Discuss the human and natural factors contributing to changes in the global energy balance of the Earth over the last 1,000 years.
The global energy balance describes the balance between shortwave radiation received from the sun and outgoing longwave radiation emitted back into space (Global Physical Climatology, 1994). The absorption of solar radiation mostly occurs at the surface of the Earth, but most emission into space happens in the atmosphere (Global Physical Climatology, 1994). The amount of solar energy absorbed and emitted by the Earth fluctuates because of different human and natural influences (Srinivasan, 2008). The Earth’s climate is largely regulated by the energy balance and these fluctuations (Wild, 2013). This paper will discuss the impact of some of the most significant regulators of the energy balance, and how they have influenced climate forcings over the past 1000 years.
Figure 1: Grobe 2000, Percentage of reflected sun light in relation to the various surface conditions of the earth.
Clouds and Albedo
Clouds play a dominant role in the radiation balance of the Earth. Clouds have a cooling effect on the surface by increasing the planetary albedo and thus lessening the incoming shortwave radiation that reaches the surface (Södergren, 2018). As shown in Figure 1, in comparison to other surfaces, clouds have a high albedo and hence when present will likely reflect more shortwave radiation than the surface beneath them (Grobe, 2000). The ice particles present in clouds disperse between 20% and 90% of the sunlight that reaches them, which explains their bright white appearance (Geerts, 2002). An Earth without clouds would absorb approximately 20% more shortwave radiation from the sun than comparative present-day values (Geerts, 2002). Studies, such as those done by Tapio Schneider, have shown that a significant reduction in stratocumulus clouds could cause an astounding 8°C jump in average global surface temperatures (Schneider, 2019). However, clouds also contribute to a warming of the surface by trapping outgoing longwave radiation and radiating it back downwards (Graham, 1999). The longwave radiation absorbed as a result of cloud re-radiation warms the Earth’s surface by around 7°C, which when coupled with the cooling effect from shortwave radiation causes a net cooling of about 5°C (Rossow, 1996). In comparison to the decadal radiative forcing caused by the increase in greenhouse gases, the net cooling effect induced by clouds is around 10 times higher, and therefore changes in cloud albedo significantly affect the radiative balance, and hence climate on a global and regional scale (Mueller, 2011).
Figure 2: Norris 2016, Locations where cloudiness has changed from the 1980 - 2000, relative to the global average change.
Anthropogenic influences can alter cloud structure, formation and geography. Monitoring of the Earth’s radiation budget between 1985 and 2012 has revealed an imbalance: there has been more radiation absorbed than emitted (Orwig, 2014). Could changes in clouds explain these changes in the radiation budget? Since the 1980s, it was discovered that clouds have gotten higher (Gramling, 2016). Assuming a constant temperature in the atmosphere, an inverse relationship is distinguishable between cloud height and outgoing longwave radiation; as the height of clouds increase the amount of longwave radiation reflected back into space will decrease, and thus over time the Earth will experience rising surface temperatures (Evan, 2012). A positive feedback loop is thus created, as the primary driver of these changes to clouds is the increasing greenhouse gas concentrations, which over the past 1000 years has soared due to human activity (Johnston, 2016). In addition to cloud height, greenhouse gas emissions have caused cloud migration (Johnston, 2016). As shown in figure 2, during the latter half of the 20th century two distinct cloud migration patterns have occurred: firstly, there has been an increase of “subtropical dry zones” towards the poles, and secondly there has been a shift of mid-latitude storm tracks towards the poles (Norris, 2016). This shift in clouds could explain the changes to the radiation imbalance as when clouds shift towards the poles, more of the ocean is exposed to incoming shortwave radiation and because oceans are darker and have a lower albedo than clouds, more radiation will be absorbed, causing faster warming, and thus preserving the energy budget imbalance (Gramling, 2016).
Figure 3: Mueller 2011, Net global temperature cooling or warming induced by clouds.
In stark contrast to the oceans, polar regions near the poles have become less exposed to incoming radiation (Mueller, 2011). At the polar regions clouds tend to be thin and low-lying, thus allowing the sun’s energy to pass through them to the surface where they are absorbed, whilst simultaneously trapping heat near the surface of the ice (Wang, 2017); these cloud changes were speculated to have caused the record-breaking Greenland ice melt in 2012 (Politics & Government Business, 2013). As ice retreats, such as in Greenland, the presence of open water increases, favouring cloud formation and therefore further amplifying sea ice melting in a positive feedback loop (Huang, 2019). This phenomenon is shown in figure 3 by the red areas near the poles (Mueller, 2011). It can consequently be argued that the increase in the global energy surplus in the last century has been concurrently caused and amplified by clouds.
Greenhouse Gases, Aerosols and Anthropogenic Influences
Figure 4: Crowley 2000, Greenhouse gas forcings over the past 1000 years and projected forcing
As depicted in figure 4 by the sharp peak around 1850, warming over the past century is unprecedented in the past 1000 years (Crowley, 2000). Infact, 41%-64% of temperature fluctuations between 1000 and 1850 can be accounted for by external factors, such as volcanism and solar irradiance (Crowley, 2000). However, over the last 100 years global temperatures have risen by 0.74°C, and most of this change can be attributed to increasing greenhouse gas emissions and their effect on the energy budget (European Environment Agency, 2006). Carbon dioxide forces an energy imbalance by absorbing thermal infrared energy radiated by the surface (Schaller, 2013). Carbon dioxide partially traps outgoing longwave radiation in the atmosphere, therefore radiation that previously would’ve left the atmosphere is now absorbed (Pease, 1987). Anthropogenic carbon dioxide emissions have been almost exponentially increasing since the Industrial Revolution, and thus observed warming of around 0.7°C between 1880 and 2003 as response to nearly 1 W/m2 of forcing can be argued to be partially caused by Co2 emissions (Wang, 2016). In addition to carbon dioxide, water vapour also plays a part in this energy balance change. Water vapour absorbs 60% of outgoing longwave radiation, which in comparison to the 26% absorbed by carbon dioxide, is significantly more potent (Kleidon, 2008). Warming creates a positive feedback of increased water vapour, as when surface temperature increases so does ocean water evaporation – intensifying the greenhouse effect further (Jacob, 2001). However, through the process of latent heating, when water vapour condenses it forms clouds creating a negative feedback as, since previously stated, clouds have a dominant cooling effect, reducing the disparity between incoming and outgoing radiation (Earth Labs, 2019).
Figure 5: Allan 2014, Changes in outgoing longwave radiation and absorbed shortwave radiation and their net effect
Another factor contributing to changes in the energy budget are the presence of aerosols. Although naturally occurring from volcanic eruptions and wildfires, the presence of aerosols has been enlarged by anthropogenic sources such as coal-fired power plants and vehicles (Russell, 2007). Aerosols reduce the amount of sunlight reaching the surface and is said to be the primary cause of ‘global dimming’ (Hansen, 2011). While greenhouse gases are thought to proliferate warming, aerosols do the opposite (Dallafior, 2015). Intense volcanic eruptions, such as Pinatubo, disperse enough aerosols into the atmosphere to cancel out greenhouse warming resulting from a doubling of carbon dioxide (Dickinson, 1996). Moreover, the addition of aerosols in the troposphere because of fossil fuel combustion may already be countering up to half of greenhouse-gas warming (Dickinson, 1996). The cooling influence of the 1991 Mt. Pinatubo eruption, which injected around 20 Tg of SO2 into the stratosphere, is estimated to be 0.5K, which was ultimately caused by the net energy balance deficit which ensued (Swingedouw, 2017). This change is illustrated in figure 5 by the peak occurring at 1991, demonstrating the dramatic impact aerosols can have on the energy balance (Allan, 2014). Therefore, although the energy budget is currently imbalanced, without the presence of aerosols this disparity could be substantially larger.
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The last 1000 years has seen many changes to the global energy balance, however it is evident that the most profound changes have occurred over the past 100 years (Crowley, 2000). The net impact of human activity such as higher concentrations of carbon dioxide and aerosols has caused a rise in surface temperatures of the earth by 0.70C over the past century (Srinivasan, 2008). Research into the global energy balance has enlightened researchers with innovative methods in which they can counteract global warming such as artificially injecting aerosols into the stratosphere to mimic volcanic eruption cooling and engineering of brighter clouds to increase reflection of infrared radiation (Baughman, 2012). Further investigation into the interactions between Earth and solar energy could help to unlock even more methods to offset climate change, before the devastating ramifications of global warming are experienced.
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