One of the effects of global warming is the rise in sea levels. Warming and melting reinforce each other through amplifying feedback mechanisms, as the surface heating of water and the troposphere increases melting and this, by reducing the albedo (the fraction reflected back into space of the solar radiation incident on the Earth), accelerates warming, in a vicious circle. Once triggered, the heating of water (that is, its thermal expansion) and melting have inertial dynamics, so that not even a radical reduction in GHG emissions within a time interval of 0 to 100 years will be able to stop them. on the horizon of historical time. As early as 2001, the IPCC Third Assessment Report (Assessment Report or AR3) stated: “sea levels are projected to continue to rise for many centuries” [I]. Figure 1 illustrates the irreversible nature of the inertial rise in temperatures and global mean sea level over the next millennium.
As can be seen, temperature stabilization is not expected to occur in the coming centuries and sea levels will continue to rise through thermal expansion and melting for at least the next millennium. Obviously, the IPCC warned in 2001, “the impacts become progressively greater with higher concentrations of CO2” [II], hence the urgent need to reduce GHG emissions, which is not occurring.
Acceleration in the recent past
How much has sea level risen in the last century? According to IPCC-AR5 (2013), it is “very likely that the average rate of sea level rise was 1,7 [1,5 to 1,9] mm per year between 1901 and 2010, for a total average rise 19 cm (17 to 21 cm). Between 1993 and 2010, the rate was most likely higher, reaching 3,2 mm per year.” Reinforcing these data, satellite measurements from GISS/NASA indicate that between 1880 and 2013 there was an average global rise in sea level of 22,6 cm, that is, 1,6 mm per year on average over 133 years. It turns out that a third of this increase (7,6 cm) occurred in just over 20 years, between 1992 and 2013 [III]. This acceleration was calculated by a work published in Scientific Reports in 2016, which establishes the following jump in sea level rise rates [IV]:
1900-1990 = +1,2 to 1,9 mm/year
1992-2015 = +3,3 ± 0.4 mm/year
In 2016, the average increase was 3,41 mm, resulting from a new acceleration from 2012 onwards, as clearly shown in figure 2
The acceleration shown in figure 2 is confirmed by a recent assessment, according to which the global average rise in sea level was 25% to 30% faster between 2004 and 2015 than between 1993 and 2004 [IN].
Uncertainty about the rate of acceleration in the near future
Qin Dahe, co-director of IPCC-AR5, states that “as the ocean warms and sea and continental ice reduces, global mean sea level will continue to rise, but at a faster rate than observed over the last 40 years” [YOU]. How much faster is still uncertain. If the estimates of the work published in the Scientific Reports, cited above, “the magnitude of the acceleration in the mid-0,12st century will be 0.12 mm per year every year [2mm yr−XNUMX], although this value will depend heavily on future losses of continental ice, which are highly uncertain ”. To try to restrict these uncertainties, Benjamin P. Horton, Stefan Rahmstorf, Simon E. Engelhart and Andrew C. Kemp published in 2014 the results of a probabilistic assessment of the average sea level rise in 2100 and 2300, as a function of two contrasting scenarios of global warming, based on a consultation with 90 experts selected from among those who have most actively published on this topic in recent years [VII]. Figure 3 summarizes these projections in a single graph, comparing them with those of the IPCC and the National Oceanic and Atmospheric Administration (NOAA).
In the blue scenario (shaded with probable and very probable confidence intervals), warming is less than 2º C in relation to the pre-industrial period and there is a slow decrease after 2050 (RCP3-PD, by “Peak and Decline”). In this case, the most likely average elevation will be 40 to 60 cm by 2100 and 60 cm to 1 meter by 2300. In the red scenario (RCP8, with the same confidence intervals), warming is 4,5º C in 2100 and 8º C in 2300, and the most likely average elevation is between 70 cm and 1,6 meters for 2100 and 2 to 3 meters for 2300 [VIII]. Compared to the opinions of these 90 experts, the IPCC projections for 2100 in relation to 2000 are represented in the vertical bars on the right, confirming the (necessarily) conservative tendency of the IPCC. In contrast, NOAA's projections, represented by the four dotted lines, reach, in the intermediate-high case, more than 1,2 meters and, in the worst case, 2 meters [IX].
Antarctica and Greenland: non-linear sea level rise
These uncertainties arise, as mentioned, above all from the rapidity of the melting of Greenland and Antarctica, which is only now beginning to be better estimated. According to the IPCC (AR5), “observations made since 1971 indicate that thermal expansion and sea ice melt (excluding peripheral Antarctic sea ice melt) explain 75% of the observed elevation (high confidence)”. But, the text continues, “the contribution of ice from Greenland and Antarctica has increased since the early 1990s, in part because of increased ice flow induced by warming of the immediately adjacent ocean.” What has already been observed is that in just 17 years (1995 – 2011), the combined action of melting ice in Antarctica and Greenland caused a rise of 11,1 mm in sea level, a rise that, especially in Greenland, is in clear acceleration during this period, as shown in figure 4
In 2009, a work published in the Proceedings of the National Academy of Sciences already detected this acceleration: “the loss of ice mass in Greenland and Antarctica is accelerating and is closer to a quadratic trend than a linear trend” [X]. As this article states, Greenland's contribution to this rise will, once again, be greater than IPCC-AR4 (2007) projections.
It turns out that, in 2016, James Hansen, led by 18 scientists, published a work whose results greatly aggravate even NOAA's most pessimistic estimate (an increase of 2 meters by 2100) [XIV]: “Our hypothesis”, states this work, “is that the loss of mass from the most vulnerable ice, sufficient to increase sea level by several meters [several meters], approximates an exponential response better than a linear response. (...) Continuous high emissions from fossil fuels this century are predicted to cause (...) more powerful storms and non-linear increase in sea level rise, reaching several meters over a time horizon of 50 to 150 years".
Anchor ice in Antarctica
It is well known that “the West Antarctic Peninsula is one of the fastest warming areas on Earth, behind only some areas in the Arctic Circle.” [XII]. And as this region warms, continental melting accelerates, giving greater concreteness to the new scenario predicted by James Hansen and colleagues. The detachment of the Larsen Sea Platform between 1995 and 2017, throwing 10.550 km2 of sea ice into the Weddel Sea (Larsen A, 1995 = 1.500 km2, Larsen B, 2002 = 3.250 km2 and Larsen C, 2017 = 5.800 km2) has no direct impact on sea level, as it is sea ice, which already occupied a place in the sea and, furthermore, “passive ice”, that is, which does not function as a support for continental ice. But the sea ice shelves over the Amundsen and Bellingshausen Seas (west and southwest Antarctica) are mainly anchor ice, and their future calving is assumed to open the way for continental ice to descend from their steep slopes into the sea. sea, as shown by the yellow spots in figure 5 [XIII].
An increase 2 and a half times to 6 times greater than in the XNUMXth century
If sea level rise by 2100 remains only between 50 cm and 1,2 meters (NOAA's intermediate-low and intermediate-high projections), this means that over the 2st century this rise will be 6 times to 19 times greater than the 1901 cm of elevation observed, as seen above, between 2010 and 2050. The consequences of this redesign of the coastline are so multiple and complex that, for reasons of space, they must be dealt with in the next article. Let us briefly review some of them here. Given that stronger hurricanes are also predicted and that most of the world's largest cities are coastal, more frequent and worse floods than those that have occurred in recent decades will be inevitable. By 10, Bangladesh is expected to lose XNUMX% of its territory [XIV]. By 2050, sea level in New York is expected to be 76 cm above the 2000-2004 average level and by 2100, 183 cm (6 feet) [XIV]. Cities such as Guangzhou, New Orleans, Miami, Mumbai, Nagoya, Boston, Shenzen, Osaka, Guayaquil, Ho Chi Minh City and, in Brazil, Santos [XVI], are also in the first line of fire of the floods. But the spectrum of consequences is much broader than the simple loss of urban infrastructure, as an increase of this magnitude (as seen, well below the worst case scenario) could flood coastal nuclear plants, salinize aquifers and deltas, erode coasts, destroy ecosystems coastal areas and impose gigantic population displacements. These impacts will be the topic of the next article.
[I] IPCC AR3 Climate Change 2001: Synthesis Report: “sea level is projected to continue to rise for many centuries”http://www.ipcc.ch/ipccreports/tar/vol4/011.htm>.
[ii] See IPCC AR3 Climate Change 2001: Synthesis Report: “the impacts become progressively larger at higher concentrations of CO2.”http://www.ipcc.ch/ipccreports/tar/vol4/011.htm>.
[iii] See NASA Goddard Institute for Space Studies:https://www.nasa.gov/goddard/risingseas>.
[iv] See JT Fasullo, RS Nerem & B. Hamlington, “Is the detection of accelerated sea level rise imminent? Scientific Reports, 10/VIII/2016https://www.nature.com/articles/srep31245#f1>.
[v] See HB Dieng et al., “New estimate of the current rate of sea level rise from a sea level budget approach”. Geophysical Research Letters, 22/IV/2017.
[vi] Cf. “Human influence on climate change, IPCC report says”, UN and climate change, 27/IX/2014:
<http://www.un.org/climatechange/blog/2013/09/human-influence-on-climate-clear-ipcc-report-says/>.
[vii] Cf. Benjamin P. Horton, Stefan Rahmstorf, Simon E. Engelhart & Andrew C. Kemp, “Expert assessment of sea-level rise by AD 2100 and AD 2300”. Quarterly Science Reviews, 83, 2014, pp. 1-6
<https://marine.rutgers.edu/pubs/private/HortonQSR_2013.pdf>.
[viii] In 2016, another work reinforces this projection for 2100, depending on the different scenarios of atmospheric GHG concentrations: RCP2,6 = 28-56 cm; RCP4,5 = 37-77 cm and RCP8,5 = 57-131 cm. See Matthias Mengel et al., “Future sea level rise constrained by observations and long-term commitment”. PNAS, 113, 10, 8/2016/XNUMX.
[ix] See NOAA, Global Sea Level Rise Scenarios for the United States National Climate Assessment 6/XII/2012https://scenarios.globalchange.gov/sites/default/files/NOAA_SLR_r3_0.pdf>.
[X] See Jonathan T. Overpeck & Jeremy L. Weiss, “Projections of future sea level becoming more dire”. PNAS, 22/XII/2009http://www.pnas.org/content/106/51/21461.full>.
[xi] See James hansen et al., “Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 °C global warming could be dangerous” (cit). See also <https://www.youtube.com/watch?v=JP-cRqCQRc8>.
[xii] Cf. Antarctic And South Ocean Coalition (ASOC)http://www.asoc.org/advocacy/climate-change-and-the-antarctic>.
[xiii] See Fürst et al. « The safety band of Antarctic ice shelves” Nature Climate Change, 8/II/2016.
[xiv] Cf. “Bangladesh risks more flooding, droughts, hurting development”. BDNews24.com, 14/VII/2017.
[xv] See “Impacts of Climate Change in New York”. New York State. Department of Environmental Conservationhttp://www.dec.ny.gov/energy/94702.html>.
[xvi] Cf. E. Alisson, “Coastal floods in Santos could cause billions in losses”. FAPESP, 7/X/2015 http://agencia.fapesp.br/inundacoes_costeiras_em_santos_podem_causar_prejuizos_bilionarios_/21997/.