There are three main factors deciding the climate of a planet:
- Solar input –insolation. The total solar influx, depending on distance from the sun, angle of the planet’s axis and solar activity
- Albedo – or reflections of solar rays from the Earth and back into space
- Chemical composition of the atmosphere
There are several other factors and mechanisms having impacts as well, such as changes in ocean currents and maybe cosmic radiation, but as a rule they are not the primary forcing factors, but rather parts of feedback systems that enforce the trend in either cooling or warming the planet. As a rule the chemical composition of the atmosphere acts as a feedback system to the two other factors, but there are exceptions to this.
Any change in the climate (not the weather!) can only be explained by a change in the radiative forcing from these three factors.
If there has been no change in the solar forcing, the change must find its explanation in one or both of the other two factors.
The output from the sun varies.
There is a regular fluctuation in the number of sunspots; darker areas with lower temperature. Solar energy output increases with the number of sunspots, since these spots are balanced by development of bright areas; facuale . The heating effect of these outweigh the cooling effect of sunspots.
The energy output from the sun as a rule rises and falls in line with an 11-year sunspot cycle. There are other cycles as well. There is e.g. a 22-year cycle that may be linked to development of magnetic fields associated with sunspots and that may affect cosmic rays penetration effect and cloud formation. (Source: WMO: Climate into the 21st Century, p.90 ).
Last time there was a high sunspot activity was in 1998. Since 2003 sunspot activity has been unusually low.
The Earth’s atmosphere mainly consists of Nitrogen, Oxygen and inert gases.
In addition there is water vapor and other greenhouse gases like CO2, methane, nitrous oxides, ozone, fluorides etc. When the amount of greenhouse gases (GHG) increases, the greenhouse effect will also increase correspondingly.
Albedo is the ability to reflect sunlight out into outer space. Ice and snow, deserts, clouds, and most aerosols (particles in the air) reflect sunlight. The higher albedo, the more solar energy reflected; the colder. When it is getting colder, ice cover expands, thus also increasing the albedo.
A cooling trend thus enforces itself, as a warming trend likewise enforces itself by causing less albedo and releasing more greenhouse gases, above all, increasing the amount of water vapour in the air.
The amount of aerosols in the air has direct effect on the amount of solar radiation hitting the Earth’s surface. Aerosols may have significant local or regional impact on temperature.
Volcanoes can inject dust and sulphur dioxide into the upper atmosphere where sulphuric acid particles act like a veil for years, reflecting solar rays in the stratosphere, cooling the air closer to Earth. The Mt. Pinatubo eruption in 1991 cooled the global temperatures for 2-3 years.
Sulphuric compounds emissions from human activities (fossil fuel combustion) may have similar effects.
Water vapour is a greenhouse gas, but at the same time the upper white surface of clouds reflects solar radiation back into space.
Albedo – reflections of solar radiation from surfaces on the Earth – makes exact predictions difficult. If e.g. the polar icecap melts, the albedo will be significantly reduced.
Open water absorbs heat, while white ice and snow reflect it.
Ice floating in seawater. When sea ice melts, the darker sea water surface
is exposed, absorbing sunlight instead of reflecting it, as the white ice does.
(Photo: Å. Bjørke).
Barren lands reflect more light than e.g. forests. Growing forests in a desert or on previously barren mountains will thus absorb carbon dioxide from the atmosphere, but the darker tree canopy may reduce albedo compared to the light desert sand or bare rocks. Initially, the net balance may actually be a slight warming in spite of a small reduction in greenhouse gases. Over time the accumulation of CO2 in the wood will outweigh the reduced albedo – unless of course the wood is burned.
Dust particles in the air may reflect light, but settled on e.g. glaciers; soot and dust particles may absorb heat and increase melting.
See video on the science of climate change (IPCC 2013)
- Climate literacy A guide for individuals and communities.( U.S. Global change Research Program )
- What is climate change? (Met office)
- The decreases in Earth’s snow and ice cover over the past 30 years have exacerbated global warming more than models predict they should have, on average, new research from the University of Michigan shows.
- IPCC (2013) Clouds and aerosols (heavy download)
- Astronomical Theory of Climate Change (Milankovich cycles)