Climate on Earth has changed on all time scales, including long before human activity could have placed a role. Great progress has been made in understanding the causes and mechanisms of these climate changes. Changes in Earth's radiation balance were the principal driver of past climate changes, but the causes of such changes are varied. For each case - be it the Ice Ages, the warmth at the time of the dinosaurs or the fluctuations of the past millennium - the specific causes must be established individually. In many cases, this can now be done with good confidence, and many past climate changes can be reproduced with quantitative models.
Global climate is determined by the radiation balance of the planet. There are three fundamental ways the Earth's radiation balance can change, thereby causing a climate change: 1) changing the incoming solar radiation (e.g., by changes in the Earth's orbit or in the Sun itself), 2) changing the fraction of solar radiation that is reflected (this fraction is called the albedo - it can be changed, for example, by changes in cloud cover, small particles called aerosols or land cover), and 3) altering the longwave energy radiated back to space (e.g., by changes in greenhouse gas concentrations). In addition, local climate also depends on how heat is distributed by winds and ocean currents.
Starting with the ice ages that have come and gone in regular cycles for the past nearly three million years, there is strong evidence that these are linked to regular variations in the Earth's orbit around the Sun, the so-called Milankovitch cycles. These cycles change the amount of solar radiation received at each latitude in each season (but hardly affect the global annual mean), and they can be calculated with astronomical precision. There is still some discussion about how exactly this starts and ends ice ages, but many studies suggest that the amount of summer sunshine on northern continents is crucial: if it drops below a critical value, snow from the past winter does not melt away in summer and an ice sheet starts to grow as more and more snow accumulates. Climate model simulations confirm that an Ice Age can indeed by started in this way, while simple conceptual models have been used to successfully 'hindcast' the onset of past glaciations based on the orbital changes.
Although it is not their primary cause, atmospheric carbon dioxide also plays an important role in the ice ages. Antarctic ice core data show that carbon dioxide concentration is low in the cold glacial times, and high in the warm interglacials; atmospheric carbon dioxide follows temperature changes in Antarctica with a lag of some hundreds of years. Because the climate changes at the beginning and end of ice ages take several thousand years, most of these changes are affected by a positive carbon dioxide feedback; that is, a small initial cooling due to Milankovitch cycles is subsequently amplified as the carbon dioxide concentration falls.
During the last ice age, over 20 abrupt and dramatic climate shifts occurred that are particularly prominent in records around the northern Atlantic. These differ from the glacial-interglacial cycles in that they probably do not involve large changes in global mean temperature: changes are not synchronous in Greenland and Antarctica, and they are in the opposite direction in the South and North Atlantic. This means that a major change in global radiation balance would not have been needed to cause these shifts; a redistribution of heat within the climate system would have sufficed. This is indeed strong evidence that changes in ocean circulation and heat transport can explain many features of these abrupt events; sediment data and model simulations show that some of these changes could have been triggered by instabilities in the ice sheets surrounding the Atlantic at the time, and the associated freshwater release into the ocean.
Much warmer times have also occurred in climate history - during most of the past 500 million years, Earth was probably completely free of ice sheets, unlike today, when Greenland and Antarctica are ice-covered. Data on greenhouse gas abundances going back beyond a million years that is, beyond the reach of antarctic ice cores, are still rather uncertain, but analysis of geological samples suggest that the warm ice-free periods coincide with high atmospheric carbon dioxide levels.
Another likely cause of past climate changes in variations in the energy output of the Sun. Measurements over recent decades show that the solar output varies slightly (by close to .1%) in an 11-year cycle. Data correlation and model simulations indicate that solar variability and volcanic activity are likely to be leading reasons for climate variations during the past millennium, before the start of the industrial era.
These examples illustrate the different climate changes in the past had different causes. The fact that natural factors caused climate changes in the past does not mean that the current climate change is natural. By analogy, the fact that forest fires have long been caused naturally by lightning strikes does not mean that fires cannot also be caused by a careless camper.
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