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Limiting global warming by the threshold of 1.5°C: The changing hydrosphere
Hydrosphere-atmosphere interactions play an integral part in the Earth system; for example, 25% of carbon dioxide (CO₂) is absorbed by the ocean alone (Canadell et al., 2007, as cited by Seijo et al., 2016, p.1). Thus water bodies on Earth act as major carbon sinks. However, global warming, caused by exponentially rising anthropogenic carbon emissions, is disrupting the equilibrium of many earth systems, including the hydrosphere. The aim of the 2015 Paris Agreement is to keep global warming below 1.5°C of pre-industrial levels and is important because this temperature is considered a ‘tipping point’ of the Earth system and for humans are irreversible effects. This essay will look into how some hydrosphere reservoirs including the oceans and cryosphere, and hydrologic fluxes are changing, using sources drawn from the 2018 IPCC Special Report.
Oceans constitute 97.5% of water reservoirs (Skinner & Murck, 2001), and play a dominant role in hydrosphere processes. Table 1 (Bittermann et al., 2017, p.5), shows that by 2050, a temperature stabilisation of 1.5°C will correspond to a global mean sea level (GMSL) rise of 19cm. In comparison, a 2°C rise in temperature would cause a 24cm rise. By 2150 however, the GMSL rise for 1.5°C and 2°C, is estimated to be 49cm and 67cm respectively, which is 1.4 times the 1.5°C projection. This variation in GMSL emphasises that global warming should be limited to 1.5°C as the initial small differences in GMSL projections increases over time, having significant implications in the future.
Table 1.Temperature projections associated with GMSL rise. Source: Bitterman et al., (2017, p.5)
With rising temperatures, the ocean is undergoing thermal expansion, which is thought to make up 55-70% of GMSL rise (Vermeer & Rahmstorf, 2009). By 2100, the resulting coastal flooding global annual flood costs under 1.5°C and 2°C scenarios could increase by US$ 1.4 trillion per year (Jevrejeva et al., 2018), and insurance may rise from 50-160% to 20-150% respectively, showing that economic and tangible losses to homes and industry could also rise considerably above the threshold of 1.5°C (Kopp et al., 2015, p.705). However, this does not include the additional increased intensity of storms. Warmer air ocean surface temperatures increase evaporation rates and produce more frequent low pressure systems, which results in storm surges in more intense storms and extreme weather events (Skinner & Murck, 2011). Lowe et al., (2001, p.186) highlights the significance of “meteorological forcing” by comparing areas of the UK where ocean-atmosphere interactions increase local storm surge height.
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Being low-lying, Small Island Developing States (SIDS) are particularly vulnerable to GMSL rise. Storm surges threaten infrastructure, and increase the risk of salt-water inundation, which contaminates the islands’ freshwater and irrigation supplies (Storlazzi et al., 2015), rendering the islands’ populations without food or water. Additionally, empirical evidence from Kopp et al., (2015) suggests that higher magnitudes and frequency of storm surges in North Carolina are likely to increase from GMSL rise and hydro-meteorological events, so after 2050 1-in-10 year floods occur annually and 4-5 1-in-100 year floods occur each century. Land within 1m of sea level houses more than 100 million people (Douglas & Peltier, 2002); so GMSL rise and associated flooding is significant. Bittermann et al., (2017) argue that the present conditions of global warming will have a long-term effect on the oceans so that coastal management will still need implementing beyond 2100. The above case studies are localised, but their wide spatiality indicates a future global increase of storm surges along coasts. Although the future ability of the oceans to store carbon is unknown, it is thought that increased temperatures may decrease its carbon-storage capacity (Holden et al., 2018). Therefore, based on ‘high confidence’ rise in current temperature and carbon absorption rates (IPCC, 2018), flooding will be felt in most coastal areas. Based on the above, it is clear that the oceans play a prominent role in the effects of hydrosphere change.
Another key impact of rising global temperatures is on the cryosphere in the form of melting ice, particularly at the poles. Analysis by Sigmond et al. (2018, p.1) suggest the difference between a 1.5°C and 2°C increase in temperature equates to an ice-free Arctic from “once in every five to once in every forty years”. The melted ice contributes to GMSL rise, which as previously discussed, threatens human populations globally, as well as disrupting the hydrologic cycle. Additionally, further melting creates a positive feedback loop as melted ice water has a lower albedo than ice, so absorbs more radiation, inducing further warming. The freshwater influx from melting ice sheets, particularly in the Northern Hemisphere affects the thermohaline circulation, and is thought to be weakening the Gulf Stream (Skinner & Murck, 2011). The significance of a thermohaline circulation shutdown has a global impact; because it moderates European climate, temperatures could reduce by 5°C (Vellinga & Wood, 2008). If this cooling effect were limited to just 1°C increase above pre-industrial levels, it would have a serious global impact. For primary industry, increased crop-failure would likely occur and by 2100, world GDP could be reduced by up to 0.08% (Tol., 2002). Furthermore, Link & Tol (2004) suggest widespread disease and other health issues may arise, resulting in a large outmigration from Europe. As these results were only modelled for a 1°C rise only, it is imperative that global warming should not exceed 1.5°C of pre-industrial conditions if such consequences are to be avoided.
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Niederdrenk & Notz (2018) conclude that ice-free Septembers in the Arctic are highly likely for rises of 1.7°C and above. Therefore, the target of 1.5°C would ensure the Arctic maintains its ice-cover year-round. However, other evidence suggests that freshwater influx from Greenland into the northern Atlantic Ocean acts as a mechanism to offset the positive feedback induced by the melting ice’s reduced albedo and to help the system return to a steady-state. The extent of this effect on future temperature increases varies according to the model being used (Palter et al., 2018). Therefore, it is unclear how higher temperatures in the Northern Hemisphere could contribute to this “regional stabilizing effect” (Palter et al., 2018, p. 823).
Global warming has localised effects that alter the hydrosphere. Extremes of flooding and drought are caused by regional changes in precipitation and evaporation fluxes. There is ‘medium’ to ‘low confidence’ concerning future trends in relation to global warming (IPCC, 2018) due to other changes in the region over time, such as urbanisation or changes in river regimes which can affect a location’s vulnerability to such events. James et al., (2017) identify that precipitation will increase significantly with a 0.5°C increase in temperature, whereas for drought, there are regional variations. Moreover, a 1.5°C temperature rise compared to 2°C reduces high magnitude flooding events by 20-80% (Rasmussen et al., 2018, as cited by IPCC, 2018, p.3-144). The current evidence points towards other global warming factors that increase vulnerabilities. For instance, isostatic readjustment and river engineering are having a negative effect in deltas of Bangladesh and India, and threaten the coastal urban populations in particular (Brown et al., 2018). Bangladesh and India’s geography and development coupled with the effects of global warming, is accelerating the rate of GMSL rise, which increases the risk of flooding. The opposite effect can be seen in the Sahel region in Africa, where repositioning of the ITCZ, caused by warmer temperatures, has reduced precipitation over the Sahel, increasing evaporation rates by up to 10% (Ngaira, 2007). Furthermore, Ngaira (2007) argues that currently, with under 1°C of warming, coupled with human influence from past decades such as overgrazing, which leads to soil erosion and further desertification; exceeding temperatures of 1.5°C, the magnitude of these changes is expected to increase. Therefore, extremes of water surplus and deficit, with naturally or human driven environmental change are affecting the hydrosphere, which varies with location.
In conclusion, it is evident that changes to the hydrosphere have occurred due to global warming. The 2018 IPCC Special Report attributes a ‘high confidence’ towards global warming as the cause of these changes, together with warning that these trends will occur in the future with increased climatic change. Oceanic GMSL rise is the current dominant change in the hydrologic cycle, caused by thermal expansion and polar ice melt. Coastal populations are most at risk from this hydrosphere alteration due to increased flooding risk. Despite the ‘low’ to ‘medium’ confidence attached to extremes of water surplus and deficit being the cause of global temperatures, observations highlight the extent to which extremities in fluxes occur, and the potential sensitivity of such extremities to small temperature rises. Therefore, populations in continental interiors also suffer socio-economic losses as a result of global warming. For these reasons, it is imperative that global temperatures must not exceed 1.5°C due to the significant changes already made to the hydrological cycle and the impact on human environments, which would likely be far greater if carbon emissions are allowed to increase at the current or at any faster rate.
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