by Judith Curry
The concern about sea level rise is driven primarily by projections of future sea level rise.
Observed sea level rise over the last century has averaged about 8 inches, although local values may be substantially more or less based on local vertical land motion, land use, regional ocean circulations and tidal variations.
Projections of future sea level rise can be made in the following ways:
- Extrapolation of recent trends
- Semi-empirical approaches based on past relationships of sea level rise with temperature
- Process-based methods using models
1. IPCC’s 21st century projections
Sea level rise projections (both semi-empirical and process-based methods) are directly tied to projections of surface temperature, which are based upon simulations from global climate models.
The climate model simulations of 21st century climate referenced in the IPCC AR5 are based on more than 30 different global climate models from international climate modeling groups. These simulations are coordinated by the CMIP (Coupled Model Intercomparison Project), under the auspices of the World Climate Research Programme (WCRP).
Chapter 11 of the AR5 describes uncertainties in the model-based projections:
Climate projections are subject to several sources of uncertainty. The first arises from natural internal variability, which is intrinsic to the climate system, and includes phenomena such as variability in the mid-latitude storm tracks and the ENSO. The existence of internal variability places fundamental limits on the precision with which future climate variables can be projected. The second is uncertainty concerning the past, present and future forcing of the climate system by natural and anthropogenic forcing agents such as GHGs, aerosols, solar forcing and land use change. The third is uncertainty related to the response of the climate system to the specified forcing agents. [Section 184.108.40.206]
Chapter 12 of the IPCC AR5 WG1 report explains the constraints, limitations and uncertainties in the 21st century projections, which assess climate change in response to the emissions scenarios:
“With regard to solar forcing, the 1985–2005 solar cycle is repeated. Neither projections of future deviations from this solar cycle, nor future volcanic RF [radiative forcing] and their uncertainties are considered.” [Section 12.2.3]
“Any climate projection is subject to sampling uncertainties that arise because of internal variability. [P]rediction of the amplitude or phase of some mode of variability that may be important on long time scales is not addressed.” [Section 12.2.3]
“Simplifications and the interactions between parameterized and resolved processes induce ‘errors’ in models, which can have a leading-order impact on projections. Also, current models may exclude some processes that could turn out to be important for projections (e.g., methane clathrate release) or produce a common error in the representation of a particular process.” [Section 12.2.3]
A key issue is the uncertainty of sensitivity of climate models to CO2. The equilibrium climate sensitivity is a measure of the climate system response to sustained radiative forcingdefined as the amount of warming in response to a doubling of atmospheric CO2 define.
“Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence).” “No best estimate for equilibrium climate sensitivity can now be given because of a lack of agreement on values across assessed lines of evidence and studies.” (AR5 WG1 SPM, page 14)
The average value of equilibrium climate sensitivity for climate models used in the 21st century projections is 3.2oC, with a range 2.08 to 4.67oC. [AR5 WG1 Chapter 12, Box 12.2] The lower part of the climate sensitivity range is not covered by the global climate models. Figure 1 of Box 12.2 in the AR5 WG1 shows that 11 out of 19 observational-based studies of ECS have values below 1.5oC in the range of their ECS probability distributions.
The IPCC AR4 (2007) made the following projection for near term warming:
“For the next two decades, a warming of about 0.2°C per decade is projected.” [IPCC AR4 WG1 SPM, p 12] [Note: for the period 2011-2030]
At the time of the IPCC AR5, this rate of warming had not been realized, and there was slowdown in warming during the period 1998-2012:
“[T]he rate of warming over the past 15 years (1998–2012; 0.05 [–0.05 to +0.15] °C per decade), which begins with a strong El Niño, is smaller than the rate calculated since 1951 (1951–2012; 0.12 [0.08 to 0.14] °C per decade).” [IPCC AR5 SPM, p 5]
Chapter 11 of the AR5 compares the near term climate model temperature projections with observations [Figure 11.25].Figure 11.25 | Synthesis of near-term projections of global mean surface air temperature (GMST). Simulations and projections of annual mean GMST 1986–2050 (anomalies relative to 1986–2005). Projections under all RCPs from CMIP5 models showing the 5 to 95% range of annual mean CMIP5 projections (using one ensemble member per model) for all RCPs using a reference period of 1986–2005 (light grey shade) and all RCPs using a reference period of 2006–2012, together with the observed anomaly for (2006–2012) to (1986–2005) of 0.16°C (dark grey shade). The maximum and minimum values from CMIP5 using all ensemble members and the 1986–2005 reference period are shown by the grey lines (also smoothed). Black lines show annual mean observational estimates. The red hatched region shows the indicative likely range for annual mean GMST during the period 2016–2035 based on the ‘ALL RCPs Assessed’ likely range for the 20-year mean GMST anomaly for 2016–2035
It is seen the observed temperatures between 2007-2012 are at the bottom of the envelope of climate model simulations”
“The CMIP5 5 to 95% ranges for GMST (global mean surface temperature) trends in the period 2012–2035 are 0.11°C to 0.41°C per decade. It may also be compared with recent rates in the observational record (e.g., ~0.26°C per decade for 1984–1998 and ~0.04°C per decade for hiatus period 1998–2012).” [AR5 Section 220.127.116.11]
The AR5 then makes the following projection, based on expert judgment:
“[I]t is likely (>66% probability) that the GMST anomaly for the period 2016–2035, relative to the reference period of 1986–2005 will be in the range 0.3°C to 0.7°C (expert assessment; medium confidence). [This range] is also consistent with the CMIP5 5 to 95% range for all four RCP scenarios of 0.36°C to 0.79°C, using the 2006–2012 reference period, after the upper and lower bounds are reduced by 10% to take into account the evidence that some models may be too sensitive to anthropogenic forcing.” [AR5 WG1 Section 18.104.22.168]
The AR5 concludes:
““However, the implied rates of warming over the period from 1986–2005 to 2016–2035 are lower as a result of the hiatus: 0.10°C to 0.23°C per decade, suggesting the AR4 assessment was near the upper end of current expectations for this specific time interval.” [AR5 WG1 Section 22.214.171.124]
The author of Figure 11.25 – Ed Hawkins of Reading University – provides an annual update of Figure 11.25. The figure below includes the surface temperature data through 2017.
It is seen that the large El Nino of 2016 has returned the observed temperature curve to near the middle of the envelope of climate model simulations; however the previous large El Nino of 1998 was at the top of the envelope of climate model simulations. The recent data since 2012 continues to indicate that the sensitivity of at least some of the climate models to CO2 forcing is too high.
The IPCC makes the following projections for the 21st century global temperatures:
“Increase of global mean surface temperatures for 2081–2100 relative to 1986–2005 is projected to likely be in the ranges derived from the concentration-driven CMIP5 model simulations, that is, 0.3°C to 1.7°C (RCP2.6), 1.1°C to 2.6°C (RCP4.5), 1.4°C to 3.1°C (RCP6.0), 2.6°C to 4.8°C (RCP8.5).” [SR5 AR5 WG1 SPM p 20]
Figure SPM.7 | CMIP5 multi-model simulated time series from 1950 to 2100 for change in global annual mean surface temperature relative to 1986–2005, Time series of projections and a measure of uncertainty (shading) are shown for scenarios RCP2.6 (blue) and RCP8.5 (red).
While the near-term temperature projections were lowered relative to the CMIP5 simulations [AR5 Figure 11.25], a note in the caption of Table SPM.2 states:
“The likely ranges for 2046−2065 do not take into account the possible influence of factors that lead to the assessed range for near-term (2016−2035) global mean surface temperature change that is lower than the 5−95% model range, because the influence of these factors on longer term projections has not been quantified due to insufficient scientific understanding.” (AR5 SPM, Table SPM.2)
Summary: No account is made in these projections of 21st century climate change for the substantial uncertainty in climate sensitivity that is acknowledged by the IPCC. Hence, there is an internal inconsistency in the IPCC AR5 WG1 Report: the AR5 assesses substantial uncertainty in climate sensitivity and lowered their projections for 2016-2035 relative to the climate model projections, whereas the projections out to 2100 that use climate models that do not include the lower values of climate sensitivity that would produce warming that is substantially smaller than the climate model values.
2.2 Sea level rise
The IPCC AR5 projections of sea level rise are indirectly based on the CMIP5 global climate model simulations [AR5, Section 13.5.1]:
- Thermal expansion of the ocean is derived directly from the CMIP5 climate model simulations
- Changes in glacier and surface mass balance are calculated based the projections of the CMIP5 climate models.
- Possible contributions from ice sheet dynamics are assessed from the published literature and are treated as independent of emissions scenario.
- Projections of changes in land-water storage due to human intervention is assessed from the published literature and is treated as independent of emissions scenario
Table AR5 WG1 SPM.2 summarizes the sea level rise projections for 2046-2065 and 2081-2100:
“In all scenarios, thermal expansion is the largest contribution, accounting for about 30–55% of the projections. Glaciers are the next largest. By 2100, 15–55% of the present glacier volume is projected to be eliminated under RCP2.6, and 35–85% under RCP8.5. SMB [surface mass balance] change on the Greenland ice sheet makes a positive contribution, whereas SMB change in Antarctica gives a negative contribution. The positive contribution due to rapid dynamical changes that result in increased ice outflow from both ice sheets together has a likely range of 0.03–0.20 m in RCP8.5 and 0.03–0.19 m in the other RCPs. There is a relatively small positive contribution from human intervention in land-water storage, predominantly due to increasing extraction of groundwater.” [AR5 Section 13.5.1]
The IPCC also provides semi-empirical projections of global sea level rise that are greater than the process-model based projections. However, the IPCC concluded that:
“Many semi-empirical model projections of global mean sea level rise are higher than process-based model projections (up to about twice as large), but there is no consensus in the scientific community about their reliability and there is thus low confidence in their projections”. [AR5 WG1, SPM p 26].
3. Possible worst-case scenarios
It is estimated that fully melting Antarctica would contribute over 60 meters of sea level rise, and Greenland would contribute more than 7 meters, with an additional 1.5 m of sea level rise from glaciers. How much of this is potentially realizable in the 21st century?
Clarifying the possible worst-case scenario for sea level rise in the 21st century is useful in context of risk management approaches. Decision makers would rarely plan for the worst-case scenario (unless you have a lot of spare money to spend on building resilience); rather you might avoid building new major infrastructure (e.g. an airport) in coastal areas that could be impacted by such a worst-case sea level rise.
With regards to the upper bound of potential 21st century sea level rise, the IPCC AR5 states:
“Only the collapse of marine-based sectors of the Antarctic Ice Sheet could cause [global mean sea level] rise substantially above the likely range during the 21st century. Expert estimates of contributions from this source have a wide spread, indicating a lack of consensus on the probability for such a collapse. The potential additional contribution to GMSL rise also cannot be precisely quantified, but there is medium confidence that, if a collapse were initiated, it would not exceed several tenths of a metre during the 21st century” [AR5 WG1, Section 13.5.3].
Since the AR5, there has been increasing interest in describing the tail of the sea level rise distribution and the worst-case scenario, referred to as the H++ range. From a review by Nicholls et al. (2013):
Globally, the upper bound for this global H++ range considers the dynamic effects of the ice sheets. On the basis of an analogue of the last interglacial (about 127,000 to 110,000 years ago) when sea level, climate, and ice masses were broadly similar to today, sea levels are estimated to have risen up to 1.6 ± 0.8 m/century [16 mm/yr] with contributions coming from both the Greenland and Antarctic ice sheets. Using a different methodology, Pfeffer et al. argue that it is physically untenable for the total rise by 2100 to exceed 2.0 m and a scenario that allows for accelerated ice melt due to ice dynamics lies between 0.8 and 2.0m.
LeBars et al. (2017) provides an updated evaluation of the H++ range:
We have constructed a new high-end projection for global sea level rise in 2100 by modifying and extending the AR5 process-based method in three ways. For the RCP8.5 scenario, the PDF obtained has a median of 184 cm and a 95% quantile of 292 cm. Other so called ‘extreme’ or ‘worst’ scenarios that are not probabilistic can be compared with our PDF. The Dutch Delta Committee projects 110 cm that is between the 10% and 20% quantiles of our PDF. The UK H++ scenario of 270 cm and the NOAA scenario of 250 cm fall between the 80% and 95% quantiles of our PDF, so these values are not radically different from our high-end estimate.
Griggs et al. (2017) prepared H++ scenarios for cities along the California coast. This analysis is illuminating because it provides rates of sea level rise corresponding to the H++ values. For Crescent City, CA, Griggs et al derived an H++ value of sea level rise by 2100 to be 9.3 ft (283 cm), a value that is close to the global H++ values cited in the above paragraph. The H++ rates of sea level rise for Crescent City are 23 mm/yr for 2030-2050 and 51 mm/yr for 2080-2100. For reference, recall that the current global rate of sea level rise is about 3 mm/yr.
Are these scenarios of sea level rise by 2100 plausible? Or even possible? Let’s look at the paleo sea level record to provide context for these rates of sea level rise.
From Chapter 5 of the AR5:
“Much of this sea level change occurred in 10,000 to 15,000 years, during the transition from a full glacial period to an interglacial period, at average rates of to 10 to 15 mm/yr. These high rates are sustainable only when the Earth is emerging from periods of extreme glaciation. During the transition of the last glacial maximum about 21,000 years ago to the present interglacial . . . coral reef deposits indicate that global sea level rose abruptly by 14 to 18 m in less than 500 years, in which the rate of sea level rise reached more than 40 mm/yr.” [AR5 WG1 FAQ 5.2]
Rates of sea level rise during the Holocene deglaciation are illustrated in this diagram by Donoghue et al. (2011):
What about Last Interglacial (LIG) period, ca 129-116 ka? IPCC AR5 Chapter 13 states:
For the time interval during the LIG in which GMSL was above present, there is high confidence that the maximum 1000-year average rate of GMSL rise associated with the sea level fluctuation exceeded 2 m kyr–1 but that it did not exceed 7 m kyr–1 . [2 to 7 mm/yr.] Faster rates lasting less than a millennium cannot be ruled out by these data. Therefore, there is high confidence that there were intervals when rates of GMSL rise during the LIG exceeded the 20th century rate of 1.7 [1.5 to 1.9] mm/yr. [AR5 WG1 Section 126.96.36.199]
Rohling et al. (2013) provide a geologic/paleoclimatic perspective on recent and possible future sea level rise (see new section on Rates of Sea Level Rise in Part II), using an empirical Monte Carlo style approach:
We consider a ‘worst case’ outlook from our natural perspective. The upper bound of our 95% probability envelope (i.e., the 97.5th percentile) implies a 2.5% chance of 1.8 m SLR by 2100. However, this trajectory requires that SLR rates develop toward an eventual value of 4.3 m/cy [43 mm/yr] roughly similar to mwp-1a [the onset of the last deglaciation], even though today’s global ice volume is only about a third of that at the onset of the last deglaciation. Most of the extra ice during glacial times existed in North America and northwestern Eurasia. These ice sheets were highly sensitive to climate change, as witnessed by the fact that they existed during ice ages and were almost entirely absent during interglacials. Both the size and sensitivity of these glacial ice masses would have been conducive to high deglacial rates of SLR. Starting from present-day conditions, rates such as those of mwp-1a would require unprecedented ice-loss mechanisms, such as collapse of a major ice sheet (e.g., the largely marine-based West Antarctic Ice Sheet). Alternatively, such rates might develop with a large increase in the amount of ‘vulnerable’ ice, by activation of major EAIS retreat. From the natural perspective, however, the latter only seems to become relevant under extreme GHG forcing, with long-term CO above 1000 ppmv or so. Without invoking such exceptional conditions or catastrophic events, our assessment supports the notion that 2 m of SLR by 2100 represents a useful upper limit.
How should these values of H++ be interpreted?
I am in agreement with Griggs et al. in that the H++ scenario is too uncertain to assign meaningful probabilities and that the H++ scenario should be regarded to have an unknown probability. I find the empirical approach used by Rohling et al. to be more believable than the process based approached typified by LeBars et al., given the uncertainties and known deficiencies of climate models (although I appreciate that LeBars approach avoids expert judgment).
However, even a relatively modest value of H++ = 2 m by 2100 implies unprecedented rates of sea level rise during an interglacial. This would require unprecedented ice-loss mechanisms, such as collapse of the West Antarctic Ice Sheet or activation of major East Antarctic ice sheet retreat. Rohling et al. makes the following argument for a possible very rapid ice sheet adjustment:
Anthropogenic climate forcing is more than an order of magnitude faster than climate forcing or major feedbacks at any known time since the Cenozoic [past 66 million years]. Key climate system components such as deep ocean temperature and ice volume respond slowly due to their large inertia. Ice-volume contributions to future SLR will therefore reflect delayed responses to GHG emissions, developing climate system feedbacks, and future emissions. The large and fast-growing disequilibrium between accelerated cli- mate forcing and slow/lagging response thus creates a strong potential for rapid sea-level adjustments.
JC query: Is the relatively modest anthropogenic radiative forcing since ~1750 (a few Watts per meter squared) really an order of magnitude faster than climate forcing or major feedbacks in past 66 million years? What does the ‘rate’ of forcing mean? Magnitude, or rate of change of the magnitude?
From the AR5 Chapter 5 (paleoclimate), atmospheric CO2 was higher in the Cenozoic prior to 25 mya. In terms of ‘rate of forcing’, surely major volcanic eruptions have had a greater ‘rate of forcing.’
Rohling’s justification for unprecedented SLR rates in the 21st century based on exceptional radiative forcing in the past 66 million years seems . . . unjustified. However, this appears to be a key issue in justifying a scenario of possible unprecedented sea level rise in the 21st century, it deserves further investigation.
4. Geologic wild cards
While on the subject of ‘possible’ future scenarios, we should not ignore potential geologic wild cards.
Sea level changes on Earth cannot be treated as occurring in a rigid ocean basin. Tectonics, dynamic topography, sediment compaction, prograding delta buildup, ocean floor height change sub-marine mass avalanche. and melting ice all trigger variations in the configuration of the basin and ultimately impact sea level. While some of these processes operate at very slow time scales others do not and may have substantial impact on local sea level at least. Surely such changes are as ‘possible’ as a collapse of the West Antarctic ice sheet in the 21st century.
In the more ‘likely’ category of geologic impacts is geothermal heat flux in the vicinity of the Greenland and Antarctic ice sheets. Here are some recent papers that I have spotted:
High geothermal heat flux in proximity to the Northern Greenland ice stream
“The Greenland ice sheet (GIS) is losing mass at an increasing rate due to surface melt and flow acceleration in outlet glaciers. A compilation of heat flux recordings from Greenland show the existence of geothermal heat sources beneath GIS and could explain high glacial ice speed areas such as the Northeast Greenland ice stream.”
A new volcanic province: an inventory of subglacial volcanoes in West Antarctic.
“We identified 138 volcanoes, 91 of which have not previously been identified, and which are widely distributed throughout the deep basins of West Antarctica, but are especially concentrated and orientated along the >3000 km central axis of the West Antarctic Rift System.“
Influence of a West Antarctic mantle plume on ice sheet basal conditions
“The experiments show that mantle plumes have an important local impact on the ice sheet, with basal melting rates reaching several centimeters per year directly above the hotspot.”
Heat Flux Distribution of Antarctica Unveiled
“Of the basic parameters that shape and control ice flow, the most poorly known is geothermal heat flux. We present a high-resolution heat flux map and associated uncertainty derived from spectral analysis of the most advanced continental compilation of airborne magnetic data.”
Evidence for elevated and spatially variable geothermal flux beneath the West Antarctic Ice Sheet. “large areas at the base of Thwaites Glacier are actively melting in response to geothermal flux consistent with rift-associated magma migration and volcanism. This supports the hypothesis that heterogeneous geothermal flux and local magmatic processes could be critical factors in determining the future behavior of the West Antarctic Ice Sheet.”
The first physical evidence of subglacial volcanism under the West Antarctic Ice Sheet. “New evidence from ice core tephra shows that subglacial volcanism can breach the surface of the ice sheet and may pose a great threat to WAIS stability.”
Melting at the base of Greenland ice sheet explained by Iceland hotspot history. “large parts of the north-central Greenland ice sheet are melting from below. It has been argued that basal ice melt is due to the anomalously high geothermal flux1,4 that has also influenced the development of the longest ice stream in Greenland”
JC query: any additional recent papers? I would be most interested to know if any of this is being factored in to the attribution of ice sheet contribution recent sea level rise.
5. Recent projections of global sea level rise
Since the IPCC AR5 was published in 2013, new scenario and probabilistic approaches have been used for 21st century sea level rise projections.
In 2017, the U.S. National Oceanic and Atmospheric Administration (NOAA) published a Technical Report entitled Global and Regional Sea Level Rise Scenarios for the United States.
“In order to bound the set of GMSL rise scenarios for year 2100, we assessed the most up-to-date scientific literature on scientifically supported upper-end GMSL projections, including recent observational and modeling literature related to the potential for rapid ice melt in Greenland and Antarctica. We recommend a revised ‘extreme’ upper-bound scenario for GMSL rise of 2.5 m by the year 2100. [We] revise Parris et al. (2012)’s estimate of the lower bound upward by 0.1 m to 0.3 m by the year 2100.” [NOAA, 2017, p. vi]
Table 4 from the NOAA (2017) Report provides probabilities of the global mean sea level (GMSL) exceeding each sea level rise scenario for each of three emissions scenarios. The closest emissions scenario for the path that global emissions appear to be following is between RCP4.5 and RCP2.6.Scenarios exceeding 1.5 m (4.9 feet) of sea level rise in the 21st century have a probability of less than 1% for RCP4.5.
The California Ocean Protection Council has published a new report (April, 2017) entitled Rising Seas in California by Griggs et al. This report has taken a slightly different approach than NOAA (2017):
- Offers probabilistic sea level rise projections.
- The maximum physically plausible extreme scenario is regarded to have an unknown probability.
The report provides the following probabilistic projections for San Francisco:
Table 1. Projected sea-level rise (measured in feet) Projections are based on the methodology of Kopp et al., 2014 with the exception of the H++ scenario. The ‘likely range’ is consistent with the terms used by the IPCC meaning that it has about a 2-in-3 chance of containing the correct value. All values are with respect to a 1991- 2009 baseline. The H++ scenario is a single scenario, not a probabilistic projection, and does not have an associated distribution in the same sense as the other projections; it is presented in the same column for ease of comparison.
Table 2 shows the rates of sea level rise associated with the above values of sea level rise.
Table 2. Projected average rates (mm/year) of sea-level rise Projections are based on the methodology of Kopp et al., 2014 with the exception of the H++ scenario. For example, there is a 50% probability that sea-level rise rates in San Francisco between 2030-2050 will be at least 3.8 mm/year. The ‘likely-range’ is consistent with the terms used by the IPCC meaning that it has about a 2-in-3 chance of containing the correct value. The H++ scenario is a single scenario, not a probabilistic projection, and does not have an associated distribution in the same sense as the other projections; it is presented in the same column for ease of comparison.
The categories (columns) used in the above tables are very useful for decision making:
- The first column (median) is useful for ‘more likely than not’ assessments of relevance for civil lawsuits, whereby SLR values lower than the median would arguably correspond to ‘more likely than not values.’
- The second column (likely) hews to the IPCC’s apparent rationale for supporting the UNFCCC CO2 emission policies by providing a range that is sufficient to trigger the precautionary principle.
- The fourth column (1-in-200 chance) corresponds to the level of financial risk taking in risk-based capital assessments used in the insurance industry.
The issue with these projections is whether they are credible, based upon our background understanding the myriad and complex processes that determine sea level change and the limitations of climate models. Not to mention concerns about whether the climate model ensemble of opportunity provides an appropriate basis for probabilities.
6. Critical assessment of sea level rise projections
Process-based sea level rise projections for the 21st century are becoming more sophisticated and no longer rely on expert judgment. However, these projections are only as valid as the climate model simulations upon which they are based.
Apart from the uncertainties in the climate models described at the beginning of this essay, there are two overarching problems with these projections:
- The scenarios of future climate are incomplete, focusing only on emissions scenarios
- The opportunistic ensemble of climate model simulations (CMIP5) do not provide the basis for the determination of statistically meaningful probabilities.
Both the IPCC AR5 and the NOAA Report acknowledge the constraints, assumptions, contingencies and uncertainties of their projections of sea level rise.
The climate model projections of 21st century surface temperature and sea level rise are contingent on the following assumptions [AR5 WG1]:
- Emissions follow the specified concentration pathways (RCP). Tone down these scenarios; RCP8.5 is completely unrealistic, we appear to on a trajectory somewhere between RCP4.5 and RCP2.6 [12.2.3]
- Climate models accurately predict amount of warming in 21st century. There is evidence that climate models are too sensitive to CO2 and produce too much warming. [Box 12.2]
- The projections assume that solar variability follows that of the late 20th century, which coincided with a Grand Solar Maximum. [AR5, Section 188.8.131.52] Some Russian and German scientists are predicting a Grand Solar Minimum in the mid 21st century.
- The projections assume that natural internal variability of ocean circulations doesn’t impact temperature or sea level rise on these timescales. [Section 12.2.3, 3.6]
- The projections ignore volcanic activity, which overall has a cooling effect on the climate. IPCC quote[Section 12.2.3, 8.4.2]
Each of these contingent assumptions, with the possible exception of natural internal variability, most likely contribute to a warm bias in the 21st century projections.
Rather than focusing on sensitivity to emissions scenarios, just focus on RCP2.6 and RCP4.6. Additional scenarios that should be considered for the 21st century (individually or in combination):
- Scenario of volcanic eruptions matching the 19th century eruptions
- Grand solar minimum in the mid 21st century
- Transition to the cold phase of the Atlantic Multidecadal Oscillation
- Transient sensitivity to CO2 of 1.3C (or a range from 1.0 to 1.9C)
Apart from volcanic eruptions, climate models don’t handle these very well (notably the solar indirect effects and phasing and cloud feedbacks associated with the AMO); hence semi-empirical approaches would be needed in generating these scenarios. While some back of the envelope scaling factors can be estimated for these, if anyone were to take on development of these alternative scenarios, it would be a nontrivial effort.
So, what are we left with in estimating the sea level rise mid 21st century and at the end of the century? The best options seem to me:
- Extrapolate the current trend of 3 mm/yr to mid century. The current rate has a bump from Greenland melting that is coincident with and likely associated with the warm phase of the AMO (see Part V). A transition to the cool phase of the AMO may occur sometime within the next few decades, which would slow the mass loss from Greenland, which would be supplemented by perhaps more rapid thermosteric component, maintaining the current rate of sea level rise to mid century.
- Use the RCP2.6 values from the IPCC/NOAA/California assessments. Even if we are not quite on the RCP2.6 path for emissions, this could be countered by lower sensitivity to CO2, so there are two paths to the sea level rise predicted by the RCP2.6 scenario.
- The other sea level rise scenarios presented by IPCC/NOAA/California are possible (whether their H++ scenarios above 2m are possible is debatable), but probabilities cannot be meaningfully provided for these scenarios.
The bottom line is that the sea level rise will continue to rise in the 21st century, probably at a rate more than 8 inches observed in the 2oth century. And there will be substantial regional and local variations in the rate of sea level rise. Reducing emissions will have little effect on sea level rise in the 21st century even if you believe the climate models; compare the difference in sea level rise for the RCP2.6 versus RCP4.5 scenarios.
And what about the wildcard events, such as collapse of the West Antarctic Ice Sheet or some major geological event? They are wildcards; the West Antarctic Ice Sheet is much more likely to collapse in the 21st century from a geological event than it is from greenhouse gas emissions.
The NOAA Report sums it up this way:
As is discussed in detail in this report, scientists expect that GMSL will continue to rise throughout the 21st century and beyond, because of global warming that has already occurred and warming that is yet to occur due to the still-uncertain level of future emissions. GMSL rise is a certain impact of climate change; the questions are when, and how much, rather than if. There is also a long-term commitment (persistent trend); even if society sharply reduces emissions in the coming decades, sea level will most likely continue to rise for centuries.
We need to figure out ways to systematically adapt to systematic sea level rise.