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It ain't necessarily so on sea rise

By Mike Pope - posted Tuesday, 16 August 2016


What you read in the IPCC Fifth Assessment Report (5AR) regarding sea level rise this century, a maximum 0.98 m rise by 2100 based on RCP 8.5, the worst case scenario, (5AR, page 1204) ain't necessarily so. There is good reason for this. The material published in 5AR is based on some of the best scientific information available prior to publication in 2013. It represents a consensus view among contributing scientists. As such, it inevitably presents a comprehensive though conservative view, providing what the IPCC believes is a sound basis for reliable public policy formulation. But is it?

Every aspect of climate change contributes to mean global sea level but the two most significant causing it to rise are melting of land-based ice and thermal expansion of seawater. Both are caused by global warming, particularly warming in polar regions where most land-based ice is located. So you would think that the IPCC would pay particular attention to ensuring the accuracy of its prognosis for global warming in general and polar warming in particular. But does it?

Can we, should we, rely on its assertion that mean global sea level will rise 0.52-0.98m by 2100,Figure SPM9 (pdf), for the worst case scenario (RCP8.5), as a sound basis for public policy, particularly regarding the location and protection of buildings and other infrastructure in coastal areas?

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Arctic Warming

The IPCC is well aware that the Arctic is undergoing abrupt climate change (abrupt even on a human life-span scale) in the form of atmosphere and ocean warming. This has resulted in accelerating ice mass loss from the Greenland ice sheet (GIS), exacerbated by aerosol deposits (soot, dust, ash etc) on both sea ice and much of the GIS. It recognises that loss of Arctic sea ice will reduce albido, increase ocean warming and further hasten melting of the GIS.

However The IPCC appears to be wrong about the speed with which sea ice loss will occur or other factors contributing to Arctic Ocean warming. These include strengthening convection currents accelerating sub-sheet penetration by warmer water and its effect on rate of ice mass loss from the GIS, the more vulnerable marine West Antarctic ice sheet (WAIS) and glaciers of the East Antarctic Ice sheet (EAIS).

Consider IPCC reliance on CMIP5, an assembly of 36 global climate models, to determine Arctic amplification which conclude that Arctic warming is likely to occur at ~x2 the rate of present average global warming. Using observed data rather than climate model (CMIP5) outcomes,(Overland et al (2014) indicate that Arctic amplification could be in the order of x3 - x4 average global surface warming possibly reaching +13°C by 2100 with business as usual (RCP 8.5) greenhouse gas emissions.

Fig. 1 Annual mean surface air temperature for 1966–2005 period based on NCEP/NCAR reanalysis of observed data (left) and ensemble mean of 36 CMIP5 models. Units are °C/decade.Source: Overland et al (2014).

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The IPCC 5AR relies on CMIP5 projections which underestimate observed warming but is nevertheless used as the basis for calculating future Arctic amplification and its effects on loss of albedo, surface air temperature, Arctic Ocean warming and their effects on ice mass loss from the GIS and consequential sea level rise.

Methane GWP

The global warming potential of CH4 is still quoted by some as being "over 20" and "25 times" that of CO2 over a 100 year period, the latter being a value routinely used in 2016 by the U.S. EPA. This is wrong, and dangerously misleading when it comes to determining the effects of methane on average global temperature and SLR. The IPCC's 5AR more accurately describes CH4 as having GWP of 34, Table 8.7 (pdf) but even this may be an underestimate. Why so?

Because CH4 in the atmosphere oxidizes to CO2 over a period of 12 years and for decades the concentration of CH4 in the atmosphere has been either stable or rising. In other words, CH4 has been and is entering the atmosphere at a rate which over short periods is equal to and over longer periods is higher than the rate of its loss due to the photochemical process resulting in its conversion to CO2. Because CH4 is being released as a result of permafrost degradation and ocean warming at an increasing rate, we know that its future concentration in the atmosphere will continue to rise and the rate may accelerate.

Accordingly at any point in time the GWP of CH4 is more accurately its GWP during its lifetime of ~12 years rather than the 100 year span most frequently cited. There is a political imperative but no scientific basis for selecting a 100 year span. The 5AR calculates the GWP as 84 over a 20 year period is much closer to the lifetime of CH4 in the atmosphere, yet is rarely used.

Carbon Emissions

None of the climate models used by the IPCC factors in the permafrost feedback.

The 5AR asserts that CH4 emissions associated with Arctic permafrost degradation have remained unchanged since 1980 and constitute <1% of global methane emissions – Table 6.8 (pdf). Indeed, the Bottom-Up measure shows that total natural emissions declined by 8 gigatonnes in the decade 2000-09 compared to the decade 1980-89. But can this be true given that Arctic permafrost is degrading and contains vast quantities of biota and methane gas?

Offshore, the Arctic continental shelf is estimated to contain over 2,200 gigatonnes of carbonincluding 500 gigatonnes in permafrost covered seabed sediments and 700 gigatonnes in gaseous form in the hydrate stability zones.

Water over the Siberian continental shelf is warming resulting in permafrost degradation. Seabed sediments at a depth of over 50 meters beneath the seabed have been found to be free of permafrost, permitting CH4 gas on and under the continental shelf to supersaturate and percolate through covering seawater.

Seawater covering the Siberian Continental Shelf is ≤ 40 meters deep so seabed emissions have no chance of being oxidized and enter the atmosphere as CH4 and this is occurring at increasing levels. CH4 in the atmosphere above the shelf has been measured at concentration regionally reaching ~8ppm compared to a global average of ~1.8 ppm.

Onshore permafrost is estimated to contain ~1,500 gigatonnes of carbon largely in the form biota which, with permafrost degradation, could result in release to the atmosphere of up to 160 gigatonnes by 2100. It is estimated that >95% will be in the form of CO2 with the balance as CH4.

Onshore estimates of the rate of permafrost degradation and CH4 release are likely to be underestimates because (a) the former excludes the warming effect of bacterial activity and phytoplankton, accelerating methanogen activity and permafrost melting and: (b) release of CH4 to the atmosphere is likely to be much higher than an estimated ≤5% of total emissions for three reasons.

1. Archea and other methanogens are active and able to produce CH4 from biota in sub-zero conditionsas well as at higher temperatures. It is unclear that bacteria (methanotrophs) can convert CH4 to CO2 in oxic conditions on or near the land surface at temperatures below 0°C.

2. CH4 gas is converted to CO2 when it percolates from anoxic production conditions into those where oxygen is abundant and bacteria can oxidize it. It is unlikely that bacteria would be able to oxidize a very high quantum of CH4 (in excess of 95%) before much of it escapes to the atmosphere – particularly in the absence of sphagnum moss (uncommon on permafrost land) with which methanotrophs have a symbiotic relationship and act most efficiently.

3. As surface permafrost melts it produces a landscape which is largely waterlogged since lower, intact permafrost inhibits drainage resulting in formation of thermokast lakes. This creates anoxic conditions essential for methanogens to produce CH4 from biota but inhibit oxidation by methanotrophs. Release of carbon from waterlogged land surfaces is likely to be in the form of CH4 rather than >95% CO2 predicted by some.

Fig. 2. Comparison between atmospheric concentration of CH4 and CO2 over the last 400,000 years and their effects on average global temperature to 2013. While most CH4 emissions result from human activity, mining and animal husbandry, emissions associated with permafrost degradation are increasing. Source: R.Morrison, Wikipedia

Analysis of foraminifera and Antarctic ice cores show that globally, CH4 concentration in the atmosphere is now at its highest level in over 800,000 years. What makes present levels so dangerous is that studies by Reisinger et al (2011) show that over a 20 year period, CH4 now has radiative forcing properties 84 times greater than CO2, a finding accepted by 5AR. Yet the 5AR dismisses the notion that CH4 emissions from Arctic continental shelves and adjacent onshore areas contribute to Arctic amplification or that this contribution can, let alone will, rapidly accelerate permafrost degradation and larger CH4 releases this century.

The result of underestimating CH4 emissions in the Arctic and their GWP makes it likely that the 5AR significantly underestimates Arctic amplification, loss of ice mass from the GIS, consequent sea level rise this century and the speed with which sea ice covering the Arctic Ocean reduces.

Albedo Loss

The 5AR defines an ice-free Arctic Ocean as sea ice extent less than 1 × 106km2for at least 5 consecutive years (5AR Ch. 11.3.4.1)and asserts that these conditions could pertain by summer 2050 (5AR Ch. 2, Fig. 2.1.b) and that consequential loss of albedo will have an important (though unquantified) effect on ocean warming, accelerating ice loss, coastal erosion and permafrost degradation. Most scientists would agree, though many are of the view that the Arctic Ocean will be ice free in summer long before 2050, possibly as soon as 2030. The 5AR is seen by many as underestimating Arctic Ocean warming, loss of sea ice and albedo.

Relatively rapid loss of sea ice, combined with ingress of warmer water from the Pacific and Atlantic will reduce formation of multi-year sea ice, making the Arctic coastline vulnerable to erosion, exposing carbon bearing ice and biota to a warming atmosphere. This is likely to result in greater release of CH4 as sub-surface ice containing biota are exposed to methanogens.

Fig. 3 Soot and aerosols produced by human activity and from natural sources contribute to atmospheric warming and when they settle on ice in the Arctic, absorb solar energy which accelerates melting. Source: Arctic News/Dark Snow Project.

Aerosol soot in the Arctic originates in human activity and, as the climate warms, a rising incidence of forest burning. It falls over a vast area of the Arctic covering large areas of sea ice and the GIS. This reduces albedo and increases absorption of solar radiation, thereby accelerating seasonal melting, increasing mass ice loss

since 2000. In the case of the GIS, surface melting produces lakes which ultimately flow to the ice sheet base, lubricating the land surface on which its rests before flowing to the ocean, enhancing ice sheet mobility.

At the same time relatively warm ocean water penetrates deep under the ice sheet, pushing back the grounding line of glaciers, enabling an increase in their outflow. The result: land based ice is simultaneously attacked from above and below, further accelerating SLR. There can be no doubt that the cryosphere, particularly in polar regions, is in serious and irreversible decline.

Hansen et al

When predicting SLR this century, the IPCC accepts the broad view that mean global sea level will rise ~90cm by 2100. It does not accept "outlaying values" (5m) provided in an essay by Hansen (2005) based on paleo-climate research of sea level during the Eemian interglacial. That research shows that with average global surface temperature 2°C above the pre-industrial and when atmospheric CO2 concentration was around 280 ppm, mean global SLR during the Eemian maximumwas 5m-9m higher than at present.

More recently, Hansen has shown that mean global SLR in the order of 5m by 2100 is supported by both paleo-climate and observed trends showing decadal doubling of ice mass loss from polar ice sheets. Hansen shows

that were that trend to persist, sea level would indeed rise ~5m. by 2100. Based on polar ice mass loss in the year 2000, actual loss from the GIS to 2009 had more than doubled in less than a decade.

When challenged on the basis that SLR could not be so rapid in such a short period (~100 years), Hansen (2005) draws attention to Pulse 1A during the Holocene when sea level rose at a rate of ~ +4m per century over 5 consecutive centuries.

Yet the analyses of eminent scientists which fall outside majority consensus conclusions published by the IPCC, are seemingly not considered sufficiently "reliable" to base its own prognosis of sea level rise this century. Even so, a growing number of scientists are coming to the view that mean global sea level rise, well in excess of 2m, is both possible and increasingly likely by 2100.

Hansen et al (2016) and 18 eminent co-authors point out that polar ice loss results in cooling of ocean surface water, while tropical warming continues, increasing surface energy imbalance. This increases the incidence and severity of destructive storms, far more severe than this, pushing sea level even higher where they occur, increasing the threat to coastal population centres, causing coastal erosion and flooding of low lying land.

Summary

The IPCC 5AR prognosis for sea level rise over the next century appears to be a dangerous underestimate for policy makers because it:

  • Relies on CMIP5 which underestimates present and future Arctic warming
  • Underestimates the Global Warming Potential of CH4
  • Ignores the effect of carbon emissions associated with permafrost degradation on Arctic amplification
  • Miscalculates the rate of albedo reduction (due to aerosols, sea ice loss and Arctic amplification) and its effect on polar ice
  • Underestimates the rate of ocean warming, particularly bottom water and its effects on polar ice sheets
  • Ignores analyses by Hansen et al indicating multi metre sea level rise and severe storm surges before 2100.

Conclusion:

The IPCC needs to regularly review and revise its advice to policy makers, having regard to the latest research and findings of climate scientists, rather than rely on data available to 2012, immediately prior to publication of the 5AR.

At present, authorities at all levels of government often cite 5AR estimates of mean SLR and use these for planning purposes. They do so in the belief that these are based on a degree of certainty that is far from true.

Planning authorities and owners of real estate located in areas which would be affected by sea level rise of 2-3 m or more should be aware that present IPCC estimates of a 0.9m sea level rise by 2100 are unrealistic.

Over coming decades, current estimates of SLR will be repeatedly revised upwards, though possibly not in sufficient time to allow for prudent and timely policy development and planning to deal with future seal level rise.

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About the Author

Mike Pope trained as an economist (Cambridge and UPNG) worked as a business planner (1966-2006), prepared and maintained business plan for the Olympic Coordinating Authority 1997-2000. He is now semi-retired with an interest in ways of ameliorating and dealing with climate change.

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