March 3, 2022 By Adem Delibas

In the March 2022 update of the En-ROADS climate solutions simulator, we have updated our global sea level rise (SLR) estimates for the 21^st century in light of the latest science regarding historical SLR since 1900 and the Working Group I contribution to the IPCC’s Sixth Assessment Report (AR6).

With these revisions, the 2100 SLR simulated by En-ROADS in the Baseline scenario is updated to 71 cm (from 115 cm before these revisions). Users can also now test different assumptions about the contributions to global SLR from ice sheet melting in Antarctica and Greenland, within an uncertainty range covering the full spectrum of AR6 scenarios. Moreover, we have introduced a new SLR map type to help users visualize the additional risk of flooding due to storm surges.

Watch this video where Climate Interactive Director Andrew Jones shares the highlights of these sea level rise updates:

Why is the estimated future sea level rise lower than before? 

Simulating 21^st century global sea level rise (SLR) has always been a part of En-ROADS. To accomplish this, the En-ROADS model first approximates the relationship between global temperature and SLR. We compare this approximation generated by the model with the historical SLR data to see how closely En-ROADS simulates history and adjust the model as needed. Then, with this calibrated model, En-ROADS estimates future global SLR for a particular scenario.

Prior to the March 2022 update, the En-ROADS SLR model used the sensitivities of SLR to an increase in global temperature proposed by Vermeer and Rahmstorf (2009)^[1] and was tested against the historical SLR data composed of a reconstruction of the 20^th century tide gauge data provided by Jevrejeva et al. (2008)^[2] and relatively limited satellite data.

With the continued scientific progress over the last decade two areas of evidence have changed:

• Recent reconstructions of the 20^th century tide gauge data, like Dangendorf et al. (2019)^[3] or Frederikse et al. (2020)^[4], showed that the SLR during the 20^th century was only 13 cm as opposed to the 19 cm previously reported by Jevrejeva et al. (2008)^[2].
• New satellite data revealed that the prior calibration of the En-ROADS SLR model based on older data overestimated the SLR during 1993-2021 (especially during 2005-2021) compared to the NASA satellite data (5.1 mm/year annual rate of change in SLR in En-ROADS as opposed to 3.4 mm/year observed by the NASA satellite altimeters).

To address these two issues, we have recalibrated the relationship between global temperature and SLR to the new historical data composed of: (1) 1900-1993 tide gauge reconstructions by Dangendorf et al. 2019; and (2) 1993-2021 NASA satellite observations. The figure below displays both the fit between the estimated SLR and the measured data for the 1900-2021 period and the estimated SLR until the end of this century, after this recalibration.

The following points are worth noting:

• With the new calibration, the annual rate of change in the simulated SLR during 1993-2021 is 3.4 mm/year—the same as the mid-range NASA satellite observations.
• Future global sea level rise estimates that are consistent with the mid-range projections for the AR6 SSP3-7.0 scenario (see Table 1 below). The SSP3-7.0 scenario of the AR6 is the closest to the En-ROADS Baseline in terms of future temperature projections.

The 21^st century SLR (compared to year 2000) estimated by En-ROADS in the Baseline scenario is now 71 cm.

Table 1. The global sea level rise estimates of the En-ROADS Baseline scenario vs. the AR6 SSP3-7.0 scenario (relative to the 1995-2014 average). Values in parentheses represent the likely range, 100-66% probability.

Year	AR6 SSP3-7.0 SLR (relative to 1995-2014 average)	Recalibrated En-ROADS Baseline SLR (relative to 1995-2014 average)
2030	0.10 m (0.08 — 0.12)	0.11 m
2050	0.22 m (0.19 — 0.28)	0.24 m
2090	0.56 m (0.46 — 0.74)	0.58 m
2100	0.68 m (0.55 — 0.90)	0.70 m

What if ice sheets melt faster than we thought?

Contributions to SLR from ice sheet melting in Antarctica and Greenland remain the largest sources of uncertainty in estimating how much the Earth’s oceans will rise in the future. While the methods outlined above give us confidence in the estimates of sea level rise from thermal expansion, the dynamics of future ice sheet melting from climate change could be very different than the past 100-150 years. To account for this uncertainty, En-ROADS includes two new Assumptions sliders to capture how much ice sheet melting in Antarctica and Greenland contributes to global SLR.

Unlike in prior versions of En-ROADS, these two sliders are connected to the SLR outputs. In other words, if users want to change the En-ROADS default assumption of a 11 cm contribution to global SLR from ice sheet melting in Antarctica by the end of the century, they can increase it and the Sea Level Rise graph, the Sea Level Rise–Flood Risk Maps, and the Population Exposed to Sea Level Rise table will respond to this change.

It is also worth noting that the same rate of increase in ice sheet melting in Antarctica or Greenland will have different effects on different parts of the world. Therefore, knowing where the accelerated ice sheet melting is expected is as important as the rate of that acceleration.

The lower and upper bounds of the sliders controlling the contribution to SLR from ice sheet melting in Antarctica (2-56 cm) and Greenland (0-59 cm) are set such that users can experiment with the uncertainty ranges of any scenarios considered in AR6. The table below—adapted from the AR6 report—lists contributions to global SLR by the end of the century (relative to the  1995-2014 average) under the range of AR6 scenarios. The associated uncertainty range is shown in parentheses next to each entry. The SSP3-7.0 scenario is highlighted as it is the most consistent scenario with the En-ROADS Baseline in terms of future temperature projections.

Table 2. Contributions to global sea level rise under different scenarios. Values in parentheses represent likely probability ranges. Source: Table 9.9 Working Group I contribution to the IPCC’s Sixth Assessment Report.

With no additional policy actions, pulling these sliders to the upper-bounds of the uncertainty indicated by the SSP5-8.5 Low Confidence scenario, 0.56 meters for Antarctica and 0.59 meters for Greenland, increases the 21^st century SLR estimate of En-ROADS to 1.64 meters – 0.93 meters above the Baseline.

Sea level rise maps in En-ROADS can now visualize the effect of storm surges

Embedded SLR maps were added to En-ROADS in October 2021. In this March 2022 release, we introduce a new map type: 21^st Century with Storm Surge. This map responds to the new Storm Surge slider in the map to depict the additional risk of flooding due to storm effects.

To access the map, open the Sea Level Rise—Flood Risk Map under the Graphs > Impacts menu and select 21^st Century with Storm Surge in the Map Type dropdown menu. Use the Storm Surge slider to add the additional risk of flooding due to storm effects.

Several points are worth noting about this new map type and storm surges:

• The height of surge caused by a storm varies from location to location, and not all coastal areas are threatened by storm surges.
• Other than a few notable exceptions, like Hurricane Katrina in 2005 or Typhoon Haiyan in 2013, most major storms across the world have caused surges of less than five meters. Below are some examples^[5]:
  • Hurricane Florence (2019, North Carolina): 1.7 meters
  • Typhoon Hagibis (2019, Japan): 1.4 meters^[6]
  • Typhoon Haiyan (2013, Philippines): 5-7 meters^[7]
  • Hurricane Sandy (2012, New York City): 4.3 meters
  • Hurricane Ike (2008, Texas): 3-4.5 meters
  • Hurricane Katrina (2005): 7.6-8.5 meters along portions of the Mississippi coast; 3-6 meters along the southeastern Louisiana coast
  • Hurricane Rita (2005, Louisiana): 3-4.5 meters
  • Hurricane Dennis (2005, Florida): 2-3 meters
  • Hurricane Opal (1995, Florida): 7.3 meters
• In En-ROADS, adding a storm surge only modifies the 21^st Century SLR Map with Storm Surge. The Sea Level Rise graph and the Population Exposed to Sea Level Rise table are not affected by the specified storm surge height.

Take a look at the En-ROADS simulator now to create your own scenario of climate action and see the difference it makes in the sea level rise and related impacts. If you would like to read about the mathematical implementation of climate impacts, see the En-ROADS Reference Guide. Please get in touch with us if you have questions, comments, or ideas.

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[1]  Vermeer, M. & Rahmstorf, S. (2009). Global sea level linked to global temperature. Proceedings of the National Academy of Sciences of the United States of America, 106 (51), 21527-21532. https://doi.org/10.1073/pnas.0907765106

[2] Jevrejeva, S., Moore, J.C., Grinsted, A., & Woodworth, P.L. (2008). Recent global sea level acceleration started over 200 years ago? Geophysical Research Letters, 35(8). https://doi.org/10.1029/2008GL033611

[3] Dangendorf, S., Hay, C., Calafat, F.M., Marcos, M., Piecuch, C.G., Berk, K., & Jensen, J. (2019). Persistent acceleration in global sea-level rise since the 1960s. Nature Climate Change, 9,705–710. https://doi.org/10.1038/s41558-019-0531-8

[4] Frederikse, T., Landerer, F., Caron, L. Adhikari, S., Parkes, D., Humphrey, V.W., Dangendorf, S., Hogarth, P., Zanna, L., Cheng, L., & Wu, Y.-H.  (2020). The causes of sea-level rise since 1900. Nature, 584, 393–397. https://doi.org/10.1038/s41586-020-2591-3

[5] National Hurricane Center. (n.d.). Storm Surge Overview: Notable Surge Events. National Oceanic and Atmospheric Administration. https://www.nhc.noaa.gov/surge/

[6] Islam, M., & Takagi, H. (2020). Typhoon parameter sensitivity of storm surge in the semi-enclosed Tokyo Bay. Frontiers of Earth Science, 14(3), 553-567.

[7] Soria, J. L. A., Switzer, A. D., Villanoy, C. L., Fritz, H. M., Bilgera, P. H. T., Cabrera, O. C., … & Fernandez, I. Q. (2016). Repeat storm surge disasters of Typhoon Haiyan and its 1897 predecessor in the Philippines. Bulletin of the American Meteorological Society, 97(1), 31-48.