The following is part of my research into climate change. I do not consider it complete; some sections need to be fleshed out a bit, but it's what I've got for now. Also, I had some formatting problems with the last half or so, hence the breaks instead of indents between paragraphs.
Climate change may be the most important scientific issue facing the global community (Maslin, 2009). In the past few decades several international organizations have formed to meet the need for scientific, political, and economic analysis. Climatology and paleoclimatology are relatively new fields of research; the development of more sophisticated global-scale observations is needed to validate and refine existing atmospheric models (Crutzen, 2000). The large uncertainties surrounding climate change remain current areas of research (Raisanen, 2007).
Global climate change is thought to be primarily due to variations in the Earth’s orbit around the Sun and varying levels of greenhouse gases (Maslin, 2009; Stanley, 1999). These influence the Earth’s energy budget by affecting, respectively, the amount of energy received from the Sun or the amount of energy lost due to radiative cooling (Maslin, 2009).
Variations in the earth’s orbit thought to be responsible for climate change include changes in the earth’s precession, obliquity, and the eccentricity of its orbit. Planetary orbits are elliptical, and the eccentricity of a planet’s orbit is the ratio of the two foci to the major axis of the ellipse (Morrison, Wolf, & Fraknoi, 1995). In other words, eccentricity describes how stretched out the oval-shaped ellipse is; this determines the range of distances between the planet and the sun that is experienced during a revolution.
Obliquity describes the tilt of the earth on its axis (~23°) and precession is the approximately 26,000 year cycle in which the earth “wobbles” on its axis like a top (Morrison, Wolf, & Fraknoi, 1995). These variations affect the amount of incoming solar radiation (or insolation) received from the sun and therefore affect Earth’s climate (Maslin, 2009).
Orbital variations (sometimes called Milankovitch cycles after the scientist who popularized the theory) are an example of external forcing on earth’s climate. Greenhouse gases are an example of an internal forcing (Maslin, 2009). When the Sun’s radiation reaches the Earth, around 30% of it is reflected back out into space and the other ~70% is absorbed. Earth’s atmosphere absorbs around 20% of the energy and the earth’s surface absorbs the remaining 50% (Karl, 2003; Maslin, 2009). The energy absorbed by earth’s surface is then re-radiated in the form of infrared light (Karl, 2003). This infrared light (or heat energy) is mostly released into space; greenhouse gases, however, absorb some of this energy and re-radiate it within the earth’s atmosphere. This is the “greenhouse effect” from which greenhouse gases (hereafter GHG) receive their name, and the presence of GHG results in an increase in the earth’s average temperatures (Maslin, 2009). The greenhouse effect keeps earth habitable, as without any GHG the earth’s average temperatures would be around -18° C, over 30 ° C colder than current averages (Ward & Brownlee, 2000, p. 207).
Examples of GHG and their relative contributions to the global greenhouse effect include water vapor (60%), carbon dioxide (25%), ozone (8%), methane, and nitrous oxides (Karl, 2003). The atmospheric levels of these gases have varied throughout the earth’s history and contributed to past climate change (Maslin, 2009; Stanley, 1999).
Orbital variations and GHG are only two of a myriad of variables involved in earth’s climate system (Rind, 2002). Others include the ocean circulation system, variations in solar output, aerosols, vegetation, and various feedback mechanisms (Rind, 2002). Scientists are divided over the relative importance of each of these mechanisms, and the sheer number and dynamicity of these variables make accurate reconstructions and models difficult (Bony et al., 2006; Maslin, 2009; Raisanen, 2007).
What is clear is that the earth has experienced dramatic climate changes in the past; these changes include natural cycles between ice ages and warmer, interglacial periods as well as the corollary changes in sea level, temperatures, precipitation patterns, ocean circulation, atmospheric circulation, and ice cover. The exact mechanisms responsible for past climate change are a source of debate and uncertainty within the scientific community (Maslin, 2009; Paillard, 2006).
Past and Current Climate Change
The global average temperature of earth has increased by approximately .75 degrees Celsius over the past 150 years (Maslin, 2009; IPCC, 2007). According to the IPCC (2007) the consensus among scientists is that the primary cause of this global warming is anthropogenic (human-caused) carbon dioxide emissions. Since the industrial revolution humans have been burning fossil fuels (e.g. coal, oil, gasoline) for energy. The combustion of fossil fuels results in the formation of carbon dioxide, which is released into the atmosphere (Eubanks et al., 2006). Pre-industrial levels of carbon dioxide (CO2) in the earth’s atmosphere were around 280 parts per million (ppm). Current levels are ~385 ppm, an increase of over 100 ppm (Maslin, 2009). Since CO2 is a greenhouse gas, rising CO2 levels result in an increase in the amount of outgoing infrared radiation absorbed within the earth’s atmosphere. This additional heat energy causes an overall warming of the earth (Eubanks et al., 2006).
In An Inconvenient Truth, the popular documentary about global warming, Al Gore makes the claim that earth’s past ice ages and intervening warm periods are due to the rising and falling of carbon dioxide (CO2) levels (Bender, 2006). Gore points to reconstructions of temperatures and CO2 levels for the last 650,000 years drawn from ice cores in
In the Miocene period, 13.9 million years before present (BP), a global cooling episode was initiated by a change in earth’s obliquity (Holbour, Kuhnt, Schulz, & Erlenkeuser, 2005). This resulted in the extensive ice sheets that continue to cover
In the past 2.5 million years orbital variation has been the dominant forcing involved in the transitions into and out of ice ages (Maslin, 2009). Over the past 423,600 years, during the late Pleistocene, Milankovitch cycles account for the majority of climate change (Meyers, Sageman, & Pagani, 2008). Both precession and obliquity cycles were involved in these changes (Huybers, 2006; Meyers et al., 2008).
During the last interglacial period, ~129,000 years BP, orbital variations caused a warming episode that resulted in extensive open water in the
The current interglacial epoch, the Holocene, began around 10,000 years BP (Maslin, 2009). There is evidence for precessional forcing of climate change during this period, including changes in both ocean hydrology and atmospheric circulation and precipitation patterns (Partin, Cobb, Adkins,
Milankovitch cycles correlate well with past climate change (Maslin, 2009; Meyers et al., 2008; Soon, 2007). The relationship between GHG levels and climate, on the other hand, is a source of controversy among scientists (Kerr, 2001). Historical records indicate a strong correlation between GHG and climate (Alley,
It is likely that both orbital variations and GHG levels have contributed to past climate change (Paillard, 2006). Other mechanisms such as varying solar output and shifts in ocean circulation also play an important role in regional and global climate change (Curry, Dickson, & Yashayaev, 2003; Rohling & Palike, 2005; Thornalley, Elderfield, & McCave, 2009). The earth is a dynamic interconnected system and further research is needed for any certain conclusions about the mechanisms that can explain past climate changes (Paillard, 2006; Rind, 2002). In particular, research into the interactions between orbital forcing and other mechanisms, such as ice-sheet or cloud feedbacks, is needed (Huyberys, 2006; Bony et al., 2006).