As detailed in the sections on the basic science of climate change, global climate is determined by the radiation balance of the planet. There are three fundamental drivers of climate in the earth system:
In addition to these factors, local and regional climates depend on how heat is distributed by winds and ocean currents.
Palaeoclimatic records from ice cores dating back 740,000 years demonstrate that the earth system has undergone a regular 100,000-year oscillation between glacial (ice age) and inter-glacial cycles. There is strong evidence linking these cycles with the 'Milankovitch cycles', which are regular variations in the earth's orbit around the sun. These variations arise from cyclic changes in eccentricity (the shape of the orbital ellipse), obliquity (tilt of the earth's axis) and precession. Precession is the slow movement of the earth's rotational axis (similar to the wobble of a spinning top), which causes the direction of the axis to change slowly at a given point in the earth's orbit. The eccentricity of the Earth's orbit varies on cycles of 100,000 and 400,000 years, while the obliquity has a cycle of 41,000 years and precession about 22,000 years.
Another significant influence on global climate is variation in energy output of the sun. Currently, we know that the sun goes through a fairly regular cycle with a period of about 11 years. This is known as the sunspot cycle because it involves changes in the number of dark sunspots visible on the sun. The sun's luminosity also varies slightly (by about 0.1 per cent ) in line with this cycle, giving a very small but predictable effect on global temperature. The cycle also brings changes in the quantity of cosmic rays reaching the earth’s upper atmosphere, and it has been suggested that this is important because it may affect cloud formation. However, this needs further research. What is clear is that, no matter what slight changes the 11-year solar cycle may bring to earth, climate change has persisted across the cycles. Satellite measurement has shown quite conclusively that the sun's output has not varied sufficiently over the last few decades to account for the observed temperature rise on earth in that time.
Volcanic eruptions can also have quite dramatic, but short-lived, effects on the climate in a region or across the entire planet. This is because eruptions can introduce reflective aerosols into the stratosphere, which reduce the amount of sunlight reaching the surface. This happened after the Mount Pinatubo eruption in June 1991, causing a decrease in global temperature particularly in the six months following but persisting for more than two years. At various times in earth’s history, it seems that prolonged periods of extensive volcanic activity may also have caused slow climate change by affecting CO2 levels over timescales of millions of years.
See Why climates change and its subsections for further information.
At regional and local scales, atmospheric circulation and ocean currents drive oscillations in climate regimes that may persist for many months or some years. One such mechanism important to Australia is the El Niño – Southern Oscillation (ENSO), which periodically causes large changes in ocean currents and ocean temperatures, affecting global weather patterns and producing significant regional impacts.
It is now widely accepted that current global climate change is occurring in response to human activities on the planet. The Intergovernmental Panel on Climate Change (IPCC) in its Fourth Assessment Report (AR4) on Climate Change concluded that global warming is unequivocal, and that almost certainly (a probability of 90 per cent) this warming is caused by human activities—chiefly the burning of carbon-containing fuels. The influence of natural factors mentioned above that drive long-term climate variability are not sufficient to explain the observed rapid increase in global mean air temperature. Burning fossil fuels (e.g. in coal-fired power plants and car engines) and other activities, such as industrial processing of minerals and deforestation, emit greenhouse gases that act to trap heat in the atmosphere by absorbing the heat radiated from the earth's surface, while allowing sunlight through to heat the surface. This is the human-caused enhancement of the natural greenhouse effect. The most abundant of these gases is carbon dioxide, but other gases, such as methane, emitted in smaller amounts, are also important because their ability to cause warming is much stronger than that of carbon dioxide for a given mass. Models demonstrate that the observed global warming can only be explained when the effect of increasing concentrations of these greenhouse gases in the atmosphere due to human activities is included together with the natural processes described above (see Is the current global warming caused by natural factors?).
Further evidence for human-induced climate change and future projections are described in detail in under the climate is changing section of this website and its subsections.