Hello and welcome to just have a think. First of all a big thank you to all those of you who
left comments on last week's programme about the NOAA report. There were some really interesting
observations and suggestions in there which we'll try and build into future content of the
channel. Several of you mentioned the blue ocean event and asked if we could talk more about this
phenomenon and explain precisely why its effects would be such a cause for concern. Now that's not
a small area of discussion so it merits some careful research on the scientific data and
some expert opinion before putting it together in a video. So look out for that later on in the
year. This week though we're going to concentrate on the parts of the report that talked about the
land-based ice up in Greenland. Just to get you warmed up let's have a look at some of
the insanely huge numbers that describe just how large this thing is. I do love a good chart and
statistic, as you know, so here's some Wikipedia action for you. The Greenland ice sheet is a vast
body of ice covering 1 million seven hundred and ten thousand square kilometres or 666 thousand
square miles - roughly 80% of the surface of Greenland. And thanks to another website called
www.thetruesize.com we can have a look at how Greenland compares in size to countries that we
may all be slightly better acquainted with. Here it is dragged across North America and then across
into Europe and for our Asian and antipodean friends just so you don't feel left out here's how
it compares to India and then to Australia. So you know, it's big! In fact, the Greenland ice sheet
is the second largest ice body in the world after the Antarctic Ice Sheet. It's 2.4 million years
old, almost 2,400 kilometres or 1,500 miles long north to south, and 1,100 kilometres or 618 miles
wide at its widest point. The ice itself is on average about two kilometres or 1.2 miles thick,
and more than three kilometres or 1.9 miles at its thickest point. Climate scientists use
two measures to assess the performance of the ice sheet on an annual basis. The first is something
called the surface ice mass balance, sometimes referred to as the surface mass budget. And the
second is the total ice mass balance or the total mass budget. So what do our friends at NOAA and in
the wider climate community mean by these two terms? There's a very thorough explanation at
the website www.polarportal.com which is linked to the Danish Meteorological Institute and is
also partnered with www.carbonbrief.org. Here's their basic summary..."glaciers and ice sheets
are vast slow-moving reservoirs of freshwater fed by precipitation particularly snowfall and losing
mass by melting and carving of icebergs. If more snow falls than is lost to melt or icebergs then
the glacier will grow and advance, and conversely if more melts and carves than accumulates it will
retreat and shrink." So far so simple. Surface mass balance in Greenland is dominated by
snowfall. Generally you get accumulation for nine months of the year and then you get snow loss,
or what the scientists call ablation, through June, July and August. Layers of snow are
gradually compressed into glacier ice and start to flow downslope, driven by surface melt at lower
altitudes and more snowfall at the higher points. So surface mass balance drives the internal flow
of glaciers, and there are several variables that go into calculating the annual increases
and decreases. You've got sublimation which means snow and ice changing into water vapour
in the air without first melting into water, and then the evaporation of the water vapour
direct from the surface, and this is an important mass loss process particularly in the winter.
In the summer though, it's good old-fashioned melting that dominates the mass loss. The Danish
Meteorological Institute or DMI used data from PROMICE and GC NET weather stations as well as
their own automatic weather stations to crunch the numbers. They use a fairly complex set of
parameters to work out the amount of melting which gives them an equation that looks like this. Yeah
I know! So this bit is net shortwave radiation, which means how much sunlight hits the surface,
minus how much sunlight is reflected back because of the albedo or reflectiveness. This next bit is
the net long wave radiation which means how much radiation comes from the atmosphere and
the clouds minus how much the surface radiates through something called surface emissivity.
QE describes the transfer of energy due to the difference in temperature between the surface
and the atmosphere - it's the thing that causes wind chill factor. QL represents something called
enthalpy. If you saw last week's video you'll remember we talked about latent heat of fusion.
Well that is enthalpy. And there's also a latent heat of evaporation. In fact enthalpy happens at
every phase change - melting, evaporating freezing and sublimation. QE and QL are strongly dependent
on the air temperature, humidity and wind speeds. GE stands for ground heat flux. Essentially this
is an energy transfer to and from the subsurface which results in the surface temperature, or skin
temperature. As the polar portal puts it... "The skin is best imagined as an infinitesimally thin
layer at the surface. On a glacier we assume that this temperature cannot reach above the melting
point and since the first law of thermodynamics applies, any additional energy is available for
melt" which is the AME. This melt calculation is added to the other variables to give an overall
calculation for surface mass balance. Surface mass balance equals precipitation minus evaporation
minus sublimation minus runoff, with runoff being melt plus rainfall minus refreezing. And the
actual results from these clever calculations? Well according to the website Science Nordic,
who get their information from the scientists at the DMI and polar portal, "for this year we
calculated a total SMB of 517 billion tons which is almost a hundred and fifty billion tons above
the average for 1981 to 2010 ranking just behind the 2016-17 season, and sixth highest on record."
Here's what the folks at Carbon brief say about 2018 "the melt season started as normal in May
but it was a relatively cold month. The summit station at the very top of Greenland even set a
new record low for the month when it dropped to minus forty six point three degrees Celsius on
the 9th of May. In addition late spring snowfall in early June restricted melting and the ablation
season when ice melts and runs off the ice sheet into the ocean didn't really get going until the
last week of June. Further fresh snow over much of southern Greenland in early July again replenished
the snow at the surface and reduced melt rates. Fresh snow not only indicates cold weather it
also reduces melting as it's very bright and reflective compared to darker old snow and
bear glacier ice which is often covered in dirt and dust and algae. Typically summer melt rates
increase as the darker snow and ice is exposed which absorbs more solar energy leading to more
melting - the so called ice albedo feedback. This summer the frequent snowstorms replenished the
bright white surface on several occasions, holding back the albedo feedback from accelerating melt
through the season." And importantly they also note "The period 2003 to 2011 has seen ice sheet
losses on Greenland averaging 234 billion tons each year. The neutral mass change in the last
two years does not and cannot begin to compensate for these losses. The comparison here does show
that in any given year the mass budget of the ice sheet is highly dependent on regional climate
variability and specific weather patterns." So to get from surface ice mass balance or
budget to total ice mass balance the scientists have to take into account some other factors,
most notably the carving of the glaciers, which in basic terms that I can understand means the
bit at the end of the glacier that drops off into the sea. The report card tells us "Marine
terminating glaciers are the outlets by which the Greenland ice sheet discharges ice mass to
the ocean. The 2017-18 net area change of the 47 surveyed glaciers relative to the previous
year is plus 4.1 square kilometres and stands alone as the only year in the survey with an
area gain. The annual area lost per year averaged over the 19 year period of record between 2000 and
2017 is minus 109.7 square kilometres per year." NOAA and the DMI and Polar Portal, and just about
anybody else all rely quite heavily on satellite data to calculate the total mass balance number.
Since 2002 this has been provided by NASA's twin Gravity Recovery and Climate Experiment
Satellites, otherwise known as GRACE, but the original NASA grace mission ended in October
2017. Those of you interested in space stuff may have watched the launch by SpaceX of the GRACE
follow-on or GRACE FO mission back on the 22nd of May 2018. The engineering accuracy involved
in these instruments really is breathtaking. Just take a listen to this commentary from the
live SpaceX launch transmission..."now the two GRACE satellites fly in orbit about 220
kilometres apart. The satellites use microwave radio frequency signals to measure the distance
between them, the measurement is accurate to about the thickness of a blood cell. It's incredibly
accurate." Absolutely incredible! Anyway the data from those two new satellites is currently being
reviewed for quality control purposes and sadly there's no data available from space between
October 2017 and May 2018. The next set of data that will come out will be for the summer 2018,
and that's going to be calibrated against the old GRACE satellites to make sure it's accurate. And
we'll do a recap video when those numbers come out. In the meantime though here's NASA's take
on the direction of travel up in Greenland. "The mass of the Greenland ice sheet has rapidly been
declining over the last several years due to surface melting and iceberg carving. Research
based on the GRACE satellites indicates that between 2003 and 2013 Greenland shed approximately
280 Giga tons of ice per year causing global sea levels to rise by nought point eight millimetres
every year." I must admit that did turn out to be a bit more complicated than I expected. I hope it
all made sense though and as always if there are some technical points of correction that you folks
need to make then feel free to leave your edits in the comment section below. That's it for this week
though. Please do give us a like and a share and most importantly, if you haven't already done so,
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