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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).