15 Global Climate Change
At the end of this chapter, students should be able to:
- Accurately describe which aspects of the environment are changing due to anthropogenic climate change.
- Describe the role of greenhouse gases in climate change.
- Describe the sources of greenhouse gases.
- Describe the causes of recent climate change, particularly the role of humans in the overall climate balance.
- Explain how we know about climates in the geologic past.
- Explain Earth’s energy budget and global temperature changes.
- Explain how positive and negative feedback mechanisms can influence climate.
This chapter describes the systems involved in regulating Earth’s temperature its temperature, geologic evidence of past climate changes, and the role humans have on today’s climate change. It is critically important to be aware of the geologic context of climate change processes if we want to understand anthropogenic (human-caused) climate change. First, this awareness helps understanding of how and why our activities are causing present-day climate change, and second, it allows us to distinguish between natural and anthropogenic processes in the climate record in the past.
In science, a system is a group of objects and processes that interact, such as the rock cycle. Earth System Science is the study of how earth systems (geosphere, atmosphere, hydrosphere, cryosphere, and biosphere) interact and change in response to natural cycles and new human- driven forces. Changes in one earth system affects another system. A significant part of this chapter introduces various processes from different earth systems and discusses how they influence each other and impact global climate. For example, global temperature largely changes based the composition of the air (atmosphere), circulation of the ocean (hydrosphere), and characteristics of the land surface (geosphere, cryosphere, and biosphere).
In order to understand climate change, it is important to distinguish between climate and weather. Weather is the temperature and precipitation patterns occurring in the short-term such as right now or later this week. Climate is the temperature and precipitation patterns and range of variability averaged over the long-term for a particular region (see chapter 13.1). Thus, a single cold winter does not mean that the entire globe is cooling—indeed, the cold winters in the US of 2013 and 2014 took place while the rest of the Earth was experiencing record warm winter temperatures. To avoid these generalizations, many scientists use a 30-year average as a good baseline . Therefore, climate change refers to slow changes in temperature and precipitation patterns over the long-term for a particular area.
15.1 Earth’s Temperature
Because the Moon doesn’t have much of an atmosphere, daytime temperatures on the moon are around 224℉ and nighttime temperatures are around -298℉. That is an astonishing 522 degrees of change between the light-side and dark-side of the Moon . This section describes how Earth’s atmosphere is involved in regulating the planet’s temperature.
15.1.1 Earth’s Energy Budget
Solar radiation from the Sun is relatively uniform. Energy (or heat) moves from the Earth’s surface and lower atmosphere back to space. This flow of incoming and outgoing energy is Earth’s energy budget. For Earth’s temperature to be stable over long stretches of time, incoming energy and outgoing energy have to be equal so that the energy budget at the top of the atmosphere balances. About 29 percent of the incoming solar energy arriving at the top of the atmosphere is reflected back to space by clouds, atmospheric particles, or reflective ground surfaces like sea ice and snow. About 23 percent of incoming solar energy is absorbed in the atmosphere by water vapor, dust, and ozone. The remaining 48 percent passes through the atmosphere and is absorbed by the surface. Thus, about 71 percent of the total incoming solar energy is absorbed by the Earth system .
When this energy reaches Earth, the atoms and molecules making up the atmosphere and surface absorb the energy and they increase in temperature. If this material could only absorb energy, then the temperature of the Earth would be like the water level in a sink with no drain where the faucet runs continuously. The sink would eventually overflow. However, temperature does not infinitely rise because the Earth is not just absorbing sunlight. The Earth is also radiating thermal energy (heat) back into the atmosphere. If the temperature of the Earth rises, the planet emits an increasing amount of heat to space and this is the primary mechanism that prevents Earth from continually heating .
Greenhouse gases act like a giant blanket for Earth. The more greenhouse gases in the atmosphere, then the more outgoing heat will be retained by Earth before this thermal infrared energy (heat) dissipates to space. The greenhouse effect is discussed in more detail in the next section.
Factors that can affect the Earth’s energy budget are not limited to greenhouse gases. Increases in solar irradiance (more solar energy) can increase the energy received by earth. However, increases associated with this are very small . In addition, less ice and snow covering land and the Arctic Sea increases the amount of sunlight absorbed by land and water (see animation below). The reflectivity of the Earth’s surface is called albedo. Furthermore, aerosols (dust particles) produced from burning coal, diesel engines, and volcanic eruptions can reflect more incoming solar radiation and actually cool the planet. The effect of anthropogenic aerosols is weak on the climate system. Thus, the net effect is warming due to more anthropogenic greenhouse gases associated with fossil fuel combustion .
An effect that changes the planet can trigger feedback mechanisms that amplify or suppress the original effect. A positive feedback mechanism is when the output or effect enhances the original stimulus or cause. Thus, it increases the effect later. For example, the loss of sea ice at the North Pole makes that area less reflective (reduced albedo). This allows the surface air and ocean to absorb more energy in an area that was once covered by sea ice . Another example is melting permafrost. Permafrost is permanently frozen soil located near the high latitudes, mostly in the Northern Hemisphere. As the climate warms, more permafrost thaws and the thick deposits of organic matter are exposed to oxygen and begin to oxidize (or decay). This oxidation process releases carbon dioxide and methane which in turn cause more warming which melts more permafrost, etc. A negative feedback mechanism is when the output or effect reduces the original stimulus or cause . For example, in the short term, more carbon dioxide (CO2) is expected to cause forest canopies to grow and absorb more CO2. An example for the long term, is increased carbon dioxide (CO2) in the atmosphere is expected to cause more chemical weathering should occur transporting ions and CO2 to the oceans and stored in sediment.
15.1.2 Composition of Atmosphere
The composition of the atmosphere is a key component that regulates the planet’s temperature. The atmosphere is 78% nitrogen (N2), 21% oxygen (O2), 1% argon (Ar), and less than 1% for all other gases known as trace components. The trace components include carbon dioxide (CO2) water vapor (H2O), neon, helium, and methane. Water vapor is highly variable, mostly based on region, but has been estimated to be about 1% of the atmosphere . The trace gases include several important greenhouse gases, which are the gases responsible for warming and cooling the plant. On a geologic scale, the source of atmospheric CO2 is volcanoes and the sink is the weathering process that bury CO2 in sediments. Biological processes both add and subtract CO2 from the atmosphere .
Greenhouse gases trap heat in the atmosphere and warm the planet. They have little effect on incoming solar radiation (which is shortwave radiation) but absorb some of the outgoing infrared radiation (longwave radiation) that is emitted from Earth, thus keeping it from being lost to space. More greenhouse gases in the atmosphere makes the planet warmer.
The most common greenhouse gases are water vapor (H2O), carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). Water vapor is the most abundant greenhouse gas but its abundance in the atmosphere does not change much over time. Carbon dioxide is much less abundant than water vapor, but carbon dioxide is being added to the atmosphere by human activities such as burning fossil fuels, land use changes, and deforestation. Further, natural processes such as volcanic eruptions add carbon dioxide , but at an insignificant rate compared to anthropogenic contributions.
However, there are two important reasons why carbon dioxide is the most important greenhouse gas. First, carbon dioxide has a long residence time in the atmosphere (meaning that it does not go away for hundreds of years). Second, most of the additional carbon dioxide is “fossil” in origin. That means that it is released by burning fossil fuels. For example, coal is a fossil fuel. Coal is made from plant material created by photosynthesis millions of years ago and stored in the ground. Photosynthesis takes sunlight plus carbon dioxide and creates the carbohydrates of plants. This occurs over millions of years, as a slow process accumulating fossil carbon in rocks and sediments. When we burn coal, we instantaneously release the stored solar energy and the fossil carbon dioxide that took million of years to accumulate in the first place.
15.1.3 Carbon Cycle
Earth has two important carbon cycles. One is the biological one, wherein living organisms — mostly plants — consume carbon dioxide from the atmosphere to make their tissues through photosynthesis, and then, after they die, that carbon is released back into the atmosphere when they decay over several years or decades . The following is the general equation for photosynthesis.
CO2 + H2O + sunlight → sugar + O2
A small proportion of this biological-cycle carbon becomes buried in sedimentary rocks: during the slow formation of coal, as tiny fragments and molecules in organic-rich shale, and as the shells and other parts of marine organisms in limestone. This then becomes part of the geological carbon cycle, a cycle that actually involves a majority of Earth’s carbon, but one that operates only very slowly .
The following is a list of storage reservoirs for the geological carbon cycle.
- Organic matter from plants is stored in peat, coal, and permafrost for thousands to millions of years.
- Weathering of silicate minerals converts atmospheric carbon dioxide to dissolved bicarbonate, which is stored in the oceans for thousands to tens of thousands of years.
- Dissolved carbon is converted by marine organisms to calcite, which is stored in carbonate rocks for tens to hundreds of millions of years.
- Carbon compounds are stored in sediments for tens to hundreds of millions of years; some end up in petroleum deposits.
- Carbon-bearing sediments are transferred to the mantle, where the carbon may be stored for tens of millions to billions of years.
- During volcanic eruptions, carbon dioxide is released back to the atmosphere, where it is stored for years to decades .
During much of Earth’s history, the geological carbon cycle has been balanced, with carbon being released by volcanism at approximately the same rate that it is stored by the other processes. Under these conditions, the climate remains relatively stable. During some times of Earth’s history, that balance has been upset. This can happen during prolonged stretches of greater than average volcanism. One example is the eruption of the Siberian Traps at around 250 million years ago, which appears to have led to strong climate warming over a few million years. A carbon imbalance is also associated with significant mountain-building events. For example, the Himalayan Range was formed between about 40 and 10 Ma and over that time — and still today — the rate of weathering on Earth has been enhanced because those mountains are so high and the range is so extensive. The weathering of these rocks — most importantly the hydrolysis of feldspar — has resulted in consumption of atmospheric carbon dioxide and transfer of the carbon to the oceans and to ocean-floor carbonate minerals. The steady drop in carbon dioxide levels over the past 40 million years, which led to the Pleistocene glaciations, is partly attributable to the formation of the Himalayan Range. Another, non-geological form of carbon-cycle imbalance is happening today on a very rapid time scale. We are in the process of extracting vast volumes of fossil fuels (coal, oil, and gas) that was stored in rocks over the past several hundred million years, and converting these fuels to energy and carbon dioxide. By doing so, we are changing the climate faster than has ever happened in the past .
15.1.4 Greenhouse Effect
The greenhouse effect is a natural process by which the atmosphere warms surface temperatures. Without an atmosphere, Earth would have huge fluctuations of temperature between day and night like the moon. Daytime temperatures would be hundreds of degrees Fahrenheit above normal and nighttime temperatures would be hundreds of degrees below normal. The greenhouse effect occurs because of the presence of greenhouse gases in the atmosphere.
The greenhouse effect is named after the same process that warms a greenhouse or a car on a hot summer day. Sunlight can pass through the glass of the greenhouse or car, reaches the surface of the interior, and changes into heat. The heat radiates upward and gets trapped by the glass windows. The greenhouse effect can be explained in three steps.
Step 1: Solar radiation from the sun is composed of mostly ultraviolet (UV), visible light, and infrared (IR) radiation. Components of solar radiation include parts with a shorter wavelength than visible light, like ultraviolet light, and parts of the spectrum with longer wavelengths, like IR and others. Some of the radiation gets absorbed, scattered, or reflected by the atmospheric gases but about half of the solar radiation eventually reaches the Earth’s surface.
Step 2: The visible, UV, and IR radiation, that reaches the surface converts to heat energy. Most students have experienced sunlight warming a surface such as a paved surface, a patio, or deck. When this occurs, the warmer surface thus emits more thermal radiation, which is a type of IR radiation. So, there is a conversion from visible, UV, and IR to just thermal IR. This thermal IR is what we experience as heat. If you have ever felt heat radiating from a fire or a hot stove top, then you have experienced thermal IR.
Step 3: Thermal IR radiates from the earth’s surface back up into the atmosphere. But since it is thermal IR instead of UV, visible, or regular IR, this thermal IR gets trapped by greenhouse gases. In other words, the sun’s energy leaves the Earth at a different wavelength than it enters, so, the sun’s energy is not absorbed in the lower atmosphere when energy coming in, but rather when the energy is going out. The gases that typically do this blocking on Earth include carbon dioxide, water vapor, methane, and nitrous oxide. More greenhouse gases in the atmosphere results in more thermal IR being trapped. Explore this external link to an interactive animation on the greenhouse effect from the National Academy of Sciences.
15.2 Evidence of Recent Climate Change
While climate has changed many times in the past (see chapter 14.5.1 and chapter 15.3), the scientific consensus is that human activity is causing climate to change today . While this seems like a new idea, it has been suggested for more than 75 years . This section describes the evidence that scientists agree is most likely a result of anthropogenic climate change, or, human-caused climate change. For more information, watch this six-minute video on climate change by two professors at a North Carolina State University.
15.2.1 Global Temperature Rise
Since 1880, average global surface temperatures have trended upward and most of that warming has occurred since 1970 (see this NASA animation). Since the ocean is absorbing a lot of the additional trapped heat, surface temperatures include both land surface and ocean temperatures . Changes from land surface or ocean surface temperatures as called temperature anomalies. A temperature anomaly is how different an average temperature is from a predetermined datum. This datum is often the average temperature of the last century (1900-2000). Therefore, an anomaly of 1.25 ℃ for 2015 means that average temperature for 2015 was 1.25 ℃ greater than the 1900-2000 average. In 1950, the temperature anomaly was -0.28 ℃, so this is -0.28 ℃ lower than the 1900-2000 average . These temperatures are annual average surface temperatures.
In addition to a rising average surface land temperature, the ocean has absorbed a lot of the heat. With oceans covering about 70% of the earth’s surface, there is a lot of opportunity to absorb energy. The ocean has been absorbing about 80% to 90% of the additional heat added due to human activities. As a result, the top 2,300 feet of the ocean has increased in temperature 0.3℉ since 1969 (external link to this 3 minute video by NASA JPL on heat capacity of the ocean) . The reason the ocean has warmed less than the atmosphere, while still taking on most of the heat, is due to water having a very high specific heat, which means that water can absorb a lot of energy for a small temperature increase. In contrast, the atmosphere needs less energy to increase its temperature.
Some scientists suggest that anthropogenic greenhouse gases do not cause global warming since surface temperatures have not increased very much between 1998 and 2013, while greenhouse gas concentrations have continued to increase during that time period. However, since the oceans are absorbing most of the heat, decade-scale circulation changes (similar to La Niña) in the ocean push warmer water deeper under the surface . Once the absorption and circulation of the ocean is accounted for and the heat added back into surface temperatures, then the temperature increases become apparent (Figure). Furthermore, this ocean heat storage is temporary, as reflected in the record-breaking warm years of 2014-2016. Indeed, with this temporary ocean storage effect, 15 of the first 16 years of the 21st century have been the hottest in recorded history.
15.2.2 Carbon Dioxide
Anthropogenic greenhouse gases, mostly carbon dioxide (CO2), have increased since the industrial revolution when the burning of fossil fuels dramatically increased. These levels are unprecedented in the last 800,000 year earth history as recorded in geologic sources such as ice cores. Carbon dioxide has increased by 40% since 1750 and the rate (or speed) of increase has been the fastest during the last decade . For example, since 1750, 2040 gigatons of CO2 have been added to the atmosphere, about 40% has remained in the atmosphere while the remaining 60% has been absorbed into the land (by plants and soil) or the oceans . Indeed, during the lifetime of most young adults, the total atmosphere has increased by 50 ppm, or 15%.
15.2.3 Melting Glaciers and Shrinking Sea Ice
Glaciers are ice on top of land. Alpine glaciers, ice sheets, and sea ice are all melting. Explore melting glaciers at NASA’s interactive Global Ice Viewer). Satellites have recorded that Antarctica is melting at 118 gigatons per year and Greenland is melting at 281 gigatons per year (1 gigaton is over 2 trillion pounds). Almost all major alpine glaciers are shrinking, deflating, and retreating and the rate of ice mass loss is unprecedented (not observed before) since the 1940’s when quality records for most began. Before anthropogenic warming, glacial activity was variable with some retreating and some advancing . The extent of spring snow cover has decreased. In addition, the extent of sea ice is shrinking. Sea ice is ice floating in the ocean (not on land like a glacier). Most sea ice is at the North Pole which is only occupied by the Arctic Ocean and sea ice . Below, the NOAA animation shows how perennial sea ice has declined from 1987 to 2015. The oldest ice is white and the youngest (seasonal) ice is dark blue. The amount of old ice has declined from 20% in 1985 to 3% in 2015.
15.2.4 Rising Sea-Level
Sea-level is rising 3.4 millimeters (0.13 inches) per year and has risen 0.19 meters (7.4 inches) from 1901 to 2010. This is thought largely to be from both melting of glaciers and thermal expansion. Thermal expansion means that as objects such as solids, liquids, and gases heat up, they expand in volume. Since 1970, the melting of glaciers and thermal expansion account for 75% of the sea-level rise .
Classic video demonstration (30 second) on thermal expansion with brass ball and ring (North Carolina School of Science and Mathematics).
15.2.5 Ocean Acidification
Since 1750, about 40% of the new anthropogenic carbon dioxide has remained in the atmosphere. The remaining 60% gets absorbed by the ocean and vegetation. Therefore, the ocean has absorbed about 30% of new anthropogenic carbon dioxide. When carbon dioxide gets absorbed in the ocean, it creates carbonic acid which makes the ocean more acidic which has an impact on marine organisms that secrete calcium carbonate shells. Recall that hydrochloric acid reacts with limestone rock made of calcite, which is calcium carbonate. Ocean acidification associated with climate change has been linked to the thinning of the carbonate walls of some sea snails (pteropods) and small protozoan zooplankton (foraminifera) and declining growth rates of corals . Small animals like protozoan zooplankton are an important component in the marine ecosystem. Acidification combined with warmer temperature and lower oxygen levels is expected to have severe impact on marine ecosystems and human-used fisheries. Possibly affecting our ocean-derived food sources .
15.2.6 Extreme Weather Events
Occurrence and intensity of extreme weather events such as hurricanes, precipitation, and heatwaves have increased . Since the 1980’s, hurricane frequency, intensity, and duration have increased and connections to a warmer climate are likely. Since 1910, average precipitation has increased by 10% in the contiguous United States, and much of this increase is associated with heavy precipitation events like storms . However, the distribution is not even and more precipitation is projected for the northern United States while less precipitation is projected for the already dry southwest . Further, heatwaves have increased and rising temperatures are already affecting crop yields in northern latitudes . Increased heat allows for greater moisture capacity in the atmosphere, increasing the potential for more extreme events .
15.3 Prehistoric Climate Change
Over Earth history, the climate has changed a lot. For example, during the Mesozoic Era, the Age of Dinosaurs, the climate was much warmer and carbon dioxide was abundant in the atmosphere. However, throughout the Cenozoic Era (65 Million years ago to today), the climate has been gradually cooling. This section summarizes some of these major past climate changes.
15.3.1 Past Glaciations
Through geologic history, climate has changed slowly over millions of years. Before to the most recent Pliocene-Quaternary glaciation, there were three other major glaciations . The oldest, known as the Huronian, occurred toward the end Archean-early Proterozoic (~2.5 billion years ago). The major event of that time, the great oxygenation event (Chapter 8), is most commonly associated with the cause of that glaciation. The increased oxygen is thought to have reacted with the potent greenhouse gas methane, causing cooling .
The end of the Proterozoic (about 700 million years ago) had more glaciation, known as the Snowball Earth hypothesis . Glacial evidence has been interpreted in widespread rock sequences globally, and even has been linked to low-latitude glaciation . Limestone rock (usually formed tropical marine environments) and glacial deposits (usually formed in cold climates) are often found together from this time in regions all around the world. In Utah, Antelope Island in the Great Salt Lake has interbedded limestone and glacial deposits (diamictites) interpreted to be formed by continental glaciation . The idea of the controversial Snowball Earth hypothesis is that a runaway albedo effect (ice and snow reflecting solar radiation) might cause a complete freezing of land and ocean surfaces and a collapse of biological activity. The ice covered earth would only melt when carbon dioxide from volcanoes reached high concentrations, due to the inability for carbon dioxide to enter the now-frozen ocean. Some studies estimated carbon dioxide was 350 times higher than today’s concentrations . The complete freezing and extent of the freezing has come into question.
Glaciation also occurred in the Paleozoic, most notably with the Karoo Glaciation of the Pennsylvanian (323 to 300 million years ago). This also was caused by an increase of oxygen and a subsequent drop in carbon dioxide, most likely caused by the evolution and rise of land plants .
During the Cenozoic Era (the last 65 million years), climate started out warm and gradually cooled to today. This warm time is called the Paleocene-Eocene Thermal Maximum and Antarctica and Greenland were ice free during this time. Since the Eocene, tectonic events during the Cenozoic caused persistent and significant planetary cooling. For example, the collision of the Indian Plate with the Asian Plate created the Himalaya Mountains increasing the weathering and erosion rates. This increased rate of weathering of silicate minerals, especially feldspar, consumes carbon dioxide from the atmosphere and therefore reduces the greenhouse effect, resulting in long-term cooling .
At about 40 Ma, the narrow gap between the South American Plate and the Antarctica Plate widened, resulting in the opening of the Drake Passage. This allowed for the unrestricted west-to-east flow of water around Antarctica, the Antarctic Circumpolar Current, which effectively isolated the southern ocean from the warmer waters of the Pacific, Atlantic, and Indian Oceans. The region cooled significantly, and by 35 million year ago (Oligocene) glaciers had started to form on Antarctica .
At around 15 Ma, subduction-related volcanism between Central and South America created the Isthmus of Panama that connected North and South America. This prevented water from flowing between the Pacific and Atlantic Oceans and reduced heat transfer from the tropics to the poles. This created a cooler Antarctica and larger Antarctic glaciers. The expansion of that ice sheet (on land and water) increased Earth’s reflectivity (albedo), a positive feedback loop of further cooling: more reflective glacial ice, more cooling, more ice, and so on .
By 5 million years ago (Pliocene Epoch), ice sheets had started to grow in North America and northern Europe. The most intense part of the current glaciation is the last 1 million years of the Pleistocene Epoch. The Pleistocene has significant temperature variations (through a range of almost 10°C) on time scales of 40,000 to 100,000 years, and corresponding expansion and contraction of ice sheets. These variations are attributed to subtle changes in Earth’s orbital parameters called Milankovitch cycles , which are explained in more detail in the chapter on glaciers. Over the past million years, the glaciation cycles have been approximately every 100,000 years with many glacial advances in the last 2 million years
Warmer portions of climate within an ice age are called interglacials, with brief versions called interstadials. These warming upticks are related to variations in Earth’s climate like Milankovitch cycles. In the last 500,000 years, there have been 5 or 6 interglacials, with the most recent belonging to our current time, the Holocene.
Two of the more recent climate swings demonstrate the complexity of the changes: the Younger Dryas and the Holocene Climatic Optimum. These events are more recent, and yet have conflicting information. The Younger Dryas cooling are widely recognized in the Northern Hemisphere , though the timing of the event (about 12,000 years ago) does not appear to be equal everywhere . It also is difficult to find in the Southern Hemisphere . The Holocene Climatic Optimum, a warming around 6,000 years ago , though it was not universally warmer, and probably not as warm as current warming , and not at the same time everywhere .
15.3.2 Proxy Indicators of Past Climates
How do we know about past climates? Geologists use proxy indicators to understand past climate. A proxy indicator is a biological, chemical, or physical signature preserved in the rock, sediment, or ice record that acts like a “fingerprint” of something in the past . Thus they are an indirect indicator of something like climate. For ancient glaciations from the Proterozoic and Paleozoic, there are rock formations of glacial sediments such as the diamictite (or tillite) of the Mineral Fork Formation in Utah. This dark rock has many fine grained components plus some large out-sized clasts, like a modern glacial till .
For climate changes during the Cenozoic Era (the last 65 Ma), there is a detailed chemical record from the coring of deep sea sediments as part of the Ocean Drilling Program. Studies of deep-sea sediment use stable carbon and oxygen isotopes obtained from the shells of deep-sea benthic foraminifera that have settled on the ocean floor over millions of years. Oxygen isotopes are a proxy indicator of deep-sea temperatures and continental ice volume .
Sediment Cores – Stable Oxygen Isotopes
Oxygen isotopes are an indicator of past climate. The two main stable oxygen isotopes are 16O and 18O. They both occur in water (H2O) and in the calcium carbonate (CaCO3) shells of foraminifera as the oxygen component of both of those molecules. The most abundant and lighter isotope is 16O. Since it is lighter, it evaporates more easily from the ocean’s surface as water vapor, which later turns to clouds and precipitation on the ocean and land.
During geologic times when the climate is cooler, more of this precipitation is locked onto land in the form of glacial ice. Consider the giant ice sheets, a mile thick, that covered a large part of North America during the last ice age only 14,000 years ago. During glaciation, the glaciers effectively lock away more 16O, thus the ocean water and foraminifera shells become enriched in 18O. Therefore, a ratio of 18O to 16O (𝛿18O) in calcium carbonate shells of foraminifera is an indicator of past climate. The sediment cores from the Ocean Drilling Program record a continuous accumulation of sediment.
Sediment Cores – Boron-Isotopes and Acidity
Boron-isotope ratios in ancient planktonic foraminifera shells in deep sea sediment cores have been used to estimate the pH (acidity) of the ocean over the past 60 million years. Ocean acidity is a proxy for past atmospheric CO2 concentrations. In the early Cenozoic, around 60 million years ago, CO2 concentrations were over 2,000 ppm and started falling around 55 to 40 million years ago possibly due to reduced CO2 outgassing from ocean ridges, volcanoes and metamorphic belts and increased carbon burial due to uplift of the Himalaya Mountains. By the Miocene (about 24 million years ago), CO2 levels were below 500 ppm and by 800,000 years ago CO2 levels didn’t exceed 300 ppm .
Carbon Dioxide Concentrations in Ice Cores
For the more recent Pleistocene climate there is a more detailed and direct chemical record from coring into the Antarctic and Greenland ice sheets. Snow accumulates on these ice sheets and creates yearly layers. Ice cores have been extracted from ice sheets covering the last 800,000 years. Oxygen isotopes are collected from these annual layers and the ratio of 18O to 16O (𝛿18O) is used to determine temperature as discussed above. In addition, the ice traps small gas bubbles as the snow turns to ice.
Small pieces of this ice are crushed and the ancient air extracted into a mass spectrometer that can detect the chemistry of the ancient atmosphere. Carbon dioxide levels are recreated from these measurements. Over the last 800,000 years, the maximum carbon dioxide concentration during warm times was about 300 ppm and minimum during cold stretches was about 170 ppm . The carbon dioxide content of earth’s atmosphere is currently over 400 ppm.
Microfossils, like foraminifera, diatoms, and radiolarians, can be used to interpret the past climate record. In sediment cores, different species of microfossils are found in different layers. Groups of these microfossils are called assemblages. One assemblage consists of species that lived in cooler ocean water (in glacial times) and another assemblage found at a different level in the same sediment core are made of warmer water species .
Every year a tree will grow one ring with a light section and dark section. Each ring will vary in width. Since trees need a lot of water to survive, narrower rings indicate colder and drier climates. Since some trees can be several thousands of years old, we can use this for regional paleoclimatic reconstructions. Further, dead trees such as those used in Puebloan ruins can be used to extend this proxy indicator, which showed long term droughts in the region and why their villages were abandoned.
Flowering plants produce pollen grains. Pollen is distinctive when viewed under a microscope. Sometimes pollen can be preserved in lake sediments that accumulate every year. Coring of lake sediments can reveal ancient pollen. Fossil pollen assemblages are groups of pollen from multiple species such as spruce, pine, and oak. Through time (via the sediment cores and radiometric age-dating techniques), the pollen assemblage will change to match the plants that lived in the area at the time. Thus the pollen assemblages will be an indicator of past climate since different plants will prefer different climates . For example, in the Pacific Northwest east of the Cascades, a region close to the border of grasslands and forest, a study tracked pollen over the last 125,000 years covering the last two glaciations. As shown in the figure (Fig. 2 from reference Whitlock and Bartlein 1997 ), pollen assemblages with more pine tree pollen are found during glaciations and pollen assemblages with less pine tree pollen are found during interglacial times .
Other Proxy Indicators
Paleoclimatologists study many other phenomena to understand past climates such as human historical accounts, human instrument record from the recent past, lake sediments, cave deposits, and corals.
15.4 Anthropogenic Causes of Climate Change
As shown in the previous section, prehistoric changes in climate have been very slow. Climate changes typically occur slowly over many millions of years. The climate changes observed today are rapid and largely human caused. Evidence shows that climate is changing, but what is causing that change? Scientists have suspected since the late 1800s that human-produced (anthropogenic) changes in atmospheric greenhouse gases would likely cause climate change, as this has been the case every time in the geologic past. By the middle 1900’s, systematic measurements began which confirmed that human-produced carbon dioxide was accumulating in the atmosphere and other earth systems, like forests and the oceans. By the end of the 1900’s and into the early 2000’s the Theory of Anthropogenic Climate Change was solidified as evidence from thousands of ground-based studies and continuous satellite measurements of our land and ocean mounted revealing the expected temperature increase. Theories evolve and transform as new data and new techniques become available, but they represent the state of thinking for that field. The Theory of Anthropogenic Climate Change is that humans are causing most of the changes to climate by burning fossil fuels such as coal, oil, and natural gas. This section summarizes the scientific understanding of anthropogenic climate change.
15.4.1 Scientific Consensus
The overwhelming majority of climate studies indicate that human activity is causing rapid changes to the climate, which will cause severe environmental damage. There is strong scientific consensus on the issue. Studies published in peer-reviewed scientific journals show that 97 percent of climate scientists agree that climate warming is from human activities . There is no alternative explanation for the observed link between human-produced greenhouse gas emissions and changing modern climate. Most leading scientific organizations endorse this position, including the U.S. National Academy of Science which was established in 1863 by an act of Congress under President Lincoln. Congress charged the National Academy of Science “with providing independent, objective advice to the nation on matters related to science and technology” . Therefore, the National Academy of Science is the leading authority when it comes to policy advice related to scientific issues.
One way we know that the increased greenhouse gas emissions are from human activities is with isotopic fingerprints. For example, fossil fuels have a ratio of stable carbon-13 to carbon-12 (13C/12C) that is different from today’s stable carbon ratio in the atmosphere. Studies have been using isotopic carbon signatures to identify anthropogenic carbon in the atmosphere since the 1980s. Isotopic records from the Antarctic Ice Sheet show stable isotopic signature from ~1000 AD to ~1800 AD and a steady isotopic signature gradually changing since 1800 followed by rapid change after 1950. These changes show the atmosphere having a carbon isotopic signature increasingly more similar to that of fossil fuels .
15.4.2 Anthropogenic Sources of Greenhouse Gases
Anthropogenic emissions of greenhouse gases have increased since pre-industrial times due to global economic growth and population growth. Atmospheric concentrations of the leading greenhouse gas, carbon dioxide, are at unprecedented levels that haven’t been observed in at least the last 800,000 years . Pre-industrial level of carbon dioxide was at about 278 parts per million (ppm). As of 2016, carbon dioxide was, for the first time, above 400 ppm for the entirety of the year, at about 402 ppm. Based on the ice core record over the past 800,000 years, carbon dioxide ranged from about 185 ppm during ice ages to 300 ppm during warm times . View the data-accurate NOAA animation below of carbon dioxide trends over the last 800,000 years.
What is the source of these anthropogenic greenhouse gas emissions? Fossil fuel combustion and industrial processes contributed 78% of all emissions since 1970. Sectors of the economy responsible for most of this include electricity and heat production (25%); agriculture, forestry, and land use (24%); industry (21%); transportation (14%); other energy production (9.6%); and buildings (6.4%) . More than half of greenhouse gas emissions have occurred in the last 40 years (Figure 1.5 p.45 of and 40% of these emissions have stayed in the atmosphere. Despite the growing amount of climate change mitigation, legislation, and international agreements that have reduced emissions in some places, the continued economic growth of the less developed world has increased global greenhouse gas emissions. In fact, the time between 2000 and 2010 saw the largest increases since 1970 .