Antarctic Ice: An 800,000 Year Record of Climate Change
Antarctic Ice: An 800,000 Year Record of Climate Change provides an extraordinary opportunity for scientists to reconstruct climate events of the past. Scientists have recently been able to assemble a continuous 800,000 year record of climate change using ice core samples from Antarctica. This record not only contains information relevant to global temperature changes but also contiguous information on tends of atmospheric greenhouse concentrations during the same period.
Other scientific sources of information exist that shed light on ancient Earth climate conditions, known as paleoclimate, at various times throughout the 4.5 billion year history of the planet. But the importance of the Antarctic ice record is that it provides a continuous and fairly fine resolution record of climate cycles throughout the entire period of existence of our species, Homo sapiens, (originating approximately 200,000 years ago) and during the approximately half-million year prior period in which evolution gave rise to the human species derived from pre-human lineages.
Human Ancestry with time period of Antarctic ice record superimposed (Source: Smithsonian Institution)
In recent decades, global warming has been shown to be at least partially attributed to human-related activities (including deforestation, peatlands destruction, rice farming and fossil fuel combustion), dating primarily back approximately 250 years to the start of the industrial Revolution. During most of the period of the 800,000 year Antarctic ice record humans and immediate pre-human ancestors were present but exhibited minimal impact on global-wide processes affecting climate. Thus global change cycles that occurred when humans existed but were not influential on climate processes can now be compared to climate changes now occurring during a period of major influence of human activity to global climate processes; this period of considerable human influence can be generally viewed as the early to mid-Holocene, when broad scale deforestation, livestock keeping, widespread crop production and extensive forest burning began to alter atmospheric levels of methane and carbon dioxide.
There are two reasons why ancient ice can serve as an important source of climatological data. First, frozen ice from glaciers and from Antarctica does not only contain water but also tiny bubbles containing a sampling of ancient atmospheres. Second, molecules within the frozen ice provide atomic signals that serve as an indicators of the global temperatures present when the water now in the ice originally fell from the sky as snow.
We tend to think of freshly fallen snow as being comprised mostly of water but in fact the density of snow may be one tenth that of water depending upon the amount of air contained within the snow. Ice cream is a man-made frozen substance with specific amounts of air purposefully incorporated to obtain desired consistency and texture. The amount of air incorporated into one's favorite ice cream can be seen by looking at the volume change of a melted bowl of ice cream.
As years, decades, centuries, and millenia of snowfall events are laid down in a very cold place like Antarctica, the fresh snow is gradually compressed into layers of ice. Some of the air that came with the originally freshly fallen snowfall events is retained in the column of ice as minute bubbles. The ice acts as a "time-machine", allowing scientists to sample the concentrations of greenhouse gas present in ancient air dating back more than three-quarters of a million years.
Ancient air bubbles in Antarctic ice. (Source: European Project for Ice Coring in Antarctica (EPICA))
Two factors contribute to allow atoms within the water molecules of Antarctic ice to serve as paleothermometers of past climate periods. First, not all water molecules are the same. While all molecules of water are composed of atoms of hydrogen and oxygen, a small but significant number of water molecules contain a "heavy" form of hydrogen called deuterium. The deuterium isotope is twice as heavy as the more common hydrogen isotope. Thus deuterated water molecules are quite a bit heavier water molecules than lighter hydrogen isotope.
Deuterium is twice as heavy as the more common hydrogen isotope.
Second, all water on Earth passes through what is known as the hydrologic cycle, in which water molecules are converted between solid, liquid, and vapor forms through physical processes including freezing, melting, evaporation, and sublimation and moved through various locations in the biosphere through physical (e.g. wind patterns, ocean circulation, and streamflow) and biological (plant root uptake and transpiration) processes. The movement of water through the hydrologic cycle is temperature dependent. As liquid water is heated, the rate of evaporation is increased.
But an elevated evaporation rate is not the only change that occurs in heated water. As the water is heated, the lighter molecules containing the common hydrogen isotope will evaporate more quickly than heavier molecules containing deuterium. Water vapor formed under higher temperatures will have less deuterium than water vapor formed under lower temperatures. This process is called deuterium fractionation. Not only does the fractionation process work at the scale of water boiling in a pot, it also works at the scale of the global hydrologic cycle. Therefore, in warmer years, water vapor and cloud water will tend to contain lower concentrations of the heavier isotope deuterium that concentrations found in cooler years. In warmer years water and cloud vapor moving over Antarctica as well as the snow derived therefrom will tend to have lower concentrations of deuterium that that of cooler years. As Antarctic snowfall events are converted to layers of ice, the relative concentrations of deuterium in each layer of ice reflect the relative global temperatures of the years in which the original snow accumulated. Thus the relative concentrations of hydrogen and deuterium isotopes (called Delta-D or δ-D) in each ice layer serve scientists as paleothermometers of the Earth's average global temperatures dating back hundreds of thousands of years.
As the result of many years of hard work by the collaborative efforts of international teams of numerous paleoclimatologists led by EPICA (European Project for Ice Coring in Antarctica), detailed data dating back 800,000 years providing direct evidence of past environmental and climatological conditions are now available. Of the panoply of components that have now been measured (from ions to isotopes, trace metals, and dust particles) the most illuminating findings come from the the analysis of hydrogen isotope rations (δ-D as an estimator of global temperatures) and the measurements of greenhouse gas concentrations (carbon dioxide, methane, and nitrous oxide) captured in the chronology of modern to ancient bubbles captured within the ice.
a, The 800,000-year Antarctic ice records of atmospheric carbon dioxide (red; parts per million, p.p.m.) and methane (green; parts per billion, p.p.b.) and a temperature reconstruction (relative to the average of the past millennium) based on the deuterium–hydrogen ratio of the ice. Concentrations of greenhouse gases in the modern atmosphere are highly anomalous with respect to natural greenhouse-gas variations (present-day concentrations are around 380 p.p.m. for carbon dioxide and 1,800 p.p.b. for methane). b, The carbon dioxide and methane trends from the past 2,000 years. (Source: Brook. Ed., Palaeoclimate: Windows on the greenhouse.Nature 453, 291 - 292 (2008).)
Temperature, carbon dioxide and methane
The 800,000 year record shows a consistent tight coupling between greenhouse gas concentrations and climate (using δ-D as an estimator of relative global temperatures). Prior to the last approximately 250 years, during the remainder of the entire 800,000 year record carbon dioxide levels never surpassed 300 parts per million and methane levels never surpassed 500 parts per billion (ppb). Current levels for carbon dioxide and for methane are now about 380 ppm and 1800 ppb, respectively. Modern-day anomalously high levels of greenhouse gasses correspond with unprecedented global levels of human activity that have occurred during the past two centuries beginning about the time of the start of the Industrial Revolution and the modern period of global warming documented in real-time temperature records collected over the last century.
Trends in the the ice core data also correlate well with the timings of the onset of multiple glacial and interglacial periods, including nine major glacial terminations, during the past 800,000 years. The last glacial period substantially ended about 11,000 years ago at the onset of the Holocene. This major shift in climate as well as the other prior global climate shifts documented in the Antarctic ice record occurred long the activities of humans or our pre-human ancestors would have be able to produce planet-wide climate impacts. Thus while recent warming has been generally linked to human activity, global climate changes that occurred throughout most of the period of the Antarctic ice record were related to extra-human factors.
Orbital drivers of climate change
In the early 1940's, Serbian scientist Milutin Milankovi? published a textbook in which he postulated that swings in climate related to glacial and interglacial periods were not only due to extra-human factors but also due to extra-terrestrial changes in the Earth's orbital motion around the sun. Now more than 65 years later, results from the Antarctic ice data have dramatically reinforced the existence of what are now called Milankovitch cycles.
Two overriding factors affect the amount of heat that accumulates and is retained at the surface and therefore the average global surface temperature of a planet such as the Earth. The first factor is the amount of solar insolation impinging on the planet's surface which can be absorbed and converted into heat. Planets closer to the sun intercept greater amounts of solar isolation. Thus, in general, planets closest to the sun exhibit greater surface temperatures than those farthest from the sun.
The second overriding factor affecting warmth of the surface of a planet is the presence and concentration of greenhouse gases in the globe's atmosphere. Like a blanket on a bed, greenhouse gases in the atmosphere intercept and absorb infrared energy radiating from the surface of a planet and re-radiate a portion of that energy back towards the surface. The net result is that planets containing significant amounts of greenhouse gases can be expected to have moderated temperatures.
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