In my astronomy class we will be discussing the origin of the universe soon; this of course includes discussion of the currently accepted theory of cosmological origins, the big bang theory. Just in mentioning some upcoming topics, I've already had one student (a Christian) raise her hand and protest. I cringe when this sort of thing happens, for two reasons. One, I don't believe Christians should be in the business of not allowing the current theories to be taught, and Two, Christians are often very misinformed about the big bang theory.
Christians often associate the big bang theory with an atheistic explanation of the universe (atheistic in the sense that the explanation does not require invoking a creator; the universe, in other words, has a naturalistic explanation). In fact, one of the big bang theory's original formulators and proponents was a Belgian priest by the name of George Lemaitre. While he based his theory on science, not his religious beliefs, there was a clear consistency between the traditional Christian view of the origin of the universe and the big bang theory. The establishment view of the time was a static, eternal universe; this preference can be traced back to the Greeks and was often considered the more appropriate, secular way of viewing the universe. Several decades after Lemaitre's initial proposal, some other astronomers had gathered some observational and mathematical support for the big bang theory. While the evidence was not conclusive, it certainly was worth paying attention to. However, this scientific work was largely ignored and ridiculed by the scientific establishment. Some scientists ridiculed the big bang as being a thinly disguised attempt to bring religion into the fold by characterizing God's initial creation event in scientific terms. The Pope also noticed the congruence between Christianity's creation event and the big bang, formally declaring that science had vindicated Christianity's long standing belief that the universe had a beginning. This of course further enraged the more zealous of the secular astronomers and probably further delayed acceptance of the theory.
So, in contrast to what people often seem to believe, the big bang theory actually can quite easily be seen to support (or at least be compatible with) the initial creation event of Christianity. In fact, there is more tension between an atheistic worldview and the big bang than there is between Christianity and the big bang (a side note: I do not consider it wise for Christianity to ally itself too closely with any specific theory). After all, an eternal universe does not require a starting point, and therefore, no creator. The big bang theory posits that the universe itself (including space, time, and all known laws of physics) originated during this singular event. The implication is that something outside of space, time, and the known laws of physics must be responsible for the initiation of this event. This has not gone unnoticed by secular astronomers, hence the efforts by scientists such as Stephen Hawking to escape the conclusions of their own work by mathematically wriggling their way out of a universe with a beginning (Hawking uses imaginary time to circumvent an actual beginning, preferring instead a self-contained universe with no possibility of a creator).
To sum up, Christians need not fear the big bang theory (or embrace it too closely). Rather, we can observe the shared characteristics that it has with Christian beliefs and watch where the science leads.
Sunday, April 26, 2009
Saturday, April 25, 2009
Child of Dust
Dear prodigal, you are my son and I
Supplied you not your spirit, but your shape.
All Eden's wealth arrayed before your eyes;
I fathomed not you wanted to escape.
And though I only ever gave you love,
like every child you’ve chosen to rebel;
uprooted flowers and filled the holes with blood;
ask not for whom they toll the solemn bells.
A child of dust to mother now return;
for every seed must die before it grows.
and though above the world may toil and turn,
no prying spade will find you here below.
Now safe beneath their wisdom and their feet,
Here I will teach you truly how to sleep.
Listen here.
Supplied you not your spirit, but your shape.
All Eden's wealth arrayed before your eyes;
I fathomed not you wanted to escape.
And though I only ever gave you love,
like every child you’ve chosen to rebel;
uprooted flowers and filled the holes with blood;
ask not for whom they toll the solemn bells.
A child of dust to mother now return;
for every seed must die before it grows.
and though above the world may toil and turn,
no prying spade will find you here below.
Now safe beneath their wisdom and their feet,
Here I will teach you truly how to sleep.
Listen here.
Sunday, April 19, 2009
Climate Change #2: Recent and Future Change
Here's another part:
Recent and Future Climate Change
While past climate change remains somewhat enigmatic, there exists a stronger consensus regarding current climate change (IPCC, 2007). As discussed above, the earth has warmed approximately .75 degrees Celsius over the last 150 years (Maslin, 2009). While around 26% of this warming can be accounted for by solar forcing, the majority of the warming is due to anthropogenic CO2 emissions (Karl, 2003). Each year human activities add around 6 gigatons of CO2 to the atmosphere (Eubanks et al., 2006). Approximately 4/5 of these emissions come from the combustion of fossil fuels, with the remaining 1/5 coming from deforestation or other land-use changes (Maslin, 2009). Anthropogenic influence on the warming of the 20th and 21st century has now been detected through the modeling of the recent climate change (Hegerl et al., 2007). Separating out anthropogenic warming from natural variability is necessary because we are currently in a naturally warm interglacial period known as the Holocene (Maslin, 2009).
The Holocene began around 10,000 years BP (Maslin, 2009). This marked the end of the last ice age, which reached its maximum around 21,000 years BP (Carlson, Clark, Raisbeck, & Brook 2007). Following the last glacial maximum (LGM) earth has been warming steadily (Maslin, 2009, p. 42). Approximately 19,000 years BP there was a 10+ meter rise in sea levels that took place within ~500 years (Clark, McCabe, Mix, & Weaver, 2004). Within a period of ~500 years at the beginning of the Holocene there was a rise in temperature of 8-13 degrees Celsius (Birks & Ammann, 2000). The Laurentide Ice Sheet, which covered much of North America, retreated between 9,000 and 8,400 years BP. This resulted in a further ~5 meter increase in sea levels within a ~1000 year period (Carlson et al., 2007). Within the Holocene there have been periodic cooling and warming episodes, some of which were dramatic, but the last 1000 years have been relatively stable (Thornalley et al., 2009; Maslin, 2009, p. 45). Starting in the late 19th century temperatures have begun to rise once again (IPCC, 2007; Maslin, 2009).
Rising temperatures will bring a host of other changes, some of which will further modify earth’s climate. These changes include melting ice cover, rising sea levels, and shifting oceanic and atmospheric circulation patterns (Maslin, 2009). Changes that could further influence earth’s climate are called feedback mechanisms; these could either accelerate or buffer global warming, and are one of the largest sources of uncertainty in future climate predictions (Bony et al., 2006; Raisanen, 2007).
Effects of Global Warming
The first effect of global warming discussed by Al Gore in the documentary An Inconvenient Truth is the melting of mountain ice caps, or glaciers (Bender, 2006). The first example given is of the retreating glacier atop Mt. Kilimajaro in Africa. Kaser (2004), however, found that this particular glacier’s retreat is due to factors other than global warming; this is in part evidenced by that fact that temperatures never rise above freezing at the glacier’s altitude. However, other glaciers are retreating due to global warming (Kaser, 2004). Approximately 67% of glaciers in the Himalayan mountain range are currently retreating (Ren, Karoly, & Leslie, 2007). According to Ren et al. (2007) glaciers in the Himalayan mountains, a source of fresh water for approximately half of the world’s population, may disappear by 2100.
Another concern as the earth warms and ice cover melts is that global sea levels will rise and threaten the coastal communities of people around the world (Alley, Clark, Huybrechts, & Joughin, 2005). From 1961 to 2003 sea level has been rising at an average rate of 1.8 mm per year; from 1993 to 2003 the average rate increased to 3.1 mm per year (IPCC, 2007). This rate is low compared to some episodes of past climate change; during the Holocene melting of the LIS (discussed above) sea levels rose at rates up to 10 mm per year (Carlson et al., 2007). Two particular melting episodes between the LGM and the start of the Holocene had peak rates perhaps greater than 50mm per year (Alley et al., 2005). While current rates are historically low, even small increases in sea level rise could have a substantial impact on coastal areas through erosion, groundwater contamination, and increased vulnerability to storm surges (Alley et al., 2005).
Approximately half of the recent sea level rise comes from thermal expansion of the water itself; the other half is due to the melting of land-based ice sheets (Alley et al., 2005; IPCC, 2007). In An Inconvenient Truth Al Gore does not mention thermal expansion, only the melting of ice caps on mountains and the ice sheets over Greenland and Antarctica (Bender, 2006). Gore uses computer models to simulate a 20 foot sea level rise to demonstrate the future effects of melting ice cover (Bender, 2006). However, current melting rate estimates for Greenland and Antarctica are, respectively, +0.5 mm per year and -0.6 mm per year; this results in a total net contribution to sea level of around zero (Alley et al., 2005). Contributions to sea level rise from mountain glaciers are projected to be < 1 mm per year through the 21st century (Raper & Braiswaite, 2006).
While current sea level change from ice sheets may be negligible, large uncertainties exist concerning ice-sheet dynamics and the possible responses to global warming (Alley et al., 2005; Huyberys, 2006). An increase of glacial earthquakes in Greenland has been detected, and overall melting rates are increasing (Alley et al., 2005; Ekstrom, Nettles & Tsai, 2006). During the last interglacial period (129,000 years BP) sea level was 4-6 meters higher than present (Otto-Bleisner et al., 2006; Overpeck et al., 2006). Greenland is thought to have contributed > 2 meters to sea level rise at that time (Overpeck et al., 2006). A current rise in sea level of that amount could cover some low-lying countries (Overpeck et al., 2006). Dramatic past changes in sea level raise the possibility of such dramatic change happening during the present warming; more research into ice-sheet dynamics is needed (Alley et al., 2005).
Another effect of global warming that has received high media attention and was featured in An Inconvenient Truth concerns changes in hurricane patterns (Curry, Webster, & Holland, 2006; Bender, 2006). Studies have shown an increase in SST over the past 50 years (IPCC, 2007). This rise in SST correlates with a rise in the frequency of intense hurricanes since 1970 (IPCC, 2007). Al Gore uses the example of Hurricane Katrina, which struck the Gulf Coast in the U.S. in 2005, to illustrate the impact of global warming (Bender, 2006). Some studies, however, indicate that the effect of global warming on hurricanes is uncertain: Landsea, Harper, Hoarau, and Knaff (2006) argue that the database of past hurricane activity is too short and unreliable to use to detect trends in intense storms. In addition, there is evidence that as temperatures increase so will wind shear over the Atlantic and Pacific oceans (Vecchi & Soden, 2007; Wang & Lee, 2008). Wind shear is an atmospheric phenomenon that could result in a decrease in hurricane activity and the number of hurricanes making landfall (Vecchi & Soden, 2007; Wang & Lee, 2008). A statistical analysis by Dailey, Zuba, Ljung, Dima and Guin (2009), however, found that increasing SST will likely increase the number of hurricanes making landfall at least in the Southeastern United States. More research in this area is needed, as storm risk is also an important policy issue for coastal communities (Curry et al., 2006).
Ocean circulation is another variable likely to be affected as global temperatures rise. The Atlantic meridional overturning circulation (AMOC), an important transport mechanism for moving heat around the globe as well as an important part of the carbon cycle, can be affected by both temperature change and salinity change as fresh glacial meltwater is added to the oceans (Thornalley et al., 2009). Changes in Atlantic currents can have significant regional climate impacts (Clark et al., 2004). In the early Holocene a regional (and possibly global) cooling event was caused by meltwater disrupting North Atlantic Deep Water (NADW) formation (Rohling & Palike, 2005). Disruption of NADW formation can paralyze the Gulf Stream, an important transporter of heat to Northern Europe (Maslin, 2009). There is evidence of meltwater and other factors already affecting the current salinity of the Atlantic Ocean (Curry et al., 2003). If meltwater disrupts the Gulf Stream, Europe could expect much colder, more extreme winter weather (Maslin, 2009). More research is needed in order to accurately model future changes in ocean circulation and its potential effects on climate (Curry et al., 2003).
Biodiversity will also be affected by climate change; some species will benefit and others will be adversely affected (NRC, 2008). Some forests may be threatened by rising temperatures (Scholze, Knorr, Arnell, & Prentice, 2006). Other plant species, such as soybeans, will have increased vulnerability to predation as CO2 levels rise (Zavala, Casteel, DeLucia, & Berenbaum, 2008). Some grasslands, on the other hand, seem to be mostly immune to climate change (Grime et al., 2008). Overall, animal biodiversity is projected to decrease due to global warming, particularly for species that cannot easily migrate or adapt to a changing climate (Maslin, 2009).
Several other areas will also be impacted by global warming, including agriculture, the spread of certain diseases, increases in wildfires, and an increase in extreme weather events such as floods and droughts (Maslin, 2009; Scholze et al., 2006). Some models and observations suggest that human influence can already be detected in a change of precipitation patterns; precipitation is increasing at middle latitudes in the Northern hemisphere, while it is decreasing in the Northern hemisphere subtropical regions (Zhang et al., 2007).
Recent and Future Climate Change
While past climate change remains somewhat enigmatic, there exists a stronger consensus regarding current climate change (IPCC, 2007). As discussed above, the earth has warmed approximately .75 degrees Celsius over the last 150 years (Maslin, 2009). While around 26% of this warming can be accounted for by solar forcing, the majority of the warming is due to anthropogenic CO2 emissions (Karl, 2003). Each year human activities add around 6 gigatons of CO2 to the atmosphere (Eubanks et al., 2006). Approximately 4/5 of these emissions come from the combustion of fossil fuels, with the remaining 1/5 coming from deforestation or other land-use changes (Maslin, 2009). Anthropogenic influence on the warming of the 20th and 21st century has now been detected through the modeling of the recent climate change (Hegerl et al., 2007). Separating out anthropogenic warming from natural variability is necessary because we are currently in a naturally warm interglacial period known as the Holocene (Maslin, 2009).
The Holocene began around 10,000 years BP (Maslin, 2009). This marked the end of the last ice age, which reached its maximum around 21,000 years BP (Carlson, Clark, Raisbeck, & Brook 2007). Following the last glacial maximum (LGM) earth has been warming steadily (Maslin, 2009, p. 42). Approximately 19,000 years BP there was a 10+ meter rise in sea levels that took place within ~500 years (Clark, McCabe, Mix, & Weaver, 2004). Within a period of ~500 years at the beginning of the Holocene there was a rise in temperature of 8-13 degrees Celsius (Birks & Ammann, 2000). The Laurentide Ice Sheet, which covered much of North America, retreated between 9,000 and 8,400 years BP. This resulted in a further ~5 meter increase in sea levels within a ~1000 year period (Carlson et al., 2007). Within the Holocene there have been periodic cooling and warming episodes, some of which were dramatic, but the last 1000 years have been relatively stable (Thornalley et al., 2009; Maslin, 2009, p. 45). Starting in the late 19th century temperatures have begun to rise once again (IPCC, 2007; Maslin, 2009).
Rising temperatures will bring a host of other changes, some of which will further modify earth’s climate. These changes include melting ice cover, rising sea levels, and shifting oceanic and atmospheric circulation patterns (Maslin, 2009). Changes that could further influence earth’s climate are called feedback mechanisms; these could either accelerate or buffer global warming, and are one of the largest sources of uncertainty in future climate predictions (Bony et al., 2006; Raisanen, 2007).
Effects of Global Warming
The first effect of global warming discussed by Al Gore in the documentary An Inconvenient Truth is the melting of mountain ice caps, or glaciers (Bender, 2006). The first example given is of the retreating glacier atop Mt. Kilimajaro in Africa. Kaser (2004), however, found that this particular glacier’s retreat is due to factors other than global warming; this is in part evidenced by that fact that temperatures never rise above freezing at the glacier’s altitude. However, other glaciers are retreating due to global warming (Kaser, 2004). Approximately 67% of glaciers in the Himalayan mountain range are currently retreating (Ren, Karoly, & Leslie, 2007). According to Ren et al. (2007) glaciers in the Himalayan mountains, a source of fresh water for approximately half of the world’s population, may disappear by 2100.
Another concern as the earth warms and ice cover melts is that global sea levels will rise and threaten the coastal communities of people around the world (Alley, Clark, Huybrechts, & Joughin, 2005). From 1961 to 2003 sea level has been rising at an average rate of 1.8 mm per year; from 1993 to 2003 the average rate increased to 3.1 mm per year (IPCC, 2007). This rate is low compared to some episodes of past climate change; during the Holocene melting of the LIS (discussed above) sea levels rose at rates up to 10 mm per year (Carlson et al., 2007). Two particular melting episodes between the LGM and the start of the Holocene had peak rates perhaps greater than 50mm per year (Alley et al., 2005). While current rates are historically low, even small increases in sea level rise could have a substantial impact on coastal areas through erosion, groundwater contamination, and increased vulnerability to storm surges (Alley et al., 2005).
Approximately half of the recent sea level rise comes from thermal expansion of the water itself; the other half is due to the melting of land-based ice sheets (Alley et al., 2005; IPCC, 2007). In An Inconvenient Truth Al Gore does not mention thermal expansion, only the melting of ice caps on mountains and the ice sheets over Greenland and Antarctica (Bender, 2006). Gore uses computer models to simulate a 20 foot sea level rise to demonstrate the future effects of melting ice cover (Bender, 2006). However, current melting rate estimates for Greenland and Antarctica are, respectively, +0.5 mm per year and -0.6 mm per year; this results in a total net contribution to sea level of around zero (Alley et al., 2005). Contributions to sea level rise from mountain glaciers are projected to be < 1 mm per year through the 21st century (Raper & Braiswaite, 2006).
While current sea level change from ice sheets may be negligible, large uncertainties exist concerning ice-sheet dynamics and the possible responses to global warming (Alley et al., 2005; Huyberys, 2006). An increase of glacial earthquakes in Greenland has been detected, and overall melting rates are increasing (Alley et al., 2005; Ekstrom, Nettles & Tsai, 2006). During the last interglacial period (129,000 years BP) sea level was 4-6 meters higher than present (Otto-Bleisner et al., 2006; Overpeck et al., 2006). Greenland is thought to have contributed > 2 meters to sea level rise at that time (Overpeck et al., 2006). A current rise in sea level of that amount could cover some low-lying countries (Overpeck et al., 2006). Dramatic past changes in sea level raise the possibility of such dramatic change happening during the present warming; more research into ice-sheet dynamics is needed (Alley et al., 2005).
Another effect of global warming that has received high media attention and was featured in An Inconvenient Truth concerns changes in hurricane patterns (Curry, Webster, & Holland, 2006; Bender, 2006). Studies have shown an increase in SST over the past 50 years (IPCC, 2007). This rise in SST correlates with a rise in the frequency of intense hurricanes since 1970 (IPCC, 2007). Al Gore uses the example of Hurricane Katrina, which struck the Gulf Coast in the U.S. in 2005, to illustrate the impact of global warming (Bender, 2006). Some studies, however, indicate that the effect of global warming on hurricanes is uncertain: Landsea, Harper, Hoarau, and Knaff (2006) argue that the database of past hurricane activity is too short and unreliable to use to detect trends in intense storms. In addition, there is evidence that as temperatures increase so will wind shear over the Atlantic and Pacific oceans (Vecchi & Soden, 2007; Wang & Lee, 2008). Wind shear is an atmospheric phenomenon that could result in a decrease in hurricane activity and the number of hurricanes making landfall (Vecchi & Soden, 2007; Wang & Lee, 2008). A statistical analysis by Dailey, Zuba, Ljung, Dima and Guin (2009), however, found that increasing SST will likely increase the number of hurricanes making landfall at least in the Southeastern United States. More research in this area is needed, as storm risk is also an important policy issue for coastal communities (Curry et al., 2006).
Ocean circulation is another variable likely to be affected as global temperatures rise. The Atlantic meridional overturning circulation (AMOC), an important transport mechanism for moving heat around the globe as well as an important part of the carbon cycle, can be affected by both temperature change and salinity change as fresh glacial meltwater is added to the oceans (Thornalley et al., 2009). Changes in Atlantic currents can have significant regional climate impacts (Clark et al., 2004). In the early Holocene a regional (and possibly global) cooling event was caused by meltwater disrupting North Atlantic Deep Water (NADW) formation (Rohling & Palike, 2005). Disruption of NADW formation can paralyze the Gulf Stream, an important transporter of heat to Northern Europe (Maslin, 2009). There is evidence of meltwater and other factors already affecting the current salinity of the Atlantic Ocean (Curry et al., 2003). If meltwater disrupts the Gulf Stream, Europe could expect much colder, more extreme winter weather (Maslin, 2009). More research is needed in order to accurately model future changes in ocean circulation and its potential effects on climate (Curry et al., 2003).
Biodiversity will also be affected by climate change; some species will benefit and others will be adversely affected (NRC, 2008). Some forests may be threatened by rising temperatures (Scholze, Knorr, Arnell, & Prentice, 2006). Other plant species, such as soybeans, will have increased vulnerability to predation as CO2 levels rise (Zavala, Casteel, DeLucia, & Berenbaum, 2008). Some grasslands, on the other hand, seem to be mostly immune to climate change (Grime et al., 2008). Overall, animal biodiversity is projected to decrease due to global warming, particularly for species that cannot easily migrate or adapt to a changing climate (Maslin, 2009).
Several other areas will also be impacted by global warming, including agriculture, the spread of certain diseases, increases in wildfires, and an increase in extreme weather events such as floods and droughts (Maslin, 2009; Scholze et al., 2006). Some models and observations suggest that human influence can already be detected in a change of precipitation patterns; precipitation is increasing at middle latitudes in the Northern hemisphere, while it is decreasing in the Northern hemisphere subtropical regions (Zhang et al., 2007).
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
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).
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