The Earth has an atmosphere which acts like a blanket, trapping heat, and making the surface life-supporting. Adding unnatural quantities of certain gases upsets this delicate balance, leading to changes in the global climate.
Feedback mechanisms in climate change
Feedback mechanisms in climate change theory are mechanisms which are secondary effects of direct anthropogenic changes to the Earth's biospheric conditions.
There are both positive and negative feedback mechanisms: the positive accelerate the global warming trend, and negative slow it down.
- Melting of tundra
- Reduction of albedo where ice caps have melted at high altitudes
- Depletion of ozone in upper atmosphere letting through more short wave radiation to ground level.
- Increased cloud cover (more reflected sunlight)
- Increase in precipitation at poles (reducing sea level rise)
- Increased CO2 uptake in oceans
- Thermal mixing in oceans slowing expansion of water
- Air pollution by certain chemicals (e.g. sulphate aerosols and SO2), reducing short wave radiation received at the surface.
- Non-linear responses by atmosphere to increasing GHG levels
- Solutions of equations in GCMs (General circulation model, or global climate model, climate modelling)
- Certain cloud types
Solar irradiation and climate
The intensity of solar radiation is a key factor in climate, and therefore has a great influence on the distribution of species.
Life is found all over the planet. It is very determined, and adapts to the conditions in which it finds itself. The most obvious factor in determining these conditions is the climate. The latitude (how far north or south) is the most important of all climatic factors, since the intensity of the Sun's radiation varies most directly with latitude.
The Earth is a sphere, so the part of the Earth receiving the most vertical sunlight (the middle, or equatorial regions) will receive the most sunlight, and be hotter. As we move further north or south, the same amount of sunlight is spread over a larger area, and therefore the temperature and irradiation (sunlight available for photosynthesis) decreases (for those who like maths: by the sine of the angle of incidence).
Seasons are caused by the angle of tilt of the Earth, making the sun's heat inconsistent at any one point on the surface
The Earth is tilted to the plane of its orbit around the Sun. This means the line between the north and south poles (axis of rotation) is not at right angles to the line Earth-Sun (in the orbital plane). The angle of tilt is 23.44°. As the Earth goes around the Sun, the tilt causes the angle the sunlight strikes the Earth's surface at (angle of incidence) to change, and therefore the amount of sunlight that hits any place on Earth will vary continuously throughout the year. This causes seasons.
A square metre of the Earth's surface at sea level receives on average 1050 watts of energy from the sun in full sunshine. One estimate (WMO) suggests that the average over the entire Earth is about 164W/m2 over 24 hours. This will obviously vary enormously depending on latitude, cloud cover, and season. 'Sunshine duration' is the cumulative time at least 120W/m2 is received by an area. [World Meteorological Organization WMO definition]
The solar constant is 1361 W/m². This is a measure of flux density of solar EMR incident to a plane perpendicular to the rays at the distance of the Earth from the Sun (1 AU). Since the atmosphere reflects and absorbs part of this radiation, the amount of energy received on the surface is more like 1050 W/m², which provides illuminance of 98,000 lux (lumens/m²). Sunlight at the Earth's surface is 52-55% Infrared (> 700 nm wavelength), 42-43% visible light (4-700 nm), 3-5% ultraviolet UV (< 400 nm ).