Before I begin discussing that which is extremely relevant to the global warming debate, I will first cover the Earth’s climate since its conception in order to develop some context. The average temperature of the Earth has been erratic since its beginning around 4.5 billion years ago.
Its temperature has swung between 54 and 72 degrees Fahrenheit, creating and destroying life on Earth. Temperatures were too high in the newly formed Earth at first to form an atmosphere; any gases that did exist would become superheated and escape from the Earth’s gravity. After enough time had passed, the Earth began to cool and hold atmospheric gases. From this creation of an early atmosphere came rain, and that rain produced oceans. The atmosphere then allowed for even greater cooling by blocking a portion of the heat irradiating from the sun and towards the Earth. From this, life was formed. About 530 million years ago, the “Cambrian Explosion” occurred, a time when complex animals began to come into existence, and when a major diversification of other organisms took place. It was around this time that the ozone layer formed, and climate as we know it today came into existence. After the Cambrian explosion, a cycle of “ice houses” and “green houses” began.
For the majority of the Earth’s post-Cambrian explosion life, it has been warmer than it is today. In fact, during these warm “green houses,” there is not a single frozen lake or snowflake on Earth. In between “green houses” exist the “ice houses,” of which there have been three, and humans inhabit the third. Ice houses last 30 to 50 million years; ours is 20 million years old, so no need to worry about that for at least another 10 million years. The graph below shows the Earth sliding out of the last “green house,” specifically the Eocene Maximum, and into the “ice house” that we now inhabit.
One theory for this 140 million year cycle is that the Earth reacts to increased cosmic ray bombardment as it proceeds through the spiral arms of the milky way. I will describe the process by which cosmic rays can influence climate in a later post, but essentially, if this theory were true, it would give more credibility to the suggestion that the sun has caused modern global warming.
In the top left corner of the graph, an acronym that reads PETM (Paleocene Eocene Thermal Maximum) is placed next to what appears to be a dramatically sharp rise and fall off of global temperature. Three theories exist for the cause of the PETM: the first theory is that reservoirs of methane were released by warming temperatures, causing a runaway greenhouse effect; the second theory is that the PETM was caused by a comet striking the Earth; and the third theory is that a large oceanic volcano erupted, spewing gases into the atmosphere. While I have absolutely nothing to contribute to that debate, it is important to note that that debate is a microcosm of the modern global warming debate surrounding “runaway” warming and climate sensitivity to greenhouse gases, such as methane.
Also, it is important to notice the erratic behavior of temperature nearing modern times. This next graph zooms in on the temperature behavior, showing that the behavior is very cyclical.
Each cycle lasts approximately 100,000 years, and climate oscillates between glacial periods and interglacial periods. These glacial periods are referred to as “ice ages” in popular culture, and they occur for the majority of time while in an ice house. During these ice ages, glaciers stretch inward towards the equator, and life is forced to adapt to frigidly cold temperatures. Every 100,000 years, climate shoots up ten degrees Celsius into an interglacial period, like the one we currently inhabit. Yet, these periods are short-lived, and there is very little agreement on when this interglacial period will end. Some say that the Earth is already heading back into a glaciation (Imbrie and Imbrie), while others contend that we have another 50,000 years of warm climate ahead of us (Berger and Loutre).
This thermal cycle can most likely be attributed to the Milankovitch Cycle, which is the collective effect of the changes in the Earth’s eccentricity, axial tilt, an precession, all of which effect the amount of solar radiation reaching the Earth. But, the cycle alone cannot explain the entirety of the warming and cooling, and so ice sheet dynamics and greenhouse gases must play a part, amplifying the direct effect of solar radiation. A question that I can pursue later when I have time is if solar wind intensity changes significantly as the Earth moves closer to the Sun, and if robust cosmic ray data exists on the 100,000-year scale. If they do change significantly, then less positive feedback from greenhouse gases would be needed to amplify the effect of the Milankovitch Cycle, and this would have major implications for the modern debate. I’m sure that some scientist is way ahead of me, so I’ll look into it and find an answer soon. I will explain the mechanics of cosmic rays and solar winds in a later post.
Within these interglacial and glacial periods, there is significant temperature oscillation on the millennial scale, but the climate science community is far from agreement on the behavior of temperature over the past millennium. While establishing paleoclimatic trends back a hundred thousand, a million, or even a billion years ago is important (and even, on occasion, can provide insight to today’s debate), the most contested issue is clearly temperature trends over the past millennium for several reasons:
1. It would shed light on the idea that today’s temperatures are out of the natural variability.
2. It would possibly correlate temperature with societal development.
3. By establishing the behavior of the curve, we would gain greater insight into the strength of solar intensity as a climate modifier. Specifically, if temperatures followed the same trend as solar intensity, then we could suggest that perhaps solar intensity has a dominant role in climate change. Yet, we have to be careful; this is merely speculation. To actually make a scientific case for a strong solar sensitivity, we would have to establish the mechanics by which such a process would work as well as the historical correlation.
My purpose in writing this post was to briefly cover all of the different events throughout the history of climate on Earth that are important in the global warming debate. The cause of ice houses and green houses, the cause of the Paleocene-Eocene Thermal Maximum, and the cause of the ice age cycle are microcosms of today’s debate on the cause of 20th century warming. Establishing the cause of the various thermal events in paleoclimate would help to determine the sensitivity of the Earth’s temperature to changes in greenhouse gas concentration.
My next post will most likely focus on the validity and implicatons of different reconstructions of the Earth's temperature over the past thousand years.