Climate Change: The “big” picture
Adapted from the work of Lee C. Gerhard, of the Kansas Geological Survey, W.E. Harrison, of the Kansas Geological Survey, and B.M. Hanson, an Independent Petroleum Producer (Deceased) first published as AAPG Studies in Geology No. 47, entitled “Geological Perspectives of Global Climate Change,” available from the AAPG online bookstore (http://bookstore.aapg.org) and also the more recent efforts of M. Ray Thomasson, a Trustee of the American Geological Institute Foundation and past president of the American Geological Institute.
Global climate has varied since the most primitive atmosphere developed on earth billions of years ago. Climate change on Earth has been continuous and has occurred on all timescales.
The sedimentary rock record reflects numerous sea-level changes, changes in atmospheric composition, and temperature changes, all of which attest to climatic variation. Past climates have varied from those that create continental glaciers to those that yield global greenhouse conditions. It gets warmer or cooler but typically does not remain the same for extended periods of geologic time. Human history shows us that in general, warmer conditions have been beneficial, and colder conditions have been less kind to society.
Today we are living through a not-yet-completed interglacial stage. We have been in this interglacial for about 10,000 years. Interglacial periods appear to last for about 11,000 years, but with large individual variability. It is very likely that warmer conditions lie ahead for humanity, with or without any direct human contribution.
Geology is a scientific discipline that routinely works backward through significant periods of time to understand the natural processes of the Earth as a dynamic system. Geology brings both data from the past history of climate and scientific methods for interpreting such data to the debate about greenhouse gases from human activities and global temperature behavior.
The earth is an integrated dynamic system, billions of years old, that has never been in equilibrium. Climate can vary rapidly and over a range that can have a profound influence on human society. There are predictable geologic effects of climate change such as sea-level changes, rates of glacial movement, ecosystem migration, methane hydrate formation, and changes in agricultural productivity.
Geologic processes are not in equilibrium and this makes assessing any cumulative human impact on climate difficult. It is difficult to determine the range of natural variation in global climate over the last 10,000 years, and even more difficult to determine the impact of human activities on such variation.
Adaptation to the changes that continually occur on Earth requires flexibility, planning, and acceptance of the earth-system constraints.
Political processes cannot change earth dynamics.
Climate drivers vary in intensity and over time. Human influences are of comparatively low intensity and take place over short time spans. The non-equilibrium systems that control natural phenomena on earth dwarf the ability of human beings to affect climatic conditions on a global scale other than by means of global nuclear or biological war.
Climate drivers: First-, second-, third-, and fourth-order climate controls
The scale of temperature changes through time compared with climate change drivers can be visualized with an ordering of climate drivers and timescales which suggests there is a direct relationship between the range of absolute temperature change and the amount of time over which the climate drivers operate.
In the following graphic, the diamond shapes are interpreted from literature-documented ranges of values and dots indicate possible ranges of values. The error bars for the interpretations are broad, but the phenomena appear to fall into significant categories of effects and time.
The blue diamonds represent the estimated range of values for the named processes which affect surface temperature on the Earth over time.
Climate drivers can be categorized by the range of temperature change forced by the driver and the length of time over which the driver operates. The vertical axis (time) is logarithmic (units are powers of 10), and the horizontal axis (temperature effect/change) is arithmetic. This permits comparison of climate drivers with their potential effects, and separation of drivers of different magnitude.
First-order climate controls
Earth has a climate which supports human life because of its distance from the sun, solar luminosity, and the evolution of a greenhouse atmosphere of water vapor, methane, CO2, and other gases that trap solar energy and make it usable.
The atmosphere of the Earth has evolved over the last 4.5 billion years and continues to evolve. Carbon-dioxide concentrations in the atmosphere have decreased over the past 600 million years from 18 times the current concentration.
The greenhouse effect itself makes the earth 20°–40° Celsius (C) warmer than it would otherwise be at ~93 million miles from the Sun.
Second-order climate controls
Second-order climate control results from the distribution of continents and oceans upon the planet which controls the major ocean currents which distribute heat. This fundamental concept explains the 15°–20°C temperature variations over hundreds of million of years.
The climate of the Earth has been continuously cycling between glacial “icehouse” and warm “greenhouse” states. The late Precambrian “icehouse” 4600–541 million years ago evolved into the Devonian “greenhouse” which lasted from 419.2–358.9 million years ago followed by the Carboniferous “icehouse” from 358.9–298.9 million years ago then the Cretaceous “greenhouse” from 145–66 million years ago which evolved to the present “icehouse” state. Glacial activity has been identified in the “record of the rocks” at various locations as early as about 3 billion years ago.
Gerhard and Harrison theorize that redistribution of heat around the earth is determined by the presence of equatorial currents that keep and thrust warm water masses away from the poles. Blockage of such currents permits the formation of gyres that move warm waters to the poles and creates the setting that allows continental-scale glaciation. When continental landmasses are positioned so that equatorial oceanic circulation patterns exist, general global climate conditions are warmer. Conversely, when landmasses are positioned so as to impede or prevent equatorial circulation, “icehouse” conditions prevail. When warm waters are moved to polar regions, high rates of evaporation create continental glaciers and facilitate widespread global cooling. Conversely, strong and persistent equatorial currents preclude heat transfer to high latitudes, and warm conditions prevail. These relationships help to illustrate that thermal energy or heat is transferred around the earth much more effectively by oceanic circulation patterns than by atmospheric circulation.
Temperature variation under this scenario ranges up to 15°C. Second-order control of global temperature is natural, driven by earth dynamics, and occurs over tens to hundreds of millions of years. For instance, the Cretaceous “greenhouse” condition changed to the modern “icehouse” condition over a period of 60 million years. There appears to be a relationship between the intensity of temperature variation and the length of time over which it occurs.
Third-order climate controls
Solar insolation variability has emerged as a major climate driver, as are the orbital variations that change the distance between the earth and the sun.
Large-scale changes in ocean circulation through changes in current structure can be significant climate drivers. Large-scale ocean tidal cycles may drive climate, including the large-scale maximum and minimum associated with the Medieval Climate Optimum and the Little Ice Age, on an 1800-year cycle with a 5000-year modulation. These drivers may cause temperature changes of 5°–15°C over hundreds to hundreds of thousands of years.
Fourth-order climate controls
There are many drivers that control small temperature changes up to 5°C over short periods of time up to hundreds of years. Many are natural phenomena, including smaller-scale oceanographic oscillations such as La Niña and El Niño; volcanic activity such as the eruptions of Pinatubo and Krakatoa; solar storms and flares; small orbital changes; meteorite impacts, and human activities such as deforestation and urban blacktop paving.
Tectonic and topographic uplift have small temperature effects and are regional rather than global. 18–, 90–, and 180–year cycles driven by ocean tides have been recognized and they drive climate in the short term by modifying heat transfer rates between the oceans and the atmosphere.
The sun is the primary source of energy for the climate of the earth. Earth’s distance from the sun, a function of the geometry of the solar system, is the major factor controlling the base temperature of the earth.
During the earliest history of the Earth, solar irradiance was low, but internal radioactive decay likely produced enough heat to melt the earth’s crust.
According to current theory based on geophysical measurements of density distribution in the earth, differentiation of the hot earth segregated a core, a mantle, a crust, and an atmosphere of light gases. These gases, which constitute the atmosphere that exists today, have varied somewhat in composition throughout geologic time, and have caused greenhouse conditions to develop. Slowly, solar irradiance has grown, perhaps by as much as 25%. Eventually, in perhaps a billion years fromm the present, the growing solar irradiance will have burned away the earth’s atmosphere and made the Earth uninhabitable.
The greenhouse envelope of the atmosphere which evolved over 4.5 billion years is the primary temperature control that permits life, as we know it, to exist. Without that envelope, it is likely that the earth’s temperature would be 15°–30°C below its present level.
About 80% to 95% of the total greenhouse gas budget is water vapor (including clouds) and the remainder consists of CO2, methane, and other gases.
Third-order temperature drivers include orbital “wobbles” of the planet earth in its annual revolution around the sun, and variability of solar energy reaching the surface of the Earth by luminosity changes in the radiation of the sun itself.
Milankovitch Cycles and Glaciation
The episodic nature of the Earth’s glacial and interglacial periods within the present Ice Age (the last couple of million years) have been caused primarily by cyclical changes among variations in the Earth’s eccentricity, axial tilt, and precession, collectively known as the Milankovitch Cycles named for Milutin Milankovitch, the Serbian astronomer who is generally credited with calculating their magnitude. Variations in these three cycles create alterations in the seasonality of solar radiation reaching the Earth’s surface. These times of increased or decreased solar radiation directly influence the Earth’s climate system, particularly the advance and retreat of Earth’s glaciers. The three Milankovitch Cycles impact the seasonality and location of solar energy around the Earth, thus impacting contrasts between the seasons.
The 100,000 year Milankovitch cycle:
Variations in the shape of Earth’s elliptical orbit (cycle of eccentricity)
The first of the three Milankovitch Cycles is eccentricity, the shape of the Earth’s orbit around the Sun. This constantly fluctuating orbital shape ranges between more and less elliptical (0 to 5% ellipticity) on a cycle of about 100,000 years. These oscillations, from more elliptic to less elliptic, are of prime importance to glaciation in that it alters the distance from the Earth to the Sun, thus changing the distance the Sun’s short wave radiation must travel to reach Earth, subsequently reducing or increasing the amount of radiation received at the Earth’s surface in different seasons.
Today a difference of only about 3% occurs between aphelion (farthest point) and perihelion (closest point). This 3% difference in distance means that Earth experiences a 6% increase in received solar energy in January than in July. This 6% range of variability is not always the case, however. When the Earth’s orbit is most elliptical the amount of solar energy received at the perihelion would be in the range of 20% to 30% more than at aphelion. These continually changing amounts of received solar energy around the globe result in significant changes in the climate and glacial regimes of planet Earth. At present the orbital eccentricity is nearly at the minimum of its cycle.
The 21,000 year Milankovitchcycle:
>Earth’s combined tilt and elliptical orbit around the Sun (precession of the equinoxes)
Axial tilt, the second of the three Milankovitch Cycles, is the inclination of the Earth’s axis of rotation in relation to the plane of its orbit around the Sun. Oscillations in the degree of the Earth’s axial tilt occur on a periodicity of 41,000 years from 21.5° to 24.5°.
Today the Earth’s axial tilt is about 23.5°, which largely accounts for our seasons. Because of the periodic variations of this angle the differences among the seasons on the Earth changes. With less axial tilt the Sun’s solar radiation is more evenly distributed between winter and summer. However, less tilt also increases the difference in radiation at the surface of the Earth between the equatorial and polar regions.
One hypothesis for Earth’s reaction to a smaller degree of axial tilt is that it would promote the growth of ice sheets. This response would be due to a warmer winter, in which warmer air would be able to hold more moisture, and subsequently produce a greater amount of snowfall. In addition, summer temperatures would be cooler, resulting in less melting of the winter’s accumulation. At present, axial tilt is in the middle of its range.
The 41,000 year Milankovitch cycle:
Cycle of the +/- 1.5° wobble in Earth’s orbit (tilt)
The third and final of the Milankovitch Cycles is precession, a slow wobble as the Earth turns on its axis.
A consequence of the precession is a changing pole star.
The precession of Earth changes the orietation of the Earth and moves the “north” star over time to a different constellation in the night sky. The current pole star is Polaris located about one degree from the pole. The brilliant Vega in the constellation Lyra was the pole star around 12,000 BC and will be the pole star again around the year 14,000, however, it never comes closer than 5° to the true pole.
This top-like wobble, or precession, has a periodicity of 23,000 years.
When the axis of the Earth is tilted towards Vega the positions of the Northern Hemisphere winter and summer solstices will coincide with the aphelion and perihelion, respectively of the elliptical orbit. This means that the Northern Hemisphere will experience winter when the Earth is furthest from the Sun and summer when the Earth is closest to the Sun. This coincidence will result in greater seasonal contrasts. At present, the Earth is at perihelion very close to the winter solstice.
These variables are important because the Earth has an asymmetric distribution of landmasses, with virtually all, except Antarctica, located in the Northern Hemisphere.
At times when Northern Hemisphere summers are coolest— farthest from the Sun due to precession and greatest orbital eccentricity— and winters are warmest due to minimum tilt of the axis, snow can accumulate and cover broad areas of northern America and Europe. At present, only precession is in the glacial mode, with tilt and eccentricity not favorable to glaciation
Even when all of the orbital parameters favor glaciation, the increase in winter snowfall and decrease in summer melt would be barely enough to trigger glaciation, not to grow large ice sheets. Ice sheet growth requires the support of positive feedback loops, the most obvious of which is that large masses of ice and snow tend to reflect more radiation back into space thus cooling the climate and allowing glaciers to expand.
Variations in solar energy reaching the earth’s surface modify the climate.
Several factors control the influx of solar energy, including variations in (1) the albedo of the Earth which is the measure of the diffuse reflection of solar radiation out of the total solar radiation received by the Earth; (2) earth’s orbit and rotation; and (3) solar energy output.
Potential temperature changes driven by these variations can be as much as 10°C and the changes may take thousands of years. Minor climate changes and those that mark the changes from glacial to interglacial may be the signature of these events.
Many of these changes take place in a more or less regular cycle. The most commonly observed solar cycle is the 11-year sunspot cycle. Statistically optimized simulations suggest that direct solar forcing can account for 71% of the observed temperature change at the earth’s surface between 1880 and 1993, corresponding to a solar total irradiance change of 0.5%. The full effects of solar irradiance changes, including Milankovitch effects, must be interpreted from imprecise historical data, because direct measurements have been systematically available only since 1978.
In the short time since 1978, direct measurement of total solar irradiance (TSI) by satellites has shown cyclical variations in solar energy of 0.1% in conjunction with the 11-year sunspot cycle. Indirect evidence from the sun and other sunlike stars indicates that TSI has had significantly greater variation as the sun goes through its energy output cycles.
The statistical correlation between climate and TSI variations seems to be significant. Small variations in TSI initiate indirect mechanisms on earth that yield climate changes greater than that predicted for the TSI change alone.
At least three solar variables are known to affect earth’s climate: (1) TSI, which directly affects temperatures; (2) solar ultraviolet radiation, which affects ozone production and upper atmospheric winds; and (3) the solar wind, which affects rainfall and cloud cover, at least partially through control of earth’s electrical field.
Each affects the earth’s climate in different ways, producing indirect effects that amplify small changes in TSI. Individually, they do not account for all observed climatic changes. Collectively, however, they create changes.
The Energy Budget of the Earth
The oceans of the Earth are vast, and owing to the specific heat of water, they contain vast amounts of thermal energy. Much of that energy is transferred around the earth through ocean currents. Some is transmitted to and from the atmosphere and controls local weather.
Ocean currents are driven by wind, the Coriolis effect, and thermohaline circulation which depends on density differences of waters entering the ocean. Fresh waters are lighter than saline waters, and warm waters are lighter than cold waters. The density of a water mass will determine whether it rises or sinks in the water column.
On possible effect of such changes in density is that warm events after glacial events deliver low-salinity, thus low-density, meltwaters into the ocean which tend to float rather than sink in areas where sinking (downwelling) normally takes place. This action interferes with total normal oceanic circulation; for instance, the Gulf Stream may be diverted eastward by meltwaters from the North American arctic, depriving England and northern Europe of heat now moderating their climate.
If massive events such as floods of fresh water backed up in proglacial lakes were to spill into the ocean, then very rapid climate changes could take place by changing the ocean circulation patterns. At least two such events took place near the beginning of the present interglacial stage, with up to 4°–5° C rapid temperature swings in the Sargasso Sea.
The Sargasso Sea is a region of the North Atlantic Ocean bounded by four currents forming an ocean gyre. Unlike all other regions called seas, it has no land boundaries. The Sargasso Sea is bounded on the west by the Gulf Stream, on the north by the North Atlantic Current, on the east by the Canary Current, and on the south by the North Atlantic Equatorial Current, all of which establish a clockwise-circulating system of ocean currents termed the North Atlantic Gyre. It lies between 70° and 40° W, and 20° to 35° N, and is approximately 1,100 km wide by 3,200 km long Bermuda is near the western fringes of the sea.
It is distinguished from other parts of the Atlantic Ocean by its characteristic brown Sargassum seaweed and often calm blue water.
A large number of smaller-scale geological climate drivers exist. Among these are volcanic eruptions, meteorite impacts, solar storms and flares, shorter solar cycles, small orbital changes, tectonism (mountain building and erosion), weathering of rocks, and small ocean-circulation changes such as La Niña and El Niño oscillations, North Atlantic Oscillation, &c.
These effects which may alter climate up to perhaps 3° or 4°C are fourth-order climate drivers. The well-known effects of the 1883 Krakatoa eruption affected climate for two years or more and the particulates blown into the upper atmosphere from the recent Pinatubo eruptions in the Philippines have also had some influence on global climate.
Fossil-fuel emissions, land-use changes, agricultural fertilization of croplands, and organic sewage discharges to aquatic systems, all coupled with a slight temperature rise, have disturbed the carbon cycle.
Land-use changes affect the uptake or release of carbon. Deforestation may result in weakening of the terrestrial carbon sink in the future. Reduction of the thermohaline current transport of CO2 into the deep world ocean could be a result of increased global temperature, and the increased temperature could then be further increased because of the reduction in sequestration of CO2 in the deep ocean. In addition, such a change in the intensity of the thermohaline circulation could increase the ability of coastal marine waters to store atmospheric CO2.
Methods of estimating ancient temperatures
The geologic record contains many clues about former climates on Earth. Most of the proxies used to interpret past temperatures agree well enough to build a consensus about the more recent geologic past, and general acceptance of large-scale changes in the more distant past.
The accuracy of interpretations of past climates declines as we go farther back into the past
Climates throughout the Pleistocene geological epoch, often referred to colloquially as the Ice Age, which lasted from about 2,588,000 to 11,700 years ago are well documented; but the geologic records become progressively destroyed by erosion and tectonic cycling as time passes. Climates of 600 million years ago are less well documented, and those of billions of years ago are poorly documented.
Reconstruction of paleoclimatic conditions based on the study of stable isotopes in ice core materials is one such proxy which is robust and not subject to uncertainties related to erosion, tectonism, or sedimentation processes. It has evolved into a well-recognized and powerful methodology.
Ice-core materials from mid- to low-latitude glaciers represent a time span of approximately 25,000 years and provide valuable insight to regional variation in climate patterns. Oxygen isotopes are used as a proxy for actual temperature.
Oxygen has three naturally occurring isotopes: 16O, 17O, and 18O, where the subscripts 16, 17 and 18 refers to the atomic mass. The most abundant oxygen isotope on Earth is 16O, with a small percentage of 18O and an even smaller percentage of 17O.
Oxygen isotope analysis determines the ratio of 18O to 16O present in a sample; then compares the calculated ratio of those masses to a standard to obtain information about the temperature at which the sample ice was formed.
The relationship between isotopes of Oxygen, temperature, and weather
Because 18O is two neutrons “heavier” (more massive) than 16O and causes the water molecule in which it occurs to be heavier by that amount, it requires more energy to vaporize H18OH (the water molecule is ofter written by organic chemists as HOH rather than H2O) H16OH, and H18OH liberates more energy when it condenses. In addition, H16OH tends to diffuse more rapidly.
Because H16OH requires less energy to vaporize, and is more likely to diffuse to the liquid surface, the first water vapor formed during evaporation of liquid water is enriched in H16OH, and the residual liquid becomes enriched in H18OH.
When water vapor condenses into liquid, the “heavier” H18OH preferentially enters the liquid, while H16OH is concentrated in the remaining vapor.
As an air mass moves from a warm region to a cold region, water vapor condenses and is removed as precipitation. The precipitation removes H18OH, leaving progressively more H16OH-rich water vapor. This distillation process causes precipitation to have a lower ratio of 18O/16O as the temperature decreases. Additional factors can affect the efficiency of the distillation, such as the direct precipitation of ice crystals, rather than liquid water, at low temperatures.
Due to the intense precipitation that occurs in hurricanes, the H18OH is exhausted relative to the H16OH resulting in relatively low 18O/16O ratios. The subsequent uptake of hurricane rainfall in trees, creates a record of the passing of hurricanes that can be used to create a historical record in the absence of human records.
When water evaporates in a time of cold temperatures relatively few atoms of the heavier 18O are picked up as compared to 16O, whereas in warm temperatures with more energy, more 18O is picked up. When that water is dropped on glaciers and preserved as ice, that ratio of isotopes is preserved. That permits the calculation of temperature from measuring the relative abundance of each isotope in ice core samples.
In geochemistry, paleoclimatology and paleoceanography δ18O or delta-O-18 is a measure of the ratio of stable isotopes oxygen-18 (18O) and oxygen-16 (16O). It is commonly used as a measure of the temperature of precipitation, as a measure of groundwater/mineral interactions, and as an indicator of processes that show isotopic fractionation, like methanogenesis. In paleosciences, 18O:16O data from corals, foraminifera and ice cores are used as a proxy for temperature.
In addition to techniques that may reveal paleoclimate information from marine settings, some lines of evidence are based exclusively on terrestrial organisms. Insects, specifically fossil beetles, provide such evidence.
A fossil beetle assemblage collected from northern Greenland demonstrates an unusual level of stasis through approximately 2 million years, even though the climate at that time was significantly warmer than that of today. The number of species remained relatively constant through several episodes of glaciation.
Using measurements of δ18O in benthic foraminifera from 57 globally distributed deep sea sediment cores taken as a proxy for the total global mass of glacial ice sheets, Lorraine Lisiecki was able to reconstruct the climate for the past five million years.
The stacked record of the 57 cores was orbitally tuned to an orbitally driven ice model, the Milankovitch cycles of 41 ky (obliquity), 26 ky (precession) and 100 ky (eccentricity), which are all assumed to cause orbital forcing of global ice volume. Over the past million years, there have been a number of very strong glacial maxima and minima, spaced by roughly 100 ky.
Beetles are highly mobile insects, and their most apparent response to variation in climate is migration from one location to another. This is well illustrated by the replacement (21,500 b.p.) of a forest fauna along the North American Laurentide ice sheet by a glacial fauna when temperatures were approximately 10°–12° C below current ones. When the ice sheet receded (approximately 12,500 b.p.), a forest fauna returned. Overall, beetles have survived global climatic changes due to their mobility, but reduction in areas suitable for habitat make future extinctions more likely.
SI (Stomatal Index) values for birch trees dated (by radiocarbon) at 10,070 years correspond to CO<sub2< sub=””> levels of approximately 240 to 280 parts per million, volume (ppmv). Birch trees that are 9370 years old have SI values indicateing CO<sub2< sub=””> concentrations of 330–360 ppmv. Thus, these findings indicate an 80–90 ppmv variation in naturally occurring CO<sub2< sub=””> levels over a 700-year period. Additionally, SI data suggest a dramatic change of 65 ppmv CO<sub2< sub=””> levels in less than a century.</sub2<></sub2<></sub2<></sub2<>
Several lines of evidence indicate that early to middle Miocene time, 23.03 to 5.333 million years ago, was one of the warmest periods of the entire Cenozoic, 66 to 0 million years ago. Fossil angiosperm materials of middle Eocene age showed elevated SI values indicating 450–500 ppmv for the CO<sub2< sub=””> level of the middle Eocene atmosphere.</sub2<>
Natural variability and records of change
Over the last 60 million years in central Europe, temperature has dropped by more than 20° C. In relatively recent times, the Medieval Climate Optimum, or Medieval Warm Event (MWE) (a.d.~1200–1350), was the time of building of castles, growing of vineyards in northern Europe, and the settlement of agricultural colonies by Vikings in Greenland. This warm period in human history was followed by the Little Ice Age (LIA), from the end of the MWE to about 1850, when temperatures plunged to isolate the Greenland colonies by sea ice and cause their demise by starvation. As Lamb (1995) pointed out, society prospered during the MWE, but suffered greatly from starvation, plague, and pestilence during the ensuing cold years.
Determination of the direction of global temperature change is a function of the time span used to make the determination. On the 10,000-year interglacial scale, the earth is cooling from the high temperatures of the early interglacial. However, on a 16,000-year record, starting in the late Pleistocene glacial episode, the overall effect is global warming. Similarly, the last 2000 years show that the earth is cooling, over the last 600 years it is slightly warming, and over the last decade it is warming. Picking the time constraint for a model determines the model’s outcome, without regard to the complexity of the model.
Geological interpretations are four-dimensional, that is, they consider time as well as space.
Sea surface level changes are measured today by specially purposed satellites.
The sea-level record approximates the “sawtooth” effect seen in the temperature records, that is, sharp warming episodes that gradually cool. The causes of this sawtooth effect are still being debated, but for glacial and interglacial episodes it may be the result of polar ocean freezing and cutting off moisture to feed continental glaciers, with rapid warming and sea-level rise as a consequence, or thermohaline circulation changes.
Thermohaline circulation (THC) is a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes.The word thermohaline derives from thermo, referring to temperature, and haline, referring to salt content, factors which together determine the density of sea water. Wind-driven surface currents such as the Gulf Stream travel polewards from the equatorial Atlantic Ocean, cooling en route and eventually sinking at high latitudes forming North Atlantic Deep Water. This colder denser water sinks and flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters with a transit time of around 1000 years eventually upwell in the North Pacific establishing that the oceans of the Earth are a global system. On their journey, the water masses of the THC transport both energy in the form of heat and matter as solids, dissolved substances, and gases around the globe.
The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyor, or the global conveyor belt.
Over most of geologic history, long-term average sea level has been significantly higher than today. In the first graphic time moves from the Cambrian ~ 542 Ma ago on the left to the relatively recent Neogene ending ~2.58 Ma ago. Ma means “million years according to ISO rules.
In this second graph covering the same period, the Cambrian is on the right and the Neogene is on the left. Otherwise the figures are equivalent.
Comparison of two sea level reconstructions during the last 500 Ma. The scale of change during the last glacial/interglacial transition is indicated with a black bar.
Various factors affect the volume or mass of the ocean and lead to long-term changes in eustatic sea level, the worldwide change of sea level elevation with time. The two primary influences are temperature because the density of water depends on its temperature, and the mass of water locked up on land and sea as fresh water in rivers, lakes, glaciers and polar ice caps.
Eustatic change occurs when the sea level changes due to an alteration in the volume of water in the oceans or, alternatively, a change in the shape of an ocean basin and hence a change in the amount of water the sea can hold. Eustatic change is always a global effect.
During and after an ice age, eustatic change takes place. At the beginning of an ice age, the temperature falls and water is frozen and stored in glaciers inland, suspending the hydrological cycle. Water is taken out of the sea but not being replaced leading to an overall fall in sea level.
As an ice age ends, the temperature begins to rise and so the water stored in the glaciers reenters the hydrological cycle, the sea is replenished, and sea levels rise.
Over long geological timescales, changes in the shape of oceanic basins and in land–sea distribution affect sea level. The shape of the ocean basins can change due to tectonic movement. If an ocean basin becomes larger, overall sea level will fall because there is no increase in the voume of ocean water. If the ocean basins become smaller, sea level rises accordingly. Orogeny (mountain building) and isostatic uplift are tectonic processes that change the sea level of an affected ocean basin
Evidence demonstrates that the last interglacial between about 135,000 and 115,000 years ago, maximum sea-level rise was about 6 meters (m) above the present sea level.
Since the Last Glacial Maximum about 20,000 years ago, sea level has risen by more than 125 m, with rates varying from tenths of a mm/yr to 10+mm/year.
During deglaciation between about 19,000 and 8,000 calendar years ago, sea level rose at extremely high rates as the result of the rapid melting of the British-Irish Sea, Fennoscandian, Laurentide, Barents-Kara, Patagonian, Innuitian ice sheets and parts of the Antarctic ice sheet. At the onset of deglaciation about 19,000 calendar years ago, a brief, at most 500-year long, glacio-eustatic event may have contributed as much as 10 m to sea level with an average rate of about 20 mm/yr. During the rest of the early Holocene, the rate of sea level rise varied from a low of about 6.0–9.9 mm/yr to as high as 30–60 mm/yr during brief periods of accelerated sea level rise.
Solid geological evidence, based largely upon analysis of deep cores of coral reefs, exists only for 3 major periods of accelerated sea level rise, called meltwater pulses, during the last deglaciation. They are Meltwater pulse 1A between circa 14,600 and 14,300 calendar years ago; Meltwater pulse 1B between circa 11,400 and 11,100 calendar years ago; and Meltwater pulse 1C between 8,200 and 7,600 calendar years ago. Meltwater pulse 1A was a 13.5 m rise over about 290 years centered at 14,200 calendar years ago and Meltwater pulse 1B was a 7.5 m rise over about 160 years centered at 11,000 years calendar years ago.
In sharp contrast, the period between 14,300 and 11,100 calendar years ago, which includes the Younger Dryas interval, was an interval of reduced sea level rise at about 6.0–9.9 mm/yr. Meltwater pulse 1C was centered at 8,000 calendar years and produced a rise of 6.5 m in less than 140 years. Such rapid rates of sea level rising during meltwater events clearly implicate major ice-loss events related to ice sheet collapse. The primary source may have been meltwater from the Antarctic ice sheet.
Other studies suggest a Northern Hemisphere source for the meltwater in the Laurentide ice sheet.
The Laurentide Ice Sheet was a massive sheet of ice that covered millions of square kilometers, including most of Canada and a large portion of the northern United States, multiple times during the Quaternary glacial epochs— from 2.588 ± 0.005 million years ago to the present.
The last advance covered most of northern North America between c. 95,000 and c. 20,000 years before the present day, and among other geomorphological effects, gouged out the five Great Lakes and the hosts of smaller lakes of the Canadian shield. These lakes extend from the eastern Northwest Territories, through most of northern Canada, and the upper Midwestern United States (Minnesota, Wisconsin, and Michigan) to the Finger Lakes, through Lake Champlain and Lake George areas of New York, across the northern Appalachians into and through all of New England and Nova Scotia.
At times, the ice sheet’s southern margin included the present-day sites of northeastern coastal towns and cities such as Portsmouth, New Hampshire, Boston, and New York City, and Great Lakes coastal cities and towns as far south as Chicago and St. Louis, Missouri, and then followed quite precisely the present course of the Missouri River up to the northern slopes of the Cypress Hills, beyond which it merged with the Cordilleran Ice Sheet. The ice coverage extended approximately as far south as 38 degrees latitude in the mid-continent.
Recently, it has become widely accepted that late Holocene, 3,000 calendar years ago to present, sea level was nearly stable prior to an acceleration in the rate of rise that is variously dated between 1850 and 1900 AD. Late Holocene rates of sea level rise have been estimated using evidence from archaeological sites and late Holocene tidal marsh sediments, combined with tide gauge and satellite records and geophysical modeling. This research included studies of Roman wells in Caesarea and of Roman piscinae (fish ponds) in Italy. These methods in combination suggest a mean eustatic component of 0.07 mm/yr for the last 2000 years.
Since 1880, however, the ocean has begun to rise briskly: a total of 210 mm (8.3 in) through 2009 causing extensive erosion worldwide.
A global experiment without a “control”
A truly global experiment is well underway.
We are all part of it and we have no choice but to participate. There is no “control” population on some other planet. The experiment will either establish that human industrial activity has a significant influence on global temperature or that it does not.
The boundary conditions for the experiment have been set.
On one side is the ever-increasing concentration of carbon dioxide in the atmosphere which is assumed to be the result of human activity and presumed to drive global temperature upwards and make global climate warmer. That is the position of the United Nation IPCC (Intergovernmental Panel on Climate Change) and is supported by numeric computer models.
On the other side is a large and growing group of natural scientists who work from data and observations who see the replication of long-term climate changes that occur in rather predictable cycles driven by natural processes, particularly solar changes. Among these somewhat regular cycles are the Millennial Cycle which includes the Roman Warm Event, Medieval Warm Event, and, perhaps, the current Modern Warm Event; the “Sunspot” (solar activity) Cycle (10–12 years), and the Gleissberg Cycle.
“The current prediction for Sunspot Cycle 24 gives a smoothed sunspot number maximum of about 69 in the late Summer of 2013. The smoothed sunspot number reached 68.9 in August 2013 so the official maximum will be at least this high. The smoothed sunspot number has been rising again towards this second peak over the last five months and has now surpassed the level of the first peak (66.9 in February 2012). Many cycles are double peaked but this is the first in which the second peak in sunspot number was larger than the first. We are currently over five years into Cycle 24. The current predicted and observed size makes this the smallest sunspot cycle since Cycle 14 which had a maximum of 64.2 in February of 1906.”
The solar cycle or solar magnetic activity cycle is the nearly periodic 11-year change in the Sun’s activity (including changes in the levels of solar radiation and ejection of solar material) and appearance (changes in the number and size of sunspots, flares, and other manifestations).
They have been observed (by changes in the sun’s appearance and by changes seen on Earth, such as auroras) for centuries.
The changes on the Sun cause effects in space, in the atmosphere, and on Earth’s surface. While it is the dominant variable in solar activity, aperiodic fluctuations also occur.
The Sunspot/solar activity Cycle has been amply substantiated, the others are apparent in observations and data. There is recent speculation that the Gleissberg Cycle is influenced or caused by long-term ocean circulation cycles rather than simple solar changes.
Solar Activity Cycles from 1749
The Gleissberg Cycle
Temperature on this Earth is a dynamic cyclical process!
The origins of the Controversy
In 1896, Svante Arrhenius, started the debate on the effect of atmospheric carbon dioxide concentration on global temperature with his original work on atmospheric gases, On the Influence of Carbonic Acid upon the Temperature of the Ground in which he stated that “if the quantity of carbonic acid [carbon dioxide] increases in geometric progression, the augmentation of the temperature will increase nearly in arithmetic progression.”
Arrhenius, who first identified carbon dioxide as a “greenhouse gas” in 1896, in the same paper also showed that carbon dioxide loses effectiveness logarithmically with increasing concentration so that the greenhouse effect attributable to the rise from 100 to 200 parts per million (ppm) is much greater than during the rise from 300 to 400 ppm. A part per million has been likened to a jigger of vermouth in a railroad car full of gin—the recipe for a very very dry martini.