Lecture 3: The interconnected Earth system

## **Lecture 3: The interconencted Earth system: Ocean, atmosphere and land** **(Katy Sheen)**

**What is this lecture looking at?**

* How is the Earth system powered?
* What factors imapct the Earth's cliamte and cause it to be how it is?

**The Earth's energy budget:**

* The main inputs and outputs of energy flux’s to the Earth’s system
* An energy flux is the amount of energy that is hitting one square meter of the Earth’s surface every second
* ^ it is always the quantity, per unit time, per unit area.
* Most of the energy that drives our Earth’s systems comes from the sun.
* The sun produces around 342W m-2- that is the average energy flux that reaches the Earth’s surface from the Sun.
* Not all the solar radiation is absorbed by the Earth’s surface - the Earth’s surface and the atmosphere reflect some of the Sun’s energy back to space.
* Exactly how much energy reflected depends on the reflectivity of the atmosphere (e.g amount of cloud cover) and the land surface (e.g oceans absorb a lot of solar energy, snow and ice and clouds reflect a lot of energy).
* The Earth’s albedo is the fraction of the incoming energy from the Sun that is reflected back into space.
* The global mean albedo is about 0.3, so around 1/3 of the Sun’s energy is reflected back into space (around 102Wm-2), with the remaining 240Wm-2 being absorbed by the land, ocean and atmosphere. Figure 1.
* Land use also affects the albedo e.g a city will have a different reflectivity to agricultural lands desert region, or a rain forest.
* Aerosols (fine solid particles suspended in the air) also affect planetary albedo.
* Aerosols, Natural (e.g dust from volcanic eruptions, sea salt, fog) and man made (e.g pollutants, dust) promote the formation of clouds as they provide particle ‘seeds’ for condensation droplets to form, and tend to increase planetary albedo. These clouds are bright so reflect more.
* For the temperature of the Earth to remain relatively constant, the overall flux of energy into the system is equal to the flux out. It is in balance. So there must be a way that the Earth emits energy back to space, to keep this equal. This comes to a theory of black body radiation.

**Black body radiation and calculating the Earth's temperature:**

* Any object which is above the temperature of absolute zero\*, emits electromagnetic radiation. This is how the Earth gets rid of energy.
* \*Temperate is a measure of how much the atoms or molecules which make up a substance vibrate. Absolute zero is defined as the point where there is no atomic vibration. It occurs at zero Kelvin.
* The total thermal energy flux is emitted from a black body per second per unit area, E, is related to it’s temperate by the Stefan-Boltzman law:
* The amount of energy emitted by an object (in terms of radiation), is equal to a constant x the temperate of that object (in degrees kelvin) to the power of 4.
* So the energy it emits is dependent on the temperature.
* The Stefan-Boltzmann constant is 5.67x10(to the power of -8)Em2 K-4
* However, if we did all of that then it shows that the temp of the Earth would come out at about -18 degrees C. SO we made some wrong assumptions.
* What we didn't take into account was that the energy coming from the sun is from a different part of the electromagnetic spectrum to what the Earth emits.
* The energy coming from the Sun is largely in the visible wavelength range (out eyes evolved to see sunlight). But because the Earth is much cooler than the Sun, the outgoing radiation from Earth is characterised by longer wavelengths (infra-red).
* Certain gases in the atmosphere (Greenhouse gases- CO2, H20 and CH4) absorb certain radiation bands in the infra-red part of the spectrum. As a result some of the energy which is emitted by the Earth as electromagnetic radiation is absorbed by the atmosphere and reemitted back to Earth, warming it up (Greenhouse effect). When they reemit it, they do so in all directions- so half of it goes to space, and the other half goes back down to earth, and warms it up.
* So we need to take into account these atmospheric gases in the Earth’s energy budget.
* However, there are more things that haven't been taken into account:
* The real atmosphere emits more energy downwards than upwards
* Not all the radiation emitted by the Earth gets absorbed by the atmosphere (greenhouse gases only absorb certain wavelengths)
* Radiative transfer is not the only means that energy escapes from the Earth’s surface: heat also reached the atmosphere by the conduction of heat (sensible heat) and by evaporation and condensation (latent heat). The total is about 100Wm-2.
* One these are accounted for, the true energy emitted as infra red radiation by the Earth’s surface is 390Wm-2. This causes the Earth to be the right temperature.
* Heat that is associated with a change in phase is known as latent heat.

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**​Upsetting the balance:**

* In the Earth’s energy balance, the amount of energy reaching the Earth’s system equals that which escapes. If the Earth’s energy budget remains balanced in this way, then the temperature of the Earth will remain relatively constant.
* However, changes in the Earth’s energy balance by increasing or decreasing the amount of energy in the system, results in a changing climate. There are different ways this occurs:
* Changes in the Sun’s output (sunspots)
* Variations in the Earths orbit (Milankovitch cycles- Eccentricity, Obliquity, Precision).
* Changing the amount of greenhouse gases in the atmosphere
* Changing the Earth’s albedo: Changing the land use, e.g significant deforestation. Changes in aerosols e.g volcanic eruptions.
* Increased aerosols from human activity are thought to cool the Earth, compensating global warming to some degree. Current estimates sugar that global cooling driven by aerosol effects is less than half as much as the warming caused by greenhouse gases.
* The global mean albedo also varies with global mean temperature. During glacial periods, more of the Earth is covered by ice and snow increasing the albedo, cooling it even further (feedback loop). The albedo varies from about 0.3 (when the Earth is warm) to around 0.7 (when it is cooler during glacier times).
* El Niño also affects the cloud cover, impacting it too.

**Monitoring the Earth's energy budget:**

* We have been monitoring it since about 1997
* We have been monitoring it by the TERRA satellite through satellite sensors know as CERES (Clouds and Earth’s Radiant Energy System). Both visible and infra red radiation can be monitored by it.

**Energy flow:**

* The energy received from the Sun is not evenly distributed, due to the Earth’s curvature it is much more intense on the Equator.
* This sets up the temperature gradient between the equator and the poles. Whenever you get a temperature gradient you get a flow (convection current).
* See global atmospheric circulation drawing.
* The Coriolis effect causes spinning in out atmospheric weather patterns.

**Water in the atmosphere:**

* An important part of atmospheric flow is the movement of water. Water has the ability yo store and transport lots of heat energy
* Evaporation of water near the Equator, takes energy from the Sun. When it condenses to clouds, usually further towards the poles, this energy is released.
* This evaporation-condensation cycle is an important mechanism for transferring heat energy around Earth.
* Most air masses have lots of energy and are thus prone to powerful disturbances: thunderstorms, tornadoes, hurricanes.
* This is the latent heat effect.

**Global warming impacting atmospheric ciirculation patterns:**

* The poles are warming faster than the other parts of the Earth. The Arctic is warming twice as fast as the global average warming, because:
* Significant changes in surface albedo (Reduced ice)
* The thinner atmosphere at the poles (less air to heat)
* The drier atmosphere at the poles (more energy in raising temperatures as opposed to evaporating water)
* The transport of energy to poles by large weather systems transport energy to the poles.
* This is going to impact het atmospheric circulation, which was dependent on the particular temperature gradient.

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**Ocean circulation:**

* The ocean currents flow to redistribute heat around the globe
* They are driven by: surface winds (to a degree, patterns reflect the surface wind field); salinity variations from the distribution of precipitation and evaporation; the Coriolis effect; and land masses.
* Global over-turning circulation: Cold, dense waters (fro the salty water) form in the North Atlantic and around Antarctica through cooling and ice formation. They are generally upwelled in the Southern ocean. Warm, surface currents flow to complete the cycle e.g the Gulf stream. A complete cycle through the ocean takes around 1000 years, so the deep oceans act as a long terms store for heat and carbon dioxide, and help to dampen the effects on global warming.
* Oceans are an important sink for atmospheric Co2- this is reducing some of the impacts of increased Co2 in the atmosphere, but it is interfering with the chemistry of the oceans.
* As oceans absorb Co2, they become more acidic.
* Increasing acidity has a dramatic erect on some calcifying species.

**Global warming's impact on the over-turning circulation:**

* It may impact on it
* Warmer temperatures are resulting in rapid melting of the Greenland ice cap
* As a result waters around Greenland are warmer and fresher endless likely to sink, slowing the southward flow of deep, cold water in the north Atlantic and hence the warn Gulf stream which flows to replace it
* The warm Gulf Stream keeps the UK from freezing over in winter.
* The North Atlantic Overturning Calculation is currently being monitored by a line of oceanographic instruments called the RAPID array.

* The Earth is a complex, interacting system. When you do one thing you affect another. It does not contact independent cycles and processes.
* Due to this, the only way that we can really study this is through climate models.

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