Water vapor is the most abundant green house gas and its concentration is controlled by the Earth's water cycle. This includes evapotranspiration from the Earth's surface and precipitation as rain or snow. Water that falls to the surface can be stored as liquid water in lakes, rivers, and ground water, or as solid water, which we know as ice, in glaciers, and ice caps. Liquid water that doesn't evaporate eventually runs off to the ocean. At any point in the cycle, liquid water, or ice, may be evaporated, transpired, or sublimated back into gaseous form. The water cycle ensures that a relatively constant concentration of water vapor, about 0.4% by volume, is in the atmosphere at any given time, although this varies greatly with location and season. Under natural conditions of the water cycle, the concentration of water vapor in the Earth's atmosphere remains essentially constant from year to year. The concentration of carbon dioxide in the atmosphere, like water, is maintained by its own cycle. In this case, the global carbon cycle. Unlike the water cycle, with its phase changes, the carbon cycle involves chemical compounds of carbon. The carbon cycle has five major reservoirs interconnected by exchange pathways, the atmosphere, the terrestrial biosphere, the oceans and their constituent living organisms, the sediments, including organic remains in the form of soil carbon and fossil fuels, and the interior of the Earth. For the interior of the Earth, its carbon interacts with other reservoirs by geological processes over very long time periods on the scale of tens to hundreds of millions of years. Most of the carbon cycle between different reservoirs in the carbon cycle on an annual basis is transferred in the form of carbon dioxide or CO2. This is the powerful greenhouse gas that keeps the Earth warm. For this course, we will focus on the so-called fast carbon cycle, which is the movement of carbon between the atmosphere, ocean, and biosphere over the lifespan of an average human or less. We have all heard that the Earth's climate is warming and that this warming is predicted to continue into the future. What effect would a warming atmosphere have on water vapor? Select all the answers you think apply. A, more evapotranspiration would take place near the equator. B, warmer air would be able to hold less moisture, so the water vapor would condense earlier. C, clouds could either promote warming or counteract warming initiated by water vapor, depending on cloud type. D, more water vapor in the atmosphere would automatically mean more atmospheric carbon dioxide and methane. Answers A and C are both correct. If the atmosphere becomes warmer, more evapotranspiration will take place, particularly near the equator. The greater capacity of warm air to hold water vapor will create a positive feedback, where the additional water vapor further enhances the greenhouse effect and increases atmospheric temperatures. Different types of clouds in the atmosphere may either promote warming by trapping outgoing long wave radiation near the Earth's surface, or counteract warming by reflecting short wave insulation. An increase in atmospheric water vapor does not necessarily mean heightened amounts of other atmospheric greenhouse gases, like methane or carbon dioxide. Within the fast carbon cycle, CO2 and other gases such as O2 are exchanged freely between the atmosphere and the ocean but only at the ocean surface. The direction of gas exchange depends on two factors, saturation solubility and the concentration of gas in the sea water. The saturation solubility is the maximum amount of gas that can be dissolved in water at a given temperature, pressure and salinity. Where seawater is undersaturated, gas moves from the atmosphere to the ocean. If seawater is oversaturated, gas moves from the ocean to the atmosphere. The solubility of gas increases with colder temperatures, decreasing salinity, and increasing pressure. Therefore, cold brackish water can dissolve more gas than can warm saltier water. In the case of CO2, the concentration typically increases quickly to about 300 meters depth then more slowly as the depth increases. In contrast, oxygen or O2, is greatest near the surface, and decreases with depth. This pattern results from photosynthetic organisms, such as microscopic algae that live near the ocean surface, where they consume CO2 and produce O2. With increasing depth, the lack of light leads to a change of the dominant biological processes to respiration and decomposition by various animal species. These species can range from bacteria through to whales that, like humans, consume O2 and expel CO2. Most of the carbon in the Earth's fast carbon cycle is stored in the ocean as dissolved CO2, with 60 times as much in the deep ocean as in the atmosphere. Some green organisms use the carbonate ion from dissolved CO2 and dissolved calcium ion to construct inorganic calcium carbonate, outer skeletons, or shells. In shallow water, where calcium and carbonate are abundant, shells, coral and other organisms can accumulate in reefs. If buried, these reefs form limestone, a very long lasting geological reservoir. But if water is undersaturated with carbonate, shells and even limestone, can dissolve and return their carbon to the ocean. Ocean carbon is also stored in living organisms' bodies. Microscopic photosynthetic organisms convert dissolved CO2 into organic carbon compounds and other organisms feed on these compounds. As organisms die, their bodies continually rain out on the sea floor leading to the storage of organic carbon in ocean floor sediments. Such buried marine organic carbon is the source of the world's petroleum reserves. When subjected to heat and pressure from burial under thick sediments, organic carbon can be converted into petroleum compounds, such as oil, gas, and bitumen. On land, plants also convert CO2 to organic carbon through photosynthesis. Respiring organisms, including humans, convert the oxygen that they breathe and organic carbon that they eat into the CO2 that they exhale. Respiring animals leave organic carbon behind in their waste, while photosynthesizing plants and respiring animals leave behind the organic matter of their bodies when they die. The largest land-based reservoir of carbon is, in fact, the soil, including frozen soils in permafrost regions where organic matter often resides after being buried or decomposed. In addition, soils contain abundant microorganisms whose bodies are built of organic carbon. Most of the carbon in Earth's fast carbon cycle is stored in the deep ocean as dissolved CO2. Which of the following statements is not true of carbon storage in the ocean? Select all the answers that apply. A, CO3 carbonate ions are necessary for the shells of many sea creatures. B, limestone is a long-lasting geological reservoir of carbon. C, organic carbon can not be dissolved in sea water. Or D, burning fossil fuels releases carbon from the slow carbon cycle into the fast carbon cycle. Only answer C is not true of the storage of carbon in our oceans. The deep ocean store most of the carbon that is active in the earth's fast carbon cycle. A portion of this storage does, in fact, consist of dissolved organic carbon in sea water. The oceans, in turn, provide the most significant input from the fast carbon cycle to the slow carbon cycle. Much of the fossil fuels we continue to extract were originally deposited as organic carbon on the floor of the Earth's oceans over 100 million years ago. Under natural conditions, such long lived reservoirs only interact with the carbon cycle over millions of years in the slow carbon cycle. Under natural conditions, the fast carbon cycle remains essentially stable. Carbon dioxide cycles between the atmosphere and ocean, and through the terrestrial and marine biospheres, with stable reservoirs and fluxes. Thus, the carbon cycle acts as the earth's thermostat and keeps global temperature fairly constant. Why are long term geological carbon reservoirs insignificant to the fast carbon cycle under natural conditions? Select all the answers that apply. A, the amounts released annually are so small as to be insignificant to the volumes involved in the fast carbon cycle. B, the amounts released annually change very little from year to year. C, the amounts released annually are offset by similar amounts added to geological reservoirs. D, the amounts released annually only move from one geological reservoir to another. Answers A, B, and C are correct, although A is the most important. Crucial to the fast carbon cycle is that under natural conditions the immense carbon reservoirs of the slow carbon cycle interact in such a minor fashion on an annual basis as to be insignificant. Although there is a total of more than 75 million billion tons of carbon contained in the Earth's interior, fossil fuels in bedrock, and deeply buried organic carbon of the Earth's sediments, their minuscule contribution does not disturb Earth's equilibrium under natural conditions. While the Earth's natural carbon cycle has maintained a relatively constant state for thousands of years, human activity has had a substantial and increasing effect. Under natural conditions, fossil fuels including oil, gas, bitumen, and coal are isolated from the fast carbon cycle, due to their storage in deep sedimentary reservoirs that are unable to interact with the fast carbon cycle. Since the onset of extensive fossil fuel use during the industrial revolution of the 18th century, humans have removed over 360 billion tons of fossil fuels from geological reservoirs. Most of this has been burned, releasing CO2 and black carbon soot into the atmosphere. With an increase in global population, human changes to the earth's landscape have added a further 30 billion tons of carbon to the atmosphere, primarily due to deforestation and land use changes. Why do you think the change in human land use has led to additional carbon in the atmosphere? Select all the answers that you think are correct. A, on deforested land, grasses remove less CO2 from the atmosphere than trees. B, urbanized and industrial land have few surfaces for plants to grow on from which to draw down CO2. C, cattle grazing on deforested land belch methane. D, mono-culture agricultural crops do not allow for efficient carbon storage in soil. Technically all of these are true. In particular, agricultural and urban land use decreases species diversity on the land, and so decreases the amount of carbon that plants can take up, especially compared to forest ecosystems. While cattle and other ruminants such as reindeer, do belch methane, they tend to do so with or without land use changes. However, land has been deforested in order to raise livestock. A problem that is ubiquitous across much of the world. So answers A, B, C and D are correct. As of 2013, the burning of fossil fuels and changes in land use have added about 240 billion net tons of carbon to the atmosphere, since 1850. This is an increase of about 43% over this time period. An additional four billion net tons of carbon are added to the atmosphere annually. This is despite land plants drawing down about 2.5 billion tons per year of carbon through photosynthesis. The oceans absorb another 2.5 billion tons per year. In the case of the oceans, this also increases their acidity, which we'll discuss in lesson four. CO2 taken up by the surface ocean is re-exchanged with the atmosphere over about one to ten years. In the deep ocean CO2 can be retained for up to 2,000 years before it returns to the atmosphere. Which of the following are true? Select all the answers you think are correct. A, the ocean is ineffective at removing CO2 from the atmosphere. B, CO2 can be stored in the ocean indefinitely to mitigate climate change. C, CO2 emitted today will still be a problem in 2000 years. D, storing CO2 in the ocean maybe an effective way to mitigate climate change for at least several centuries. C and D are true. The ocean is effective at removing CO2 from the atmosphere, but is not able to store it over the long term. This means that even if humans stop burning fossil fuels today, carbon in the deep ocean will eventually return to the atmosphere. As a result, we will face higher than natural carbon dioxide concentrations for thousands of years to come. In the atmosphere, CO2 is commonly measured in parts per million volume or ppm, giving us about 400 ppm of carbon dioxide today. Today, the human influenced atmosphere contains both 0.04% CO2 by volume or about 800 billion tons of carbon. Carbon dioxide levels in the atmosphere have been measured continuously since 1958, when CO2 was only about 320 parts per million. It is traditionally measured at the summit of Hawaii's Mauna Loa volcano as well as other stations worldwide. Atmospheric CO2 concentration varies seasonally. It's lowest during the Northern Hemisphere summer when plant photosynthesis is most active. It's about five ppm higher during Northern Hemisphere winter, cuz there's less land area in the Southern Hemisphere occupied by photosynthesizing plants. Other than the seasonal variability the CO2 concentration in the atmosphere has been continuously increasing since measurements began. On top of the increasing trend, the rate of increase is larger year over year. As recently as 150 years ago, the concentration of CO2 in Earth's atmosphere was only 280 parts per million, 43% lower than today's 400 parts per million. Before about 1850, atmospheric CO2 was relatively constant for about 10,000 years. This period of stable climate is known as the Holocene epoch. Going back 800,000 years, atmospheric CO2 only varied between 180 parts per million and 300 parts per million. This variation occurred over cyclical 100,000 year cycles a continent wide glaciation or ice ages. Similarly, atmospheric methane concentration varied between 350 and 750 parts per billion volume over the same 800,000 years but are now over 1,800 parts per billion today. This is more than double the pre-industrial level. Scientists have made actual measurements of the past atmosphere by analyzing bubbles that are trapped in glacier ice in the enormous Antarctic ice sheet. Eight major ice cores have been collected from the Greenland and Antarctic ice sheets. The ice core records are known to overlap and show similar patterns of CO2 and methane gas concentrations. Which of the following can you infer from this information? Select all the answers you think are correct. A, if the patterns are similar it provides confidence the records are correct. B, if the patterns are similar then the climate was identical in Greenland and Antarctica when the air bubbles were frozen in. C, if the patterns show some differences, it shows that ice cores are unreliable. D, if the pattern shows some differences, it may reflect poor preservation of annual layers. Answers A and D are correct. Where the overlapping records are similar, it provides confidence in the accuracy of both ice cores. However, sometimes annual ice layers are very thin and poorly preserved or difficult to measure, leading to discordant sections. The level of CO2 and methane in the gas bubble reflect global not local climate. And so cannot be used to tell if Greenland and Antarctica experience similar or different climate. We've looked at how human activity, in particular fossil fuel burning has dramatically increased the amount of carbon dioxide in Earth's atmosphere and oceans. When scientists realized that human carbon emissions were changing the Earth's system United Nations member governments requested to set up a panel to discuss the issue. Founded in 1988, this is the intergovernmental panel on climate change or IPCC. The main task of the IPCC is to provide reports and support of the United Nations framework convention on climate change. The IPCC reports are designed to help government understand the scientific basis and impacts of human induced climate change and the options for adaptation and mitigation. Rather than conducting its own research, the IPCC relies upon peer reviewed scientific literature, the gold standard of scientific information. This information for many scientific fields is comprehensive, objective, open, and transparent. The IPCC uses this information to assess the scientific, technical, and socioeconomic information relevant to understand the risks of human induced climate change. They look at its potential impacts and discuss options for adaptation and mitigation. The IPCC brings together and summarizes research conducted in many diverse fields including climatology, meteorology, oceanography, plant and animal biology, geology, physics, computer science and many others. Based on data summarized in the IPCCs 2013 assessment report five. The Earth's global average temperature over land and ocean has increased by 0.85 degrees Celsius between 1880 and 2012. Nearly the entire planet experienced warming during this period, particularly over land. Earth's oceans have also warmed by about 0.1 degrees Celsius. While this may seem small, the immense size of Earth's oceans. And the very high heat capacity of water means that this 0.1 degree Celsius change accounts for 90% of additional energy added to the Earth system since 1970. Coupled climate ocean models, run on supercomputers, have been able to replicate the warming ocean and atmosphere during the entire period of recorded temperatures going back to 1850. These models have only been able to simulate this warming with the addition of fossil fuel derived greenhouse gases. Where only natural factors are invoked, models have been unable to reproduce the observed warming. The IPCC considers that human-induced greenhouse gases are extremely likely, a probability greater than 95%, to be responsible for the majority of all climate warming since 1951. IPCC reports conclude that there is greater than 95% probability that human-induced greenhouse gases are responsible for the majority of climate warming. Which of the following statements are true? Select all that apply. A) The IPCC represents one side of an ongoing debate with the scientific community. B) The IPCC reports are exhaustively reviewed and represent the consensus of the scientific community. C) Increased greenhouse gases represent one of many viable reasons for climate change. D) The only models that can accurately simulate ocean and atmospheric climate warming use fossil fuel-derived greenhouse gases. Both B and D are correct. It is the scientific consensus that human-induced greenhouse gases are responsible for climate change. 97.1% of scientific studies published between 1991 and 2011 that expressed a position on climate change agreed that it is occurring and is the result of human activities. To give a sense of the scale of the influence of each greenhouse gas, scientists have converted their heat retaining potential into terms of watts per square meter, as is used to measure incoming solar radiation. In Lesson 1, we saw that an average of 238 watts per square meter of solar radiation is received at the Earth's surface. Comparing today's atmosphere to pre-industrial times, CO2 adds about 1.68 watts per square meter of additional heat. Methane adds another 0.97 watts per square meter, and other greenhouse gases 0.35, for a total of 3 watts per square meter. The natural variation of solar radiation over the same period is only responsible for 0.05 watts per square meter. Human activities also slightly reduce the solar radiation reaching Earth by about 0.7 watts per square meter. The strongest influence is the emission of dust and aerosol particles including black carbon soot. Even accounting for this influence, human-produced greenhouse gases are still responsible for a net 2.3 watt per square meter increase, 1% more than pre-industrial times. While one percent may seem small, in a system as finely balanced as the Earth's climate, that one percent makes an enormous difference.
