The Earth s Atmosphere and Climate

Transcription

1 The Earth s Atmosphere and Climate After Earth formation The process of forming the Earth was complete by about 4.5 billion years ago. The history of the atmosphere. What controls Earth s climate? How has climate changed in the past? Cretaceous to the present Permian and Carboniferous Precambrian Snowball Earth after Snowball Earth Earth experienced a period of internal melting (due to initial high temperatures and heat from radioactive decay). Dense elements (especially iron and nickel) sank to the centre of the Earth, forming the metallic core. As the Earth cooled and differentiation continued the crust became thicker and continents began to "grow" due to plate tectonics. Lighter material rose to the surface (forming a thin, weak, crust). Initial crust was likely too thin to support plate tectonics as we know it today. First crust likely had a basaltic composition (like modern oceanic crust) and lacked continents. At zones of subduction, intrusion of magma into overlying crust would have caused thickening to form continental crust. Oldest continental igneous rocks are 4.02 billion years old. Oldest sedimentary rocks (sandstones) contain 4.2 billion year old minerals. Therefore, granitic continental crust must have been present by 4.2 billion years ago. The continents evolved over the past 4.2 Billion years due to plate tectonics. Repeated collisions between early continents constructed the modern continents over time. 1

2 Earth s atmosphere and early life Initial atmosphere may have had a "primitive" composition, like that of the sun (gases derived from the initial nebula). E.g., much more Ar 40 compared to other isotopes of Ar in the Earth s atmosphere. Ar 40 formed by decay of K 40, therefore, was derived from the Earth after it formed. The modern atmosphere has a composition that is very different from that of the sun. Other Argon isotopes were removed with the early atmosphere and the Argon 40 formed by subsequent K 40 decay. Solar wind likely drove off the primitive atmosphere early in Earth s history when all of the inner planets lost their initial atmosphere. The modern atmosphere resulted from "outgassing" of the Earth: emission of gases during volcanic eruptions. The combined composition of the modern atmosphere, biosphere, hydrosphere and sediments is very similar to that of volcanic gas. Over time the Ar 40 was formed in the Earth by decay of K 40 and released to the atmosphere with the volcanic eruptions. Note: there is growing evidence that the bulk of the atmosphere (including water) was delivered by millions of comets that collided with Earth during its very early history. By 4 billion years ago the early atmosphere had the following composition: Principle gases: Carbon dioxide (CO 2 ) Water vapour (H 2 O) Nitrogen(N 2 ) Minor gases: Hydrogen (H 2 ) Hydrogen chloride (HCl) Some free Oxygen was produced by dissociation of water in the upper atmosphere due to ultraviolet radiation: 2H 2 O + ultraviolet rays = 2H 2 +O 2 Light H 2 is lost to space and heavier O 2 remains in the atmosphere. However, the rate at which O 2 appears to have increased could not have been produced by dissociation alone. Sulfur dioxide (SO 2 ) Note: there was virtually no free oxygen (which makes up about 20% of the modern atmosphere). 2

3 Photosynthesis is likely responsible for more rapid production of Oxygen: CO 2 + H 2 O + light (+ chlorophyll) = (CH 2 O) + O 2 The oldest organisms that could produce chlorophyll are cyanobacteria: single celled organisms that lacked an organized nucleus and lived in the oceans. Photosynthesis requires chlorophyll, a complex chemical that is produced by some organisms (e.g., plants). First cyanobacteria appeared about 3.5 billion years ago and were anaerobic (did not require free Oxygen). They are very common in rocks younger than about 2.5 billion years old. As Oxygen levels increased as aerobic organisms developed (that used free Oxygen) and these, in turn produced even more Oxygen. There is strong correlation between O 2 levels in the atmosphere and the development of life, on Earth. Cyanobacteria began the process of increasing atmospheric Oxygen. Oxygen levels became high enough to support more complex life, which in turn, produced more oxygen. By 600 million years ago Oxygen levels had almost reached the modern concentration of about 20% of the Earth s atmosphere. The evolution of land plants, resulted in a modest increase in O 2. Variation in O 2 levels over the past 500 million years reflect changes in plant cover on Earth. Climate versus Weather Weather: the condition of the atmosphere at a particular point in space and time. Carboniferous: warm, moist, tropical settings predominated on land and land plants thrived. O 2 levels almost doubled. Permian and Triassic: arid conditions were extensive on land and the abundance of land plants diminished. O 2 levels dropped to below 15%. 3

4 Weather changes over short periods of time and is described in terms of: Air temperature Air pressure Humidity Cloud cover Precipitation (rain, snow) Visibility Climate: the average weather for a defined region. Based on long-term, accumulated data derived from weather observations. Long term average temperature, humidity, cloud cover, etc. Climate (our perception of long term average weather) generally changes relatively slowly as average conditions change due to some long-term changes in factors that control climate. Current focus on climate change is on the long term change in global temperature (Global Warming). Wind Climate Change in North America? Predicted summer temperature change. Predicted winter temperature change. Climate varies with the long term radiative balance: The balance between incoming solar radiation and radiation emitted back to space from Earth. 4

5 α = albedo, reflectivity of a surface (atmosphere or land). The albedo of the atmosphere varies with the amount of cloud cover and the concentration of atmospheric aerosols (solid dust and tiny liquid particles). Surface α Fresh snow Old snow Sandy soils Clay soils Peat soils Water surface (albedo) Greenhouse gases: absorb longwave radiation and emit some of it back to the Earth as heat. Water vapor and CO 2 are important greenhouse gas, others include: Methane Nitrous oxide Chlorofluorocarbons On average, 30% of incoming solar radiation is reflected back to space. The role of water vapour as a greenhouse gas is poorly understood. As earth warms more water vapour will be added to the atmosphere. Water vapour both increases greenhouse effect AND increases atmospheric albedo. Evaporation from the oceans acts to cool the earth. Water vapour is added by the breakdown of methane. Greenhouse Gas Atmospheric lifetime; GWP CO 2 tens of thousands of years; GWP of 1 over all time periods. Methane 12 ± 3 years; GWP of 72 over 20 years, 25 over 100 years and 7.6 over 500 years. The decrease in GWP at longer times is because methane is degraded to water and CO 2 through chemical reactions in the atmosphere. Nitrous oxide 114 years; GWP of 289 over 20 years, 298 over 100 years and 153 over 500 years. CFC 100 years; GWP of over 20 years, over 100 years and 5200 over 500 years. Atmospheric Lifetime: the average time that a molecule of the greenhouse gas will remain in the atmosphere. Global warming potential (GWP): measure of how much a given mass of greenhouse gas is estimated to contribute to global warming. It is a relative scale which compares the gas in question to that of the same mass of carbon dioxide (whose GWP is by definition 1). A GWP is calculated over a specific time interval. 5

6 Overall, there is a balance between incoming energy and energy emitted to space; over the long term they are equal. Climate changes as any component of the system changes. Especially: The amount and distribution of incoming energy from the Sun. The reflectivity of the Earth (changes in snow cover and atmospheric aerosols). The concentration of greenhouse gases in the atmosphere. Changes in any of these factors cause an imbalance in the radiative balance and the response is a change in climate. Radiative Forcing is any change in the average net radiation (incoming minus outgoing) reaching the top of the Earth s atmosphere. Positive radiative forcing (outgoing radiation is less than incoming) causes a net increase in global temperature. Negative radiative forcing (outgoing radiation is more than incoming) causes a net decrease in global temperature. The temperature of the Earth controls the amount of outgoing radiation: As temperature increases so does the amount of outgoing radiation. As temperature decreases so does the amount of outgoing radiation. For example: If the amount of incoming radiation is reduced, global temperature drops so that outgoing radiation is reduced to return to a radiative balance. The amount of radiation sent back to space from Earth can be reduced by increasing the concentration of greenhouse gases. More heat remains on Earth, the global temperature increases such that the amount of radiation from Earth to space increases to return to the radiative balance. 387 PPM The current concentration of CO 2 in the atmosphere is 370 ppm. Northern Hemisphere Growing Season Short-term variation in climate In geological terms, short-term refers to hundreds to thousands of years. Variation in greenhouse gases accounts for some variation in global temperature. Variation in incoming solar radiation also accounts for some of the global temperature change. Over the past 155 years the Earth s temperature has increased by 0.8 degrees C. Prior to 1800 temperatures were low (known as the Little Ice Age). 6

7 Why has Earth s temperature been increasing? Carbon dioxide 35% Proportion of Greenhouse Gases (GHG) in the atmosphere. Methane 88% Fluorocarbons Rising GHG concentrations correspond reasonably well with rising temperatures. 10% Incoming solar radiation varies with an 11 year cycle. Variation in incoming solar radiation also results in a change in the radiative balance and global temperature. Increased radiation: a positive radiative forcing (Earth becomes warmer). Decreased radiation: a negative radiative forcing (Earth becomes cooler). Variation in the amount of solar radiation reaching the top of the atmosphere over two years. 0.2% The number of sunspots also varies with an 11 year cycle (9.5 to 11 years, averaging 10.8 years over the last 150 years). Sunspot activity has an important impact on the amount of radiation that reaches the Earth (and it s the opposite of what you might think!). The sun rotates with an average period of 27 days; 30 days at its poles and 24 days at its equator. Average diameter of sunspots is about 37,000 km and they are regions on the sun s surface that are cooler than average (sun s diameter: 1,390,000 km). The outer layer of the sun is on average 6,000 degrees Kelvin Sunspots have an average temperature of about 4,600 degrees Kelvin. Sunspots form in regions of strong magnetic force intersecting the sun s surface. 7

8 Sunspots are darker, cooler regions on the sun. Associated with sunspots are faculae, brighter, hotter regions on the sun. Overall, the sun is brighter when there are many sunspots. Overall, the combination of sunspots and faculae result in a net increase in the average solar radiation. faculae From 1650 to 1700 AD there were almost no sunspots and the global temperature was particularly low. Since 1700 AD the number of sunspots has been increasing. The most recent sunspot maximum was in 2001 and we have passed the trough of the cycle and numbers are on the rise The solar magnetic field reversed in 2001 and will do so again in 2012 (as it always does at the sunspot maximum). The pattern of increasing global temperature over the past few hundred years corresponds reasonably well with the rise in the number of sunspots. Variation in Greenhouse Gases (GHG) accounts for 51% of the recorded variation in temperature. Variation in Solar Radiation accounts for 71% of the recorded variation in temperature. Combined Solar Radiation and GHG explain 92% of the variation in temperature. 8

9 The fact is, the Earth is 18,000 years into a warming trend that began when the glaciers began to retreat northward. 57% of the global warming over the last century is attributed to variation in solar radiation. 43% of the global warming over the last century is attributed to variation GHG. The relative role of greenhouse gases and solar variability is the topic of an ongoing debate. The CO 2 levels are certainly much higher than any time over the past several hundred thousand years, certainly due to addition CO 2 produced by human activity. Industrial revolution Long term variation in global temperature Climate Change Over Geologic Time The geologic record shows a wide range of variation in global temperature on a variety of scales. Radiative forcing due to a variety of causes has been postulated for many extreme climatic events. Tertiary and Quaternary periods have experienced wide fluctuations over relatively short spans of time with overall cooling towards the present. The Quaternary is characterized by repeated continental glaciation. The Cretaceous Period was the warmest time in all of geological history (average global temperature is estimated to have been 20 degrees C). 9

10 Over two different spans of time the Earth was much cooler than any other time in geologic history: The end of the Carboniferous Period. The end of the Precambrian Eon. From the Cretaceous to the present. Conditions in the Cretaceous: 5 degrees warmer than today (average global temperature = 20 degrees C). Ice caps were not present. Sea level was higher than any time in geological history. Due to melted ice caps and rapid sea floor spreading (twice the modern rate). Why was the Cretaceous so warm? A period of Greenhouse Earth. Sea levels were so high that over much of the Cretaceous the central portion of North America was occupied by the North American Interior Seaway. Rapid rates of seafloor spreading suggest high rates of magma delivery to the Earth s surface via volcanoes; including large volumes of CO 2 that accumulated in the atmosphere. Elevated CO 2 concentrations caused a positive radiative forcing. Temperatures rose to the level required for outgoing longwave radiation to match incoming solar radiation (same level as today). Volcanic activity peaked in the mid-cretaceous and diminished to the present..resulting in a long period of global cooling. Climatic fluctuation over the Tertiary and Quaternary periods. Through the Tertiary climate varied from colder to warmer in regular cycles. Glaciers: more-or-less permanent bodies of ice and compacted snow that have reached a thick enough accumulation to flow under their own weight. Glaciers currently cover 10% of the current Earth s surface. Average global temperature was warmer during the Tertiary Period so that continental glaciations did not take place. By the Pleistocene Epoch average global temperature had been reduced so that cold periods resulted in spreading of ice sheets over the continents on four different occasions. 10

11 The Water Budget Total Water on Earth 1,360,000,000 km 3 Oceans and Seas 1,331,746,800 km 3 (98%) Glaciers and Ice Sheets 24,000,000 km 3 (<2%) Groundwater 4,000,000 km 3 Lakes and Reservoirs 155,000 km 3 Soil Moisture 83,000 km 3 Vapor in the atmosphere 14,000 km 3 The distribution of modern glaciers Glaciers cover 15,800,000 km 2 of the Earth s surface (almost the area of South America). Approximately 97% of glaciers are in regions of high latitude (near the north and south poles). Remaining 3% of glaciers are at low latitudes but high altitudes. Location Area of glaciers Km 2 Rivers 1,200 km 3 The elevation above which glaciers will form (i.e., the snowline) depends on local climate. Southern British Columbia: approximately 1500 metres above sea level. Central Africa: approximately 5000 metres above sea level. The glacier ice cap on the top of Mt. Kilimanjaro is expected to disappear between 2010 and 2020 due to global warming as the snowline rises to higher elevations. Antarctica: snow line is at sea level. 18,000 years ago 32% of the land surface and 30% of the ocean surface were covered by glaciers. Canadian landscape was shaped by glaciers. 11

12 18,000 years ago 10 million square kilometres of North America was covered by a continental ice sheet up to 3 km thick. The centre of greatest ice thickness was near Hudson Bay. Isostatic subsidence depressed the land surface by approximately 380 metres. The region has rebounded by about 300 metres and continues to rise at 2 cm/yr. We currently live in a glacial period over which several continental glaciations have taken place. Sea level rose and fell over each period of glaciation. Growth of ice sheets has a dramatic impact on sea level. 18,000 years ago during the peak of the last continental glaciation sea level was approximately 140 metres lower than today. Sea level has been rising since that time but the rate has become progressively slower as the continental glaciers disappeared. At the glacial maximum 18,000 years ago the shoreline was up to several hundred kilometres offshore of the modern shoreline. If all of the ice sheets and glaciers were to melt sea level would rise by an additional 80 metres. Heavily populated areas worldwide would be flooded by a sea level rise on this scale. 12

13 Modern rates of sea level change are known from long term records. At present sea level is rising by 2 mm per year. Global warming is expected to cause an increase in the rate of sea level rise: Animated Earth with changing sea level. ( 18 cm above present by 2030 (5.6 mm/yr, average) 44 cm above present by 2070 (6.1 cm per year average) Disintegration of Modern Ice Sheets Ice sheets are the most extensive glaciers (>50,000 km 2 ). Flow outward, from the centre of thickest ice. At the shores of the land mass the ice sheet may flow onto the ocean to form an extensive, floating ice shelf. x Ice Shelf y A continental ice sheet Ocean x Ice sheet Ice y Greenland: 80% of total land mass is covered by an ice sheet. Average thickness 1.5 km, 3 km locally. Antarctica: Ice sheet covers 13.5 million square kilometres. Exceeds 4 km in thickness. Bounded by extensive Ice Shelves; e.g., Ross Ice Shelf: total area of 500,000 km 2 13

14 Larsen Ice Shelf: a relatively small ice shelf that is breaking up. Regional temperature has increased by 2.5 degrees C over the past 50 years. In February 1995 a major storm initiated the breakup of Larsen A Ice Shelf. The largest iceberg was 70 km long and 25 km wide. Hundreds of icebergs 1 2 km in size were liberated by the breakup. Larsen Ice Shelf continues to break up. March 2002 Larsen B disintegrated. 14

15 Over the year following the disintegration of Larsen B the glaciers that fed the shelf increased their flow rates from 1.7 m/day to 3.1 m/day (a 180% increase). Implications: In March, 2003 Iceberg B-15 broke off the Ross Ice Shelf. One of the largest icebergs ever seen, it was 300 km long and 40 km wide (11,000 km 2 ). Suggests that global warming may lead to breakup of other shelves. Such breakup leads to an increase in the rate of flow of glaciers into the Ocean. This, in turn, will increase the rate at which sea level rises with global warming. There is evidence that just such an event took place in the geologic past, when average global temperature was a few degrees warmer, raising sea level by more than 6 metres. The Ward Hunt Ice Shelf on the coast of Ellsmere Island (the Arctic s largest ice shelf) broke up in 2003 due to climatic warming. The Point: With global warming the ice shelves are breaking up. This is expected to increase the rate at which glacier ice flows into the world s oceans. This will lead to an increase in the rate of sea level rise. 15

16 18,000 years ago Melting of the Western Antarctic Ice Sheet Images are from William Haxby at the Lamont-Doherty Earth Observatory Melting of the Eastern and Western Antarctic Ice Sheet 18,000 years ago Melting of the Western Antarctic Ice Sheet Melting of the Eastern and Western Antarctic Ice Sheet 16

17 18,000 years ago Melting of the Eastern and Western Antarctic Ice Sheet Melting of the Western Antarctic Ice Sheet Melting of the Western Antarctic Ice Sheet 18,000 years ago Melting of the Eastern and Western Antarctic Ice Sheet 17

18 What caused the Pleistocene continental glaciations? Many periods of continental glaciation have taken place over the Earth s history. 4 glacial periods occurred in recent succession, approximately 100,000 years apart. Between each glacial period is an interglacial period when conditions are like modern conditions. Most theories of the continental glaciation involve changes in global climate. Average global temperature today is 15 degrees C. A reduction in temperature by 2 to 4 degrees C may cause the continental ice sheets to extend across the continents. Over the past 200 years the Earth s temperature has increased by about 1 degrees C (little chance of glaciation). Past speculation for causes of continental glaciation: Large volumes of volcanic ash in the upper atmosphere would increase the global albedo and reflect solar radiation back into space and cooling the Earth. Large asteroid impacts throw dust into the atmosphere, reducing receipt of solar radiation (and result in cooling). Such events have caused considerable cooling but there is no correlation between continental glaciations and such events. These mechanisms cannot explain the 100,000 year cycles of glaciation over the recent past. What appears to be the correct answer to the question of the cause of continental glaciations came from Mulatin Milankovich..Milankovich Cycles Milankovitch conducted a mathematical study of the effect of the Earth s orbit on climate. He identified three components of the Earth s orbit and rotation that would affect climate: Eccentricity Obliquity Precession Eccentricity: variation in the form of the Earth s orbit about the sun. Low eccentricity High eccentricity High eccentricity increases differences between seasons, low eccentricity decreases difference between seasons. 18

19 Obliquity: variation in the angle between the axis of rotation of the Earth and the plane of the orbit about the sun. Precession: a wobble of the Earth s rotational axis. Influences the difference in sunlight reaching the poles in winter and summer (high angle, more summer sunlight, less winter sunlight). Slightly influences amount of sunlight in summer and winter in polar regions. At present the angle is 23.5 degrees. All three components vary in a cyclical manner. Duration of each cycle: Eccentricity: 100,000 years Obliquity: 40,000 years. Precession: 26,000 years. All three cycles are superimposed and cause variation in the distribution of solar radiation over the seasons. During parts of the cycle: Mild winters, abundant snow and cool summers. Conducive to development of glaciers. When eccentricity is low widespread glaciation is most likely (more uniform seasons) Seasonal extremes (hot summers and cold winters) associated with high eccentricity are less likely to see the spread of glaciers. Cold, dry winters, hot summers. Glaciers will not develop. The 100,000 year cycle of eccentricity has the greatest effect and corresponds to continental glaciations. Once glaciers begin to develop they increase global albedo, reducing incoming solar radiation and causing further cooling of the Earth. 19

20 In the 1970s a means of testing the hypothesis was developed based on the chemistry of shells produced by organisms. The correlation between eccentricity and global temperature was striking and supported Milankovich s ideas. The concentration of certain isotopes that are incorporated into shell material was found to be a function of seawater temperature. Periods with smaller seasonal differences in temperature cause the beginning of glacier advance away from the poles to extend across the continents. Once the glaciers begin to advance they increase global albedo (reflecting more incoming solar radiation away from the Earth). This reduces the average global temperature which, in turn, causes the further advance of glaciers. Carboniferous/Permian The Permian Period began with the lowest temperatures of the Phanerozic Eon. It coincided with a marked decrease in O 2 concentration following the peak in the Carboniferous. The glacial advance ends when seasonal differences increase and the glaciers retreat back towards the poles. The Permian was a time when all of the major continents were grouped into a supercontinent called Pangea. Much of the land surface was exposed to a very dry, inland climate, much like central Canada or Asia today. Much of the land surface extended north and south of the equator, in climatic zones that experience desert-like conditions. Extensive arid conditions reduced plant life which, in turn, led to a reduction in atmospheric O 2. With the widespread, arid land mass, continental weathering of existing rocks took place at a greater rate than in the past. Weathering of rocks can remove CO 2 from the atomosphere: Carbonic acid is important for several weathering reactions. 20

21 E.g., weathering of limestone (made up of the mineral Calcite) Snowball Earth An excellent article is located at Paul Hoffman s site at: The end result is the loss of CO 2 from the atmosphere. Lower CO 2 results in a negative radiative forcing and global temperature falls. The subsequent increase in temperature was likely due to increased volcanic activity that continued through to the Cretaceous. Hypothesis: That million years ago the Earth was effectively covered by glacial ice, including 500 to 1500 m thick sea ice cover. Evidence: Glacial deposits (e.g., Tillite.the rock form of Till) that were laid down at low latitudes, apparently at the equator and near sea level on several continental masses of the same age. Tillite: a sedimentary rock that is made up of a wide range of sizes of materials ranging from fine mud to huge boulders. Such deposits are laid down from retreating glaciers. How do we know the latitude of ancient tillites? By measuring the magnetic inclination of the weak magnetic field associated with the deposits. Near horizontal inclination indicates deposition near the equator. There was considerable skepticism that these were glacial deposits because it seemed impossible for glaciation to proceed all the way to the equator. How would such glaciation come to be? Mikhail Budyko (USSR) undertook climatological modeling to determine whether or not a condition could exist that would cause glaciers to extend to sea level at the Equator. Based on a climatic energy balance. Simplest case: Net Energy Reaching Earth = incoming radiation (I) reflected radiation (α x I) Important factors: Albedo: The higher the global albedo the more incoming radiation is reflected and the less is absorbed to warm the Earth. Greenhouse gases: The more greenhouse gases are in the atmosphere, the more heat is retained (temperatures rise). Budyko found that as glaciers advance southward from the poles that global albedo increases, cooling the Earth and enhancing formation of glaciers. 21

22 If sea ice and glaciers come to extend to within 30 degrees North or South of the Equator then global albedo becomes high enough so that the glaciers will proceed to cover the Earth to the equator (forming a Snowball Earth). Such freezing of the Earth would drastically reduce the amount of water vapor, reducing the greenhouse effect even further, causing even further cooling. What would cause such a glaciation 700 million years ago? 1. Sun s energy was 6% less than today (less incoming heat). 2. With the evolution of algae CO 2 uptake by photosynthesis may have decreased the amount of CO 2 in the atmosphere: lessening Greenhouse Effect. But, if this were to ever had taken place, why does the Earth not continue to be a snowball? Joe Kirschvink: Found a possible cause of the destruction of such global glaciers. Even with a cover of ice plate tectonics will continue to be active. Volcanism would continue to add gases to the Earth s atmosphere, including CO 2. Over time, the CO 2 concentration would increase at a constant rate. CO 2 would not be lost to weathering and the formation of carbonate rocks (ice covered continents don t weather). To melt the glaciers and sea ice, given the high albedo, would require 350 times as much CO 2 as there is in the current atmosphere. It would take 10 million to 40 million years for sufficient CO 2 to accumulate to the required levels. Average global temperatures would rise to almost 50 degrees Celsius and it would take only a few thousand years for sea ice to melt.reducing the global albedo. 22