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

Sea level change since 2002

Just a short post today... This interesting video from NASA shows cumulative SLC since 2002:

It is very interesting as it shows meltwater fingerprinting particularly off Greenland and Antarctica and simply shows the recent trends in SLC, showing quite how non uniform it is across the globe and also a possible acceleration in recent years. For further discussion on meltwater fingerprinting Mitrovica et al., (2011) provide a good overview.

How do we measure Sea Level Change?

In the last post I made some assertions that global mean sea level (GMSL) is rising, but making me ask myself how do we know this? Physically measuring the rate of sea level change (SLC) is not a simple process, and as such there are different methods for doing so:

Tide Gauges

In terms of directly measuring SLC there have been approximately 300 years of coverage in an initially very limited spatial area. Tide gauges were the first form of SLC measurement, and work quite simply by constantly measuring the water level at a fixed geographical point. The first tide gauge was constructed in Amsterdam in 1700, and across Europe most major ports had them by the end of the 18^th Century.

Tide gauge stations with >40 year records. Source

Current global coverage. Source

However as the maps above show, tide gauges have lacked global spatial coverage, with a considerable Northern Hemisphere bias until the last 40 years or so.

Therefore for long term SLC they lack consistency but if you choose the most accurate and consistent gauges can still find a reasonable fit for recent SLC. This new paper from Thompson et al., (2016) shows that even when using the best quality tide gauge records it remains difficult to validate the recent observed sea level trends due to systemic underestimation of melt from 20^th century ice cap loss. This is likely due to local trends at each tide gauge station for example melting of ice caps from different parts of the world produce a variable sea level trend due to differing distribution of the meltwater across the globe. 

Meltwater impact on regional SLC from Greenland (above) and Antarctica (below). Source

These local signals imprint on the global signal and therefore tide gauges records need to be selected very carefully in estimating global SLC. As you can see from some tide gauge data I collated (below), there is much noise in the data but most show an overall trend of slow rise through the 20^th Century. 

Self- collated SLC at sites from across the world for the last 100 years. Source

The attachment of tide gauges to land means vertical land motion is another complicating factor, therefore has to be accounted for when doing reconstructions. This is a complicated process requiring data from models that is being constantly updated and therefore past sea level trends may have been poorly estimated due to issues with vertical land motion. This explains the relative sea level fall at Stockholm. Fingerprinting the difference between local/regional and global sea level trends appears to be crucial in accurately working out the SLC signal. 

Satellite Altimetry

Satellite altimetry is the more recent, more accurate form of measuring SLC. It uses satellites (with the catchy names of JASON and TOPEX…) to measure the height of the sea surface compared to a reference and therefore you can look at its change over time to get GMSL change. 

How satellite altimetry works. Source

However you still need to consider ocean basin volume changes and changes in pressure that lead to differing regional levels. This technology is only approximately 25 years old so gives a shorter term record but will be most used to assess future SLC, considering the revolution it has led to in the understanding of SLC since its inception (Milne et al., 2009). To compare GMSL change for the differing instrumental eras gives an indication of the recent acceleration in GMSL, from tide gauges 1900-2012 it is 1.7mm/yr, from altimetry 1993-2010 3.2mm/yr.  Does this reflect differing accuracy or a clear acceleration is SLC in the late 20^th Century? These questions are critical to estimating future SLC and the potential magnitude of its effect. Altimetry appears limited in measuring local changes and therefore a combination of the two major methods (tide gauges for local, altimetry for global) appears to often be the best for working out recent SLC and looking towards the future.

Another way of measuring SLC is to work out the magnitude of differing contributions to SLC. Therefore the next couple of blogs will look at the major contributions to 20^th Century SLC. Steric changes of ocean water, ice melt, changes in terrestrial water storage and groundwater depletion all contibute to SLC. These will be discussed in the coming posts...