This week instead of a normal Sunday School, I'm doing something different. I'll be exploring the science behind global warming. Fair warning: this is long. Over 4500 words long. I've done my best to distill the information from the sources from each section, but that doesn't mean you shouldn't read them, too!
There are three basic principles that have to be understood first to understand the greater topic of global warming. They are the greenhouse effect, the carbon cycle, and radiative forcings.
In the 1820's Joseph Fourier first began to understand that solar energy coming from the sun would be partially absorbed by the earth, but that some of it would be reflected back into the atmosphere as infrared radiation. The air in turn absorbs some of this energy and radiates back to earth effectively trapping it from heading back out into space. This was then called “the greenhouse effect.” While scientists at the time could not determine how much energy was absorbed vs released, they could determine that if the earth had no atmosphere, then it would be much cooler.
The greenhouse effect was first related to carbon dioxide in the atmosphere by John Tyndall. Through laboratory test he was able to determine that water vapor and carbon dioxide were responsible for trapping heat. Svante Arrhenius, a Swedish chemist who would be awarded the Noble Prize in Chemistry in 1903, was able to calculate the infrared absorption of carbon dioxide and water vapor in 1896. He realized that fluctuations of water vapor in the atmosphere changed daily and geographically, while concentrations of carbon dioxide was fairly stable amd changed over a geological time scale. Arrhenius concluded that slight changes to concentrations of atmospheric CO2 would change temperatures which would change the amount of water vapor in the air, thus amplifying the temperature changes no matter the way they shifted. Warmer air would increase the amount of water vapor that could be held, which would lead to more energy reflected back to the earth which would lead to warmer temperatures and vice versa.
In a nutshell, the greenhouse effect is the reflection of different bands of solar radiation back to the earth and thus heating it instead of letting it back out into space. The Green House Gases (GHGs) that have the greatest effect are (in no particular order) CO2, CH4 (methane), H2O, N2O (nitrous oxide), and the halocarbons (CFCs, HFCs and similar gases).
“Because scientists know which wavelengths of energy each greenhouse gas absorbs, and the concentration of the gases in the atmosphere, they can calculate how much each gas contributes to warming the planet. Carbon dioxide causes about 20 percent of Earth’s greenhouse effect; water vapor accounts for about 50 percent; and clouds account for 25 percent. The rest is caused by small particles (aerosols) and minor greenhouse gases like methane.
Water vapor concentrations in the air are controlled by Earth’s temperature. Warmer temperatures evaporate more water from the oceans, expand air masses, and lead to higher humidity. Cooling causes water vapor to condense and fall out as rain, sleet, or snow.
Carbon dioxide, on the other hand, remains a gas at a wider range of atmospheric temperatures than water. Carbon dioxide molecules provide the initial greenhouse heating needed to maintain water vapor concentrations. When carbon dioxide concentrations drop, Earth cools, some water vapor falls out of the atmosphere, and the greenhouse warming caused by water vapor drops. Likewise, when carbon dioxide concentrations rise, air temperatures go up, and more water vapor evaporates into the atmosphere—which then amplifies greenhouse heating.
So while carbon dioxide contributes less to the overall greenhouse effect than water vapor, scientists have found that carbon dioxide is the gas that sets the temperature. Carbon dioxide controls the amount of water vapor in the atmosphere and thus the size of the greenhouse effect.”
So if carbon dioxide controls the amount of warming, why hasn't there been a runaway effect already. The answer is: The Carbon Cycle
Greenhouse Effect Sources:
The carbon cycle is the movement of carbon from the atmosphere to the lithosphere (rocks essentially). There are two different speeds to the cycle and are are descriptively named the Fast Carbon Cycle and the Slow Carbon Cycle. “On average, 10^13 to 10^14 grams (10–100 million metric tons) of carbon move through the slow carbon cycle every year. In comparison, human emissions of carbon to the atmosphere are on the order of 10^15 grams, whereas the fast carbon cycle moves 10^16 to 10^17 grams of carbon per year.”
The fast carbon cycle exists in a period of a lifetime. Carbon is absorbed from the atmosphere by plants and phytoplankton. The carbon used in their bodies are either used down the food chain or released in decomposition. The Fast Carbon Cycle peaks and troughs seasonally. Most of the land mass and therefore most of the vegetation in the world is in the northern hemisphere. During the winter months and into late spring the carbon dioxide concentrations are higher and by late summer they plummet to a yearly low. Some plants and animals are buried in sediment or build shells and skeletons made of calcium carbonate and thus become part of the Slow Carbon Cycle.
The Slow Carbon Cycle is the movement of carbon dioxide over a period of millions and millions of years. When rain interacts with carbon in the atmosphere, it creates trace amounts of carbonic acid. The acid acts on rocks to break them down in a process of chemical weathering, which release ions like calcium that flow with water out into the oceans. Organisms in the ocean like coral, plankton and other shell building animals used calcium and carbon to create shells of calcium carbonate. Eventually, new layers build on the ocean floor consisting of sediments and the remains of animals. It gets crushed together by pressure and time to become limestone and other types of sedimentary rocks. If you didn't know, anywhere with large amounts of limestone rock was once underwater and it's not uncommon to find marine fossils.
“Only 80 percent of carbon-containing rock is currently made this way. The remaining 20 percent contain carbon from living things (organic carbon) that have been embedded in layers of mud. Heat and pressure compress the mud and carbon over millions of years, forming sedimentary rock such as shale. In special cases, when dead plant matter builds up faster than it can decay, layers of organic carbon become oil, coal, or natural gas instead of sedimentary rock like shale.”
The way that this carbon eventually gets back into the atmosphere is through volcanic eruptions. Subduction zones pull the crust downward, heat it up to a liquid form, and spit it out again through volcanoes. “At present, volcanoes emit between 130 and 380 million metric tons of carbon dioxide per year. For comparison, humans emit about 30 billion tons of carbon dioxide per year—100–300 times more than volcanoes—by burning fossil fuels.” As carbon levels in the atmosphere increase and increase temperature and humidity, there is also an increase in precipitation. More rain lead to more chemical weathering which leads to the storage of more carbon in the ocean floor as sedimentary rock, but this works on a geological scale of hundreds of thousands of years.
There is also a transition effect between the ocean and the atmosphere. Some CO2 sinks down into the ocean depths with ocean currents and good, old-fashioned diffusion while some is released back into the atmosphere. As more CO2 is absorbed into the oceans, the oceans become more acidic, which has knock on effects for marine organisms
“Ocean acidification affects marine organisms in two ways. First, carbonic acid reacts with carbonate ions in the water to form bicarbonate. However, those same carbonate ions are what shell-building animals like coral need to create calcium carbonate shells. With less carbonate available, the animals need to expend more energy to build their shells. As a result, the shells end up being thinner and more fragile.
Second, the more acidic water is, the better it dissolves calcium carbonate. In the long run, this reaction will allow the ocean to soak up excess carbon dioxide because more acidic water will dissolve more rock, release more carbonate ions, and increase the ocean’s capacity to absorb carbon dioxide. In the meantime, though, more acidic water will dissolve the carbonate shells of marine organisms, making them pitted and weak.”
The oceans have been pulling in more carbon dioxide in the past century then they were in the previous centuries causing a slight, net storage. While it is likely that the oceans will continue this trend as long as atmospheric carbon remains high. It will still take thousands and thousands of years before it can help remove most of the carbon dioxide that we have put into the atmosphere.
By burning fossil fuels which are essentially giant carbon-sinks created by organic matter trapped in sediment, we have been interefering with the Slow Carbon Cycle. We've been freeing carbon from the soil and deep in the crust and burning it which puts it back into the atmosphere well before it would be done by the natural process of volcanism. Because of this, the concentrations of carbon dioxide have been rising faster than the carbon cycle can remove it from the atmosphere and store it, not that the earth hasn't tried to keep up. Approximately 55% of all CO2 emmisions from human sources have been absorbed through the Fast Carbon Cycle. 30% of that is due to the oceans and 25% of that is through vegetation, but we are still contributing more than the planet can readily remove. Because there is more carbon dioxide in the atmosphere, there is more warming due to an effect that all GHGs have called “radiative forcing.”
Carbon Cycle Sources:
“Radiative forcing is a measure of how the energy balance of the Earth-atmosphere system is influenced when factors that affect climate are altered. The word radiative arises because these factors change the balance between incoming solar radiation and outgoing infrared radiation within the Earth’s atmosphere. This radiative balance controls the Earth’s surface temperature. The term forcing is used to indicate that Earth’s radiative balance is being pushed away from its normal state.”
The planet is constantly being bombarded by solar radiation. Some of it is reflected out into space, and most of it is absorbed by the earth. Like a warm drink placed in a fridge, the earth gives off some energy into space. It does so as infrared radiation. The total energy flowing into the system minus the energy flowing out of the system is the total radiative forcing measured in Watts per square meter (W/m2). If that number is positive then the earth is warming. If it's negative, then the earth is cooling. Different parts of the system have different radiative forcing values.
Albedo, the reflectivity of light, has a negative radiative forcing. It's a force for cooling the planet. Ice, snow, clouds are all highly reflective and send energy back into the atmosphere rather than absorbing it. Land use also changes albedo. Forests and other forms of vegetation absorb energy preventing it from being released, but when people clear the land and expose rock and soil with reflective minerals, it increases reflectivity. Volcanoes are also a source of cooling. They spray particulate matter into the atmosphere which helps cool the world for a short time until the particles fall back to earth. The last time there was a major cooling event due to a volcano was in 1991 with the eruption of Mount Pinatubo.
Greenhouses gases have a positive radiative forcing which creates warming. Each gas has a different radiative forcing based on its concentration and the amount of energy it prevents from exiting the atmosphere. The radiative forcings of these gases don't give the whole picture. At equal concentrations, methane has a much greater effect than carbon dioxide. Another measurement was created to use CO2 as a standard called the Global Warming Potential (GWP). It is the comparison of the amount energy trapped by certain mass of a GHG to the amount of energy trapped by the same mass of carbon dioxide. Due to the longevity or lack thereof of some of these gases, the GWP is calculated at intervals of 20, 100, and 500 years. Methane remains for about 12 years in the atmosphere before it decays, but it has a GWP 21 times that of carbon dioxide. Sulfur Hexafloride remains in the atmosphere for 3,200 years and has a GWP of 23,900. Thankfully, some of these molecules are in low concentrations otherwise the effect would be much greater. The greatest issue for greenhouse gases it not the radiation that it prevents from escaping earth; it's the current concentration. Of those concentrations, CO2 has been growing at an unprecedented rate due to human consumption and that is why its radiative forcing is cumulatively greater than that of the other GHGs. “Carbon dioxide is the single most important greenhouse gas emitted by human activities. It is responsible for 85% of the increase in radiative forcing over the past decade.”
Radiative Forcing and Global Warming Potential Sources:
To determine how much of an effect humans are having presently, we first have to know what conditions have been like in the past. How do we know what the past history of carbon dioxide concentrations were? Scientist can measure the concentrations of gases caught in tiny bubbles in ice cores taken from Antarctica up to a depth of 3 kilometers. Due to new layers of snow and ice added every year, there is an excellent indication of time much like the rings of growth in a tree. The first measurements came from Lake Vostok which measured levels of GHGs back 450,000 years ago. The second team was European Project for Ice Coring in Antarctica (EPICA) which was able to get data for about 650,000 years ago. To determine what concentrations were greater than a million years ago, several methods are used.
“Two proxies apply the fact that biological entities in soils and seawater have carbon isotope ratios that are distinct from the atmosphere (Cerling, 1991; Freeman and Hayes, 1992; Yapp and Poths, 1992; Pagani et al., 2005). The third proxy uses the ratio of boron isotopes (Pearson and Palmer, 2000), while the fourth uses the empirical relationship between stomatal pores on tree leaves and atmospheric CO2 content (McElwain and Chaloner, 1995; Royer, 2003).”
Using these proxies, scientists have been able to determine roughly what the atmosphere was like back to some 500 million years ago. What has really been focused on has been the last few glacial and inter-glacial periods. The data from EPICA gives a very clear view of carbon dioxide concentrations through this time. At no point from 1900 to 650,000 years ago have carbon dioxide concentrations been above 300 parts per million (ppm) and the lowest they've gone is about 180ppm, but since about the mid 1900's CO2 concentrations have been above 300ppm and growing exponentially. Lead researcher for EPICA, Thomas Stocker, has stated that the last 50 years have seen the growth of carbon dioxide 200 times faster than any growth in the last 650,000 years and that current levels are 30% higher than at any point in that period. The most recent measured concentration at Mauna Loa Observatory in Hawaii was 396.79ppm on 02 AUG 2013. It was only a few months ago in May that concentrations were measured at over 400ppm, which is higher than any period in at least the last 2 million years and possibly the last 15 million years.
“At the end of the Ice Age, it took 7,000 years for carbon dioxide levels to rise by 80 parts per million, Tans said. Because of the burning of fossil fuels, carbon dioxide levels have gone up by the same amount in just 55 years.
The speed of the change is the big worry, said Pennsylvania State University climate scientist Michael Mann. If carbon dioxide levels go up 100 parts per million over thousands or millions of years, plants and animals can adapt. But that can't be done at the speed it is now happening.”
While CO2 is the most abundant GHG and the one that is driving global warming, it is not the only one scientists are concerned with. Methane as was pointed out earlier has a far greater effect than carbon dioxide. Methane is naturally produced from the decay of plants and animals as well as by animals themselves in small amounts. Humans have been contributing more and more to a rise in concentrations to methane through industrialized agricultural uses, manufacturing, burning of biomass, energy use (natural gas is primarily methane), and landfills. Approximately 40% of the atmospheric concentration is by natural sources and 60% is by human sources.
Methane concentrations were determined for the past through the Vostok and EPICA ice cores as well as ice cores from Greenland glaciers as well. “Ice core records have shown that the CH4 concentration had remained between 350 and 800 parts per billion (ppb) for the past 650 thousand years (kyr) (Brook et al., 2000; Spahni et al., 2005); whereas presently global mean CH4 concentration is 1775 ppb (Forster et al., 2007).” “Atmospheric methane reached a new high of about 1813 parts per billion (ppb) in 2011, or 259% of the pre-industrial level, due to increased emissions from anthropogenic sources.”
While methane concentrations have been rising exponentially with carbon dioxide, they have notably slowed down since about 1990.
“In results published Aug. 23  in the journal Nature, the team led by UC Irvine chemistry professor Donald Blake reported that the observed leveling-off in atmospheric methane is largely a result of changes in fossil fuel use - specifically, reductions in fugitive emissions of natural gas that can occur during fossil fuel exploration. Fugitive emissions include venting and flaring, evaporative losses, and equipment leaks and failures, but exclude combustion of fuels. The study finds these measures probably account for up to 70 percent of the slowing growth in atmospheric methane levels observed at the end of the 20th century.
Blake and his team, including co-author Mads Sulbaek Andersen, formerly of UCI and now with NASA's Jet Propulsion Laboratory in Pasadena, Calif., used nearly three decades of air samples of methane and ethane collected at remote locations across the globe for their analysis. Ethane is the most abundant non-methane hydrocarbon in Earth's atmosphere, and shares its major emission sources with methane. Because methane and ethane are emitted from fossil fuel sources with characteristic emission ratios, the scientists were able to use their long-term ethane records to investigate quantitatively the slowing growth rate of atmospheric methane.”
Because methane is so much more potent than CO2, scientists have been keeping a close eye on permafrost in northern latitudes now that the earth is warming. Plant and animal matter that were frozen in the tundra may soon start to decay as those areas thaw. This could lead to a potentially massive amount of methane escaping into the atmosphere and drastically increasing the effects of an already warming world.
“Over hundreds of millennia, Arctic permafrost soils have accumulated vast stores of organic carbon - an estimated 1,400 to 1,850 petagrams of it (a petagram is 2.2 trillion pounds, or 1 billion metric tons). That's about half of all the estimated organic carbon stored in Earth's soils. In comparison, about 350 petagrams of carbon have been emitted from all fossil-fuel combustion and human activities since 1850. Most of this carbon is located in thaw-vulnerable topsoils within 10 feet (3 meters) of the surface.”
NASA is currently running a program, Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE), to determine what the emissions are from these areas. It is expected that the drier the area gets the more carbon dioxide will be released than methane, but the wetter it is the more methane will be released. What those proportions are is essential to find out for models to predict future warming. Since Methane is about 20 times more potent than CO2, so knowing how much is released it vital.
Concentrations of GHG Sources:
Finally, there's the matter of temperature change on earth. In 1998 Mann, Bradley, and Hughes reconstructed temperature data for the northern hemisphere for the last 1,000 years using proxies such as tree rings, lake sediments, ice cores, corals, and historical records. Their work showed an unusual spike in warming around the 1900s to present and that no temperature had been that high since the 1400s. The work was added to the IPCC third assessment and became the infamous “hockey stick” graph. Over the course of 15 years, their work has been validated time and time again and through the use of different proxy data.
In March of this year a new study was released using date from 73 sites from around the world. Using ice cores date, sediment cores from lake beds and determining the ratios of certain ions in the shells of microscopic organisms, and using a few other methods, they were able to reconstruct a global temperature record dating back 11,300 years ago. After the last ice age, temperatures warmed, plateaued, then fell. The first decade of the 1900s, 1900-1909, was colder than 95% of the years in the study, but since 1910 to the present, we have gone from near the lowest temperatures in the last 11,000 years to a temperature that is 75% higher than any seen in that same time frame. “What that history shows, the researchers say, is that over the past 5,000 years, the Earth on average cooled about 1.3 degrees (Fahrenheit) – until the past 100 years, when it warmed ̴ 1.3 degrees (F).”
“Today's study should help debunk the common climate change denial argument that recent warming is simply part of a long-term natural trend. Indeed, Marcott says, the earth should be nearing the bottom of a several-thousand year cool-off (the end-point of the rainbow arc in (B) above), if natural factors like solar variability were the sole driving factors. Instead, temperatures are rising rapidly.”
The Milankovitch cycles, periodic cycles of eccentricity in earth's orbit and the movement of it's axis, are drivers of solar forcings. They change the seasonal amount of energy received by the earth and because of this they lead to the natural periods of warming and cooling that lead to glacier formation and recession. The longest one of these, changes to the earth's orbit, happens at a period of 100,000 years.
The EPICA ice cores did not just determine historic concentrations of GHGs, but also temperature based on ratios of isotopes found in the gas. There is good, though not perfect, correspondence with the Milankovitch cycles and temperature variability, but there is a far greater correspondence between levels of CO2 and temperature.
“This is entirely consistent with the idea that temperature and CO2 are intimately linked, and each acts
to amplify changes in the other (what we call a positive feedback). It is believed that the warmings out of glacial periods are paced by changes in Earth’s orbit around the Sun, but the tiny changes in climate this should cause are amplified, mainly by the resulting increase in CO2, and by the retreat of sea ice and ice sheets (which leads to less sunlight being reflected away). Looking at the warming out of the last glacial period in detail, we can see how remarkably closely Antarctic temperature and CO2 tracked each other.
It is often said that the temperature ‘leads’ the CO2 during the warming out of a glacial period. On the most recent records, there is a hint that the temperature started to rise slightly (at most a few tenths of a degree) before the CO2, as expected if changes in Earth’s orbit cause an initial small warming. But for most of the 6,000-year long ‘transition’, Antarctic temperature and CO2 rose together, consistent with the role of CO2 as an important amplifier of climate change.”
What we are seeing in the last century is completely unprecedented in geological history. Never have GHG levels risen so dramatically so quickly and independent of natural processes. The natural way that the climate warms is by slight upticks in temperature over long periods of time, followed by a greater movement in GHGs. In this way, they move together. Over the last 100 years because of human activity and disruption of the carbon cycle, we're forcing the cycle to increase in ways unknown to history by forcing carbon dioxide levels up before natural warming would do so. This means that geochemistry of the earth will catch up. It will do so more slowly, but it is inevitable that the world will continue to warm even if all human induced emissions stop this very instant.
Rate of Natural Temperature Increase Sources:
Richard Muller was a notable climate skeptic funded by the very conservative Koch brothers to investigate whether climate change was really man-made or not. After a thorough investigation on his own and using his own sources, he could come to no other conclusion than global warming was happening, that people were responsible for it, and that there is no other explanation for it other than an increase in human created GHGs.
“My total turnaround, in such a short time, is the result of careful and objective analysis by the Berkeley Earth Surface Temperature project, which I founded with my daughter Elizabeth. Our results show that the average temperature of the earth’s land has risen by two and a half degrees Fahrenheit over the past 250 years, including an increase of one and a half degrees over the most recent 50 years. Moreover, it appears likely that essentially all of this increase results from the human emission of greenhouse gases.
These findings are stronger than those of the Intergovernmental Panel on Climate Change, the United Nations group that defines the scientific and diplomatic consensus on global warming. In its 2007 report, the I.P.C.C. concluded only that most of the warming of the prior 50 years could be attributed to humans. It was possible, according to the I.P.C.C. consensus statement, that the warming before 1956 could be because of changes in solar activity, and that even a substantial part of the more recent warming could be natural.
How definite is the attribution to humans? The carbon dioxide curve gives a better match than anything else we’ve tried. Its magnitude is consistent with the calculated greenhouse effect — extra warming from trapped heat radiation. These facts don’t prove causality and they shouldn’t end skepticism, but they raise the bar: to be considered seriously, an alternative explanation must match the data at least as well as carbon dioxide does. Adding methane, a second greenhouse gas, to our analysis doesn’t change the results. Moreover, our analysis does not depend on large, complex global climate models, the huge computer programs that are notorious for their hidden assumptions and adjustable parameters. Our result is based simply on the close agreement between the shape of the observed temperature rise and the known greenhouse gas increase.”
Richard Muller Source:
To sum it all up: More greenhouse gases = more energy trapped on earth = more heating. There is not a natural source for where these gases are coming from and the energy heading to earth is not increasing. We are wholly responsible for increasing trends in global warming and the effects thereof. The real question now is what do we do about it?