Contributions to Sea Level Rise: Antarctic Ice Sheet

The Antarctic Ice Sheet is by far the largest body of ice in the world, and therefore has the potential to cause massive SLR. However during the 20^th Century its contribution to SLR has been fairly limited considering its massive volume. This is due to Antarctica having not experienced significant temperature rise due to its isolation by the massive and cold Southern Ocean.

The volume of ice in Antarctica is equivalent to 60 m of SLR

Map of Antarctica, including major bases and ice shelves. Source

Current Situation

Antarctica is separated into two major ice sheets by the Transantarctic Mountains; the East and West Antarctic Ice Sheets. Overall there has been negligible thinning over the majority of the Ice Sheet in recent years explaining the limited contribution to SLR. Recent suggestions are that in terms of SLR the West Antarctic (~7 m SL equivalent) is most likely to lead to a large contribution as there has been evidence of increased ice stream velocity and ice loss in recent years. This is most often seen on floating ice shelves in West Antarctica. This is also shown by the thinning in the West Antarctic Ice sheet as shown by Chen et al., (2009).

Accelerated thinning in West Antarctica. Source

In addition catastrophic events of ice shelf collapse have been seen on the Antarctic Peninsula. The Larsen B Ice Shelf (3,250 km2) collapsed in 2002 due to warming of the Peninsula and meltwater ponding. Following that there was significant glacier speed up and thinning (Rignot et al., 2004). This suggests a possible acceleration in SLR if large parts of the many Antarctic ice shelves collapse. 

Location of major ice shelves (left) Catastrophic collapse of Larsen B Ice Shelf (right). Source

The Pine Island Glacier in West Antarctica is second in speed to Jakobshavn in Greenland in terms of speed of retreat. It drains 20% of the West Antarctic Ice Sheet, and has been estimated to possibly contribute 10 mm of GMSL rise in the next 20 years (Favier et al., 2014). Similar to Jakobshavn the cause of this rapid retreat and ice loss by discharge is attributed to melting as a result of a warmer ocean. If this picture is repeated across a number of the major outlet glaciers then the contribution to SLR could rapidly increase.

Speed of Pine Island Glacier as it reaches the sea. Source

Future

GCMs project an increase in precipitation in the Antarctic region that could actually increase SMB across the region and therefore contribute negatively to GMSL rise. Surface melt in Antarctica is minimal due to the consistently cold temperatures and is likely to remain so for the foreseeable future as a massive rise in temperatures would be required to have any effect. The worrying areas are glaciers that are grounded in the ocean, warming ocean temperatures are the biggest threat to Antarctic Ice Sheet stability, particularly in the West. It is possible on longer timescales that the East Antarctic could contribute significantly to SLR because of marine ice melting however this is very unlikely to occur before 2100 (Mengel & Leverman, 2014).

Contributions to Sea Level Rise: Greenland Ice Sheet

Having focused in the last post on the contributions of glaciers and ice caps to SLC, this week focuses on the one of the two largest stores of freshwater the Greenland Ice Sheet. Along with the Antarctic Ice Sheet it holds 98% of the freshwater stored in ice and so its stability or otherwise could be crucial to SLC in the future. The behaviour of ice sheets creates the most significant uncertainty for future SLC, as their response to climate change is poorly understood and they are notoriously difficult to model.

The volume of ice in Greenland is equivalent to 7 m of SLR

From this statement it is clear that melting of ice from Greenland (below) has potential to catastrophically affect the world. However what is the likelihood of significant ice loss from this source?

The Greenland Ice Sheet with some major outlet glaciers indicated. Source

Current situation

The Greenland Ice Sheet, despite being significantly smaller than the Antarctic Ice sheet has in recent decades contributed more to global SLR due to increased surface melting (Rignot et al., 2011).  Straneo & Heimbach (2013) suggest mass loss has quadrupled in the last 20 years and contributes up to 25% of global SLR. This is attributed to increased surface runoff due to summer surface melting (see below) as a result of the warming temperatures in the region. Temperatures have increased rapidly by up to 5 degrees in recent years which increases the propensity to melt (Applegate et al., 2014). Until recent years this has been offset by increases in precipitation (Hanna et al., 2007).

Increased area of surface melting in the last 10 years. There is some interannual variability but a trend for increased melting. Source

However the increased run off has been linked to a subsequent increase in sliding at the base of the glacier which increases velocity and therefore calving in the ocean terminating glaciers (Zwally et al., 2002). The graph below summarises the recent situation of the Greenland Ice Sheet, with the decrease in Surface Mass Balance (red) particularly worrying and suggesting SLR.

Graph showing mass flux of Greenland Ice Sheet. Blue line (Precipitation), Green line (Melting), Red line (Surface Mass Balance), Orange Line (Run off). Dashed lines are trends since 1990. Source

Greenland has only a few outlet ice streams that reach the sea, and act as the major deliverer of melt to the sea. Indeed some of Greenland’s outlet glaciers such as Jakobshavn have been dubbed ‘the fastest glaciers in the world’. As the image below shows, Jakobshavn has retreated rapidly in the past 150 years. This is worrying as Jakobshavn drains 6.5% of Greenland’s Ice sheet area. This picture is repeated across a number of outlet glaciers with rapid retreat and increasing contribution to SLR.

The retreat of the Jakobshavn Glacier since 1950. Source

Increasing ocean temperatures around Greenland result in rapid calving at the front of the glaciers as this video shows, attributed to rapid submarine melting due to the increased ocean temperatures (Rignot et al., 2010). This speedup continues with it estimated to move up to 46 m per day! As the clip from the film 'Chasing Ice' shows, calving can be rapid and dynamic causing SLR.

The Future

The recent acceleration in ice loss from the Greenland Ice Sheet means it is contributing more and more to GMSL rise. This contribution is projected to increase as more surface melt due to occurs over more sections of the Greenland Ice Sheet, which increases ice loss. The IPCC suggests it will contribute 0.01- 0.17 m to GMSL rise (or 0.2 - 2 mm/yr) by 2100 but these estimates may be an underestimation due to limits in the understanding of glacier flow, leading to uncertainty as shown in the graph below. 

Projections of Greenland's contribution to SLR by 2100. C is Solid Ice Discharge (ie. calving), D is Surface Mass Balance (melting). Source

Irreversible Loss?!

Some GCMs have projected a non-linear response to warming for the Greenland Ice Sheet; as the surface melts it lowers and this warms the near surface which would further melt the ice sheet. This could lead to vastly increased SLR as the Ice Sheet dynamically thins. This is as yet uncertain and projected as unlikely in the 21st Century but further forward could lead to massive reduction in size of the Greenland Ice Sheet and significant contribution to SLR.

Contributions to Sea Level Rise: Mountain Glaciers & Ice Caps

In this momentous week where Donald Trump has been elected as President of the USA; Climate Scientists, and all those who believe in the argument that frankly among climate scientists is no longer an argument, it is worrying to point out this video where Trump dismisses Climate Change as a hoax or just ‘weather’. A good point of view piece is from Green MP Caroline Lucas on why Trump's election is a worry. On the off chance Donald is reading this blog (…) I will point out the Climate Central website which projects the impact of SLC on coastal cities including his beloved New York… 

But I shall not dwell on this and move onto one of the major contributors to SLR, that from glacier melting. To clarify this I am following the IPCC in distinguishing between the Antarctic and Greenland Ice Sheets (see map below) and all other glaciers and ice caps, this post focuses on these smaller glaciers and their contributions to SLR both during the instrumental period and in the future.

Location of the major mountain glaciers and ice caps (1-18) that contribute to SLC, Greenland & Antarctica (19 & 20) are not discussed in this post. Source

Mountain glaciers and regional ice caps have been observed to be one of the most important recent contributors to SLC. The video below shows how Glacier National Park, USA could become no Glacier National Park by 2050. 

This picture is repeated across mountain glaciers and ice caps around the world, dynamical and extreme retreat has occurred in most mountainous regions since the 1850s, producing meltwater that contributes to SLC. As the graph below shows recent SLC contribution is similar to that of the Greenland and Antarctic Ice Sheets combined.

This paper from Jacob et al., (2012) analyses the contribution of Glaciers and Ice Caps to SLC and suggests a value of 0.41±0.08 mm/yr as its contribution to the GMSL rise. This paper heralded a distinct advancement in the understanding of these smaller ice sources and their contribution to SLC by using GRACE altimetry to constrain mass balance (whether a glacier is advancing or retreating). It shows an acceleration across a number of regions including Alaska but suggests a decreased influenced from the Himalayas. This is due to enhanced spatial resolution as they took sub regions of the Himalayas compared to seeing the Himalayas as one region as done by Matsuo & Heki (2010), which resulted in a perceived overestimate of Himalayan contribution. This is a forward step for understanding contribution to SLC from these smaller, spatially disparate sources that have had significant impact in recent times to SLC. Further support for the large contribution to recent SLC is given by Gardner et al., (2012) who suggest current contribution of 29±13% to global SLR is from these Glaciers and Ice caps. Ice loss is particularly strong in the Andes, and Arctic Canada and Alaska (see below) and as my post from a couple of weeks ago suggests, this could cause far greater local and regional SLC.

Contributions of different glaciated regions to SLC. Source

Future Contribution

As significant ice mass loss continues this will contribute to global SLR, however as glaciers shrink in size they are likely to come to a point when they reach a balance. Some glaciers such as those remaining in Africa are likely to disappear completely although their contribution to SLR is likely to be negligible. Radic & Hock (2011) present a study suggesting many will have experienced serious loss but some may only lose 20% of ice mass. GMSL contribution by 2100 is expected to be 0.124±0.037m yet variable contributions and uncertainty mean that this is by no means certain. Therefore by 2100 the contribution may be almost negligible as melt may have stabilised due to lack of available ice to melt. In addition the effects on water availability, for instance billions of people rely on the Himalayas as the primary source of water could have severe consequences.

Projected volume (left axis) and Sea Level equivalent (right axis) of glaciers for a series of models until 2100. Source

Contributions to Sea Level Rise: Thermosteric change

The IPCC suggests there are three major factors that have contributed to the observed 20^th Century rise in SLC: Thermal expansion of the ocean, ice loss from glaciers and ice sheets and changes in terrestrial water storage. Other factors (see below) also contribute but those 3 are the most important for GMSL change. This blog will focus on thermal expansion’s impact on recent SLC and also future implications.

Causes of SLC. Source

Simple physics suggests that as water is warmed, it expands due to having increased energy: Thermal Expansion (see video...)

For the altimetry record (1993-2010) thermal expansion is calculated to have contributed 1/3 of the total GMSL change, while the graph below records estimates for its contribution to GMSL rise for the past 50 years.

Thermal expansion in the upper 700m is in red, in the deep ocean is orange. Source

The world’s oceans are the key sink of anthropogenic climate warming, estimated at having absorbed 93% of the warming of the earth’s system since 1950, and although this has been beneficial in checking the levels of anthropogenic warming on the atmosphere it has had an effect on SLC by raising ocean temperatures and subsequently causing thermosteric SLR (Sabine et al., 2004). This has been mainly in the upper section of the ocean (0-2000m). Although upper ocean warming is well constrained, thanks to the ARGO float scheme of measuring steric changes in the oceans, the deep ocean warming remains poorly understood.

The Argo Float network. Source

Studies have started to unlock this such as this Johnson & Doney, 2006 who showed recent abyssal South Atlantic warming but it is unsure over the longer timescales whether this can be applied to the whole ocean, although Johnson et al., (2007) showed a similar trend in the Pacific. Both these studies used robust methods and returned good confidence intervals that this deep ocean warming is observable. However a lack of spatial coverage of sampling from both of these studies of the deep ocean makes rigorous conclusions about temperature changes hard to apply to the wider ocean. There is not as yet a sampling system similar to ARGO (above), and therefore deep ocean warming continues to be a relatively poorly understood mechanism of thermosteric SLR. The combination of deep ocean and upper ocean warming acceleration in thermal expansion has been observed during the 20^th Century and is included in climate models to increase in the future (Church et al., 2006).

Future thermosteric rise

Projected SLR as a result of thermal expansion for three separate climate scenarios. RCP 45 is considered most likely at present whereas RCP 85 is a worst case scenario. Source

The question of whether thermosteric sea level rise will continue to increase seems clear. It is highly likely to and the rate of rise is also projected to increase. The ocean should still be able to act as a sink for some of the Global Warming. So should we be worried? This is unfortunately one of the most consistent contributors to SLR as the above graph shows and thermal expansion will continue to affect SLR it is now more of a question of how much it will increase in the future..