The last post

Well, if nothing changes in the future, this would be the post, nearly done! I want to use this last post to share something I have learnt and thought while writing these posts.This blog is about ice melting over the world, and can be splinted into three major parts, which are sea ice, glaciers and ice sheets. Under each major topic, contents generally follows the order of current melting, future projections and major effects. While writing the blogs, I recognized that ice, particularly in the northern hemisphere, has melt significantly during the last few decades, and is likely to continue melting over this century.

 Arctic sea ice might reach near ice free status in the second half of this century! Global climate, ocean circulation and local species have already affected by the decline of sea ice, and the situations could delegate in the future. Meanwhile, losing ice from glaciers is also a serous issue over the world. In mountain regions, melted water forms glacier lakes which have potential to cause local flood, threatening the survival of local population. Once a glacier has totally melt, drought and water shortage will become problems, particularly for Asian region as many major rivers originated from glaciers. Melting from glaciers and ice sheets also contribute to sea level rise, though not much up till now, it does have potential to rise rapidly and submerge many coastal areas. 

The changes are dominantly caused by global warming. Humans are to blame for the current  global warming due to GHGs emissions since industrial revolution. The atmospheric concentration of carbon dioxide has exceeded 400 ppm. The last year, 2016, becomes the warmest year in history, beyond our prediction. Even if we stopped all emission immediately, co2 would remain in the atmosphere and kept warming the Earth.

Paris Agreement was signed to control the temperature rise within 2 C. This is a significant development in resisting global warming with governments involved globally. However, uncertainty still exist in the future, for instance, the attitude from the US government.

What we can do is to save energy and resources to reduce GHGs emissions in over daily life. The Earth is not only the home to us, but also the home to all livings. We share the Earth, and we should not damage their habitats or threaten their survivals to meet our development. We also have the responsibility to provide an Earth suitable to survive to future generations. 

Ice melting and sea level rise

So far, we have talked about ice melting from sea ice, glaciers and ice sheets, and have mentioned some serious impacts. This time, we are going to systematically look at one significant global issue derived from ice melting, i.e. sea level rising. Will the world be submerged in the future?



Figure 1. Processes relevant to sea level change in IPCC. (Source: IPCC, 2013)

There are three factors contribute to sea level rise, including thermal expansion, added water from land ice (glaciers and ice sheets) and change in basin depth. Thermal expansion is the biggest contributor to sea level rise (IPCC, 2013), and the topographic change currently contribute to 3 mm/yr drop in sea level due to glacial isostatic adjustment. I will only discuss the contribution from ice melting below, which is sea level change from changes in mass of ocean. The unit is sea level equivalent (SLE), mentioned in previous posts, i.e. sea level equivalent to a mass of water (mass divided by density (1000kg/m³) and area (362500 billion m²) Approximately, adding 362.5 Gt water will lead to 1 mm rise in sea level. Figure 1 shows how IPCC AR5 assessed the processes involving in modelling sea level change, and the linkage between the relevant sections. Sea level change can be modelled via several types of model, while Atmosphere-Ocean General Circulation Models (AOGCMs) provide most comprehensive simulations. Regional Climate Models (RCMs) with information form AOGCMs are important to simulating changes in glaciers and ice sheets.

Ice losses from glaciers and ice sheets have been mentioned in early posts (the first and the last three posts) with unit in Gt. Convert them to SLE:

• Total ice loss from glaciers in SLE was 0.62 ± 0.37 mm/yr during 1971-2009, 0.76 ± 0.37 mm/yr during 1993-2007, and 0.83 ± 0.37 mm/yr during 2005-2009;
• During 1992-2001, ice loss was -0.02 to 0.20 mm/yr from Greenland ice sheet and -0.10 to 0.27 mm/yr form Antarctica ice sheet, and during 2002-2011 the rate was 0.43-0.76 mm/yr from Greenland and 0.20-0.61 mm/yr from Antarctica.

Figure 2 shows the contribution to sea level change up to 2100 under different scenarios. Major contribution would come from ice melting from glaciers, contributing to 0.08-0.38 m rise by the end of this century, while 0.10-0.19 m mean rise under 4 RCPs (IPCC, 2013) . Based on modelled future projections, Greenland ice sheet would contribute to a rise below 0.11 m, while Antarctica ice sheet would likely to drop sea level with predicted increase in precipitation, according to IPCC (2013).

Figure 2. Future projections of sea level rise under 4 RCPs. (Source: IPCC, 2013)

 



















 

Ice loss from ice sheets 2: Antarctica ice sheet

Hope you have enjoyed your Christmas holidays!


Following last post, we are going to look at Antarctica ice sheet.  About 90% of ice on the Earth, which is the largest potential contributor to sea level rise in the future. Antarctica ice sheet consists of three parts, West Antarctica, East Antarctica and the Antarctica Peninsula. Figure 1 show a schematic structure of Antarctica.


Observations and scientific understanding of Antarctica remain at low level (IPCC, 2013), which cause poor understanding of ice loss at Antarctica ice sheet. Overall, ice has kept losing over the last two decades though with large uncertainty, as showed in figure 2. The loss rate had increased from -135 to -58 Gt/yr during 1993-2010 to -221 to -74 Gt/yr during 2005-2010 (IPCC, 2013). The loss was caused by warming in temperature, as well as warming in tropical sea surface temperature (Ding et al., 2011). Shepherd et al. (2012) found that ice loss occurred at the Antarctica Peninsula and the West Antarctica with accelerating rate, while the East Antarctica had net gain ice. The three maps in lower Figure 3 show these changes in ice. 


Figure 1. Cross section of Antarctica ice sheet. (source: LIMA, NASA)



Figure 2. Cumulative ice loss from Antarctica ice sheet. (Source:IPCC, 2013)






Figure 3. Ice loss rate (Source: IPCC, 2013)


However, a relative new study by NASA indicated that Antarctica ice sheet gains more ice than losses. Though this study agreed with IPCC (2013) and Shepherd et al. (2012) that
ice loss occurred at the Antarctica Peninsula with accelerating rate, it showed that
East Antarctica and the interior of West Antarctica had net gain of ice with thicker at the rate of 1.7 cm/yr, compensating ice discharge over the whole ice sheet and even reducing sea level rise at the rate of -0.23 mm/yr. This challenges the statement that Antarctica ice sheet currently contributes to sea level rise at the rate of 0.27 ±0.11 mm/yr over 1993-2010 and 0.41 ±0.20 mm/yr over 2005-2010 (IPCC, 2013). Future studies are required here in order to solve the accurate contribution of Antarctica ice sheet on sea level rise.

Figure 4. Changes in ice by NASA's study.




Ice loss from ice sheets 1: Greenland ice sheet

A ice sheets is a enormous glaciers that cover an area over 50,000 km². Two.ice sheets are Greenland ice sheet (7 × 10⁶ km²⁾ and Antarctica ice ice sheet (1.4 × 10⁷ km²). They, together,  contain about 99% freshwater ice over the world. 2/3 of sea level rise is contributed by ice melting from the two sheets (IPCC, 2013), the major source of freshwater to the ocean. Figure 1 shows how the two ice sheets form and flow.

Figure 1. Form and flow of Greenland and Antarctica ice sheets. (Source: adopted from  LIMA, NASA)

Greenland

Change in mass of ice sheets is the net mass gain over precipitation and melt and runoff, and it can be measured by mass budget, repeated altimetry and temporal variations in Earth gravity field (IPCC, 2013). Over the last few decades, Ice mass loss has occurred in Greenland with decline rate accelerated: the average loss rate had accelerated from -121 Gt/yr over 1993-2010 to -229 Gt/yr over 2005-2010 (IPCC, 2013). Up till now, the highest ice loss occurred in 2012, as simulated by model MARv3.5.2 (Figure 2). In this year (2016), the loss is estimated about -144 Gt/yr extra with respect to 1981-2010 average loss, according to the model. The melt season occurred earlier than previous years. 

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Figure 2. Anomalies of surface mass balance, snowfall and runoff form the Greenland ice sheet, simulated by model MARv3.5.2.(Source: adopted from Laboratoire de Climatologie et Topoclimatologie).

Fettweis et al. (2012) used the MAR model to estimate the contribution of surface mass balance from the Greenland to global sea level rise in the future. The model is a regional climate model (RCM) forced by general circulation models (GCMs) from CMIP5 and ICE2SEA under scenario RCP4.5 and RCP8.0. Their study showed that models used in the study simulated continuous ice loss from Greenland within this century (Figure 3), positive correlated to temperature rise but with non-linear relationship (Figure 4). 

Figure 3. Annual changes in surface mass balance, snowfall, rainfall, runoff and contributions to sea level rise. (Source: adopted from Fettweis et al. (2012))

Figure 4. Annual changes under RCP4.5 and RCP8.0. (Source: adopted from Fettweis et al. (2012))

 

 

Melting glaciers

Last time, we looked at two major issues from glacier melting at Himalayas. Himalayan glaciers are important, because they are sources to rivers and ‘water tower’ of Asia to support Asian population. However, these glaciers only take small proportion of glaciers worldwide. This time, we will look at the fate of glaciers over the world. Though difficult to estimate glaciers due to uncertainties in measurement, a global glacier mass is likely in the range of 114,000-192,000 Gt (IPCC, 2013).

Figure 1. Global distribution of glaciers, based on RGI regions. (Source: adopted from IPCC, 2013).

Figure 1 shows the global distribution of glaciers, divided into 19 Randolph Glacier Inventory (RGI) regions. Large proportion of glaciers are distributed in high-latitude regions, which are dominant sources contributing to sea level rise caused by glacier melting. Changes in glaciers can be measured via mass, volume, length and area. Table 1 shows these methods and their characteristics.

                             Table 1. Measurement methods, adopted from IPCC (2013)

Glaciers are sensitive to climate change (temperature and precipitation in particular), and they adjust to new equilibrium to balance the change. Over the last few decades, global glaciers have retreated overall with considerable mass loss in order to balance global warming, e.g. all RGI regions lost glacier mass during 2003-2009 with rate at –259 ± 28 Gt/yr in total (Gardner et al., 2012). Figure 2 shows the glacier mass budget at RGI regions during the period. Large mass loss occurred in Alaska (–50 ± 17 Gt/yr), Greenland periphery (–38 ± 7 Gt/yr), Arctic Canada North (–33 ± 4 Gt/yr), Southern Andes (–29 ± 10 Gt/yr), Arctic Canada North(–27 ± 4 Gt/yr) and High-Mountain Asia (–26 ± 12 Gt/yr) (Gardner et al., 2012). 

Figure 2. Global mass budget in Gt/yr, based on GRI regions. (Source: adopted from Gardner et al., 2012)

Marzeion et al. (2012) used CRU-forced model to simulate glacier mass change 1901-2010 (Figure 3), using unit SLE. SLE represents sea level equivalent to glacier mass. Their simulations showed that glacier mass gradually lost during the period, with peak loss rate occurred in 1930s caused by glacier melting at Greenland. Peak loss rate of glacier mass at Russian Arctic occurred between 1950 and 1960, as well as Arctic Canada. After low loss rate in 1970s, the rate increased again.

Figure 3. Cumulative global surface mass balances relative to the 1986–2005 mean, and rates from the CRU-forced model. (Source: adopted from Marzeion et al., 2012)

Future projection (Marzeion et al., 2012) shows gradual loss in glacier mass in all RCPs towards 2100 (Figure 4), with rate increasing until the middle of this century . After mid-century, the rates under RCP2.6 and RCP4.5 will slow down, while the rates under RCP6.0 and RCP8.5 will still increase, according to Figure 4.

Figure 4. Cumulative global surface mass balances and rates from the model forced with CMIP5 projections under 4 RCPs (RCP2.6 in red, RCP4.5 in green, RCP6.0 in blue and RCP8.5 in pink) toward the end of this century. Solid lines represent the mean of simulations from individual models in CMIP5. (Source: adopted from Marzeion et al., 2012)

 There are about 170,000 glaciers, and they have their own characteristics and climate conditions, leading to their different response to climate change with various time scales. For example, some glaciers at Alaska have retreated quickly (Gardner et al., 2012), while some at Karakoram mountain range have been stable or even have advanced (Bolch et al., 2012) Meanwhile, many glaciers are still poorly known. These result in considerable uncertainties in future projections, particularly in mountain regions like the Karakoram-Himalaya mountain range with complex glacier properties and climate conditions (IPCC, 2013).

Melting Himalayas: Flood and freshwater shortage

Though melting at Arctic sea is a serious issue, melting at Himalayas is more concerned by public. Himalayan mountains contain 40% of freshwater over the world and thousands of glaciers, and they are the source of the seven greatest rivers in Asia. 20% of the world’s population rely on Himalayan glaciers as water sources. Large proportion of glaciers here are fed by snowfall from summer monsoon, however, global warming reduces precipitation from monsoon and extends melting period (Bolch et al., 2012). This leads to many serious problems. There is a nice video called “Himalayan Meltdown” introducing these problems. I just put its trailer here for you to get a feel for how serious the melting at Himalayas is.

One major issue threatening Asia is glacier lakes. Melted water forms thousands of glaciers lakes in the region, and water in glacier lakes can accelerate melting rate via transmit heat efficiently to glacier in contact with glacier ice (Bolch et al., 2012). Meanwhile, many of these lakes are likely to burst at the seams. For example, Glacial Lake Outburst Floods (GLOFs) Assessment (2010) identified  more than 200 lakes in the region likely to burst. When they burst, villages at downstream are threatened and may destroyed  by floods. Governments put a lot of effort into reduce the water level of glacier lakes. Bhutan Government had successfully reduced water level of Lake Thorthormi. However, this is not a permanent solution as long as glaciers keep melting and feed the lake. 

Another problem is drought. Since global warming reduces precipitation from seasonal monsoon, the proportion of runoff based on glacier melted water increase (Bolch et al., 2012). When glacier dries up, there will be serious issues in water supply. According to the video, some villages have faced water shortage. Downstream discharge is significantly affected by upstream discharge. Immerzeel et al. (2010) used A1B SRES scenario to simulated mean upstream discharge (Figure 1), and found decreases in major Asian rivers. They concluded that 4.5% of the total population will have food security problem due to decreasing water supply.

Figure 1. Simulated mean upstream discharge.

Ice melting at Arctic Sea 4: effects, bad or good?

Continuing the previous series, this will be the last post about sea ice melting. From the early posts, both observations and simulations show that sea ice at Arctic sea has declined significantly with melting rate accelerated. Sea ice would be likely to decrease continuously within this century, and might reach nearly ice-free status. So, what are the effects?

 Climate 

Arctic sea ice affects global climate change through complex processes. Firstly, there is positive feedback between warming and ice melting (as showed in Figure 1, Dessler, 2011). Current global warming has caused and will cause more sea ice melting at Arctic sea. Decreasing ice cover while increasing sea surface leads to the decrease in albedo, as sea surface reflects less and absorbs more solar radiations than ice surface. More absorbed energy leads to extra warming, thus, melting more ice. Therefore, ice melting at Arctic sea will amplify the global warming in this century. 

Meanwhile, sea ice also insulates the below sea water from the atmosphere, preventing thermal exchange and gas exchange between the ocean and the atmosphere. Sea ice loss at Arctic sea changes the interaction between the ocean and the atmosphere. Melting water decreases the salinity of sea water as well, which changes the density of sea water affecting ocean circulation. This will affects the climate at global scale.

Figure 1. Ice-albedo feedback

 Ecosystem

Sea ice is the habitat to many species, from fungus to large mammals like polar bears. However, sea ice loss has threatened their survival, affecting the population of the species. The US Geological Survey studied how the Pacific walrus and polar bears response to the rapid decrease of sea ice at Arctic sea. They found that stampeding events (Oakley et al., 2012) could be to blame for the high mortality in young walruses, as walruses had to crowd living on the shores of Alaska and Russia with less sea ice available. They also found that longer swimming distance could be blame for the decrease in the survival of polar bears (Oakley et al., 2012), with less sea ice available as well. Loss of hunting habitats could also cause the decline in polar bear's population (IPCC WG2, 2014). An earlier study (Stirling and Parkinson, 2006) found that the decline could be substantially. 

Source: USGS

Sea ice loss has also challenged the local biodiversity. One problem is hybrid. A hybrid of a grizzly and a polar bear was hunted in the Arctic in 2006 (Kelly et al., 2010). The explanation could be that polar bears live in the same area as grizzlies because of the loss in their habitats, sea ice.


 Human activities

Though ice melting leads to many bad impacts, there is a good news for human. Nearly ice-free status can provide open Arctic ocean. Ships may be able to navigate through Arctic ocean within this century, saving lots of time. Melia et al. (2016) simulated routes through Arctic, and concluded that shipping from Europe to Asia through the Arctic could become more than 10 days faster, and could save 4 days from N. America to Asia.