifest on Earth. The entire planet reverts to one of two extremes in icehouse conditions; either frozen tundra, or desert on most continents. Habitable ecosystems capable of supporting large populations are uncommon. Somewhat paradoxically, seasonal influence on life is dampened as the effects of active glaciation across the planet take precedence over minor fluctuations.

Life forms were specially adapted to the extreme cold and heat. There were scare resources - brutal competition taking place between apex predators and pack animals. Mass migration was a seeminly continuous phenomenon in most land dwelling animals as herds of herbivores relying on plant life for sustenance were often cut off from their food supply abruptly. Predators rely on herd animals for hunting - as such they migrate with the herd. Throughout the cenozoic, the geography of Earth was transforming due to the influence of glaciation. A displaced continent meant that life ecosystems which were previously in a state of relative homeostasis underwent rapid transformation. Populations relying on a moderate climate may be confined to an extremely limited territory - one unable to support pre-stadial populations. Thus the most delicate species were prone to mass-die of or extinction. The Cenozoic era and ice ages generally, are the ecological precedent for several important selective axioms in life evolution. Namely, species most suited to adaptation tend to survive, while the most novel and exotic die off. The Darwinian process in the early Cenozoic (34 MYE) operated at a pace which would quickly eradicate many species which thrive on Earth today.

At the beginning of the Cenozoic, Milankovitch cycles were operating at around 40,000 year intervals around the Earth. As mammals evolved, they did so within an unstable ecosystem. This made it difficult for more "sensitive" organisms to gain a foothold as populations were subject to regular extinction events. Around 2.58 million years ago, the pace of the ice age decreased. Instead of cycling at 40,000 year intervals, stadials shifted to 100,000 year events. The shift marked the beginning of the Quaternary period within the Cenozoic age. With it, the first indications of genus Homo - modern man's most recent ancestor.

The Quaternary period is further subdivided into the Pleistocene and Holocene. The vast majority of the Quaternary falls within the Pleistocene, which stretches from 2.58 MYE until 11.7 MYE. During this extended period, the direct ancestors of modern man began appearing around the world. From an extremely early period in in the Pleistocene, members of genus Homo gained several advantages over the climate which had never before existed on Earth. The discovery of fire and clothing allowed early man to endure when all other species had been driven away. This alone may account for the dramatic rise in populatiotn of our ancestors across the Earth since the start of the Quaternary period. Some evidence suggests that large groups of early human ancestors had adapted even to the most harsh regions of glaciation during stadial periods. The Cenozoic ice age no longer commanded the power of extinction over life on Earth.

While anatomically modern humans are catagorized as Homo Sapiens it's clear that we share significant overlap with our most recent ancestral relatives. The cross-species overlap isn't confined to genetic inheritance. The fossil record indicates a cultural overlap. Throughout the Paleolithic, the various species of Homo which evolved independently from one another appear to have been in communication. Thus, to obtain an accurate picture of human culture in its infancy it isn't sufficient to begin with Homo Sapiens in Africa. A complete explanation can only be realized by considering the pre-existing culture that Homo Sapiens migrated into.

In every case, modern man's predecessor possesses at least a rudimentary culture, often referred to by archaeologists as "industry" in its early stages. Some evolutionary biologists argue that the cognitive capacity to construct tools goes hand in hand with language formation.^[i] If their hypothesis holds true, that could theoretically push the beginning of spoken language back a staggering 2.4 million years. In the case of the more primitive members of genus homo from the Oldowan industry, it seems unlikely that they have contributed to our modern vocabulary in any significant way. While they may have settled in a region that shared an overlap with more modern ancestors, their anatomical characteristics likely isolated them from the larger Erectus culturally. Members from the Acheulean onward however, likely played an important role in the formation of early speech and culture.

Neanderthal in particular seems to be greatly overlooked in their role. From the evidence currently available, it seems Neanderthal were matching Homo Sapiens 1:1 in terms of technological and societal development. The two shared overlapping territory - particularly in Europe. Furthermore, the two species were apparently interbreeding. Many modern humans to this day posses Neanderthal DNA. It's plausible that a cross-species transmission of DNA, technology, and language may have taken place among earlier proto-human ancestors with overlapping territory, culture, and anatomy to the earliest stages of the Acheulean industry (1.9 Million Years Before Present). Obviously, this is true genetically as modern human is after all the result of precisely this sort of relationship. When considering the implications of such a transmission culturally or linguistically however, our conceptual model of early history can easily be upturned.

At the very least, early Homo Sapiens did endure the presence of several pre-human cultures in their immediate environment. Migration out of Africa didn't take place within an unoccupied landscape. Neanderthal disappeared from Earth very suddenly around 40K years before present and modern researchers aren't certain about why this mass extinction took place. One of the prevailing theories is that they were simply absorbed into the larger gene pool of Homo Sapiens as they expanded north into Europe.^[i]

This issue alone makes it difficult to accurately talk about human origins - culturally, genetically, or linguistically. Based on the information available to us, it seems less likely that these things emerged instantaneously along with man, from zero to one. Instead, it seems a sort of "proto-cultural" period occurred - possibly over the course of hundreds of thousands of years. During this time, it's likely that the foundation for the things we regard as strictly "human phenomenon" (language and culture) was laid out in Africa, Europe, and Asia prior to modern humans' arrival. Our Homo Sapien forefathers did not "invent the wheel" - they only improved upon it exponentially. This fact may easily be overlooked. The cultural revolution that took place at the dawn of "civilization" caused such a dramatic transformation in social structure that it may easily be mistaken as a qualitatively different thing when contrasted with the the ones that preceeded it, but this is almost certainly not the case.

Homo Erectus first appears in the fossil record around 2 millions years ago. The belonged to the Acheulean stone tool industry. Their territory stretched from Africa to East Asia. The species appears to have had a substantial population until around 100k MYE. The Acheulean industry appear to be the first human ancestor capable of building fire. They made stone cleavers and cutting tools. They were arguably capable of seafaring. Subject to some debate among archaeologists is the extent to which they communicated verbally using language. They were the first of the members of genus Homo to possess human proportions - long legs and shorter arms, indicating an evolutionary commitment to walking upright for the first time.

Homo Heidelbergensus is a direct ancestor of Homo Erectus. Their approximate habitation of Earth spanned from 800k - 200k years before present. Most hypotheses assert that Homo Heidelbergensus split roughly into two branches. The European branch went on to become Homo Neanderthalensis while the African branch evolved into Homo Sapien.

Neanderthal belonged to the Mousterian industry which spanned the Eurasian continent. Unlike other species, the fossil record indicates no presence in Africa. Settlement seems to extend as far south as present day Israel. Neanderthal had the most advanced culture of all pre-human ancestors. They wore clothing, made jewelry, created art, made advanced stone tools, and most likely practiced religion. Their presence began around 400,000 years before present and ended abruptly 40,000 years ago. Most likely they interbred with Homo Sapiens as they entered Europe from Africa.

Homo Sapiens first appear in the fossil record 300k years ago. They evolved in Africa and spread across the world in a series of migrations. Wherever they went, they altogether replaced their predecessors culturally and genetically over the course of time. They possessed all the technology of previous cultures. However, where previous species had developed simple stone tools as the pinnacle of their progress, Homo Sapiens from a very early period set themselves apart in their capacity for innovation. As they migrated, they interbred with their genetic ancestors. This may have the cause for their eventual distinction, or may have only played a small role.

Every glacial period is unique in its consequences. The contients on Earth are continuously drifting. The tectonic plates deep beneath the surface often collide with one another, causing an increase in volcanic activity. Once cooled, the magma and lava ejected during volcanic activity forms the foundation of new land mass. The position and motion of the tectonic plates is a key aspect of ice formation. Fault lines are not only the focal point for volcanic and seismic activity, but often produce terrain high above sea level as plates collide. In the initial stages of a stadial period, mountaintop glaciers begin at the point of highest elevation called an "accumulation zone". As the cold period progresses, ice spreads to other parts of the range. Each major mountain range on Earth produces an ice cap corresponding glacier during stadial periods. What distinguishes one stadial from another are the mountain ranges actively producing the global cooling effect throughout its duration, and to what extent.

Our most current ice age marks the end of the Pleistocene, and ends around 12k years before present. Most glaciation effecting the continents and life habitats occurred in the northern hemisphere. The Greenland ice sheet and the Arctic sea are the most direct factor in producing lasting ice deposits in the north, acting as a sort of reservoir for other ice formations. However, major glacial deposits accumulated around all the major mountain ranges in the northern hemisphere.

Glaciation throughout the Eurasian continent was dramatic. The Finnoscandian ice-sheet extended from the Scandinavian Mountains as far east as Moscow. Southward, it extended to the 48th degree latitude. The major cause of glaciation can be directly linked to the eastward progression of the Greenland ice-shelf. Decreasing temperatures and excess moisture caused the ice caps on the Scandinavian Mountains to accumulate and initiate a positive feedback loop in the region. Mountainous ice caps caused moderate glaciation in the Himalaya Mountains to a lesser extent. The Swiss Alps were home to a large glacial formation separate from the much larger Scandinavian shelf. Siberia and Asia were too dry to support glaciation to the same extent, although mountaintop glaciation did occur.

North America was a nexus of glacial activity during the last glacial maximum in the western hemisphere. Ice from Greenland and the Arctic region fragmented into glaciers which drifted southwest to the Americas. The Laurentide Ice-Sheet was a monumental glacier roughly 2 miles thick. It covered all of present-day Canada and the United States as far south as the Ohio River (38 degrees latitude). The Great Lakes in the US are the drain water from the Laurentide as it receded at the end of the Pleistocene. In the west, the Cordilleran ice sheet stretched from just south of Alaska into the north-western US. South America experienced glaciation around the Venezuelan Andes mountain range, but its effects were much less pronounced than in the north.

Across the planet, sea levels were much lower. Many continents have considerably large continental "shelves" which are regions that have been submerged in shallow ocean water. During stadial periods, these shelves may be exposed due to changes in sea level. The exposed shelves may create a "corridor" that allows passage by land between two previously isolated continents. Land bridges can be found all over the planet when sea levels lower; in Europe, Great Britain is joined with the European continent, Australia is connected with Papua New Guinea during periods of low sea level. Perhaps the most prominent instance is in a region geologists refer to as "Beringia" located in the far northeast of Asia. The Bering Strait links Siberia with Alaska during stadial periods. Although much debate surrounds the topic, many archaeologists agree that this was the means by which early human populations entered the Americas around 15,000 years ago.

Around 11.7 thousand years ago, the ice around the world started to recede. Across the northern hemisphere, the massive Laurentide and Finnoscandian shelves were sent into remission. The meltwater from the Laurentide became the Great Lakes in the United States. The Finnoscandian shelf was integrated into the Arctic. Life was once again able to comfortable inhabit Canada, the Northern United States, and Eurasia. The Earth was renewed - global populations surged. The long winter had ended and spring had begun.

Doubtless, it was a time for celebration. For 103,300 years the Earth had been a frozen tundra - having succumb to the previous stadial period. For nearly 1,500 generations men endured the tundra. To the early humans who emerged into the modern age, the notion of a moderate climate must have long become the stuff of legend. When the clouds finally broke and the sunshine emerged bright and clear for the first time, those weary men must have been in disbelief. They may have imaged that the warmth would be a fleeting thing and that soon enough, the monumental ice shelves would be heard grinding, crunching, and popping as they carved their way back across the barren tundra.

In a way, the skeptics were right. The warmth truly is a fleeting thing. The modern scholarly community has yet to provide us with a clear understanding of our cirumsmtances at present. There's much debate about when the last ice age began, although we do know with confidence that it "ended" in 11.7 MYE. The first problem is that there isn't a clear line of demarcation between the "ice age" and the "present age". The transition was a gradual one. It wasn't precipitated by a single event but a set of interconnected incremental shifts in the Earth's position. The best course of action may be to consider Earth's current location, relative to where it was during the ice age.

As a reminder, Earth's orbit around the Sun varies between being nearly circular to being more elliptical in cycles of 90,000 to 100,000 years. Planetary orbits have a perihelion, which is the closest point on its orbital path to the Sun, and an aphelion which is the furthest point in its orbit from the sun. When eccentricity is near circular, the difference in distance between aphelion and perihelion are negligible. A near circular orbit means the temperature difference between seasons is very low. When eccentricity is high, the difference in distance between aphelion and perihelion are substantial, causing dramatic yearly change in temperature.

Currently Earth's eccentricity is 3%: near circular. When the eccentricity is at its greatest, there's a shocking difference of 20% to 30% in sunlight exposure between seasons. Changes in eccentricity alone are thought to produce climate change which may account for periods of glaciation. However, eccentricity's overall effect is either compounded or reduced by other factors in Earth's motion.

Obliquity refers to Earth's tilt on its axis. A common misunderstanding is that the seasons are primarly caused by changes in distance from the sun throughout the course of the year. While distance does play a role, obliquity is far more impactful. The amount of heat transfer that occurs between Earth and the Sun is primarily determined by the angle of intercept of in coming light on the face of the planet. When the surface is perpendicular to the direction of light travel, heat transfer is at its greatest. On Earth, the Sun's rays are at their most potent near the equator, although not identical to it. As the angle of intercept increases beyond 90 degrees (perpendicular), more of the light is reflected off the surface instead of being absorbed. This effect occurs most strongly at the poles.

Obliquity (tilt) determines the habitable zone of Earth. Just as important however, is its role in seasonal change and glaciation. When tilt is at is greatest, each of the poles receives more sunlight than when tilt is decreased. It's easiest to conceptualize a scenario where the tilt were zero degrees. In such a case, neither pole would ever receive any direct sunlight. This would quickly cause ice to build at the poles which would cool the entire planet.

On Earth, the poles experience very irregular sunlight exposure. North and south poles each experience a period of extended darkness with zero sunlight - while the other pole experiences the opposite: a long day without night. In the same way that the length of day and night changes throughout the year in response to the seasons, the poles experience extended periods of darkness and light. They are much more strongly effected by Earth's position relative to the Sun.

Obliquity on Earth oscillates between 22.1 to 24.5 degrees and has a periodicity of 40,000 years. Today Earth's axis is tilted 23.5 degrees. Our current obliquity is near ideal. It allows each of the poles to experience enough exposure to light to prevent further freezing during its warm period, and enough of a cool period to prevent large scale deglaciation. As Earth's tilt nears it's least oblique however, sunlight reaching both poles is lessened substantially year round. Instead of Earth allowing each a period of warm and cool, both are almost always cool-ish.

As the Earth spins (daily) on its tilted axis revolving (yearly) around the Sun, it also rotates (or precesses) in a cycle that lasts 25,722 years. Precession has been one of the most important ways for humans to reckon time and to judge location and position for a very long time. Precession alters what's referred to as the "celestial dome"; The celestial dome is the "night sky" as seen from Earth. Precession is caused by the tidal forces of the Moon and Sun on Earth as it spins on its axis. In short, it's being "pulled" off center in proportion to its obliquity and distance from other gravitational bodies.

The effects of precession on climate require a working understanding of the other mechanisms at play. Earth's revolution around the Sun varies in eccentricity. Eccentricity is measured by the difference in aphelion and perihelion; the furthest and closest points from the Sun during a yearly revolution, respectively. Earth's obliquity (tilt) means that either the northern or the southern hemisphere receives more direct sunlight during certain times of year. When the northern hemisphere is tilted toward the Sun it experiences summer while the southern hemisphere (tilted away) experiences winter.

At present, the southern hemisphere is tilted toward the Sun during Earth's perihelion (closest orbital period). The southern hemisphere is tilted away from the Sun during aphelion (furthest orbital period). The result of this is that the southern hemisphere experiences summers that are warmer because they take place while Earth is closest to the Sun, and winters that are colder because they take place while Earth is furthest from the Sun. Simultaneously, the northern hemisphere experiences a dampening of the seasons. Summers begin while Earth is furthest from the Sun, and winter begins when it's closest. The overall effect is that the climate in the northern hemisphere is more moderate and invariant, while the southern hemisphere experiences more dramatic seasonal shifts.

The scenario outlined above is all a product of our current precessional period. As the precessional cycle continues, the relationship between Earth's tilt, eccentricity, and their combined effect on the hemispheres will cycle through the full gamut of possibility. 13,000 years from now, Earth will tilt in the opposite direction. The northern hemisphere will endure the more pronounced seasonal shift for a time. Similarly, there will be periods where neither hemisphere experiences pronounced seasonal changes.

The scenario outlined above is all a product of our current precessional period. As the precessional cycle continues, the relationship between Earth's tilt, eccentricity, and their combined effect on the hemispheres will cycle through the full gamut of possibility. 13,000 years from now, Earth will tilt in the opposite direction. The northern hemisphere will endure the more pronounced seasonal shift for a time. Similarly, there will be periods where neither hemisphere experiences pronounced seasonal changes.

There are a few ideas about what comes next. Milankovitch (the man who first described and measured these cycles) believed that obliquity played the most important role in Earth climate. More specifically, he believed that changes in the amount of light received in the northern hemisphere during summer months played a pivotal role in climate change - predicting a glacial period every 41,000 years. However, by taking samples of ice cores modern researchers have been able to demonstrate that glaciation is occuring regularly at 100,000 year intervals and has been doing so for the last couple million years.

There is no scientific consensus on the subject and surprisingly little research is being done to address the issue. Uncovering the truth requires a more complete data set. A more accurate picture of Earth's orbital cycles along with its relationship to other planets within the solar system may provide helpful clues. We may very well be approaching the next stadial period. It may be in the next decade or it could be 200 years from now.

The last stadial period of active glaciation on Earth came to a close 11.7K years before present. It became evident early on in the age that this inter-stadial was different than the ones that came before it. At the start of the cycle, one of the main distinctions was the apparent lack of competition among archaic humans. As Homo Sapiens emerged into the new age, we did so unrivalled in our supremacy over the animal kingdom and possessing an unmatched mastery over the forces of nature. The cultural and technological innovation in human society at the dawn of the Holocene are by all accounts, the greatest leap forward made by any species during any other period in Earth's history. The Holocene had begun.

Anatomically modern humans at the start of the Holocene had already migrated across the globe. The shift in climate allowed humans to sustain greater populations, and begin establishing permenant settlements. Humans began organizing communities in larger groups than previously seen at key areas. These were usually areas that offered easy access to water or abundant resources. The most desireable areas also offered a strategic advantage geographically. Key areas in the Middle-East within the Fertile Crescent became central trade hubs. The Fertile Crescent offered not only a moderate climate with access to water and food, it was also a natural corridor for groups of humans migrating from Africa and a logical place to consider settling. Consequently, it grew in population and became one of the earliest examples of a true city-state during the stone age.

There were a series of advances that made the Holocene unlike any other period in history. Perhaps the most radical was the development of agriculture. This by definition was the advance that allowed the transition away from hunter-gatherer communities. While the change may intially seems somewhat trivial, it enabled groups of early humans to free themselves from the reliance on nature. Prior to agriculture, human settlements were temporary. Hunter communities often followed herd animals in their yearly migration. Gathering from nature was an unreliable means of sustenance. Providing adequate nutrition for even moderately sized groups of humans required grazing and hunting over an enormous territory. Consequently, prior to agricultre, groups of hunter-gatherers occupying a shared territory were extremely competitive and hostile toward one another. The human population only grew as much as the environment allowed.

Just as important was the development of animal husbandry in early people. In early stone age communities males in nearly every case were hunters. It was such a central occupation that coming of age ceremonies seem to have been exclusively concerned with young males stepping into the role of hunter. Nonetheless, food was scarce. Males in hunter-gatherer communities spent the vast majority of their time hunting and it still wasn't enough. Animal husbandry was revolutionary for these communities.

These two innovations seem to be the primary catalyst for the transition into the stage of humanity we refer to as "civilization". Their significance really can't be overstated - mankind effectively transcended the Darwinian arena through their discovery. For every species on Earth, their population and survival were a direct reflection of their environment. Life had always been defined by the natural world. Humanity in discovering husbandry and agriculture superceded nature. Men were no longer at the mercy of climate change or competing for the resources offered up by nature. Men became like gods - free to define nature and to define themselves for the first time.

The adoption of agriculture and husbandry didn't occur simultaneously across the globe - discovery took place many times in many different places. Human population boomed as a result. More specifically, population density increased. Large populations of settlers could now be supported in a region without much issue. In practically every community across the globe, soon after agriculture and husbandry came hierarchical social organization. Freed from the constant demands of hunting and gathering, early settlers began adopting specialization.

As the age of hunter-gatherer societies drew to an end around the world and the human population began to increase, a somewhat counter-intuitive consequence could be observed among the now isolated communities. Although their permenant settlements were often in contact with one another via-trade, they began to deviate from one another culturally and linguistically. Small groups had less reliance on trade hubs and larger villages. Regular communication lessened over time as agriculture and husbandry enabled small groups to sustain life without the need for outside support. What occurred at the beginning of the Holocene can be likened to the opposite of modern day "globalisation".

The further back in the archaeological record we look, the more similar are the symbols, icons, tools, and technologies among distant populations. Linguistically, the same effect can be observed. It's likely that Homo Sapiens in Africa possessed language. As groups migrated across the planet they may well have initially spoken the same or very similar languages. However, over the long course of time, isolated settlements each developed their own distinct dialect. By the time the last ice age had ended, populations of humans which had been isolated geographically were no longer capable of understanding one another in speech. It's clear that for most of the present age, groups of humans separated geographically have been growing further apart - becoming more different in time.

Whether or not cultural similarities are coincidental, negligible, or altogether inaccurate is the topic of heated debate among anthropologists from every discipline today. Some researchers dismiss cultural similarities outright. In their view, early civilizations emerged without any pre-existing framework to build upon. Early human progress occurred, more or less, in a regional vacuum in every case. As humans migrated and settled outside of Africa during the Pleistocene they possessed no blueprint for progress - they inherited little to no information from their ancestors in Africa or elsewhere. Around the world, human civilization began with a "blank slate". Contact between disparate human settlements was extremely limited. In this view, any similarity between isolated populations had arisen organically - asserting that progress itself has an inherent pattern of organization as it occurs in time, giving rise to the appearance of cultural overlap where none exists.

The ideological opposite to this view is less accepted in academia, but is nonetheless present. On the more conservative end of the spectrum, cultural similarities in isolated groups may be explained through shared observation. Perhaps groups of humans migrating along the same path may have encountered civilizations who shared information and offered a model, which was then put into practice once they had arrived at their destination. A more aggressive explanation is that a well organized culture existed within Africa throughout the Pleistocene. This hypothetical culture could have played an integral role in the migrations out of Africa around the world as they occurred. Things such as language, caste, religion, agriculture, animal husbandry, taboo, and astrology in this view were not local advancements made by distinct groups of humans around the world repeatedly. Instead, humans in Africa or elsewhere prior to the end of the ice age had made these advancements previously, and the information travelled with humans as they migrated into previously glaciated regions. Cultural similarity in this view occurs because most early communities were merely applying knowledge they had obtained prior to spreading out geographically.

In any case, there were apparent similarities between isolated groups. The only thing we know for sure is that if the similarities were more than coincidence, early humans failed to provide us with an explanation. On the contrary, in almost every case, a historical record of the origin of things like language, writing, or early technologies (fire, husbandry, agriculture) was veiled in mythology, providing only a cursory or somewhat misleading account. Modern anthropologists hypothesize that primitive societies were especially prone to describing poorly understood phenomenon through story telling.

As previously outlined, social organization seems to have existed deep into prehistory. Neanderthals occupied a vast territory in Europe and the Middle East for millenia. We have almost no way of knowing what the structure of their early culture was. Similarly, Homo Sapiens occupied Africa for 300,000 years before the end of the ice age. Their culture spanned the globe - including the Americas, prior to the end of the last ice age. We know for certain that these primitive cultures possesed religion, language, and art. It may well be the case that an established form of governance and social order may have been in use during the Pleistocene, having left no material evidence. What we can be certain of is that as more advanced cultures begin to appear, they organized themselves according to a strikingly similar pattern.

At every stage of social development, groups of early humans organized themselves into a roughly 3-fold hierarchical system. The castes equated to the first three "professions" or "specializations" that appeared after the most primitive hunter-gatherer stage. First, the ruler - early priest-kings were responsible for determining law, overseeing public dispute (judging), and interpretting signs (divination). Next were the warrior class. In the most primitive societies, the warrior class inherited heavily from the earlier hunter class in "hunter-gatherer" societies. Last were the tradesmen and craftsmen who produced trade goods which were often used to barter with neighboring communities.

The three-fold organization of early societies is ubiquitous throughout the planet. As population grew, the basic three tiered system was expanded to meet local needs. The ruling class was the first expand to include its own sub-hierarchy with a single king and several subordinate priests or a pupils. Many cultures over time developed a royal "court". The other castes did in time come to develop a more detailed sub-structure to suit their needs, but not to the same extent as the ruling class in the early days of civilization.

There are many factors to take into account when approximating Earth's climate in the future. The calculation is greatly complicated by humanities influence on the environment. As far as we know, no other species has ever had the ability to interfere with the natural periodicity of the ice age, or produce greenhouse gasses to any measurable extent. Without human interference, we would likely continue in our path within the Cenozoic ice age. Earth is due to return into a long stadial period lasting around 100k years in the relatively near future. Since the beginning of the Industrial Age however, mankind has been steadily increasing in its carbon emissions. Carbon greatly accelerates the greenhouse effect which may have catastrophic consequences for the atmosphere over the coming decades.

Global warming has been a hot topic politically the last several decades. It seems that mankind is now capable of interfering in the timeline put forward by nature to determine our fate. Ironically, rather than choose to use our technological prowess to circumvent our seemingly unavoidable apocalypse, we may be shortening it. Carbon emission is a chemical by-product of the combustion engine - the capstone of industrialization. Chemical energy is converted into mechanical energy through controlled combustion. Gas state carbon is released just as it is in a traditional fire - as smoke.

We've come to rely heavily on combusion engines, and with our booming population our collective output is beginning to have an effect on the atmosphere. Global temperature is steadily increasing. Glaciers around the world are melting. Sea-levels are rising as a consequence. While the short-term effects of glacial recession and rising sea-levels may be trivial, long-term repercussions may prove to be life altering. There are several glaciated regions on Earth in our present inter-stadial period. These areas remain frozen throughout the duration of inter-stadial periods. During winter months, they expand. During the summer months they generally recede slightly. They are measured by researchers by recording the maximum extent of their expansion during the winter, and recession during the summer.

Greenland lies at the center of glacial formation in the northern hemisphere. The entire continent is covered by an "ice shelf", while it also features several distinct glaciers in addition to the larger sheet formation. Greenland is encircled by the Arctic Ocean (and partially by the Atlantic). Consequently it plays an important role in global climate as regional temperature is greatly influenced by oceanic conditions. Greenland has been rapidly warming since the beginning of industrialization.

Sea level has risen 8-9 inches globally since 1880. This may not seem like much, but small change in sea level makes an enormous difference. 30% of the global population lives in a coastal area susceptible to rising sea level. "Nuisance Flooding" is a temporary flood that destroys entire cities, although generally doesn't result in loss of human life. Flooding of this sort has increased 300% - 900% in the last 50 years due to sea level rise.

The effects of large scale deglaciation would change the face of the Earth. It's extremely difficult to predict all the outcomes, but a few things are understood. An unexpected consequence is that Earth would rotate more slowly about its axis due to the dynamic interaction of liquid water compared to solid ice. If all the ice on Earth melted, the sea level would rise 230 feet from present. In addition to the total landmass lost, the climate would enter into a greenhouse climate - the entire planet would become more moderate and warm. Greenland and Antarctica are two enormous land masses which have been covered by permafrost for many milenia. Both of these would become habitable regions. The most difficult part of the shift would be the intial coastal recession. Presently, the coastlines around the planet are the most valuable and desireable places to live. This would quickly change. The east coast of the United States would suffer particularly devistating recession - the loss of property and life is difficult to comprehend. If such a transition were to occur without our foreknowledge well in advance, the process would be terrible. Adjusting to such a dramatic global change seems incomprehensible to us at present. However, it would absolutely be conceivable for humanity to navigate the transformation with sufficient preparation and adequate planning.