Palaeoclimate and Proximate Parameters
The inexorable warming of the earth is now generally accepted by most people; the readily discernible changes in temperature, glaciers and speciation can no longer be attributed to random variation. Whether or not these changes in climate are anthropogenic, i. e. caused by humans, is still subject to debate.; it is rightly pointed out that there have been periods of relative cold and periods of relative warm over the millions of years of Earth history due to the fundamental physics of the solar system. It is thus argued that these natural cycles are responsible for the current warming trend – and that we should therefore do nothing. This makes the subject of the geologic history of the climate, or palaeoclimate, vitally important – for only in comprehending the science of long-term climate variation can the anthropogenic question be addressed.
Global climate is at its most basic level the earth’s reaction to the energy radiating from the Sun. There are three ways that the Sun’s radiant energy balance with the earth can change. The orbit of the Earth around the Sun can (and does) change, the amount of radiant energy reflected back into space due to dust or ice (called albedo from albus, Latin for white) can change, and the amount of infrared radiation energy absorbed in the gases of the atmosphere can change. Global warming is due to the third effect – when the incoming radiation is trapped by the so-called atmospheric greenhouse gases, the atmosphere must heat up like a greenhouse to radiate enough heat back into space for the system to be balanced. The most pernicious greenhouse gas is carbon dioxide (CO2) some of which is produced by the burning of fossil fuels to generate energy. The other two major greenhouse gases are methane (CH4) mostly from cattle and landfills and nitrous oxide (N2O) mostly from fertilizer. It is contended that the current concentrations of CO2 (379 parts per million or ppm and rising) are unprecedented in the last 16,000 years. It is the veracity of this assertion that must be substantiated by the facts and derivative theories of palaeoclimatology.
The Earth has been around for some 6 billion years, the universe for over twice that – the “Big Bang” is currently thought to have occurred about 13.7 billion years ago (bya). The constituents of the atmosphere during the early phases of Earth’s evolution are mostly speculative. The first life (Archaebacteria – single cells with no nuclei) appeared about 3.5 bya. They were (and are – they are still around) anaerobic – they do not use oxygen which is toxic to them – the early atmosphere likely had only trace amounts of oxygen. Cyanobacteria (which is more widely known as blue-green algae) evolved shortly thereafter to produce oxygen as a waste gas. The availability of oxygen contributed to the evolution of the first eukaryotes (single cells with nuclei) which appeared about 1.8 bya; the first multi-cellular animals appeared 600 million years ago (mya) – the so-called Cambrian explosion. The atmosphere was clearly capable at that point of supporting life, so it is pertinent to consider the constituency of the atmosphere from that point forward. Since CO2 is the issue today, CO2 is the relevant parameter of the palaeoclimate.
During the geologic periods preceding the current Quaternary Period which started 1.8 mya, it is postulated that CO2 levels were higher than current levels and that global temperatures were therefore elevated – this due to the same heat balance effect that is the basis for the current climate concerns. The basis for the notion that CO2 levels were higher is from proxy scientific evidence. A proxy is an authority to act for another – like a stock proxy; it is used in this case to indicate that there are measurements that can be made on physical evidence extant in the present geologic deposition that indicate the level of CO2 at the time they were formed. The most prevalent proxy measurement takes advantage of the different isotopes (same element, different weight) of carbon that are present in nature: carbon 12 (6 protons and 6 neutrons) comprises about 99 percent and carbon 13 (6 protons and 7 neutrons) comprises about 1 percent (there is a trace of unstable Carbon 14 that is used in radiographic – or carbon – dating). Since it has been shown that the ratio of carbon 13 to carbon 12 (called d13C) is different for soil and seawater than it is for the atmosphere, then one can infer the amount of carbon (and therefore CO2) in the atmosphere from the d13C of the sample. If you know the age of the substrate from which the sample was extracted, then you can roughly correlate CO2 levels and time – a proxy. There are also proxies based on boron isotope ratios similar in nature to d13C and the size of the stomata (pores) on the undersides of leaves – more CO2 means fewer and smaller pores since less are needed for plant respiration.
CO2 levels were substantially higher earlier in earth’s history – the estimated peak was about 1100 ppm in the middle Cretaceous Period of the Mesozoic Era about 100 mya; this does not mitigate concerns about the present CO2 levels. For one thing, the planet was warmer – simply due to cooling down from its tempestuous volcanic origins; for most of geologic history, there was little to no ice and periods of extensive glaciation were few and far between. That has not been the case for the last 50 million years or so – glaciation has been the norm and periods of relative warmth known as interglacials have been the exception. Since there is a dichotomy between the Mesozoic warmth and the Cenozoic cold and the latter is the era we live in, it is more relevant to understand the more recent past in order to contemplate the near future. It may seem counterintuitive to address cyclic glacial cold periods to understand global warming – it is, however, the prevailing climate pattern of our most recent past.
Geologic evidence indicates that there have been five relatively long (about 100,000 years) glacial periods punctuated by five relatively short (about 30,000 years) warmer interglacial periods over the last million years or so. We are in an interglacial now, and have been for about 10,000 years – it is not expected to end until about the year 32008, the three hundred and twenty first century. The cyclic nature of the temperature swings incident to glaciation that occur over thousands of years suggests a cosmological provenance. The eponymous Milankovitch theory is attributed to the Serbian astronomer Milutin Milankovitch; the theory postulates that changes in the Earth’s eccentricity, obliquity or tilt, and precession change the amount of sunlight reaching the earth – called insolation – sol is Latin for sun. These three parameters account for changes in the Earth’s orbit of the Sun according to the theory as follows:
• Eccentricity – The earth orbits the Sun in an ellipse; eccentricity is a measure of the relative roundness of the ellipse. The eccentricity varies on approximately a 100,000 year cycle from 0 percent (nearly round) to 5 percent (very elliptical). At present the difference between the maximum distance (aphelion) and the minimum distance (perihelion) is only about 3 percent – i. e. the orbital distance between the earth and the sun varies from about 91 million miles to 94.5 million miles. At maximum eccentricity, this distance can vary by up to 30 percent – clearly this would have a profound effect on insolation – and thus glaciation.
• Obliquity – Normally, the earth’s axis has a tilt of 23.5°, which is why there are seasons. This tilt, known as obliquity, varies from 22.05° to 24.5° every 41,000 years. At a lower tilt, the warming effect of the sun would not reach as high a latitude in summer – this would contribute to the buildup of snow and ice in glaciers during the more profound winters.
• Precession – The precession of the equinoxes (roughly March 21 and September 21 when the Sun “crosses” the equator) was first discovered in about 125 BCE by the Greek astronomer Hipparchus – as the apparent shift of the stars visible at the moment of equinox from year to year. The reason for precession is that the earth is not perfectly round, but rather is flattened at the poles so that the axis about which the earth “wobbles” like a spinning top when it slows down. This cone-shaped precession of the earth’s axis occurs about every 25,800 years in what is called the Platonic year. Over time, this causes the pole star to change – it is Polaris or the North Star now – in 12,000 years it will be Vega, a bright star in the constellation Lyra.
The net effect of eccentricity, obliquity and precession is that the amount of absorbed sunlight or insolation varies according to how the three cycles are modulated (operate in concert) to produce a net effect. The Milankovitch Theory of glaciation is that a period of minimal insolation occurs on a periodic basis. Due to this lower heat loading, the northern hemisphere becomes cold enough for ice to persist through the summer; the albedo effect of the expanded ice causes a reflection of the sun’s rays during subsequent years to create a positive cooling feedback. Ultimately, the annual accumulation of snow results in extensive glaciation and a prolonged period of cold – an ice age. It is the ice of the last several ice ages that provides the most directly measurable estimates of CO2 and temperature.
In 1993, the Greenland Ice Sheet Project 2 (GISP2) completed drilling through 3053.44 meters of ice with a hollow pipe until it hit bedrock. The ice core that was extracted for scientific analysis extends back through the last two ice ages. Its validity was correlated with a previous ice core (GRIP for Greenland Ice Core Project) that was 30 kilometers distant and comparably deep. To provide an accurate measure of the relevant climate conditions from the ice cores, one must know the CO2 level, the time or date of the sample (i.e. 1200 BCE) and the corresponding temperature. The CO2 level is the most straightforward, as air bubbles are trapped in falling snow that is ultimately compacted into glacial ice. Other greenhouse gases can also be measured. Ice core data has shown that CO2 levels ranged from 180 to 300 ppm over the last 650,000 years (methane or CH4 levels were 320 to 790 parts per billion – compared to 1,774 ppb today). The current level of 379 ppm is clearly an anomaly in being well outside the historical range. The time and temperature are a little more difficult to estimate, as they cannot be measured directly.
As was the case with d13C, there is a d18O, the ratio of the heavy isotope of oxygen with 8 protons and 10 neutrons or O18 to the normal weight oxygen with 8 protons and 8 neutrons or O16. On average the amount of heavy oxygen is about 0.2 percent of total oxygen but this amount changes with temperature; it is this factor that is used in paleoclimate determinations. The water in the ocean (H2O) also has the mix of some heavy and some light oxygen. As the water nearer to the equator is evaporated, it moves north with the prevailing winds, where it cools. The water vapor with the O18 condenses first, since the molecule is heavier. Therefore, as the air with the entrained water vapor moves north, it gets depleted of heavy oxygen, and when it condenses and falls as snow on the glaciers the d18O ratio is fixed. When temperatures are cooler, the heavy oxygen condenses faster and the d18O ratio is lower. When the temperatures are warmer, the d18O ratio is higher. Since there is a seasonal variation in temperature, the d18O ratio changes from summer to winter; it is these fluctuations that allow for the determination of the age of the location along the ice core. The net information from an ice core sample is therefore a CO2 level, a time and a temperature. The temperature is the least credible of the three paleoclimate correlated parameters.
In general, temperature proxy correlations are tenuous and should not be overly emphasized. Relatively detailed global temperature measurements only go back about 150 years; the first thermometer, a sealed tube partially filled with alcohol, was not invented until 1641. From 1781 to about 1850, there were only 23 European temperature measuring stations and 1 North American station. The first Asian station did not commence recordings until the 1820’s. This hardly constitutes a global record. The temperature record before the 1700’s relies on a number of proxies – notably tree rings, ice cores, and ocean sediments. Dendroclimatlology is the sesquipedalian name for the science of using tree rings to evaluate climate. The basic process is to use modern tree ring records with known temperature variations to develop statistical equations that use physical tree ring parameters to correlate to temperature. These equations are then extrapolated to older trees that grew in the same location. Since most trees only live for several hundred years, the process can only be extended back to about a 1,000 years by using tree ring patterns that can link one generation of tree to the generation preceding (overlapping sequences in the outer rings of the older tree being the same as the inner rings of a younger tree).Ice core temperature proxies extend the record back to 650,000 years ago using the d18O ratio as described above. Ocean sediment core proxies extend the record back to about 3 million years, but with an even more convoluted methodology. Plankton shells that twist in different directions according to temperature are counted and used to correlate to the water temperature in which they grew. Even the IPCC report concludes that “temperature is a more difficult variable to reconstruct than CO2.” It is not a good metric on which to base a controversial public policy that would advocate strictures in resource allocations in an austerity mandate.
A more mundane problem that accrues from the use of temperature as a global warming metric is the cultural historical record; there have been periods of relative warmth and cold that are comparable to the magnitude of temperature averages we now experience. The most well known of these periods are the euphemistic Medieval Warm Period (MWP) and the subsequent Little Ice Age (LIA); they were certainly of manifest import to the populations of their respective eras – the prevailing weather trends in a subsistence agricultural economy are a matter of life and death. From about 1000 to 1200 CE, temperatures in the northern hemisphere were probably several degrees warmer than the preceding centuries (the uncertainties of temperature determination elucidated above inform the debate about how warm the MWP really was) . The MWP has some interesting historical ramifications. The increasing populations of the area that constitutes modern Norway and Denmark precipitated the Diaspora of Viking ships in search of sustaining provender; they extended their reach to Danelaw in the British Isles, to Normandy in France, and to the colonization of Iceland and then Greenland. They eventually reached the mainland of North America at L’Anse aux Meadows in about 1000 CE. As an indication of the relatively moderate temperatures during the MWP, one of the areas explored by the Norse in North America was called Vinland, for the grapes that grew there.
The Greenland colony succumbed when the Medieval Warm Period ended and the Little Ice Age began, presumably due largely to the climate change as chronicled by geographer Jared Diamond in his seminal environmental book Collapse. Although many cultural factors contributed to the demise of Greenland’s Western and Eastern Norse Settlements, the cooling climate was likely a key factor; archeological evidence indicates that the last inhabitants starved or froze to death at the end of what was likely a harshly cold winter. A radiocarbon date of 1435 from a woman’s dress excavated from the churchyard at Herjolfsnes provides a benchmark; the LIA is estimated to have extended from between 1350 or 1450 CE to about 1900 – the correlation is probably not coincidental. Brian Fagan in The Little Ice Age records the period between 1680 and 1730 as the coldest, when “temperatures plummeted, the growing season in England was about five weeks shorter than during the twentieth century’s warmest decades.” While it is important to study and understand the impact of climate on human culture due to its profound effects on the sustaining factors of food and fuel, it is equally important to understand the historical randomness of climate. Temperatures have been hotter and colder than the present, and have changed absent substantive burning of fossil fuel and increases in CO2 levels. What is different now is not the magnitude of the temperature but the rate of change of the temperature.
It was long thought that climate change was inexorably slow, on the order of centuries. The syllogistic logic was that if climate change was occurring, then we would have plenty of time to react. However, analysis of the Greenland ice cores revealed that climate change could occur much more rapidly – raising the chimerical specter of radical changes in temperature – on the order of a decade instead of a century. When the last ice age ended about 14,000 years ago, the climate became warmer and populations consequently migrated to the now accessible northern regions (the migration across the Bering land bridge that initiated the human habitation of the Americas occurred at this time according to recent DNA evidence). A rapid return to glacial conditions occurred about 13,000 years ago and lasted for the thousand years of a period known as the Younger Dryas , named for the wildflower Dryas octopetala commonly called Mountain Avens which grows in alpine tundra habitats; the prevalence of its pollen in the layers of the ice core were indicative of significantly lower prevailing temperatures. About 12,000 years ago, the climate returned to the moderate warmth – in about three years; this is the prevailing temperature that still prevails as it marks the beginning of the current interglacial. It is interesting to note that Dr. Sharon Moalem speculates in the book Survival of the Sickest that this is the provenance of diabetes in humans – the higher level of blood sugar providing protection against freezing – testing has shown that the level of blood sugar in diabetics goes up in the winter.
Since climate change has otherwise been slow, it is well to speculate as to the proximate cause of the rapid temperature changes that mark the beginning and the end of the Younger Dryas. It is thought to have been caused by the shutdown of the “conveyor belt,” a euphemism for the thermohaline circulation (THC) that results in Atlantic meridional overturning circulation (MOC). The warm Gulf Stream waters flow past the eastern shore of North America and gradually lose their heat content (thermal) due to convection (wind) and their salt content (halide) due to evaporation as they near the Arctic regions. When the Gulf Stream gets cold enough and salty enough, it sinks to the bottom and returns to the southern or meridional direction to complete the loop, an overturning (thermohaline) circulation much like a conveyor belt. It is the Gulf Stream flow that brings warmth to Europe – London is on the same latitude as Labrador. If the MOC were to stop, the temperature of Europe would drop precipitously, as it is believed to have done in the Younger Dryas. It is postulated that an inflow of melting Arctic ice discharged into the North Atlantic and disrupted the circulation. That global warming could melt the Arctic ice and trigger a similar period to the Younger Dryas is justifiably of great concern to humanity, most especially those who live in Europe.
The concerns of global warming are valid, but the metrics are wrong. The debate concerning the veracity of the claim that global temperatures are rising is focused on the paucity and complexity of the historical temperature record. It is fraught with large margins of error that weaken the IPCC assertion that temperatures have risen by 0.74 °C over the last 100 years; temperatures can vary by tens of degrees according to geography, topography and altitude. When temperatures are elicited as the key parameter on which to base decisions that have profound economic implications, there is a legitimate case to be made that the data are inconclusive. There is not as strident an argument concerning CO2 levels – it is difficult to challenge the actual samples that the ice cores provide. It is equally difficult to challenge the magnitude of the CO2 levels, as they represent a relatively global parameter. Rising CO2 levels cause increased temperatures due to the greenhouse effect. Rising temperatures cause ice to melt and run into the ocean to wreak havoc on the delicate balance of the MOC – and this the issue.