Sea Level History Lesson
We are sure you’ve heard that sea level is rising? We conducted a web search on “Global Warming and Sea Level” and nearly 3.5 million websites are immediately located. And before you conduct the search yourself, you already know what you will find. The earth is getting warmer due to the buildup of greenhouse gases, the warmer sea water expands causing sea level to rise, and most of all, you will read all about the ice melting throughout the world pouring fresh water into ocean basins causing sea level to rise far more. Alarmists insist that the worst is just around the corner, and the sea level rise will accelerate or even quickly jump to a new level given some catastrophic collapse of large sheets of ice near the fringes of the polar areas. Coastlines will be inundated, the human misery will be on a Biblical scale, ecosystems will be destroyed … this goes on for millions of websites!
But things aren’t really so simple.
The United Nations’ IPCC group presents the graph below (Figure 1) regarding eustatic (or global) sea level over the past 125 years, and as noted by the IPCC and by many others, the rate of rise is definitely higher in the most recent 50 years than the first 50 years of the record. So, it becomes quite possible to suggest that sea level rise is accelerating, and may continue to accelerate in the future. Alarmists can certainly find material in the IPCC document to bolster their claim that sea level is not only rising, but the rate of the rise is increasing.
Figure 1. Annual averages of the global mean sea level based on reconstructed sea level fields since 1870 (red), tide gauge measurements since 1950 (blue) and satellite altimetry since 1992 (black). Units are in mm relative to the average for 1961 to 1990. Error bars are 90% confidence intervals. (figure source IPCC)
Back in August of 2008, scientists from all over the world attended a workshop entitled “Empirical Constraints on Future Sea-Level Rise” and they just published a summary of their findings in the Journal of Quaternary Science. Somewhere along the way, they decided to refer to the group as “PALSEA” for PALeo SEA level working group.
The PALSEA group begins their article noting:
The eustatic sea-level (ESL) rise predicted for the 21st century represents one of the greatest potential threats from climate change, yet its magnitude remains a subject of considerable debate, with worst-case scenarios varying between 0.59m and 1.4m. In general, the basis for this debate revolves around the uncertainties in the dynamical behaviour of ice sheets (such as loss of buttressing through ice shelf break-up or enhanced ice flow through water lubrication of the ice sheet base), which may lead to a nonlinear sea-level response to climate change.
Note that the authors are talking about worst-case scenarios leading to “0.59m and 1.4m”; if the trend of the past 50 years continues (from Figure 1), sea level will rise around 0.20 meters (around 8 inches) by 2100. The PALSEA team notes that measuring sea level can be tricky “Because changes in ice mass will also cause changes in regional (due to gravitational and rotational feedbacks) and global (due to volume) sea level, the changes in sea level at a particular coastline record the difference between vertical motions of the land and sea, commonly referred to as relative sea-level (RSL) changes. Such isostatic effects are a function of the distance from the large ice sheets.”
Now for the good stuff! The PALSEA team states that
Given a broad range of emission scenarios the IPCC AR4 predicted global warming of between 1.18C and 6.48C during the 21st century. The last time that a global warming of comparable magnitude occurred was during the termination of the last glacial period (TI).
Furthermore, they write
Given this evidence for periods of rapid warming during TI, at least some of this warming occurred on decadal to centennial timescales. Because of the general similarity between the magnitude and rate of warming predicted for the 21st century and the warming that occurred during certain periods of TI, it is interesting to consider rates of sea-level rise during TI as a case study of the response of sea level to climate change.
The PALSEA group presents the graphic below (Figure 2) showing three different rates of sea level rise following an increase in temperature. As seen there, sea level could rise exponentially (as suggested by many climate change alarmists), it could rise linearly, or it could rise and then level off (the “asymptoting” curve).
Figure 2. An illustrative sketch of three models (black) for the time-dependent response of sea level to a perturbation in temperature (red) (from PALSEA, 2010).
Here’s what they conclude:
Therefore, we suggest that option 1 (exponential sea-level rise) is extremely unlikely. …An exponential increase in rates of sea-level rise with respect to temperature would result in 21st-century sea-level rise an order of magnitude larger than estimates using alternative patterns of response – it is an important result that the palaeo-sea-level data rule out such a response.
Finally, they write “the palaeo sea-level data suggests that sea-level rise related to current warming may be rapid at first and slow over time.”
Basically, their analysis of what happened in the past favors the “asymptoting” curve that is quite different from the exponential curve favored by those proclaiming the worst is yet to come! Mother Nature showed us in the past how sea level responds to warming – we at World Climate Report are listening!
PALSEA (the PALeo SEA level working group: Abe-Ouchi, A., Andersen, M., Antonioli, F., Bamber, J., Bard, E., Clark, J., Clark, P., Deschamps, P., Dutton, A., Elliot, M., Gallup, C., Gomez, N., Gregory, J., Huybers, P., Kawamura, K., Kelly, M., Lambeck, K., Lowell, T., Mitrovica, J., Otto-Bleisner, B., Richards, D., Siddall, M., Stanford, J., Stirling, C., Stocker, T., Thomas, A., Thompson, W., Torbjorn, T., Vazquez Riveiros, N., Waelbroeck, C., Yokoyama, Y. and Yu, S.) 2009. The sea-level conundrum: case studies from palaeo-archives. Journal of Quaternary Science, 25, 19-25.