Sunday, April 19, 2009

Climate Change

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

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.[1] 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[2] 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 Antarctica. In this ice core record there is a clear correlation between high CO2 levels and high temperatures and vice versa (Maslin, 2009, p. 7). Gore claims that these records offer evidence that CO2 levels have driven past climate change; therefore, the dramatic anthropogenic increase in CO2 levels seen over the last century will have a correspondingly large increase in global temperature. In the ice core records, however, a clear cause and effect relationship between CO2 levels and temperatures does not exist (Caillon et al., 2003). In fact, the Vostok ice cores from Antarctica show that in the past temperatures have risen around 800 years before the CO2 levels rise (Caillon et al., 2003; Maslin, 2009; Soon, 2007). The rising CO2 levels are thought to amplify the warming that is already taking place, but are not the ultimate cause of the fluctuations between ice ages (Caillon et al., 2003; Maslin, 2009). Past ice ages and interglacial periods were caused by orbital and solar variations (Maslin, 2009).

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 Antarctica today (Holbour et al., 2005). Sea surface temperatures were high during this ice buildup; this is consistent with an orbital variation being the cause rather than a change in GHG levels (Holbour et al., 2005). CO2 levels did decrease during this cooling episode, but this was due to orbital influences on the carbon cycle; this in turn led to further expansion of the Antarctic ice sheets (Holbourn et al., 2005).

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 Arctic (Otto-Bliesner et al., 2006; Overpeck et al., 2006).

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, Clark, & Ferndandez, 2007; Shin, Sardeshmukh, Webb, Oglesby & Barsugli, 2006).

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, Clark, Huybrechts, & Joughin, 2005). A reduction in GHG may have caused glaciation during the Carboniferous period 354-290 million years BP (Came et al., 2007). There is evidence that ~35 million years BP a reduction in CO2 levels may have contributed to global cooling (Garzione, 2008; Stanley, 1999). Retallack (2007) found paleosol evidence indicating that CO2 levels were a controlling factor throughout the Cenozoic (~65 million years ago - present). In contrast to these findings, reconstructions by Rothman (2002) show that CO2 levels do not correlate well with warm or cool periods over the past 500 million years.

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).

[1] E.g., the International Panel on Climate Change (IPCC), Alliance of Small Island States (AOISIS), and the United Nations Framework Convention on Climate Change (UNFCCC).

[2] An Inconvenient Truth won multiple Oscars and is currently the 4th highest grossing documentary

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Brad Wright said...

Ah, for those of us who don't speak science, in your best guess, what % of global warming is due to humans?

(Question probably phrased wrong)

Jeff L said...

According to one source, around 75% of current warming is anthropogenic and 25% is due to variation in the sun (see follow up post). The IPCC now says, in its own special terminology, that current warming is "very likely" due to human influence.

For myself, I am convinced that we are having some impact on climate; however, we are in a naturally warm period. From what I've read, it is currently impossible to separate out all of the factors involved in climate change and pin the cause down...I'm uncomfortable throwing out a percentage, but my guess would be up to maybe far. It seems clear that human influence will only increase over time.