Sea Level Change

Full Briefing available in PDF format.

Preface

Good afternoon. What I would like to discuss during the next 45 minutes or so is the question of sea-level change. I’ve altered the title of my talk from Sea-Level Rise to Sea Level Change, partly because the IPCC changed from referring to the phenomenon as sea-level rise in its 1990 report to sea-level change in its 1995 report. The main reason, however, is that we are concerned here both in terms of rise in the recent past, but over the long term we have to think about the fact that sea level has both risen and fallen.

I’m going to try to approach this today by looking at five questions.

First, what is sea level, how does one define it? I will try to show that in some ways “sea level” is a misnomer. The sea is not level. Secondly, I will briefly discuss how we measure sea level, and talk a bit about some of the data issues involved. Then I’ll address the primary question. What do the facts say: is sea level rising, and, is that sea level rise accelerating, with the rate of change increasing through time? Then I’ll shift gears again, and discuss whether or not we can explain the sea level rise that has been observed during the last 100 years or so in the instrumental record. Finally, based on that assessment, I will examine what we can look at in terms of future projections and predictions.

I mentioned that “sea level” is in some ways a misnomer This slide [Figure 1] from the TOPEX/Poseidon satellite illustrates that point. TOPEX/Poseidon uses radar altimetry to measure sea surface height in real time. This satellite has some uncertainties related to its ability to resolve sea-level height, but a lot of the problems have been worked out, and it allows you, on a global scale, to measure the level of the sea surface. Across the bottom of the graph is a legend showing the height of the sea level in centimeters, around an average of zero. The warmer colors —the reds and the oranges—indicate high sea levels; bluer colors show lower sea level. You can see that at any one time—this is data from August 1995—sea level on a global basis can differ by as many as 30 centimeters.

—Marlo Lewis Vice President for Policy and Coalitions