The melting index is computed from passive microwave satellite measurements and it can be seen as a measure of the ‘strength’ of the melting season: the higher the index the more melting occurred.  With more melting yet to come during August, 2012 will position itself way above the old records, likely becoming the 'Goliath' of the melting years during the satellite record (1979 - to date).  From the map below, we see that the cumulative melting index record is due to extensive increased melting occurring all over Greenland, especially at high elevations where melting lasted up to 50-60 days longer than the average. This means that some of the areas at high elevations in south Greenland are generally subject to a few days of melting (if it happens at all) and this year they underwent melting for more than 2 months (so far). 

Melting was extreme also in the west, northwest and northeast regions. Along the southwest coast melting does not appear to have been extreme. However, care must be taken in interpreting the passive microwave results over this area. Indeed, the exposure of bare ice might 'blind' the microwave data and preclude them from detecting melting. Extensive bare ice (above the average for 2012 !) has been suggested by model results (e.g., see the link to Xavier Fettweis' MAR results analysis, consistently with a reduced albedo observed by satellite sensors (see post one Jason Box analysis of MODIS, as well as our papers on the role of albedo in the Scientific Literature section). In 2010, we identified in warming surface temperatures, reduced accumulation and albedo reduction (following premature exposure of bare ice) some of the major potential drivers for the melting record (see here). 

So, how is this record different from the one that  happened in mid July of 2012 and received so much coverage ?

The extreme melting detected at high elevations in mid July (covering ~ 97 % of the Greenland ice sheet, see image on the left) generated liquid water that refroze after a few days, changing the physical properties of the snowpack but very likely not contributing to the meltwater that run offs from the ice and can potentially contribute to sea level rise. The event was exceptional in the sense that it is a rare event (imagine a postcard of Rio de Janeiro under a thin layer of snow !) but it happened in the past, according to the research of colleagues from the Dartmouth College at Summit. The record set by the overall melting has implications on the meltwater that goes into the ocean and it can impact ice dynamics, through basal lubrication or through its impact on the subglacial and englacial drainage systems. Also, the increased melting at higher elevations might remove the seasonal snow and expose more bare ice. The removal of bare ice (which is darker and the absorb more solar radiation and it is therefore more prone to melting than snow), is actually contributing to the net mass loss of Greenland. Seasonal snow is indeed part of the annual cycle (water from the ocean goes into the atmosphere which turns into cloud and it is released as snow which melts again and goes back to the ocean) where ice has been sitting there for decades or hundreds (and more) of years and it is therefore adding new 'material' to the cycle (e.g., the ocean). 

An important point to mention is that the passive microwave sensors cannot tell us about the amount of surface mass loss from melting. They can only 'see' when and where melting is occurring but they become 'blind' when it comes down to estimating how much water was produced from that melting. This is why we use the 'melting index', in order to have an idea of the 'strength' of the melting season. In general, there is a good correspondence between the melting index and the surface mass loss on a seasonal basis. Said this, we use models to complement the lack of information from remote sensing tools. Models have their own intrinsic limitations but they can provide estimates of physical quantities (such as the mass loss). The model we use has been widely tested vs. ground and satellite observations and we are confident about its results.  The preliminary analysis of model outputs (the Mode´le Atmosphe´rique Re´gional, a.k.a. MAR) that we obtained in collaboration with Xavier Fettweis from the University of Liege in Belgium (see link in the Main page, on the right), confirms the extreme melting year for 2012. Modeled results also indicate that the trend for the annual cumulative surface mass balance (SMB) for 2012 until the end of July is the minimum since 1958 (the model is run back to 1958, differently from satellite data that is available starting in 1979). The modeled results for the SMB until the end of July is ~ - 90 Gt (using January 1st as a starting value at zero Gt). The same values for 2010 over the same period was ~ -60 Gt and about 0 Gt for 2011. The modeled loss just for the month of August (from end of July to the end of August) in the case of 2010 was ~ - 60 Gt and it was ~ -100 Gt in the case of 2011. So, whether or not the SMB record will be set depends on how things will proceed during August. We are currenlty running the model and we might have some preliminary results by the next week. In general, we are planning to write a paper about this as soon as the season is over and we will be submitting by mid September. However it goes (whether 2012 is going to be a record or not in terms of SMB), this year will be likely be amongst the top years. Less than one month to get a response ...

Investigation is currently going on to address some of the causes of the melting record through model results and observations. The analysis of the combined remote sensing data and model outputs is carried out within the framework of a project sponsored by the National Science Foundation (NSF) to improve estimates of the surface mass balance of the Greenland ice sheet and by the NASA Cryosphere Program to collect data on ground for the validation of the results. Preliminary analysis indicates in above-normal surface temperatures and a reduction of albedo the potential major drivers for the new melting record. This is suggested by model results and other satellite observations. Let us see what August is going to reserve for us ...

A link to the ground data collected along the K-transect by the Institute for Marine and Atmospheric Research Utrecht, Utrecht University, the Netherlands can now be accessed through a new link under the Data section. An example of the available plots is reported on the left. The institute has been operating several automatic weather stations on the western Greenland ice margin (K-transect) for many years and the data of the current calendar year can be viewed from the link under Data or here. Thanks to Carleen Tijm-Reijmer (IMAU) for sending the link. 

Here's a link to Jason Box 2012 analysis of albedo over the Greenland ice sheet. The reduction of the albedo is a critical factor for enhancing melting over the Greenland ice sheet. The image on the left shows the ice sheet albedo time series updated through July 2012 by Jason Box. 

To better understand the importance of this mechanism you can check a recently published paper on the Cryosphere Discussion by Box et al. (2012) in which we focus on this problem or another paper (Tedesco et al., 2011) that we published on Environmental Research Letters focusing on the role of albedo and accumulation on the 2010 melting recording in Greenland. Both papers are also available in the Scientific Literature section of this site. 

The plot also shows that the event of July 2012 s unique for the satellite era (1979 - to date) and it lasted for a few days. For those who are not expert in passive microwave remote sensing: as the liquid water appears, the recorded signal jumps to high values, suddenly. This kind of change (sudden and strong) can be related only to the appearance of  liquid water within the snowpack as other changes in the physical properties of the snowpack would produce either a weaker signal or would have a different timescale. Note: this sensitivity of microwaves to water is the same physical principle through which we are able to cook our food using microwave ovens !