The IPCC and climate tipping point: Revised trajectories for the 21-23rd centuries

“The science that we are doing is a threat to the world’s most powerful and wealthiest special interests: the fossil fuel industry. They have used their immense resources to create fake scandals and to fund a global disinformation campaign aimed at vilifying the scientists, discrediting the science, and misleading the public and policymakers. Arguably, it is the most villainous act in the history of human civilization, because it is about the short-term interests of a small number of plutocrats over the long-term welfare of this planet and the people who live on it.” (Michael Mann).

Climate developments over the last decade raise questions regarding the mostly linear to curved IPCC model trajectories proposed for the 21st century and beyond. Reports of the International Panel of Climate Change (IPCC), based on thousands of peer reviewed science papers and reports, offer a confident documentation of past and present processes in the atmosphere, as well as approximate future model projections (Figure 1). However, when it comes to estimates of future ice melt rates and sea level change, these models contain a number of significant departures from conclusionsbased on the paleoclimate record, and current observations. This includes climate change feedbacks from land and water, ice sheets melt rates, temperature trajectories, sea level rise rates, methane release rates, the role of fires, and observed onset of transient stadial (freeze) events. Present indications of progressive stadial event/s are manifest by the build-up of a large pool of cold water in the North Atlantic Ocean south and east of Greenland and along the fringes of the Antarctic continent(Rahmstorf et al. 2015[1])  (Figure 2A).

Two major issues arise regarding the IPCC linear climate projectionsmodel:

  1. The scale and rate of global warming as a function of amplifying feedbacks from land, oceans and melting ice sheets.
  2. The role of ocean cooling events (stadials) consequent on flows of cold ice melt water from the large ice sheets,Greenland and Antarctica, into the oceans.


Figure 1.Time series of global annual mean surface air temperature anomalies relative to 1986–2005 from CMIP5 (Coupled Model Inter-comparison Project) concentration-driven experiments. Projections are shown for each RCP for the multi model mean (solid lines) and the 5–95% range (±1.64 standard deviation) across the distribution of individual models (shading).[2]

  1. Climate feedbacks. It is far from clear to what extent the IPCC linear temperature rise modelstakeaccount of amplifying climate feedbacks?“Fast” feedbacks involve changes in water vapor[3], clouds and sea ice extent.“Slow” climate feedbacks involve longer term changes in the land ice sheet disintegration, reduced vegetation cover, warming oceans, GHG release from soils, tundra or ocean sediments.Further positive feedbacks occur due to the release of methane from Arctic permafrost (~900 GtC; high-latitude peat lands ~400 GtC; tropical peat lands ~100 GtC). Hudson (2011) estimatesthe rise in radiative forcing due to total removal of Arctic summer sea ice as 0.7 Watt/m2, close to the total of methane release since 1750(~0.5 Watt/m2), although this amount would be in part offset by increased summercloudiness. The subsequent increase in evaporation is leading to the advance of coldvapor-laden Arctic fronts (Figure 2B) and thereby snow storms in the north Atlantic, North America, Europe and Siberia, producingshort-lived negative feedbacks.

Anthropogenic activities such as land-use change, agriculture and waste management have altered terrestrial biogenic greenhouse gas fluxes, and the resulting increases in methane and nitrous oxide emissions in particular can contribute to climate change. The cumulative warming capacity of concurrent biogenic methane and nitrous oxide emissions is a factor of about two larger than the cooling effect resulting from the global land carbon dioxide uptake from 2001 to 2010[4].

Transient ocean cooling (stadial) events

As the Earth continues to heat, transient temperature reversals (stadials) accentuate temperature polarities between warming land and ocean regions cooled by the flow of cold ice melt water from the melting ice sheets. Increasing polar temperatures allow cold air masses to breach the boundary moving south, and warm air masses to shift north, further heating the Arctic (Figure 2B).  The contrast between these air masses enhances the intensity of extreme weather events. These developments are consist with Pleistocene (2.6-0.01 million years ago) interglacial events peak temperatures which were succeeded by transient freeze events (stadials), such as the Younger Dryas and the 8.5 thousand years-old Laurentide ice melt, attributed to cold ice melt water flow from the polar ice sheets into the North Atlantic Ocean.

Hansen et al. (2016) (Figure 2, 3) used paleoclimate data and modern observations to estimate the effects of ice melt water from Greenland and Antarctica, showing cold low-density meltwater that tend to cap increasingly warm subsurface ocean water, affecting an increase ice shelf melting, accelerating ice sheet mass lossand slowing deep water formation. Ice mass loss would raise sea level on meters to tens of meters scale in an exponential rather than linear response, with doubling time of ice loss of 10, 20 or 40 years yielding multi-meter sea level rise.


Figure 2.(A) 2055-2100 surface-air temperature to +1.19oC above 1880-1920 (AIB model modified forcing, ice melt to  1 meter) (Hansen et al. 2016)[5](B)


Figure 3. Global surface-air temperature to the year 2300 in the North Atlantic and Southern Oceans, including stadial freeze events as a function of Greenland and Antarctic ice melt doubling time (Hansen et al. 2016).

Concomitantly with the weakening of the polar jet stream the North Atlantic overturning circulationis reduced[6]. The current warming to >1oC drives an increase in extreme weather events, analogous to the early HoloceneWarm Period (~10,000-8500 years ago) and the Eemian interglacial(130,000 – 115,000 years ago).The probability of a future transient freeze event (stadial) triggered by the flow of cold ice melt water into the North Atlantic and sub-Antarctic oceans has major implications for modern and future climate change trends, including a growing contrast between cooling subpolar oceans and warming continents and between polar-derived and tropics-derived air masses. These factors need to be taken into account in planning adaptation efforts.

Linear to curved temperature trends portrayed by the IPCC to the year 2300 (Figure 1) are rare in the Pleistocene paleo-climate record, which includes abrupt warming and cooling variations during both glacial (Dansgaard-Oeschger cycles; Ganopolski and Rahmstorf 2001[7];Camille and Born, 2019[8]) and interglacial (Cortese et al. 2007[9]) periods.

Temperature and sea level rise relations during the Eemian interglacial[10] (~115-130 kyr ago), when temperatures were about +1oC or higher than during the late stage of the Holocene, and sea levels were +6 to +9 m higher than at present, offer an analogy for present developments. During the Eemian overall cooling of the North Atlantic Ocean and parts of the West Antarctic fringe ocean due to ice melt led to increased temperature polarities and to storminess (Roverea A et al. 2017; Kaspar et al. 2007)[11], underpinning the danger of global temperature rise to +1.5oC.

These projections differ markedly from the IPCC linearmodel trends(Figure 1) which state:“For the 21st century, we expect that surface mass balance changes will dominate the volume response of both ice sheets (Greenland andAntarctica). A key question is whether ice-dynamical mechanisms could operate which would enhance ice discharge sufficiently to have an appreciable additional effect on sea level rise” (IPCC 2016)[12].Thisconclusion is difficult to reconcile with studies by Rignot et al. (2011) reporting that in 2006 the Greenland and Antarctic ice sheets experienced a combined mass loss of 475 ± 158 Gt/yr, equivalent to 1.3 ± 0.4 mm/yr sea level rise.”[13]. For the Antarctic ice sheet the IEMB team (2017) statesthe sheet lost 2,720 ± 1,390 billion tonnes of ice between 1992 and 2017, which corresponds to an increase in mean sea level of 7.6 ± 3.9 millimeter (IMBIE team 2017)[14]

A non-linear climate warming trend, including stadial freeze events, bears significant implications for planning future adaptation efforts, including preparations for transient deep freeze events in parts of Western Europe and eastern North America, for periods lasting several decades (Figure 3) and coastal defenses against enhanced storminess arising from increased temperature contrasts between the cooled regions and warm tropical latitudes.















Andrew Glikson, Earth and paleo-climate scientist, Australian National University


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