# Total mass change

This figure shows the total change in accumulated mass of the Greenland ice sheet over the year. The mass change is presented both as amount of water (km3) and how this amount of water influences global sea level (mm).

The grey line shows the average values from 2003-2012,- the red line shows the development in 2012,- and the blue line shows the development this year, 2013.

In this figure all components of the mass balance are included: the mass gain from precipitation and the mass loss from melt water runoff and iceberg discharge.

The red line shows why 2012 was a (negative) record year. From January to mid-may 2012 resembles average from the previous years – mass is accumulated on the ice sheet due to mainly snowfall precipitation. By early April 100 km3 had accumulated, equivalent to a global sea level drop of 0,2 mm.

When the melting season starts in May/June the mass output exceeds the input. Up to mid-July the year 2012 follows the average, but from then on the loss of mass accelerates. By the end of September 2012, where the melting season normally ends, the total loss is 500 km3, or an increase in global sea level of just below 1,4 mm. During the early winter month mass is again accumulated.

If the ice sheet is in balance, meaning that the input (precipitation) equals the output (ice melt and iceberg discharge) the end point of the grey line should meet the starting point. In other words, the difference between the two points shows that the average annual loss of mass from the Greenland ice sheet during 2003-2012 has been approximately 234 km3 of water,- or approximately 0,65 mm in average annual contribution to global sea level. Barletta et al. 2012

In the figure two different methods are used. Read more about these methods under the next figure.

This figure shows the total change in accumulated mass of the Greenland ice sheet  using two different methods.

Method 1 is based on monthly measurements of changes in gravity. Gravity changes as the amount of ice changes, and this can be detected by satellites (in this case the GRACE mission).

However the processing of the data from the satellites takes 2 to 3 months, and scientists at GEUS have therefore developed Method no 2: combining gravity measurements from GRACE with measurements of ice-reflection (“albedo”) it’s possible to get daily updates on the total mass change. The data on ice-reflection is retrieved from the MODIS satellite.

The grey line and grey area shows the average in the period 2003-2012 (with 95% confidence) using Method 1.
The blue dots represent the processed data sets from GRACE in 2013.

The red and blue lines uses method 2, as in the previous figure. The blue line (2013) is updated on a daily basis, based on the latest measurements of ice-reflectivity. The changes are shown in millimetres pr. month so it can be compared to Method 1.

When the lines are above zero, mass is lost. When below zero mass is accumulated.

Gravity data are from the NASA and German Aerospace Center (DLR) GRACE mission processed after Barletta et al. (2012)

The reflectivity-based mass change is based on the monthly relation between NASA MODIS MOD10A1 albedo data versus the same GRACE data (Colgan and others, in prep). The GRACE mass change is calculated using the difference of month i + 1 and month i – 1. The reflectivity-based mass change estimate is only valid from early April to September.
Albedo data processing are after Box et al. (2012).