Drivers of Climate Change
There are many causes for climate change. The most common explanation for climate change on the long timescale of glacial-interglacial cycles is the variation in the Earth's orbit around the sun. Over hundreds of thousands of years, variations in the sun's solar ouput may fit observed climate trends. The warming that the Earth has experienced in the last few decades (global warming) is increasingly seen as driven by the human pollution of the atmosphere. There is also evidence that volcanic activite can alter the climate, but usually only for a few years.
Climate forcing = Any mechanism that alters the global energy balance and 'forces' the climate to change in response.
The Milankovitch Cycles (orbital eccentricity, axial precision and axial tilt) are generally accepted as the base cause of long-term climate change. These variations in the Earth's orbit around the sun cause a change in the amount, distribution and seasonal timing of solar radiation, which affects the surface temperature of the Earth.
Over 100,000 years, the Earth's orbit changes from circular to elliptical and back again (orbital eccentricity). This changes the amount of radiation received from the sun. Over 41,000 years, the Earth's axis tilts from 21.5 to 24.5 and back again (axis tilt). This changes the seasonality of the Earth's climate. The smaller the tilt, the smaller the difference between summer and winter. In addition, on a 22,000 year timescale, the Earth's axis 'wobbles' and this changes the point in the year that the Earth is closest to the sun (axial precession).
The evidence for this theory comes from the regular ice age and interglacial pattern. The actual impact of these orbital changes is enough to change global temperatures by 0.5C, whereas records show that ice ages were about 5C colder than the interglacials. The cyces may have been enough to trigger these more substantial climate changes, but probably aren't enough to sustain it. Feedback mechanisms (e.g. greater reflection of solar radiation from increased snow cover that has a higher albedo) are needed to sustain major global climate change.
Medium-term climate change has been linked to changes in solar output at timescales longer than the regular 11-year sunspot cycle. Sunspots (dark spots on the sun's surface caused by intense magnetic storms) have been well recorded over the last 400 years. It would seem that the Little Ice Age could be linked to a long period with almost no sunspot activity.
The amount of energy emitted by the sun varies as a result of sunspots. The effect of them is to blast more solar radiation towards the Earth. The total variation in solar radiation caused by sunspots is about 0.1%. Some scientists have suggested that ~20% of 20th century warming may be attributed to solar output variation.
Key Concept: Climate Feedback
Feedback effects can either amplify a small change and make it larger (positive feedback) or diminish the change and make it smaller (negative feedback).
An example of positive feedback is snow and ice cober. Small increases in snow and ice dramatically raise surface albedo (reflectivity), so more solar energy is reflected back into space. This contributes to further cooling, which might encourage further snowfall. This may be how the 0.5C cooling identified by Milankovitch is amplified into a 5C global cooling.
An example of negative feedback is cloud cover. As global warming occurs, more evaporation will occur and this may increase global cloud cover. Increasingly cloudy skies could reflect more solar energy back into space, and diminish the effect of the warming.
Volcanic and Cosmic Causes
Volcanic activity at super-volcano scale can alter the global climate, because volcanoes eject huge volumes of ash, water vapour, sulphur dioxide and carbon dioxide into the atmosphere, where high level winds distribute it around the globe, which reduces the amount of sunlight received at the Earth's surface.
High in the atmosphere, sulphur dioxide forms a haze of sulphate aerosols, which reduces the amount of sunlight received at the Earth's surface. However, the temperature changes are short lives (1-2 years), as the sulphate aerosols only persist for 2-3 years. Similar cooling effects would be felt following a major asteroid strike on the Earth as dust and debris thrown up by the impact blanketed the planet. Cooling would be short term unless feedback mechanisms occured to amplify the change.
Patchy historic records do suggest increased volcanic activity during the Little Ice Age, but is unlikely to have been its cause.
Atmospheric Forcing is the enhanced greenhouse effect, which is caused by a huge rise in greenhouse gas emissions. Concentrations of carbon dioxide, ozone, methane, nitrous oxide and chlorofluorocarbons in the atmosphere have grown significantly since pre-industrial times as a result of human activity, which contribute to the enhanced greenhouse effect. Atmospheric forcing is seen as the prime cause of the recent climate change as the atmospheric pollution is trapping more heat at the Earth's surface than before.
Atmospheric pollutants (e.g. soot-> suspended particulate matter) and sulphur dioxide reflect solar energy back into space and so have a net cooling effect.
Greenhouse gases reflect outgoing radiation back to Earth and so have a net warming effect.
It is possible that human pollution is warming and cooling the planet at the same time, and that some pollutants are actualy reducing the full impact of global warming. In North America and Europe, soot and sulphur dioxide pollution has fallen dramatically since 1990, as a result of attempts to reduce acid rain. Is it a coincidence that the global warming trend has accelerated at the same time?