It is known that shortly after its formation the Earth's atmosphere contained far higher levels of carbon dioxide than today, but little oxygen. In fact, that early atmosphere would not have supported oxygen-breathing mammals such as ourselves. Plant life, through photosynthesis, transformed the atmosphere into one with about 20% oxygen content, and low levels (less than 1%) of carbon dioxide. This arose because carbon dioxide is a necessary component of plant life, absorbed by vegetation, whilst oxygen is a waste product. Thus, it is true to say that oxygen-breathing animals could never have existed were it not for the 'terraforming' action of plants on the primitive atmosphere. Much of the abundant early carbon dioxide has been sequestered deep underground in the form of the beds of decomposed vegetation -coal and oil fields- from which we extract our fossil fuels.
An interesting point arises out of this consideration, and that is whether the present-day carbon dioxide level is rather on the low side for plant life to achieve its best performance. Have plants extracted carbon dioxide up to the point where no more can be extracted without rendering the planet uninhabitable to themselves, and then by a process of self-regulation of their numbers, prevented the level from dropping lower? There is in fact evidence of this being the case.
Throughout its history, the Earth has undergone cyclic changes in its surface temperature. The cold intervals are known as ice ages, the warmer periods as interglacials. No-one knows exactly why these alternate cold and warm intervals arise, but Milankovitch cycles (perturbations of the Earth's orbit) are thought to be a prime candidate. The factor which interests us here is that carbon dioxide levels were known to have varied cyclically along with the temperature changes of the successive ice ages and interglacials.
This cyclic temperature change is best shown by reference to ice cores drilled from the deepest and most stable Arctic or Antarctic ice-pack. Such ice contains minute bubbles of the air present at the time the ice formed, and so an analysis of such bubbles gives us a 'snapshot' of the gas content of the air at that time in history. There is some controversy over whether the content of these bubbles is an accurate representation of the gas mix in the ancient atmosphere after being 'on ice' for so many years, but it is at least thought to give a reasonable measure. In any case, here we are mainly interested in the relative timing of changes, thus the actual proportions of each constituent gas are not so important.
Probably the best-known ice core data comes from the Vostok Station in the Antarctic, operated by Russia. This data, as originally presented, appeared to show a cause and effect relationship in which changes in carbon dioxide concentration might well have driven the temperature swings of the ice ages. From this it was hypothesised that carbon dioxide played a key role in the ice age temperature cycle, and therefore would very likely also be a driver for global warming in the present era. This data and the conclusions drawn from it formed a central part of Al Gore's film, An Inconvenient Truth, to the extent that he is seen examining a giant copy of the ice-core graph from a cherry-picker.
However, astute scientists pointed out that the original Vostok data gave no clue as to the relative timing of carbon dioxide increase and temperature rise. The early measurements were simply too coarse to resolve such a small time difference as a few hundred years over the course of millennia. When new ice cores were drilled to a much higher precision, an inconvenient truth certainly did emerge, namely that the temperature upswings had always preceded the carbon dioxide increases. Usually, by a few hundred years. Thus, Mr Gore's premise that carbon dioxide had somehow been involved in causing ice ages was shown to be false. The reverse was true!
The relationship of the changes is in fact easily explained by the fact that carbon dioxide is more soluble in cold water than in warm. Thus, it is unsurprising that after climatic warming, a time lag is caused by the thermal inertia of the oceans followed by a release of CO2 to the air. No greenhouse effect involved at all, just the same physics as in a soda bottle.
In spite of this revised understanding of the data (which is accepted by a large consensus of scientists as proven fact) you will still see publications quoting ice core data as proof of climate change theory. Some will even go to the lengths of concocting implausible stories as to how the delayed CO2 release didn't cause the original warming, but still nevertheless caused feedbacks which amplified it.
Conclusion: Not much doubt about this one. Ice core data does not support the climate change hypothesis in any shape or form. It does not refute it either. In fact, its implications are entirely neutral as regards modern-day climate change. Those who still perversely maintain that it does validate climate change in spite of our better understanding of how that data came about, are simply denying the truth because it doesn't fit in with their world-view.
Historical and Investigative Research (Sceptic, 2014)