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Wednesday, June 27, 2018

Global Heat Budget #3 – Ice


This is the third in a series of posts about the global heat budget. 

Ice is melting around the world. 

Greenland’s ice cap is melting.  Antarctica’s ice cap is melting.  Arctic sea ice is melting. Continental glaciers are melting.  Arctic permafrost is melting.  The melting is happening at a rate that is readily visible to people who live near natural ice.  From decade to decade and year-to-year, glaciers are visibly retreating, and can be directly verified by the most casual observer.   

Melting ice is the second most important heat sink on the planet, after the ocean (albeit a distant second).  Melting ice accounts for about 3% of anthropogenic heat retained in the atmosphere.  Melting ice is the second most significant proof that human-caused climate change is happening.  Melting ice may be the most significant consequence of climate change in terms of costs and damage to humanity. 

The data is unambiguous and irrefutable.  The volumes of melted ice have been measured by a variety of methods, including high accuracy satellite measurements.  The heat required to warm and melt this volume of ice can be calculated and compared to the heat trapped in the atmosphere by greenhouse gases, and the rising heat content of the oceans.  The volume of meltwater entering the ocean can also be compared to measurements of rising sea level.   The rate of sea level rise is already 3 times the rate of the past 7500 years, and accelerating.  The observed volumes of melting ice and the measurement of rising sea level provide unambiguous proof that climate change is real. 

Greenland
Ice on Greenland is melting.
Greenland covers an area one-fifth the size of Australia.  Almost all of Greenland is covered by ice, ranging between 1 and 2 miles of ice thick.  The Greenland Ice sheet contains more than 2.8 million cubic kilometers of ice.  That is enough to make sea level rise by 20 feet if it all melted.

NASA’s GRACE (Gravity Recovery and Climate Experiment) satellites have monitored the mass of the Greenland ice cap since 2002.  Gravity observations were supplemented by altimetry and radar data from overflights and satellites.  Other satellite observations include NASA’s early ICESat, and the ESA’s currently operating CryoSat2.

There is a strong seasonal signal in the history of ice loss from Greenland, with a slight build in ice mass during the Northern Hemisphere winter, and a stronger decline in the summer.  From 2002 to 2016, Greenland lost about 3900 gigatonnes of ice due to melting.  Each gigatonne is a little more than one cubic kilometer of ice by volume, and produces one cubic kilometer of fresh water when it melts.  Altimetry data show that most of the melting was concentrated near the coast, particularly on the western side.  Six feet to fourteen feet of ice has melted around the edges of the entire island. 

The GRACE satellite, designed for only a five-year life, actually worked for nearly fifteen years.  The last data was recorded in June 2017.  The replacement mission, GRACE Follow-On, is scheduled to be launched in five days, on May 19th, 2018.  

NASA’s IceBridge is an airborne project using laser altimetry and ice-penetrating radar data to measure the elevation, snow cover, and total thickness of Greenland and Antarctic ice.  IceBridge will provide data to connect and calibrate data from the new GRACE satellites.   IceBridge was originally designed to replace data from the ICESat satellite, which failed after seven years of service.  ICESat-2 is planned to be launched in September, 2018, to replace ICESat.

Antarctica
Ice on Antarctica is melting.
Antarctica is about seventeen times larger than Greenland, and nearly twice as large as Australia.  Ice covers 98% of the continent, to an average thickness of over a mile.  Antarctica holds about ten times the volume of ice as Greenland.  If all of the ice on Antarctica melted (which would require centuries to occur), it would raise sea level by about 200’, placing most of the world’s major cities and human habitation under water. 

The GRACE data for Antarctica is noisier than the data for Greenland and shows a weaker seasonal cycle.  It seems likely that the melting season over Antarctica is not (yet) as profound as over Greenland. 

As with Greenland, the Antarctic ice sheet has been monitored by NASA’s Grace and ICESat satellites, and the IceBridge aerial observation program.  Earlier observations were integrated by the European Space Agency’s IMBIE (ice sheet mass balance comparison exercise) to provide the ice balance record from 1992 to 2010.  The chart showing both IMBIE data and GRACE data is shown below.  From 1994 through 2017, at least 2450 gigatonnes of ice on Antarctica melted.

I should note that gravity methods will not detect ice loss on the portions of the ice sheet that are floating (and more susceptible to ice loss).  Ice floating on water will have the same net density as ice-free water.  So, altimetry methods must be combined with gravity methods for a full determination of ice loss on Antarctica.  The East Antarctic (Filchner-Ronne) and West Antarctic (Ross) ice shelves are approximately 900,000 square kilometers in area.  Dozens of smaller ice shelves also exist.  The actual loss of ice from Antarctica may be greater than 2450 gigatonnes, because losses from these floating ice shelves are not detected by gravity. 

While melting of floating ice shelves is difficult to observe, it is also true that the melting of floating ice will not cause sea level to rise, at least as a first-order consequence.  The same buoyancy of ice shelves that makes sea-ice loss invisible to gravity detection means that sea level does not change when a volume of ice is converted to water.  Melting ice shelves matter to the earth’s heat budget, but not (directly) to sea level. 

The rate of future ice loss in Antarctica depends on feedback mechanisms.  The principle feedback mechanism is the restraining force that ice shelves exert on flowing glaciers.  Ice shelves impede the flow of ice from the continent and into the ocean; when those shelves melt, the rate of ice loss will accelerate.  The timing and amount of acceleration are unpredictable, so the best estimates of future sea level rise are uncertain on the high end.  We are fairly certain about the minimum expected sea level rise, but the maximum possible sea level rise is very uncertain.

Antarctic Sea Ice
For many years, Antarctic sea ice was not subject to the declines seen in Arctic sea ice (seen in the next section).  This was often referenced in commentaries on climate-change deniers’ web sites.  In recent years, Antarctic sea ice has declined, but it is likely to continue to show an irregular response to climate change.  The reason is simple.  Antarctic sea ice is regularly replenished by calving from Antarctic glaciers and ice shelves.  Anyone with tour-boat experience in Alaska knows that sea ice actually increases following calving events.  So, with a huge reservoir of ice in the Antarctic ice cap, Antarctic sea ice is likely to fluctuate, but not disappear, as the mother-lode of ice continues to flow and break apart, feeding the sea ice around the continent.


 Arctic Sea Ice
Arctic Sea Ice is melting.
In contrast to Antarctica, the Arctic has no mother-lode of ice feeding the polar sea ice.  The sea ice freezes and melts in a seasonal cycle.  For the past forty years, each cycle has ended with less ice, on average, than the previous cycle.  The loss of ice has accelerated over that period.   There was about 250,000 square miles less sea ice in the 1990s than during the 1980s.   From the 1990s to the 2000s, the decadal average showed a loss of about 500,000 square miles of sea ice.  The annual data from the current decade suggests an even greater rate of loss. 


Area is not the only measure of sea ice.  Some sea ice persists through multiple seasons, gaining thickness from season to season.  However, against the background rate of general melting, less and less ice persists from season to season, and the overall thickness of Arctic sea ice is also declining.   Between 1984 and 2016, 94% of the sea ice more than four years old had disappeared.
Image by M Tschudi and S. Stewart of the University of Colorado, and W. Meier and J. Stroeve of NSIDC.

Some researchers have attributed 30 percent to 50 percent of the loss of Arctic sea ice to natural variability, and 50 to 70 percent to anthropogenic influences, including direct warming by greenhouse gases, and the second-order influence of atmospheric circulation patterns. 

Overall, Arctic sea ice has declined by 12,000 cubic kilometers since 1980.   As noted in the section about Antarctica, there is no sea level impact due to the melting of floating ice, but there is an impact on the earth’s heat budget. 

The loss of sea ice in some of the peripheral seas of the Arctic Ocean (Chukchi Sea, Bering Sea, Barents Sea, and others) is more evident.  
Image Credit, Rick Thoman, National Weather Service, Fairbanks


Continental Glaciers
Continental glaciers are melting. 
People who live near glaciers are well aware of the historical and current melting of glaciers.  In Alaska and Western Canada, popular glaciers often have signposts or old photographs showing the earlier extent of the glaciers.  Some examples are the Columbia Ice Field in Alberta between Banff and Jasper National Parks, and in Alaska, Root Glacier near Kennecott mine, Exit Glacier near Seward, Portage Glacier and associated glaciers near Anchorage, and the many tidewater glaciers along the Alaskan coast, including Glacier Bay near Juneau, College Fjord near Valdez, the glaciers of Kenai Fjords National Park, and Columbia Glacier near Valdez.  Glaciers are in retreat, on a scale which is noticeable from year to year and dramatic over the course of decades. 

The UN Glacier Monitoring Service and its predecessor organizations have measured the melting of continental glaciers, other than Greenland & Antarctica.  WGMS issued major reports in 2008 and 2015; each report shows overwhelming evidence of melting of glaciers worldwide.  WGMS includes data on about 100,000 glaciers, with digital outlines of about 62,000 glaciers; data on glacier fluctuation includes over 35,000 length observations for nearly 2000 glaciers (as of 2008).  Detailed mass balance observations are conducted on a smaller number of reference glaciers (including the most volumetrically significant glaciers).  Data from reference glaciers are extrapolated to other glaciers on the basis of regional association, altitude and latitude.

At any given time, a small number of glaciers are growing, due to natural fluctuations of snowfall, warmth, and air circulation. But the great majority of glaciers worldwide are melting. 

Annoyingly, the WGMS does not report summary ice loss in terms of cubic kilometers or gigatonnes.  Glacial Mass Balance is reported in terms of meters of water equivalent, a vertical measure of average ice melted.  Volumes of melted ice can be calculated from the reported total area of glaciers under study.  Those volumes can then be used for purposes of understanding the global heat budget and sea level rise. 

Sea Level
The earth entered a period of cyclic ice ages about 3 million years ago.  The last 600,000 years have been characterized by ice age cycles of about 100,000 years, apparently triggered by variations in earth’s orbit.  The influence of the orbital cycles is enhanced by feedback mechanisms, including CO2 and the reflectivity of ice.  The peak of the last glacial cycle occurred only about 20,000 years ago, and remnants of ice may have persisted in Ontario until about 8,000 years ago.

Deglaciation following the last ice age was mostly complete by 8000 years ago.  We know this from studies of sea level and sediment cores.  Sea level rose by about 80 meters between 14,000 years ago and 8000 years ago, an average rate of 1.3 cm/year.  From 7500 years ago to the 20th century, sea level rose only 5 meters, a rate of 0.07 cm/year.   Through the 20th century, sea level rose at about 0.2 cm/year, a significant increase over the background rate.  Satellite data over the past 25 years shows that sea level rise has accelerated to 0.35 cm/year, five times the rate of sea level rise for the past 7500 years.  This is a clear indication that global warming from human greenhouse gases is contributing to melting ice.
Additional heat retained by greenhouse gases will result in a faster rate of melting ice, and higher sea level rise.  Current forecasts of sea level rise range from about 2 feet to 8 feet by the end of the century.  Sea level rise of only 4 to 6 feet would seriously damage some coastal communities around the world, including the inundation of barrier island and low-island communities.

Heat Budget

From 2003 to 2016, greenhouse gases retained 1.6 x 1023 joules of heat in the atmosphere, according to tables of radiative forcing published by NOAA (https://esrl.noaa.gov/gmd/aggi/aggi.html).  This figure for anthropogenic heat does not include the effect of cooling or warming aerosols, primary heat from fossil fuels and deforestation, or other minor sources of heat.  Estimates for some of these other anthropogenic disturbances are found in the IPCC 5 report, but only through the year 2011.

We have good estimates of the cumulative ice lost from Antarctica, Greenland, Arctic sea-ice and Continental Glaciers from 2003 to 2016, due to high-quality satellite observations.  About 10,400 gigatonnes of ice was lost over that period.  The heat required to warm (+10 C) and melt that volume of ice is 3.7 x 1021 joules, or about 2.3% of the total heat retained by greenhouse gases.  The allocation of heat to warm the ice by 10 degrees C was to reflect heating of an equivalent amount of ice, which has not yet melted.  Average temperatures of -10 C from core-holes in Greenland and Antarctica were taken as the ambient temperature of ice before melting. 

Considering that about 2.9% of the earth’s surface is covered by ice, this seems like a reasonable distribution of greenhouse heat which is going to warm and melt ice.  Looking forward, if a higher percentage of heat goes towards melting ice, sea level will necessarily rise faster.  Possible reasons for faster melting of ice could be more rapid ice flow from Antarctica and Greenland.  This might occur as the base of the ice is lubricated by meltwater, or when restraining ice shelves are lost around Antarctica.


References

Antarctica and Greenland
NASA GRACE Ice Mass, Antarctica and Greenland

Grace Data
Wiese, D. N., D.-N. Yuan, C. Boening, F. W. Landerer, and M. M. Watkins (2017) Antarctica Mass Variability Time Series Version 1 from JPL GRACE Mascon CRI Filtered. Ver. 1, PO.DAAC, CA, USA. Dataset accessed [2017-06-07] at http://dx.doi.org/10.5067/TEMSC-ANTS1

IMBIE:  Ice Sheet Mass Balance Inter-Comparison Exercise
Integrated Methods Measuring Ice Mass

Arctic Sea Ice
Arctic Sea Ice Volume
Chart with relative volume loss (km3) to 1980. 

Charts of Arctic Sea Ice Extent by Month

Image of declining multi-year Arctic Sea Ice.  Image by M Tschudi and S. Stewart of the University of Colorado, and W. Meier and J. Stroeve of NSIDC.

Arctic Sea Ice
Multi-year ice grows up to 4 meters thick, while single-year ice is 2 meters thick at most.

the area covered by Arctic sea ice at least four years old has decreased from 1,860,000 square kilometres in September 1984 to 110,000 square kilometres in September 2016.

Continental Glaciers
World Glacier Monitoring Service bi-annual update, 2015

Volume estimate for Glaciers and Ice sheets (other than Antarctica and Greenland.


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