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21st Century Sea-Level Rise: More Than IPCC 4AR Projected

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New research was published a few months ago that provides additional evidence that sea-level projections made by the IPCC’s 4th Assessment Report are likely too conservative: sea-levels are more likely to be 1 meter higher than they were in 1990 (Vermeer and Rahmstorf), rather than only 0.5m higher, as projected by the IPCCs multi-model ensembles.

There was nothing inherently bad about the IPCC’s 4AR; I and others simply feel that their final report had to include more conservative estimates and projections in order for world governments to sign off on its language.  That does the world’s citizens an injustice, however.  In order to correctly assess risk, people need best- and worst-case scenarios available to them.  The most likely amount of sea-level rise by 2100 provided by the 4AR came out to between 0.2m and 0.6m.  Those estimates have implications to world societies, conservative though they were.  Additional implications will enter into our lives if there is 0.4-0.8m additional rise.

I want to stay on the IPCC projections for another moment.  The 2007 estimates included rates of sea level rise between 1980 to 1999 and 2090 to 2099 in metes and mm/yr.  The mm/yr rates in particular interest me because they allow for both the IPCC projections and the updated projections from the Vermeer and Rahmstorf paper to be placed in context with actual observations.  The six emissions scenarios examined by the IPCC had rates of 1.5, 2.1, 2.1, 1.7, 3.0 and 3.0mm/yr.  Satellite observations indicate that there has been approximately a +3.2mm/yr change in sea level (linear fit since 1993).  Only two IPCC emissions scenarios are close to the observed rate, and both of them underestimate them, albeit very slightly.  It is worth pointing out that the IPCC wrote:

The global average sea level rose at an average rate of 1.8 [1.3 to 2.3] mm per year over 1961-2003.

Averages over longer periods of time like this will, by nature of the averaging, tend to reduce extreme values that are small in number, as in the case of sea-level rise in the past 10 years or so.

Onto the new research findings.  Vermeer and Rahmstorf developed and tested an updated methodology to project future sea levels based on projected changes in temperature that was originally presented by Rahmstorf in a separate paper.  The original technique was based on the assumption that the sea level response time scale was long compared to the time scale of interest.  The updated technique allows for some sea level components to change quickly to a given temperature change.  The updated technique is shown to agree very well with historical data (82% of sea-level rate variance from the year 1000 to 2000).

Applying the technique to future conditions provides another potential case against which real-world observations can be compared.  By the year 2100, three different IPCC emissions scenarios generate a range of sea level projections: 1.0m, 1.2m and 1.4m, as the figure below (from the paper) shows.  That’s a big difference between the AR4 projections, using the same emissions scenarios, of 0.2-0.4m.  That extra potential meter of sea level rise will indeed have large implications across the world.

The figure shows the possible range of sea level rise values for 3 emissions scenarios considered by the IPCC: B1 in green, A2 in blue and A1F1 in yellow.  The observed emissions to date is represented by the red curve.  One important detail to note is our actual emissions rate is currently at the high end of all those considered by the IPCC (Copenhagen Diagnosis Figure 1, after Le Quere et al. 2009).

The IPCC projected a higher future rate of sea-level rise than was observed from 1961-2003.  1.8mm/yr equals 0.18m after a century (by linear extrapolation), slightly below the 0.2 minimum projected by all emissions scenarios.  Recent observations of 3.2mm/yr equals 0.32m after a century – well within the IPCC range, but well below the Vermeer and Rahmstorf range.  So what will it take to get 1.0-1.4m of sea level rise by 2100?  10mm-14mm/yr or 3-4X as much per year as is currently being observed.  There are some important details involved with that projection.  First of all, sea level change is not linear.  It varies year to year and decade to decade.  There has to be a transition from today’s 3.2mm/yr to the 10mm/yr necessary to achieve 1m sea level rise by 2100.  The rate of sea level rise would therefore have to increase over time.

Given the state of today’s atmosphere, oceans and cryosphere, a drastic change would have to occur for sea levels to rise by 10mm+ per year by the end of this century.  It is widely known that the IPCC’s science basis did not include a number of processes and feedbacks to the globes’ continental ice sheets, glaciers and sea ice (cryosphere).  Again, that wasn’t their fault – it just happens to be a weak link in the climate community’s research.  Work has been conducted since the IPCC 4AR to rectify those shortcomings.  Much more work will have to be done in the future.  Once that area is fleshed out further, I expect the IPCC’s projections to be more closely aligned with the leading research of today.

h/t RealClimate

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