September 2013

An “Interpretation for the Rest of Us” of the Latest IPCC Report – Cryosphere Sections

On September 27, Working Group 1 (WG1) of the Intergovernmental Panel on Climate Change (IPCC) issued its Summary for Policymakers after over a week of negotiations between scientists and governments on the final text in Stockholm. This is part of the overall IPCC “Fifth Assessment” that will be released next year from all the various working groups – but “WG1” is always the first out with its conclusions, as it covers the “physical science” basis on which much of the other work (on mitigating or adapting to climate change) is based.

The language in these summaries comes from extremely exact, sometimes hard-fought negotiation, yet can be difficult for lay readers to interpret. Here then, with great apologies to the delicate balance sought by WG1 members, is the original Cryosphere language (in italics) and a less formal interpretation of WG1’s conclusions (bolded), sometimes with added contextual background.

To read the entire original text – which I would strongly encourage readers to do, and draw your own conclusions – it can be downloaded at http://www.climatechange2013.org.

OBSERVED CRYOSPHERE-RELATED CHANGES:

“Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and northern hemisphere snow cover have continued to decrease in extent.” (NB: no interpretation needed here).

• The average rate of ice loss8 from glaciers around the world, excluding glaciers on the periphery of the ice sheets9, was very likely 226 [91 to 361] Gt yr-1 over the period 1971-2009, and very likely 275 [140 to 410] Gt yr-1 over the period 1993-2009

Glaciers around the world have lost ice faster in the past two decades, accelerating by about 20%.

• “The average rate of ice loss from the Greenland ice sheet has very likely substantially increased from 34 [–6 to 74] Gt yr–1 over the period 1992–2001 to 215 [157 to 274] Gt yr–1 over the period 2002–2011.[1]”

[1] 100 Gt yr of ice loss is equivalent to about 0.28 mm of global mean sea level rise per year.

Greenland, which last century was thought to be gaining ice, instead lost about 34 gigatons of volume per year in the last decade of the 20^th century, and in the first decade of this century that loss has accelerated by over six times that rate, or over 600%.

• “The average rate of ice loss from the Antarctic ice sheet has likely increased from 30 [–37 to 97] Gt yr–1 over the period 1992–2001 to 147 [72 to 221] Gt yr–1 over the period 2002–2011. There is very high confidence that these losses are mainly from the northern Antarctic Peninsula and the Amundsen Sea sector of West Antarctica.”

Antarctica also has lost ice mass, probably accelerating from about 30 to about 150 Gigatons per year, or nearly 5x greater loss (500%), with this loss occurring in the northern Antarctic Peninsula and from the West Antarctic ice sheet.

• “The annual mean Arctic sea ice extent decreased over the period 1979–2012 with a rate that was very likely in the range 3.5 to 4.1% per decade (range of 0.45 to 0.51 million km2 per decade), and very likely in the range 9.4 to 13.6% per decade (range of 0.73 to 1.07 million km2 per decade) for the summer sea ice minimum (perennial sea ice). The average decrease in decadal mean extent of Arctic sea ice has been most rapid in summer (high confidence); the spatial extent has decreased in every season, and in every successive decade since 1979 (high confidence). There is medium confidence from reconstructions that over the past three decades, Arctic summer sea ice retreat was unprecedented and sea surface temperatures were anomalously high in at least the last 1,450 years.”

Arctic sea ice extent overall decreased by about 3-4% per decade from 1979-2012, but by around 9-13% per decade for the summer minimum.  Such low ice coverage and high ocean surface temperatures probably have not been seen for at least 1,450 years.

• “It is very likely that the annual mean Antarctic sea ice extent increased at a rate in the range of 1.2 to 1.8% per decade (range of 0.13 to 0.20 million km2 per decade) between 1979 and 2012. There is high confidence that there are strong regional differences in this annual rate, with extent increasing in some regions and decreasing in others.”

Antarctic sea ice overall has increased by about 1-2% during this period, but with strong regional differences at different sites around the continent.

• “There is very high confidence that the extent of Northern Hemisphere snow cover has decreased since the mid-20th century (see Figure SPM.3). Northern Hemisphere snow cover extent decreased 1.6 [0.8 to 2.4] % per decade for March and April, and 11.7 [8.8 to 14.6] % per decade for June, over the 1967–2012 period. During this period, snow cover extent in the Northern Hemisphere did not show a statistically significant increase in any month.”

Snow cover in the northern hemisphere has decreased by about 2% per decade in early spring, and about 12% per decade in June, with unusually high scientific confidence in these results.

• “There is high confidence that permafrost temperatures have increased in most regions since the early 1980s. Observed warming was up to 3°C in parts of Northern Alaska (early 1980s to mid-2000s) and up to 2°C in parts of the Russian European North (1971–2010). In the latter region, a considerable reduction in permafrost thickness and areal extent has been observed over the period 1975–2005 (medium confidence).”

Permafrost temperatures have increased in most permafrost regions since the early 1980’s, from 2-3 degrees C in some regions of Alaska and European Russia, with considerable thinning and loss of permafrost in the latter region.

• “Multiple lines of evidence support very substantial Arctic warming since the mid-20th century.”

The Arctic has warmed substantially since 1950, and this has been shown by a number of different measures.

• “The rate of sea level rise since the mid-19^th century has been larger than the mean rate during the previous two millenia (high confidence). Over the period 1901-2010, global mean sea level rose by 0.19 [0.17 to 0.21]m.”

Sea level has risen faster since 1950 than at any time during the previous 2000 years.

• “There is very high confidence that maximum global mean sea level during the last interglacial period (129,000 to 116,000 years ago) was, for several thousand years, at least 5 m higher than present and high confidence that it did not exceed 10 m above present. During the last interglacial period, the Greenland ice sheet very likely contributed between 1.4 and 4.3 m to the higher global mean sea level, implying with medium confidence an additional contribution from the Antarctic ice sheet. This change in sea level occurred in the context of different orbital forcing and with high-latitude surface temperature, averaged over several thousand years, at least 2°C warmer than present (high confidence).”

Scientists are extremely sure that during the last warm period on Earth (the Eemian, about 125,000 years ago), sea level was at least 5 meters higher than today but probably not above 10 meters higher, and that most of this came from the West Antarctic ice Sheet rather than Greenland.  Temperatures in the Arctic and Antarctic at that time were at least 2 degrees higher than today.  (Note: The Arctic and Antarctic have already warmed by between 1.5-2 degrees and are expected to warm an additional 2 degrees already by 2050-60 if current rates continue.)

FOR THE FUTURE:

“It is very likely that the Arctic sea ice cover will continue to shrink and thin and that Northern Hemisphere spring snow cover will decrease during the 21st century as global mean surface temperature rises. Global glacier volume will further decrease.” (NB – no interpretation needed here.)

• “Based on an assessment of the subset of models that most closely reproduce the climatological mean state and 1979 to 2012 trend of the Arctic sea ice extent, a nearly ice-free Arctic Ocean in September before mid-century is likely for RCP8.5 (medium confidence) (see Figures SPM.7 and SPM.8). A projection of when the Arctic might become nearly ice-free in September in the 21st century cannot be made with confidence for the other scenarios.”

If current warming continues, the Arctic is likely to be nearly ice-free in September by 2050, with the definition being ice-free for five consecutive years (NB – that latter qualification appears in a footnote, meaning that the first ice-free September may well come even earlier.)

• “By the end of the 21st century, the global glacier volume, excluding glaciers on the periphery of Antarctica, is projected to decrease by 15 to 55% for RCP2.6, and by 35 to 85% for RCP8.5 (medium confidence).”

Global glacier volume excluding Antarctica (which remain too uncertain) will shrink by 35-85% by 2100 if current warming trends continue, and by 15-55% even if greenhouse gas emissions are cut as much as possible.

• “It is virtually certain that near-surface permafrost extent at high northern latitudes will be reduced as global mean surface temperature increases. By the end of the 21st century, the area of permafrost near the surface (upper 3.5 m) is projected to decrease by between 37% (RCP2.6) to 81% (RCP8.5) for the model average (medium confidence).”

Scientists gave their highest stamp of certainty to their projection that the extent of permafrost is going to shrink by the end of this century as temperatures increase (NB: which means there will be some degree of release of CO2 and methane from these regions).  If current rates of warming continue, the upper 3.5 meters of permafrost will decrease by about 80%, and will still decrease by 37% under the most ambitious reductions in CO2.

• “Global mean sea level rise for 2081–2100 relative to 1986–2005 will likely be in the ranges of 0.26 to 0.55 m for RCP2.6, 0.32 to 0.63 m for RCP4.5, 0.33 to 0.63 m for RCP6.0, and 0.45 to 0.82 m for RCP8.5 (medium confidence). For RCP8.5, the rise by the year 2100 is 0.52 to 0.98 m, with a rate during 2081–2100 of 8 to16 mm yr–1 (medium confidence).These ranges are derived from CMIP5 climate projections in combination with process-based models and literature assessment of glacier and ice sheet contributions.”

If current rates of ocean warming and ice loss continue, sea level will rise by at least 4-8 decimeters (16 inches-32 inches) by 2081 and 5-10 decimeters (20 inches-3 feet) by 2100.  If greenhouse gas emissions are cut extensively, this rise by 2081 would be 2.5-5.5 decimeters (10-22 inches).

• “Based on current understanding, only the collapse of marine-based sectors of the Antarctic ice sheet, if initiated, could cause global mean sea level to rise substantially above the likely range during the 21st century. However, there is medium confidence that this additional contribution would not exceed several tenths of a meter of sea level rise during the 21st century.”

Based on what we know today, the only thing that could cause sea level to rise by greater than 1 meter would be the collapse of the Antarctic ice sheet.  This event could cause sea level to rise by several more decimeters (1-2 feet) by 2100.  (NB- From what we understand of such a collapse, it would also commit us to further melting of Antarctic glaciers “held back” by the WAIS that would raise sea levels by 3-6 meters over coming centuries, unless temperatures can be returned to much lower levels.)

A final note: perhaps the most carefully couched and nearly-hidden conclusion from AR5 is this last one.  Placed in “would not exceed several decimeters” language, it is easy to miss.  Put together however with the language on past collapses of the West Antarctic Ice Sheet (WAIS) at temperatures within range of projected warming by 2050 – if not slowed – it is perhaps the most sobering conclusion from AR5 of all.