17 The Climate History of Earth

Debasish Chakraborty

1. Learning Outcomes
2. Introduction to Paleoclimatology
3. The Geological Time Scale
4. Source of information for Paleoclimatologic research
5. Methods for Paleoclimatic dating
5.1. Radioisotopic methods
5.2. Paleomagnetic methods
5.3. Chemical methods
5.4. Biological methods
6. Causes of change in Climate
6.1. External forcings
6.2. Internal forcings
7. Climate History
7.1. Evolution of Earth’s Atmosphere
7.2. Climate of Precambrian Eon
7.3. Climate of Phanerozoic Eon
8. Summary

1. Learning Outcomes

The module will help its readers to:

• Get an overview of past climate of earth
• Know different methods for conducting these studies
• The causes associated with the changes in climate during its past
• Appreciate the difference between present & past changes in climate

2. Introduction

Climate, describing the normal state of atmosphere, is probably the most influencing factor dictating the ecological distribution over the earth. Understanding the dynamics & changes in it remains the key to unravel the changes in complex ecosystem. We are too enthusiastic to know about the future of our earth’s ecosystem & environment, especially at the present era of high concerns about climatic changes & variability. Modelling the climate remains the only option to the above concern. It needs enough scientific data to build a robust model. But instrumental records span only a tiny fraction of the Earth’s climatic history and so can only provide a totally inadequate period of data for building a model which can provide proper perspective on climatic variation and the evolution of climate today. Hence studying the past climate or its history can provide us with a continuous, longer and evidence data of climate which can serve our purpose.

The Science dealing with the earth’s climatic history over the entire time scale of earth is termed as Paleoclimatology. Its unique identity is derived due to its inclusion of many fields like Climatology, the study of the earth’s present climate system, geological sciences, stratigraphy (branch of geology concerned with the order and relative position of strata and their relationship to the geological timescale), geochemistry, glaciology, as well as the newer sub-disciplines like ice core paleo-atmospheric studies, endro-climatology (which uses tree rings) and sclera-climatology (which uses tropical corals). Apart from all these, organic remains and biological processes play a major role both in the reconstruction of past climates and in regulating earth climate.

3. The Geological Time Scale

Paleoclimatology is firmly rooted in the standard geologic timescale accepted by most geologists who study earth history. Table 1 lists the major subdivisions of the geological stratigraphic period. Most of  the time we discuss about the climates of the Cenozoic – the last 66 million years of earth history, especially the past 5 million years of Pliocene and Quaternary history.

4. Sources of information for Paleoclimatic research

Evidence of past climatic conditions is commonly preserved in natural archives—marine and lacustrine sediments, loess, ice, cave deposits (speleothems), sub-fossil biological material and in geomorphological features (glacial deposits, erosional features, paleosols and periglacial phenomena). These provide materials that are indirect indicators or proxies of past climatic conditions. By definition, such proxy records of climate contain a climatic signal, but that signal may be relatively weak, embedded in a great deal of extraneous “noise” arising from the effects of other (non-climatic) influences.

For extraction of the paleoclimatic signal from these proxy data, the record must first be  interpreted or calibrated. Calibration involves using modern climatic records and proxy materials to understand how and to what extent proxy materials are climate-dependent. It is assumed that the modern relationships observed have operated, unchanged, throughout the period of interest (the principle of uniformitarianism). All paleoclimatic research, therefore, must build on studies of climate dependency in natural phenomena today. Dendroclimatic studies, for example, have benefited from a wealth of research into climate-tree growth relationships which have enabled dendroclimatic models to be based on sound ecological principles. Significant advances have also been made in palynological research by improvements in our understanding of the relationships between modern climate and modern pollen grain. It is apparent, therefore, that an adequate modern database and an understanding of contemporary processes in the climate system and in the proxies are important prerequisites for reliable paleoclimatic reconstructions.

Major types of proxy climatic data available are listed in table 2. Each line of evidence differs according to its spatial coverage, the period to which it pertains and its ability to resolve events accurately in time. For example, ocean sediment cores are potentially available from 70% of the Earth’s surface and may provide continuous proxy records of climate spanning many millions of years. However, these records are often difficult to date accurately; commonly, there is a dating uncertainty of ±1% of a sample’s true age (the absolute magnitude of the uncertainty thus increasing with sample  age). However, tree rings from much of the continental land mass can be accurately dated to an individual year and may provide continuous records of more than a thousand years in duration. With a minimum sampling interval of one year, they provide primarily high-frequency (short-term) paleoclimatic information.

The value of proxy data to paleoclimatic reconstructions is very much dependent on the minimum sampling interval and dating resolution, since it is these that primarily determine the degree of detail available from each record. At the present time, annual and even seasonal resolution of climatic fluctuations in the timescale of 101-103 years is provided by ice-core, coral, varved sediments (annual layer of sediment or sedimentary rock), tree-ring studies and some speleothems. Detailed analyses of pollen in varves (an annual layer of sediment or sedimentary rock) may provide annual data, but it is likely that the pollen itself is an integrated measure of the pollen rain over a number of prior years. On the longer timescale (>106 years) loess and ocean cores provide the best records at present, though resolution probably decreases to plus or minus a few thousand years in the Early Quaternary. Historical records have the potential of providing annual (or intra-annual) data for up to a thousand years in some areas, but this potential has been realized only for the last few centuries in a few areas.

5. Methods for Paleoclimatic dating

Accurate dating is of fundamental importance to paleoclimatic research. Without reliable estimates on the age of events in the past, it is impossible to investigate if they occurred synchronously or if certain events led or lagged others; neither is it possible to assess accuratelythe rate at which past environmental changes occurred. The commonly used methods are given in table 3. Along with these methods, it is equally important that the assumptions and limitations of the dating procedure used are understood so that a realistic interpretation of the date obtained can be made.

Dating methods fall into four basic categories: (a) radioisotopic methods, which are based on the rate of atomic disintegration in a sample or its surrounding environment; (b) paleomagnetic methods, which rely on past reversals of the Earth’s magnetic field and their effects on a sample; (c) organic and inorganic chemical methods, which are based on time-dependent chemical changes in the sample and (d) biological methods which are based on the growth of an organism to date the substrate on which it is found. Depending on the time period of interest, different dating methods will be more suitable than others.

5.1. Radioisotopic methods

Several naturally occurring, unstable isotopes of certain elements are frequently used to determine the age of climate-related phenomena. The age of geological material can be computed from the ratio of parent to daughter isotopes using the radioisotopes’ half-life, the time it takes for half the original number of atoms to decay. Radioactive isotopic decay occurs when an isotope of an element undergoes spontaneous emission of either electromagnetic radiation or particles.

Radioactive isotopes produced through cosmogenic bombardment of earth’s atmosphere are one of the key dating tools in paleoclimatology. The three most important cosmogenic isotopes are those of the elements carbon (14C, radiocarbon), beryllium (10Be) and chlorine (36Cl). The14Ccarbon atoms form carbon monoxide, then CO2 molecules and eventually are taken up into the global carbon cycle through various pathways, including biological routes such as photosynthesis, precipitation by carbonate-secreting organisms, and incorporation into other organic forms. Biogenic material is often the source material for radiocarbon dating, which is the most widely used method to date climate events back to about 40 ka (Thousand years ago from present time or thousand years).Organisms take up 14C/12C in proportion to the ratio in the atmosphere; upon death, the 14C begins to decay to 14N and the ratio of14C/12C in the dated material is used to compute its age, relative to1950 A.D., using the half-life of 5730 years.

Uranium-series age dating involves one of the most important radioactive decay series used in paleoclimatology: the progressive decay of uranium to thorium to lead. This decay series results in the conversion of 238U to 206Pb (lead), which is useful overtime scales of 10 Ma to 4600 Ma (millions of years ago from present time). U-series dating, where the 234U is converted to 230Th, is a commonly used technique to date Quaternary corals younger than about 400 ka. Corals take up uranium from sea water when they build their skeletons. Mollusks, teeth, carbonates, and phosphates are also used for uranium-series dating. Reef-building hermatypic corals meet the criteria of U-dating more often than most other material and U-series–dated corals are used extensively in climate and sea-level research. The corals should retain100% of their original aragonitic skeleton; even a small percentage of calcite would suggest that the coral has been altered and the uranium series age might be a suspect.

5.2. Paleomagnetic methods

Variations in the Earth’s magnetic field, as recorded by magnetic particles in rocks and sediments, may be used as a means of stratigraphic correlation. Major reversals of the Earth’s magnetic field are now well known and have been independently dated in many localities throughout the world. Consequently, the record of these reversals in sediments can be used as time markers or chrono-stratigraphic horizons. In effect, the reversal is used to date the material by correlation with reversals dated independently elsewhere.

5.3. Chemical methods

There are two general categories of dating methods based on chemical changes. The first involves amino acid analysis of organic samples, generally used to assess the age of associated inorganic deposits. The method may also be used to estimate paleo-temperatures from organic samples of known age. The second category encompasses a number of methods, which assess the amount of weathering that an inorganic sample has experienced. They are primarily used to assess the relative age of freshly exposed rock surfaces in episodic deposits such as moraines or till sheets. One of the most widespread and well-tested methods is obsidian hydration dating, an example of the general group of methods that involve measurements of weathering rinds.

Another method involves the chemical “fingerprinting” of volcanic ashes that often blanket wide areas after a major eruption. Chemical analyses of tephra deposits have proved to be successful in identifying unique geochemical signatures in ashes of different ages. Where the age of a tephra (rock fragments and particles ejected by a volcanic eruption) layer has been independently determined, the ash may be used as a chronostratigraphic marker to date the associated deposits and to correlate events over wide areas.

5.4. Biological methods

Biological dating methods generally use the size of an individual species of plant as an index of the age of substrate on which it is growing. They may be used to provide minimum-age estimates only, since there is inevitably a delay between the time a substrateis exposed and the time it is colonized by plants, particularly if the surface is unstable (as for example, in an ice-cored moraine).When the objective is simply establishing a relative age this method can be used as the delay is generally short and not significant. Lichenometry, study of lichens and dendrochronology, the study of annual growth rings in timber and tree trunks are widely used in these cases.

6. Causes of change in Climate

The processes that cause climate to change are both external and internal to the earth (figure 1). The term forcing is often used to refer to these processes. Climate forcing mechanisms external to the earth include solar radiation, changes in earth’s orbit and other extraterrestrial phenomena; those internal to the earth include volcanic activity, ocean-atmosphere coupling and atmospheric greenhouse gas concentrations among others.

Many forcing mechanisms act simultaneously upon earth’s climate systems and hence it is very challenging to separate the effects of one factor from the others. Solar energy is the predominant source of energy. It drives much of earth’s climate system through its influence on atmospheric circulation, evaporation and precipitation. But the other processes listed in figure 1 modify the effects of solar energy. Paleoclimatologists often try to isolate the dominant cause of reconstructed climate  change by looking for characteristic climate patterns that should, in theory, characterize climate change caused by a particular mechanism.

6.1. External Forcings

Until recently, many scientists thought that the Sun’s output of radiation only varied by a fraction of a percent over many years. However, measurements made by satellites equipped with radiometers in the 1980s and 1990s suggested that the sun’s energy output may be more variable than was once thought. Scientists have long tried to also link sunspots to climatic change. Sunspots are huge magnetic storms that are seen as dark (cooler) areas on the Sun’s surface. The number and size of sunspots show cyclical patterns, reaching a maximum about every 11, 90 and 180 years.

Computation of the orbital solution of the Earth is complex because the Earth’s motion is perturbed by Moon and all the other planets of the Solar system. Milankovitch established the relationship of earth’s orbital characteristics variation with its climate on the basis of mathematical computations. He calculated the amount of solar radiation for each latitude for different times of the year and came up with the idea that the summer insolation not the winter insolation was the main driving factor for glaciations. Orbital characteristics such as eccentricity, axial tilt and precession are the main variables. Eccentricity, the phenomenon controlling shape of the Earth’s orbit around the Sun changes over a time. The orbit gradually changes from being elliptical to being nearly circular and then back to elliptical in a period of about 100,000 years. Axial tilt or obliquity is the inclination of earth relative to its plane of travel about the sun. And, this is the main reason of seasons to occur in our Earth. The earth wobbles in space so that its tilt changes between about 22 and 24.5 degrees during the last 15Ma with a cyclic period of about 40,000 years. The current value of this ‘Obliquity’ is about 23.440 and it is decreasing. Precession originates from the fact that as the Earth rotates on its polar axis it wobbles like a spinning top changing the orbital timing of the equinoxes and solstices. This effect is also known as the precession of equinox. The precession of the equinox has a cycle of approximately 23,000 years.

6.2. Internal Forcings

Apart from external factors there are several internal factors which may have contributed in the changes of the earth’s climate. Volcanic activity is an important natural cause of climate variations because tracer constituents of volcanic origin impact the atmospheric chemical compositions and optical properties. Several other factors like plate movements of the earth which governs mountain building & ocean-volume, changes in ice sheets and sea ice along with its feedbacks such as albedo alteration and ocean-atmosphere feedbacks also play important role. Biosphere-atmosphere gas exchanges and several anthropogenic impacts, recently in the form of increased concentration of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), halocarbons, tropospheric nitrogen oxides, carbon monoxide (CO), sulfate aerosols are proved to be significantly contributing to the changes now a days.

7. Climate History

7.1. Evolution of Earth’s Atmosphere

Earth has been ever evolving & dynamic. Same is true to its atmosphere or its normal state i.e. climate. The atmosphere of our earth which we presently see has evolved over billions of years mainly through three era or stages;

Primitive or first atmosphere

Earth is believed to have formed about 5 billion years ago. In the first 500 million years a dense atmosphere emerged from the vapour and gases that were expelled during degassing of the planet’s interior. These gases may have consisted of hydrogen (H2), water vapour, methane (CH4) and carbon oxides. Prior to 3.5 billion years ago the atmosphere probably consisted of carbon dioxide (CO2), carbon monoxide (CO), water (H2O), nitrogen (N2) and hydrogen.

Reducing or Second Atmosphere

The hydrosphere was formed 4 billion years ago from the condensation of water vapour, resulting in oceans of water in which sedimentation occurred.The most important feature of the ancient environment was the absence of free oxygen. Evidence of such an anaerobic reducing atmosphere is  hidden in early rock formations that contain many elements, such as iron and uranium, in their reduced states. Elements in this state are not found in the rocks of mid-Precambrian and younger ages, less than 3 billion years old.

Oxidizing or Present Day Atmosphere

About 2.5 billion years ago, early aquatic organisms called blue-green algae began using energy from the Sun to split molecules of H2O and CO2 and recombine them into organic compounds and molecular oxygen (O2). This solar energy conversion process is known as photosynthesis. Then oxygen started accumulating in the atmosphere. High in the atmosphere, some oxygen (O2) molecules absorbed energy from the Sun’s ultraviolet (UV) rays and split to form single oxygen atoms. These atoms combining with remaining oxygen (O2) to form ozone (O3) molecules, which are very effective at absorbing UV rays. The thin layer of ozone that surrounds Earth acts as a shield, protecting the planet from irradiation by UV light. Prior to this period, life was restricted to the ocean. The presence of ozone enabled organisms to develop and live on the land. Ozone played a significant role in the evolution of life on Earth, and allows life as we presently know it to exist. With the emergence of higher life forms on land, plants added to the atmospheric O2 while animals used it up for breathing and the O2 level stabilized around the 21%, as we know today.

Scientific evidences indicate that through the past about 2 billion years earth’s climate has altered from a cool or ice house to warm or hot house or greenhouse condition (figure 2). During warm or greenhouse conditions, there is little, if any, permanent ice on either pole. Warm temperate climates are found at high latitudes. Whereas, in cool or icehouse times, global climate is cool enough to support large ice sheets at one or both poles. Earth’s climate has transitioned between these two categories only a few times in the past. The most recent transition occurred during the Cenozoic Era.

7.1. Climate of Precambrian Eon

The Precambrian time accounts for more than 80% of Earth’s history and is called the “Age of Early Life”. Climate varied widely during different periods of the Precambrian. Shallow seas—like today’s Caribbean—covered some of the early continents. During warm climates, mats, or mounds of algae called stromatolites grew in those shallow seas. Shark Bay, Australia is famous for its modern stromatolites. Billion-year-old stromatolites are found in Glacier National Park (Montana).These times had at least two and possibly more major glaciations. The more recent of these ice ages, encompassing the Marinoan &Varangian glacial maxima (about 560 to 650 million years ago), has been proposed as a snowball Earth event with continuous sea ice reaching nearly to the equator. This is significantly more severe than the ice age during the Phanerozoic. Because this ice age terminated only slightly before the rapid diversification of life during the Cambrian explosion, it has been proposed that this ice age (or at least its end) created conditions favourable to evolution.

7.2. Climate of Phanerozoic Eon

This eon, covering the last 542 million years encompasses almost the entire time of origination of complex multi-cellular life. These has more generally been a period of fluctuating temperature between ice ages, such as the current age (Holocene epoch) and “climate optima”, similar to what occurred in the Cretaceous. In between these cold periods, warmer conditions were present and often referred to as climate optima. However, it has been difficult to determine whether these warmer intervals were actually hotter or colder than occurred during the Cretaceous optima.

7.2.1. Climate of Paleozoic Era

The Paleozoic era was relatively cool during its many periods. It is also called the “Age of Fishes or Amphibians”. In these times, North America was located near the equator and experienced generally warm climates. Shallow seas advanced and retreated over vast areas of North America, depositing immense amounts of limestone and other marine sediments. These sediments contain a fantastic fossil record of evolving sea life from ubiquitous trilobites to vertebrates. Vertebrates first evolved in the oceans. Land plants and animals first appeared about 350 million years ago.

7.2.2. Climate of Mesozoic Era

Mesozoic Era climates were much warmer than today. It is called the “Age of Reptiles”. These were the times when dinosaurs lived on the earth. Carbon dioxide was likely many times higher than today contributing to a “greenhouse” planet. During the early part of the Mesozoic, all of Earth’s continents were assembled into a supercontinent called Pangaea. Pangaea began to break apart during the Mesozoic and the continents began moving towards their present locations. Lush forests, including newly-evolved flowering plants (angiosperms) blanketed much of North America. During the later portion of the Mesozoic era i.e. Cretaceous period, from 66 to 144 million years ago, average global temperatures reached its highest level during the last ~200 million years. This is likely to be the result of a favorable configuration of the continents during this period that allowed for improved circulation in the oceans and discouraged the formation of large scale ice sheet.

7.2.3. Climate of Cenozoic Era

This Era, also called as the “Age of Mammals” was a time of climatic transition. The early Cenozoic Era was a time of “greenhouse” climates like those which dinosaurs experienced. By 34 million years ago, permanent ice sheets were present at the South Pole ushering in “icehouse” conditions. Climate started warming during the Miocene as mammal populations reached their greatest diversity. Climate then cooled again. In North America, the lush “greenhouse” fossil forests were replaced by open grasslands. Grassland ecosystems are better suited for a cooler, drier “icehouse” climate. By 2 million years ago, Earth’s climate was cold enough to support large ice sheets on both poles.

Eocene period has seen a series of abrupt thermal spikes, lasting for a few hundred thousand years. The most pronounced of these, the Paleocene-Eocene Thermal Maximum (PETM). These are usually interpreted as caused by abrupt releases of methane from clathrates (frozen methane ices that accumulate at the bottom of the ocean). During these periods, the global mean temperature was as high as 23°C, in contrast to the global average temperature of today at just under 15°C. Geologists and paleontologists think that during much of the Paleocene and early Eocene, the poles were free of ice caps and palm trees and crocodiles lived above the Arctic Circle. Last 2 million years have also seen glacial and interglacial cycles within a gradually deepening ice age. Currently, the Earth is in an interglacial period, beginning about 20,000 years ago (20 ka). Such cycles ranged like 21 ka, 41 ka and 100 ka years. These were mainly governed by predictable changes in the Earth orbit known as Milankovitch cycles.

8. Summary

• Weather is the instantaneous physical condition or state of the atmosphere at a given time and place and its short-term variation in minutes to weeks.
•  Climate is the statistical description of weather pattern over a region in terms of its mean and variability over a period of 30 yrs or more.
• Climate is what you expect, like a very hot summer, and weather is what you get, like a hot day with pop-up thunderstorms or a cool evening due to sudden summer rains.
•   Atmospheric state variables or elements generally used to describe weather and climate over a  place are temperature, humidity, precipitation, visibility, atmospheric pressure and wind. These atmospheric variables vary from once place to another, with the height and with time.
• The climate of a place is determined by influence of several natural factors called climate controls. 19 Environmental Sciences Atmospheric Processes Weather and Climate
• The climate is also influence by the human activities. However, it is not very clear how much of the climate variability is caused by the natural factors and how much is caused by the human activities.
• Weather prediction is an “initial value problem” or dependent on the three dimensional real time observed values of atmospheric parameters such as temperature, wind, humidity, pressure etc. that are specified in the computer model to initiate the prediction. The more accurate the estimate of the initial conditions, the better the quality of the forecasts.
• Climate prediction is a “boundary value problem’ or depends on several boundary conditions,· both natural and that influenced by human