Topic 8: Greenhouse Effect & Global Warming 8.5 & 8.6 Allen High School IB Physics SL Source: Chris Hamper Physics

Disclaimer This section is the most political and opinionated for the whole year. Please focus on the tested material, not your opinion. There will be questions on the May test, the answer will not necessarily match your opinion.

Greenhouse Effect This is the warming of a planet due to its atmosphere absorbing ultraviolet radiation from the Sun and trapping the infrared radiation emitted by the warm Earth. Two foundation principles: How does the energy of the Sun get to Earth? How does this energy interact with the atmosphere?

Solar Radiation As previously discussed, the power per unit area that reaches Earth is a constant: 1380 W/m2 The intensity is related to its distance from the Sun. We learned in Topic 7 that the electrons of atoms can be excited and move to a higher energy level. Light can excite electrons if the energy of the photon is exactly the same as the required amount. The reverse is true too: If an excited electron drops down to a lower level, then it can release a photon. Different level jumps/drops can result in different frequencies of light. This is what happens when the atmosphere scatters light.

Ionization & Excitation Ionization happens when the photon of high frequency/energy is absorbed and causes an electron to be ejected from an atom. Molecules can become excited when it absorbs a photon of the same resonant frequency. This absorption causes the molecules to move more, thus a higher temperature. Typically, it is the infrared region (lower frequencies) of the spectrum that cause this sort of vibration.

Albedo The Sun’s electromagnetic radiation shines on a surface, it is either absorbed (causing surface to get warmer) or it is reflected. The RATIO of reflected to absorbed radiation is called Albedo. Snow has a high albedo b/c most radiation is reflected than absorbed. (albedo = 90%) Dark forests have a low albedo (10%). The average for Earth is 30%. This means it is not reflecting most of the radiation.

Albedo Chart

Blackbody Radiation A heated solid will radiate (emit) a wide range of frequencies/wavelengths (colors), however not every wavelength will be equally intense. Blackbody radiation is the radiation emitted by a “perfect” emitter. A Blackbody absorbs all wavelengths, and can emit all too.

Stefan-Boltzmann Law As seen in the intensity distribution graph, as the temperature of a black body increase, the intensity of the radiation also increases. Or, the amount of energy emitted from the surface increases. The S-B Law relates the power emitted per unit area to the temperature of the surface: Stefan-Boltzmann Constant, σ = 5.67 x 10 -8 W/m2K4

Emissivity Interesting: If the Earth had no atmosphere, the calculated temperature of Earth would be -18C! However, Earth is much warmer (thank goodness) because Earth’s atmosphere absorbs some of the radiated radiation. This is the Greenhouse Effect. Earth does not radiate as well as a true blackbody. Emissivity is the ratio of the energy radiated by a body to the energy radiated by a blackbody of the same temperature. (e values are usually decimals.)

Absorption Sun’s rays beam down on Earth (100%) The outer atmosphere (ozone) absorbs high energy (UV & X-rays) parts of the spectrum (20%). Lower layers absorb infrared radiation via water vapour and CO2 (remember resonance) (30%). That leaves 50% of the sun’s energy arriving to us on Earth. At Earth’s surface, some energy is reflected, which goes back through the atmosphere, not absorbed again. The remaining is absorbed (visible) and increases the temperature of the ground. Energy is then re-emitted from the ground in the IR region.

Surface Heat Capacity The temperature increase of the ground can be calculated based on the heat absorbed. (Don’t confuse this with Topic 3’s Heat capacity or Specific heat capacity!) Surface Heat Capacity is the amount of heat required to raise the temperature of 1m2 of the ground by 1K. For Earth, Cs=4 x 108 J/km2 Equations:

The Greenhouse Effect The IR radiated from the ground travels upwards through the atmosphere and as it does it is absorbed by CO2 and H2O. As a result, the molecules become excited, raising the temperature of the atmosphere. The excited molecules then radiate IR radiation in all directions, some traveling back to Earth. The Earth’s net temperature is based on the radiation leaving and arriving. By reducing the amount leaving, the temperature at which this balance will be achieved will be higher.

Sankey Diagrams for Greenhouse Effect

Global Warming The increase of Earth’s temperature until equilibrium is restored. Models are developed to help scientists predict future outcomes. Causes: Radiation from the Sun is not constant. Due to solar flares/sunspots Due to Earth’s orbit The amount of Sun radiation absorb by Earth’s atmosphere can vary depending on the composition of gases in it. Greenhouse gases include H2O, CO2, Methane, and nitrous oxide (in order of contribution).

Ice Core Data Antarctica ice has been growing (new layers atop old layers) for thousands of years. Drilling into these layers and taking samples from different layers has allowed scientists to examine the concentration of different isotopes of hydrogen. The varying concentrations help to determine the temperature of the layers. Heavier isotopes mean the temperature was colder. The layers of ice also contain air bubbles that tell us the composition of the atmosphere at the time.

Ice Core Data Let’s compare the temperature of Earth with the concentration of atmospheric CO2.

The Last 50 Years Data

Volume Expansion As the temperature of a liquid increases, it expands. The Coefficient of Volume Expansion is the fractional change in volume per degree change in temperature. Applying this to the oceans (water), if the average temperature increases then they will expand. Over the last 100 years, sea level has risen by 20 cm. Another cause of the rise is the melting of the ice caps (covers land in Antarctica). This does not include floating ice (glaciers), they displace their own mass, melting has no effect.

What causes the changes in CO2? The finger is pointed at Human Activity. Burning of fossil fuels = pollution of greenhouse gas Deforestation = less plants to absorb CO2 as normal What can WE do? Greater efficiency of power production Replacing the use of coal and oil with natural gas Use of combined heating and power systems Use exhaust heat from power plants to heat homes Increase use of renewable energy sources and nuclear Use of hybrid vehicles Carbon dioxide capture and storage

This is an International Issue IPCC (Intergovernmental Panel on Climate Change), 1988 Assess all available information relating to human induced climate change. Kyoto Protocol, 1997 Countries committing to reduce their greenhouse gas emissions by given percentages. By May 1997, 117 countries signed. Eairheart thoughts: We should all do what we can to reduce emissions, it doesn’t hurt to have less pollution!! Why not?!