We have just seen that Earth's climate has gone through rather large changes throughout its history. Virtually all of these changes occurred prior to the arrival of humans. One questions which we are seeking to answer is
Earth's inhabitants, far from being passive guests, have influenced conditions on this planet throughout its history. We are indebted to earlier life forms for contributions that range from the oxygen in the air we breathe to fossil fuels on which our civilizations have become dependent. During the evolution of the conditions that suit us so well, many species became extinct. The geologic record provides ample evidence of catastrophic extinctions of numerous species on several occasions. Some probably contributed to their own demise. Certain primative forms of life that thrived in Earth's original atmosphere without oxygen produced oxygen as a waste, to such an extent that enormous amount accumulated in the atmosphere, [eventually killing many of the primative life forms, but allowing oxygen breathing life forms to develop]. We, too, could be causing ourselves considerable inconvenience... For example, we are modifying the composition of the global atmosphere significantly because of the fossil fuels we are burning at a furious rate. Soon our actions will cause the atmospheric concentration of greenhouse gases to increase, not by a tiny percentage, but by a factor of two at least.
It is unlikely that the biosphere as a whole will be endangered by our actions; it has survived bigger calamities in the past. The global warming that we are likely to cause has been exceeded in earlier epochs. The present cold era our planet has been experiencing for some two million years [pleistocene epoch] was preceded by a period during which the poles were free of ice. At different times in the past, temperatures have been much higher and much lower than they are today, but because of fortuitous factors such as our distance from the Sun and the size of the Earth, temperature extremes on this planet never approached those that prevail on our neighbors Venus and Mars. In the long run, these factors will enable our planet to continue maintaining habitable conditions; our actions will be of little consequence over the next thousands and millions of years.
That is scant comfort to humans, however, because we are vulnerable to even modest climate changes that persist for only a few years or decades. We homo sapiens can ill afford something as trivial as an increase in the frequency of hurricanes, or prolonged droughts in some regions and repeated devastating floods in others [or even modest sea level rise]. Our planet may seem robust from the perspective of the entire biosphere -- life has been on Earth for more than three billion years -- but it can nonetheless appear fragile from the perspective of individual species, especially us. That is why there is cause for concern about the global environmental consequences of our agricultural and industrial practices. The matter has generated lively debates, but they often end in stalements; for example, some insist that the Earth is robust, others that is is fragile. In reality it is both!
Climate change reflects significant shifts in the mean state of the atmosphere-ocean-land system that results in shifts in the atmosphere and ocean circulation patterns, which in turn impacts regional weather.
From the paleoclimate perspective, climate change is normal and part of the Earth's natural variability related to interactions among the atmosphere, ocean, and land, as well as changes in the amount of solar radiation reaching the earth. The geologic record includes a plethora of evidence for large-scale climate changes. Evidence for climate fluctuations over all scales of time is conclusive.
There are many possible and causes of long-period climatic variations and many have been proposed by climate scientists. We are going to go over one, which almost undoubtedly has played a major role in past climate changes on Earth, and that is rather small natural fluctuations in the Earth's orbit about the Sun. These fluctations result in changes in solar radiation available at different latitudes and seasons. In terms of the magnitude of the change in the energy budget of the climate system, these are very small indeed, but have been positively linked to the current ice age cycles that have been occurring over the past two million years on Earth. Thus, this may be an example that indicates that even small changes in the energy budget of the Earth can result in very significant changes in global climate: from ice ages to warmer interglacial periods.
The changes in the distribution of solar energy at the top of the atmosphere caused by slight variations in the orbit of the Earth about the Sun were first computed by the a Serbian scientist Milutin Milankovitch about 1930. He was convinced that these orbital changes were the cause of the Ice Ages. There is supporting evidence for this claim. Past climatic fluctations in temperatures inferred from ice core and ocean sediment core data are very well correlated with the fluctuations in the Earth's orbital parameters worked out by Milankovitch
However, today scientists believe that these changes alone are not sufficient to explain the occurrence of the ice ages, although it is difficult to deny that these orbital changes are responsible for "triggering" the ice age cycles. In other words, the orbital changes push the climate system in a particular direction. Once pushed, positive feedbacks in the climate system may then result in full blown ice age cycles. What does this mean for us? Doubling the concentration of greenhouse gases is a larger forcing of the Earth's energy budget than is caused by these slight orbital variations. Will the ultimate effect on climate be larger than occurs over an ice age cycle? This is a difficult question to answer because the feedback mechanisms related to changes in greenhouse gases will be different than the feedback mechanisms related to orbital variations (among other differences).