1. INTRODUCTION

The mastery of fire by Homo sapiens about 800,000 years ago or earlier, a power not possessed by other species, has led inexorably to large scale modification of the planetary environment, culminating in geological scale transformation defined in terms of a new era—the Anthropocene, defined alternatively since the down of agriculture (Ruddimann, 2005) or of the industrial age (Steffen et al., 2008). This article compares aspects of geological crises and their driving forces with current climate changes. Of all the factors which underlie mass extinctions of marine and continental plants and animals, sharp rises in atmospheric greenhouse gases (carbon dioxide, methane, nitric oxide) are probably the most important in view of their centuries to millennia or longer-term consequences. Closely associated with runaway climate change are acidification and anoxia of the hydrosphere. A release of carbon on a scale equivalent to the world’s known fossil fuel resources (>6000 billion ton carbon [GtC]) triggering CO₂ and methane feedbacks from the ocean, the biosphere and melting permafrost, will accentuate the current mass extinction of species toward the scale of the Paleocene-Eocene thermal maximum (PETM: c.55 million years (Ma) ago when c. 2000 GtC were released as methane (Zachos et al., 2008). Potentially the scale of the runaway greenhouse effect may reach that of the K-T impact boundary 65 million years ago, when 4600 GtC are estimated to have been released in connection with an asteroid impact (Beerling et al., 2002).

2. CO₂ AND THE ATMOSPHERE

Lost all too often in the climate debate is an appreciation of the delicate balance between the physical and chemical state of the atmosphere-ocean-land system and the evolving biosphere, which controls the emergence, survival and demise of species, including humans. By contrast to Venus, with its thick blanket of CO₂ and SO₂ greenhouse atmosphere, exerting extreme pressure (90 bars) at the surface, or Mars with its thin (0.01 bar) CO₂ atmosphere, the presence in the Earth’s atmosphere of trace concentrations of greenhouse gases (CO₂, methane, nitric oxides, ozone) modulates surface temperatures in the range of -89 and +57.7 degrees Celsius, with a mean of 14 degrees C, allowing the presence of liquid water and thereby of life. Forming a thin breathable veneer, only slightly more than one thousandth the diameter of Earth and evolving both gradually as well as through major perturbations with time, the Earth’s atmosphere acts as a lungs of the biosphere, allowing an exchange of carbon gases and oxygen with plants and animals which, in turn, affect the atmosphere, for example through release of methane and photosynthetic oxygen (Figure 1). As testified by the geological record nearly all of the previous mass extinctions of species through the history of Earth have been associated with a rise in CO₂ (Figure 2), methane or H2S, injection of aerosol and dust, acidification of the oceans and anoxia (Stanley, 1987; Ward, 1994, 2007; Sepkoski, 1996; Keller, 2005; Zachos et al., 2001, 2008; Glikson, 2005, 2008; Veron, 2008).

During most of Earth history the oxygen-poor composition of the atmosphere resulted in dominance of reduced carbon species in the air and the oceans, including methane, carbon monoxide and hydrogen sulphide, restricting habitats to algae and bacteria (Kasting, 2005). An oxygen enrichment pulse occurred about 2.4 billion years ago (Ga), likely oxidizing atmospheric methane and leading to the Huronian ice age. About 0.7 Ga, in the wake of the Marinoan glaciation (so-called “Snowball Earth”) (Hoffman, 2009), an oxygen-rich cold hydrosphere allowed development of oxygen-binding proteins and thereby of multicellular animals, followed by development of a rich variety of organisms in the so-called “Cambrian explosion” c. 542 million years ago (Ma) (Gould, 1989). The concentration of greenhouse gases in the atmosphere exerts radiative forcing which modulates temperatures and, in turn, generates feedbacks from the hydrosphere and the biosphere. The “albedo-flip” process (Hansen et al., 2007, 2008) involves release of CO₂ from warming water, ice melt/warm water interaction, decline of ice reflection (albedo) and increase in infrared absorption by exposed water. Further release of CO₂ from the oceans and from drying and burning vegetation shifts global climate zones toward the poles, warms the oceans and induces ocean acidification (Hansen et al., 2007, 2008; Veron, 2008). The essential physics of the infrared absorption/emission resonance of greenhouse molecules, indicated by observations in nature and laboratory studies, is portrayed in the relations between atmospheric CO₂ and mean global temperature projections (Figure 2).

CO₂ is 28 times more soluble in water than is oxygen, where declining pH below 8.2 results in replacement of the carbonate ion (CO3-2) with bicarbonate ion HCO3-1 and carbonic acid (H2CO3), which marine organisms such as benthic fauna and corals can not use for calcification. Above critical threshold CO₂ becomes toxic for certain organisms, excess CO₂ reduces the ability of respiratory pigments to oxygenate tissues, and makes body fluids more acidic, thereby hampering the production of carbonate hard parts like shells. Relatively modest but sustained increases in CO₂ concentrations hamper the synthesis of proteins, reduce fertilization rates, and produce deformities in calcareous hard parts. The observed pattern of marine extinctions is consistent with hypercapnia (excessive levels of CO₂), with related extinction events (Ward, 2007). Close correlations are recorded between peak CO₂ events and mass extinction of species through time (Figure 3).

3. CAINOZOIC CLIMATES AND HUMAN PREHISTORY

The present state of the biosphere, allowing the flourishing of large mammals and of humans on the continents, developed when CO₂ levels declined gradually through the upper Eocene (50-34 Ma) and fell sharply to below about 500 ppm by the end of this period at 34 Ma (Royer, 2006; Zachos et al., 2001, 2008) (Figure 4), triggering formation of the Antarctic ice sheet. A significant factor in the formation of the latter is the opening of the Drake Passage between South America and West Antarctica, which allowed the establishment of the circum- Antarctic current, isolating the continent from warm currents emanating at low latitudes. This period saw a large impact cluster, including the Popigai, Chesapeake Bay and Mount Ashmore mega-impacts (Koeberl and Montanari, 2009). Global thermal rises occurred in the Oligocene (c. 25 Ma) and mid-Miocene (c. 15 Ma), when large parts of the Antarctic ice sheet melted. In the wake of a thermal rise about 3.0 Ma (mid-Pliocene) of c. 3–4 degrees C (relative to preindustrial), further global cooling resulted in formation of the Greenland ice sheet and the Arctic Sea ice, with further decline in global temperatures (Figure 4).

The increase in the size of ice sheets during the Pleistocene (1.8 Ma to 10 kyr) was intimately associated with increasing amplitude of glacial-interglacial cycles (Milankovic cycles; Roe, 2006), the main period of advanced human evolution.

The change about 1.0 Ma from cycle frequency of about 41,000 years to a frequency of about 100,000 years (Figure 5) resulted in extreme glacialinterglacial cycles of +/-5 degrees C, with glacial terminations triggered by orbital forcing peaks of 40 – 60 Watt/m2 at mid-northern latitudes. Inherent in the Milankovic cycles are strong ice melt feedbacks which amplify glacial melting through the albedo-flip process. This involves reflection of total radiative spectra and concomitant absorption of the infrared thermal spectra by melt water which form above ice as well as at lateral ice-water interfaces (Hansen et al., 2007). Detailed studies of Greenland and Antarctic ice cores indicate rapid albedo-flip process down to time scales of decades and even a few years (Steffensen et al., 2008). The reverse, ice freezing events, can occur equally fast. The warming of the oceans and release of CO₂ from water and from drying biosphere lag behind albedo-flip events by about 800 years (Hansen et al., 2007). This ensues in oscillation of CO₂ levels between glacial states of c. 180 ppm and interglacial states of c.280-300 ppm, corresponding mean global temperature changes of +/- 4 to 5 degrees C and sea level changes on the scale of +/-100 meters (Figure 5). Human evolution during the Pliocene-Pleistocene periods is closely controlled by climate changes, including an overall cooling trend inherent in which is an increase in climate variability following the stages (Figure 6: deMenocal, 2004):

The transitions through the Pliocene and Pleistocene from tropical to savannah environments in Africa, accompanied with faunal diversification from tropical species toward arid-zone type species, including Antelopes (Bovids), about 2.8 Ma, 1.8-1.7 Ma and 0.8-0.7 Ma, signify an increase in climate variability and enhanced pace of evolution. Human evolution accords with these transitions in terms of variability selection, diversification and appearance of Olduvan stone tools from about 2.7 Ma and Acheulean stone tools from about c.1.7 Ma (deMenocal, 2004) (Figure 6).

Survival stresses associated with extreme glacial-interglacial climate contrasts from about 0.9 Ma saw near-doubling of Homo cranial cavity (from c. 450 cc to c. 1200-1500 cc), mastering of fire and, from about 160 kyr, cultural developments including burial, ornamentation and rock painting. Stabilization of current interglacial from the last termination c. 11.7 kyr reaches the Holocene Optimum, followed by incipient effects of land clearing, land cultivation, domestication of animals and burning, regarded by Ruddiman (2003) as the early Anthropocene, expressed by limited rise of methane.

Small human clans post-3 million years-ago responded to changing climates through migration within and out of Africa. Homo sapiens emerged during the glacial period preceding the 124 thousand years-old Emian interglacial, when temperatures rose by about 1 degree C and sea levels by 4 - 6 meters relative to pre-industrial (Overpeck et al., 2006; Blanchon et al., 2009). The development of agriculture and thereby human civilization had to wait until climate stabilized about 8000 years ago, when large scale irrigation along the great river valleys (the Nile, Euphrates, Hindus and Yellow River) became possible.

4. THE ANTHROPOCENE CARBON OXIDATION EVENT

Since the down of the industrial age c. 1750, anthropogenic emission of carbon gases in excess of >370 GtC have increased the atmospheric inventory of 590 GtC to near-800 GtC, progressively changing the composition, infrared radiative forcing and thermal properties of the atmosphere. Rising at about 2 ppm CO₂/year, a pace unprecedented in the geological record, the late Anthropocene carbon oxidation event has already added more than one half of the original carbon inventory of the atmosphere and is triggering a fundamental shift in the state of the atmosphere-ocean-biosphere system. By the end of the first decade of the 21st century the deleterious effects of pollution and deforestation have reached a geological scale, tracking toward conditions which existed on Earth in the mid-Pliocene, about 2.8 million years ago (Haywood and Williams, 2005; Dowsett et al., 2005; Pagani et al., 2009).

Figure 7 and Table 1 provide estimates of carbon mass values of the atmosphere, ocean, plants, soil, methane deposits and fossil fuel reserves. By 2007 near 46% (4.2 GtC/year) of CO₂ emitted from fossil fuel burning (7.5 GtC/year) and land clearing (1.5 GtC/year) were retained in the atmosphere, while the rest were sequestered by the oceans and vegetation (Figure 8). The scale of emissions to date (370 GtC) equals c. 5% of the carbon content of the combined plant and soil inventory (6850 GtC), near-6% of the known global fossil organic carbon reserve (>6000 GtC), c. 8% of the carbon released to the atmosphere during the K-T impact event 65 Ma ago (c. 4600 GtC) which caused c. 47% mass extinction of genera, and c. 18% of the 2000 GtC released at 55 Ma during the Paleocene- Eocene thermal maximum (PETM) (Zachos et al., 2001, 2008). Using the PETM and KT events as benchmarks of low (<10% of genera) and high (47% of genera) levels of mass extinction, from Table 1 current fossil fuel reserves and methane deposits in permafrost and shallow sediments (Figure 9) are of a magnitude whose release would trigger consequences on a scale analogous to levels which triggered abrupt climate changes and mass extinctions in the history of Earth (Figure 3).

From Table 1 oxidation of c. 6000 GtC fossil fuel, vulnerable vegetation and about 50% of methane pools in permafrost and peatland would consume c. 2.10¹³ tons O₂, i.e. about 1 percent of the atmospheric inventory of 1.4.10¹⁵ ton O₂. However, given an oxygen atmospheric residence time of 4500 years and an oxygen cycle in the order of 3.10¹⁴ ton O₂ per year for the atmosphere and biosphere, long term decline in photosynthesis would result in several percent decline in atmospheric O₂ similar to geological greenhouse states (Berner et al., 2007) (Figure 10).

Since the 18th century mean global temperature has risen by about 0.8^oC. Another 0.5^oC is masked by industrial-emitted sulphur aerosols, and further rise is inherent in current melting of the ice sheets and sea ice. The polar regions, acting as “thermostats” of the Earth as the source of the cold air current vortices and cold ocean currents, such as the Humboldt and California current, are warming at a rate 3 or 4 times as fast as lower latitude zones (http://data.giss.nasa.gov/gistemp/). The most detailed satellite information available shows that ice sheets in Greenland and western Antarctica are shrinking and in some places are already in runaway melt mode.

A new study, using 50 million laser readings from a NASA satellite, calculates changes in the height of the vulnerable but massive ice sheets and found them especially worse at their edges, where warmer water eats away from below. In some parts of Antarctica, ice sheets have been losing 30 feet a year in thickness since 2003.

A rise of atmospheric CO₂ concentration triggers feedback effects due to warming, desiccation and burning of vegetation, further releasing CO₂. The onset of methane release from polar bogs and sediments is of major concern. Because CO₂ is cumulative, with atmospheric residence time on the scale of centuries to millennia, stabilization of the climate through small incremental reduction in emission may not be sufficient to avoid runaway climate change and possible tipping points (Lenton et al., 2008).

Sea level rise constitutes the definitive parameter reflecting all other components of climate change. Since the early 20th century the rate of sea level rise increased from about 1 mm/year to about 3.5 mm/year (1993 – 2009 mean rate 3.2+/-0.4 mm/year) (Rahmstorf, 2007), representing a nearly 4-fold increase in the rate of global warming since the onset of the industrial age. According to Overpeck et al. (2008) “Sea-level rise from melting of polar ice sheets is one of the largest potential threats of future climate change. Polar warming by the year 2100 may reach levels similar to those of 130,000 to 127,000 years ago that were associated with sea levels several meters above modern levels; both the Greenland Ice Sheet and portions of the Antarctic Ice Sheet may be vulnerable. The record of past ice-sheet melting indicates that the rate of future melting and related sea-level rise could be faster than widely thought.”

The world is in a lag period, when increasing atmospheric energy is expressed by intense hurricanes, increased pressure at mid-latitude high pressure zones and shift of climate zones toward the poles. With ensuing desertification of temperate zones, i.e. southern Europe, southern Australia and southern Africa, the desiccated forests become prey to firestorms, such as in Victoria and California. Feeble attempts by civilization to mitigate the climate are drowning in a tide of medieval conspiracy theories by vested interests and fundamentalist man-overnature ideologues (Hamilton, 2010; Hoggan, 2009). There is nowhere the 6.5 billion of contemporary humans can go, not even the barren planets into the study of which space agencies have been pouring more funding than governments allocate for environmental mitigation to date. At 460 ppm CO₂- equivalent, the climate is tracking above conditions which existed during the early and mid-Pliocene, raising sea levels by 25+/-12 meters (Haywood and Williams, 2005) and close to the upper stability limit of the Antarctic ice sheet, defined at approximately 500 ppm (Zachos et al. 2001, 2008; Royer, 2006). Once transcended, mitigation measures would be unable to re-form the cryosphere. Summing up, Hansen et al. (2008) state ”If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO₂ will need to be reduced from its current 385 ppm to at most 350 ppm. The largest uncertainty in the target arises from possible changes of non-CO₂ forcings. An initial 350 ppm CO₂ target may be achievable by phasing out coal use except where CO₂ is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO₂ is not brief, there is a possibility of seeding irreversible catastrophic effects.”