There is very little appreciation among the general public (and even among many scientists) of the great complexity of the mechanisms involved in climate change. Climate change significantly involves physics, chemistry, biology, geology, and astronomical forcing. The present political debate centers on the effect of the increase in the amount of carbon dioxide in the Earth’s atmosphere since humankind began clearing the forests of the world and especially began burning huge quantities of fossil fuels, but this debate often ignores (or is unaware of) the complex climate system that this increase in carbon dioxide is expected to change (or not change, depending on one’s political viewpoint).
The Earth’s climate has been changing over the 4.5 billion years of its existence. For at least the past 2.4 million years the Earth has been going through regular cycles of significant cooling and warming in the Northern Hemisphere that we refer to as ice age cycles. In each cycle there is a long glacial period of slowly growing continental ice sheets over large portions of the Northern Hemisphere (which are miles thick, accompanied by a sea level drop of 300+ feet), eventually followed by a rapid melting of those ice sheets (and accompanying rise in sea level) which begins a relatively short interglacial period (which we are in right now). For about 1.6 million years a glacial-interglacial cycle averaged about 41,000 years. This can be seen in the many excellent paleoclimate data sets that have been collected around the world from ice cores, sediment cores from the ocean bottom, and speleothems in caves (stalactites and stalagmites). These data records have recently become longer and with higher resolution (in some cases down to decadal resolution). Changes over millions of years in temperature, ice volume of the ice sheets, sea level, and other parameters can be determined from the changing ratios of various isotope pairs (e.g., 18O/16O, 13C/14C, etc.) based on an understanding on how these element isotopes are utilized differently in various physical, chemical, biological, and geological processes. And changes in atmospheric carbon dioxide and methane can be measured in ice cores from the Greenland and Antarctic ice sheets.
In these data the average glacial-interglacial cycle matches the oscillation of the Earth's obliquity (the angle between the Earth's rotational axis and its orbital axis). The Earth's obliquity oscillates (between 22.1 and 24.5 degrees) on a 41,000-year cycle, causing a very small change in the spatial distribution of insolation (the sunlight hitting and warming the Earth). However, beginning about 0.8 million years ago the glacial-interglacial cycle changed to approximately 82,000 years, and then more recently it changed to approximately 123,000 years. This recent change to longer glacial-interglacial cycles has been referred to by many scientists as the "100,000-year problem" because they do not understand why this change occurred.
But that is not the only thing that scientists do not understand. They still do not understand why a glacial period ends (an interglacial period begins) or why a glacial period begins (an interglacial period ends). The most significant changes in climate over the Earth's recent history are still a mystery.
For a while scientists were also not sure how such a small variation in insolation could lead to the build up of those major ice sheets on the continents of the Northern Hemisphere. Or how it could lead to their melting. They eventually came to understand that there were “positive feedback mechanisms” within the Earth's climate system that slowly caused these very dramatic changes. Perhaps a better understanding of these positive feedback mechanisms will communicate to the public a better appreciation of the complexity we are dealing with in climate change.
The most agreed upon and easiest to understand positive feedback mechanism involves the reflection of sunlight (albedo) from the large ice sheets that build up during a glacial period. Ice and snow reflect more light back into space (causing less warming) than soil or vegetation or water. Once ice sheets begin to form at the beginning of a glacial period more sunlight is reflected back into space so there is less warming of the Earth and the Earth grows colder and the ice sheets expand (covering more soil, vegetation, and water) and thus further increasing reflection into space, leading to further cooling and larger ice sheets, and so on. There is also a positive feedback during interglacial periods, but in the warming direction, namely, as ice sheets melt, more land or ocean is exposed, which absorbs more light and further warms the Earth, further reducing the ice sheets, leading to more warming, and so on.
Carbon dioxide in the atmosphere also varies over a glacial-interglacial cycle. Its concentration is lower during cold glacial periods and higher during warm interglacial periods. There are various processes which can cause carbon dioxide to decrease during cold periods and increase during warm periods (involving temperature effects on carbon dioxide solubility in the ocean, changes in biological productivity in the ocean and on land, changes in salinity, changes in dust reaching the ocean, etc.). So the debate has been whether carbon dioxide causes the warmth that produces an interglacial period or whether something else causes the initial warming which then leads to an increase in carbon dioxide. Either way there is another important positive feedback here involving carbon dioxide because the temperature changes and the changes in carbon dioxide are in the same direction.
There are other possible positive feedback mechanisms and much more detail that cannot be included in such a short essay. But the point here is that to some degree climate scientists understand how, through various positive feedback mechanisms, glacial periods get colder and colder and the ice sheets expand, and interglacial periods get warmer and warmer with melting ice sheets. But the big question remains unsolved. No one has explained how you switch from glacials to interglacials and then back to glacials. No one knows what causes a glacial termination (the sudden warming of the Earth and melting of the ice sheets) or what causes a glacial inception (the not quite as sudden cooling of the Earth and growing of ice sheets). And if you don't understand that, you don't completely understand climate change, and your climate models are lacking a critical part of the climate change picture.
Carbon dioxide may have been higher at various points in the Earth’s history but there is evidence that it has never risen to the its present levels as quickly as it has during humankind’s influence. Is it the quantity of carbon dioxide in the atmosphere that is important or is it the speed at which it has increased to that high level? And what influence does such an increase in carbon dioxide occurring at the end of an interglacial period have on the ice age cycle? The key to accurately assessing the degree to which humankind has affected climate change (and especially what the future consequences will be) is to make the climate models as accurate as possible. Which means including all the important physical, chemical, biological, and geological processes in those models. The only way to test those models is to run them in the past and compare their predictions to the paleoclimate data sets that have been meticulously acquired. Right now these models cannot produce glacial terminations and glacial inceptions that accurately match the paeloclimate data. That is the next big hurdle. It would be helpful if those debating anthropogenic global warming could gain a little more understanding about the complexity of what they are debating.