About two weeks ago, NOAA released the annual version of their latest Arctic Report Card. Of course, the usual media outlets peddled the requisite doomsday headlines when describing the report’s contents, deploying words like grim and dire and pedantically issuing a failing grade so as to paint a picture of an Arctic climate teetering on the brink of catastrophe.
It was on page 33 that the authors divulged the Greenland Ice Sheet’s mass balance statistics for 2015-’16.
2016 Arctic Report Card “Between April 2015 and April 2016 (the most recent period of available data) there was a net ice melt loss of 191 Gt. This is about the same as the April 2014-April 2015 mass loss (190 Gt).”
So the ice sheet reportedly lost 191 gigatonnes (Gt) of water between April 2015 and April 2016. Interestingly, last year’s report card (2015) had the April 2014 to April 2015 loss pegged at -186 Gt, not -190 Gt. Somehow another 4 Gt were added to the total loss for 2014-’15 between then and now.
2015 Arctic Report Card “Ice mass loss of 186 Gt over the entire ice sheet between April 2014 and April 2015 was 22% below the average mass loss of 238 Gt for the 2002- 2015 period.”
For 2013-’14, the mass loss for the Greenland Ice Sheet was even lower: just -6 Gt. In other words, the ice sheet was essentially in balance.
2014 Arctic Report Card “A negligible ice mass loss of 6 Gt between June 2013 and June 2014″
To review, that’s -6 Gt, -186 Gt, and -191 Gt for the 2013 to 2016 mass balance records. Averaged together, the loss was –128 Gt per year for 2013-’16, which is a substantially slower rate of loss relative to previous years.
A Rapid Deceleration In Greenland Ice Sheet Mass Loss Since 2012
Now here’s where it gets interesting. For their 2013 report card, NOAA authors wanted to accentuate just how profound the loss of ice mass had been for the 2008-2012 period compared with the 2002-2006 period. This way, they could point to a rapid acceleration of ice sheet mass loss.
2013 Arctic Report Card “The rate of mass loss has accelerated during the period of observation, the mass loss of 367 Gt/y between September 2008 and September 2012 being almost twice that for the period June 2002-July 2006 (193 Gt/y)“
As some may have noticed, NOAA curiously uses different starting and ending months for each demarcated period, making direct data comparisons difficult (due to different gain/melt rates depending on the month). Instead of comparing September to September every year (as they did in 2008-’12), they’ve compared April to April in some years, June to June in another year, and June to July in still another 4-year-long selection. And they’ve ignored the melt record for 2007 altogether in the above analysis when comparing 2002-2006 to 2008-2012. NOAA wouldn’t dare “move the goal posts” by cherry-picking different start and end points depending on the melt rates for specific months so as to bolster their claims of “acceleration” would they? That would imply operating with a tendentious agenda rather than objectively reporting the data. Would they do that?
But instead of digressing to discuss their odd “selectivity” with regard to choosing some months or years to start and end with instead of others, we will just accept NOAA’s methodology and analyze the yearly averages as originally reported.
So according to present and historical NOAA Arctic Report Cards we have the following rates of annual Greenland Ice Sheet mass loss for 2002-2016:
–193 Gt/yr for 2002-2006
-367 Gt/yr for 2008-2012
-128 Gt/yr for 2013-2016
We can even go a step further and add the 1990s to the record. Despite of a highly increasing rate of human CO2 emissions for the 1992-2002 period, NASA’s Zwally et al. (2005) reported an average net gain of +11 Gt/yr for the Greenland Ice Sheet during those 10 years.
Zwally et al., 2005 “Changes in ice mass are estimated from elevation changes derived from 10.5 years (Greenland) [1992-2002] … of satellite radar altimetry data from the European Remote-sensingSatellites ERS-1 and -2. The Greenland ice sheet is thinning at the margins (–42 ± 2 Gt/yr–1 below the equilibrium-line altitude (ELA)) and growing inland (+53 ± 2 Gt/yr–1 above the ELA) with a small overall mass gain (+11 ± 3 Gt/yr–1; –0.03 mm a–1SLE (sea-levelequivalent)).”
So, adding the 1992-2002 NASA values to the 2002-2016 NOAA values, here is what the overall trend in Greenland Ice Sheet Mass Balance looks like during 1992-2016 using these as-reported mass balance values:
Notice the dramatic rate of deceleration (by two-thirds) in Greenland Ice Sheet mass loss averages for recent years on the graph above, contradicting the reports of a perpetually increasing ice loss acceleration.
And the slowing mass loss rate has continued in 2016-’17. According to DMI monitoring, the Greenland Ice Sheet is gaining ice substantially above the long-term average in the last 3-4 months. It is highly likely that by the end of the record, the 2016-’17 mass loss will be significantly less than 200 Gt/yr, and perhaps less than 100 Gt/yr again.
The Context Of The Insignificant 1992-2016 Greenland Ice Sheet Mass Change
Some may counter the above analysis by claiming that even losing 100 to 200 Gt of ice per year is still quite substantial and concerning, and thus the alarmist headlines are merited. But this perspective ignores history.
First, estimates of surface mass balance for the Greenland Ice Sheet almost invariably use the 1961-1990 period as a baseline, as it is assumed that the ice sheet was essentially in balance (no net losses or gains) for those 30 years. As records have shown, though, the 1961-1990 baseline period was the coldest decadal-scale stretch since the 1800s for Greenland, which means that the 1995-to-present Arctic warming trend is being directly compared to a very cold period for the Greenland ice sheet rather than a more representative period. In fact, as indicated by several recent papers, the Greenland ice sheet’s surface mass balance was similar to or even lower during the as-warm-as-now 1920s to 1940s than it has been during the last few decades. If the baseline period were to include Greenland’s early 20th century warm years (1920s to 1940s), the recent losses would likely be substantially smaller.
Also, let’s consider what losing, say, 150 Gt per year actually means in terms of its environmental impact. According to Shepherd et al. (2012), the Greenland Ice Sheet lost an average of -142 Gt/yr (with substantial uncertainty of ±49 Gt/yr) during the years 1992-2011. This translates into an average of 0.4 mm/yr of sea level rise contribution from the Greenland Ice Sheet during this high-melt period. In other words, over the entire 20-year record (1992-2011), the Greenland Ice Sheet contributed to sea level rise at a rate of 4 centimeters (1½ inches) per century. That’s it. That’s what a loss of -142 Gt per year multiplied by 20 years translates into.
Even if this alleged modern rate of loss were doubled to -300 Gt/yr, we are still only talking about 3 or 4 inches of sea level rise contribution from the Greenland Ice Sheet every century if that rate of loss could be sustained for 100 years. As indicated above, that rate of loss couldn’t even be sustained for more than a few years in the last few decades.
Finally, as mentioned above, scientists presume the Greenland Ice Sheet was in balance during the 1961-1990 period, which is why it is used as the baseline reference period for surface mass balance estimates. If that’s the case, there is no reason why estimates of the human impact on the Greenland Ice Sheet’s mass balance should not extend back to 1961 too. Or even 1951. After all, the IPCC has indicated in their latest (2014) synthesis report that the overwhelming (“more than half”) anthropogenic influence on climate commenced that particular year:
“It is extremely likely more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together.”
So if we were to include the ~40 years of majority anthropogenic influence between 1951 and 1991 that apparently saw no net change in Greenland Ice Sheet mass balance with a (rounded up) combined mass loss rate of -150 Gt/yr between 1992 and 2016, the total human impact on the mass balance of the Greenland Ice Sheet might amount to an annual loss average of -55 to -60 Gt for the entire 65-year period. That’s a rate of contribution to sea level rise of about 1 ½ centimeters per century since 1951, or since humans allegedly began causing the majority of global warming.
Since a headline that reads “Humans Have Caused Greenland’s Ice To Melt At Rate That Adds 1 ½ Centimeters Per Century To Sea Levels!” probably wouldn’t be effective in grabbing readers’ attention, it’s understandable why journalists and advocates for the cause would prefer to use doomsday language instead. It’s also understandable why they’d prefer to gloss over the recent deceleration in Greenland Ice Sheet mass loss found in the latest Arctic Report Card. It sounds so much scarier to write about the hundreds of billions of tons of ice lost instead.