The Earth’s Atmosphere and Climate 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
After Earth formation…… Earth experienced a period of internal melting (due to initial high temperatures and heatfrom radioactive decay). The process of forming the Earth was complete by about 4.5 billion years ago. Dense elements (especially iron and nickel) sank to the centre of the Earth, forming the metallic core.
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.
As the Earth cooled and differentiation continued the crust became thicker and continents began to "grow" due to plate tectonics. 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.
Earth’s atmosphere and early life The modern atmosphere has a composition that is very different from that of the sun. Initial atmosphere may have had a "primitive" composition, like that of the sun (gases derived from the initial nebula).
E.g., much more Ar40 compared to other isotopes of Ar in the Earth’s atmosphere. Ar40 formed by decay of K40, therefore, was derived from the Earth after it formed. Other Argon isotopes were removed with the early atmosphere and the Argon40 formed by subsequent K40 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 combined composition of the modern atmosphere, biosphere, hydrosphere and sediments is very similar to that of volcanic gas.
The modern atmosphere resulted from "outgassing" of the Earth: emission of gases during volcanic eruptions. Over time the Ar40 was formed in the Earth by decay of K40 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 (CO2) Water vapour (H2O) Nitrogen(N2) Minor gases: Hydrogen (H2) Hydrogen chloride (HCl) Sulfur dioxide (SO2) Note: there was virtually no free oxygen (which makes up about 20% of the modern atmosphere).
Some free Oxygen was produced by dissociation of water in the upper atmosphere due to ultraviolet radiation: However, the rate at which O2 appears to have increased could not have been produced by dissociation alone. 2H2O + ultraviolet rays = 2H2 +O2 Light H2 is lost to space and heavier O2 remains in the atmosphere.
Photosynthesis is likely responsible for more rapid production of Oxygen: • CO2 + H2O + light (+ chlorophyll) = (CH2O) + O2 Photosynthesis requires chlorophyll, a complex chemical that is produced by some organisms (e.g., plants).
The oldest organisms that could produce chlorophyll are cyanobacteria: single celled organisms that lacked an organized nucleus and lived in the oceans. 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. There is strong correlation between O2 levels in the atmosphere and the development of life, on Earth. Cyanobacteria began the process of increasing atmospheric Oxygen.
As Oxygen levels increased as aerobic organisms developed (that used free Oxygen) and these, in turn produced even more 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 O2. Variation in O2 levels over the past 500 million years reflect changes in plant cover on Earth. Carboniferous: warm, moist, tropical settings predominated on land and land plants thrived. O2 levels almost doubled. Permian and Triassic: arid conditions were extensive on land and the abundance of land plants diminished. O2 levels dropped to below 15%.
Climate versus Weather Weather: the condition of the atmosphere at a particular point in space and time.
Weather changes over short periods of time and is described in terms of: Air temperature Air pressure Humidity Cloud cover Precipitation (rain, snow) Visibility Wind
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).
Climate varies with the long term radiative balance: The balance between incoming solar radiation and radiation emitted back to space from Earth.
a = 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 a Fresh snow 0.8-0.95 Old snow 0.42-0.70 Sandy soils 0.25-0.45 Clay soils 0.20-0.35 Peat soils 0.05-0.15 Water surface 0.05-0.08
On average, 30% of incoming solar radiation is reflected back to space. http://www.physicalgeography.net/fundamentals/7f.html
Greenhouse gases: absorb longwave radiation and emit some of it back to the Earth as heat. Water vapor and CO2 are important greenhouse gas, others include: Methane Nitrous oxide Chlorofluorocarbons
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 CO2tens 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 CO2 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 11000 over 20 years, 10900 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.
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: 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. The current concentration of CO2 in the atmosphere is 370 ppm. If the amount of incoming radiation is reduced, global temperature drops so that outgoing radiation is reduced to return to a radiative balance.
Northern Hemisphere Growing Season
Short-term variation in climate 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). 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.
Why has Earth’s temperature been increasing? Carbon dioxide Proportion of Greenhouse Gases (GHG) in the atmosphere. 35% Methane 88% Rising GHG concentrations correspond reasonably well with rising temperatures. Fluorocarbons 10%
Variation in the amount of solar radiation reaching the top of the atmosphere over two years. 0.2% 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).
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.
Associated with sunspots are faculae, brighter, hotter regions on the sun. faculae Sunspots are darker, cooler regions on the sun. Overall, the combination of sunspots and faculae result in a net increase in the average solar radiation. http://earthobservatory.nasa.gov/Library/SORCE/sorce_03.html
Overall, the sun is brighter when there are many sunspots. http://earthobservatory.nasa.gov/Library/SORCE/sorce_03.html
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).
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 pattern of increasing global temperature over the past few hundred years corresponds reasonably well with the rise in the number of sunspots.
http://www.john-daly.com/forcing/moderr.htm 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.
http://www.john-daly.com/forcing/moderr.htm 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 fact is, the Earth is 18,000 years into a “warming trend” that began when the glaciers began to retreat northward.