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New research quantifies what's causing sea level to rise

Posted on 30 October 2014 by John Abraham

There have been a number of studies that have come out recently on ocean warming and sea-level rise. Collectively, they are helping scientists coalesce around an emerging understanding of climate change and its impact on the Earth. Most recently, a study by scientists Sarah Purkey, Gregory Johnson, and Don Chambers was published. This team was responsible for a 2010 paper that was groundbreaking in that it quantified very deep (abyssal) sea warming. This latest paper is, in some respects, a continuation of that work.

The researchers recognized that changes to the sea levels are mainly caused by thermal expansion of ocean waters as they heat, changes to the saltiness of water, and by an increase in ocean waters as ice melts and flows into the sea. The total annual sea level rise is about 3 mm per year – the question is, how much of that is from expansion and how much is from melting?

Greg Johnson Greg Johnson

The researchers used a few tools to answer this question. One tool was ocean bottom pressure measurements. If you can measure changes to ocean pressure, you can deduce how much water is in the ocean. Another tool is through an inventory approach. This inventory method quantifies how much glaciers retreat, polar ice melts, and changes to water storage on land. The paper reports that both methods agree with each other. They conclude that increased water in the oceans is causing about 1.5–1.8 mm per year of sea level rise. The actual value depends, in part, on which years are under consideration.

The authors don’t just consider the ocean as a whole. They break the ocean regions into seven different sections. The reason for this subdivision is that the change to ocean levels is not uniform. In some reasons, waters are rising quickly, in other regions, the rise is much slower or zero. One region for regional variability is that the Earth’s gravity is changing.

For instance, there is so much ice in Greenland and Antarctica that is melting and flowing into the ocean, the mass of these two regions is being reduced; therefore, the pull of gravity toward Greenland and Antarctica is changing. As a result, we expect water levels near Greenland and Antarctica may actually fall as those ice sheets melt.

But, ocean levels elsewhere, particularly the USA coastlines, will rise more than average because of this same effect. I have a paper in press with Ted Scambos on this very topic that should be published in a few weeks. Another reason sea level rise isn’t uniform is that there are local changes to heat and salt which can increase or decrease water density in certain regions, causing local changes to sea level. A third reason is that changes to wind patterns can slosh water around, causing it to build up in one area, fall in another.

In each of the seven ocean regions, the researchers collected temperature and salt measurements at carefully distributed sections. These measurements allowed them to calculate how much of the ocean rise is due to heat/salt effects. They compared the expected sea level rise to actual satellite measurements. The difference between expansion sea level rise and actual sea level rise is the contribution by melt water which flowed into the ocean. This method they call the residual measurement.

Then, the collected measurements from special satellites (GRACE) which measure local fluctuations in ocean mass. They compared the GRACE results with the residual measurement. It turns out they were in near perfect agreement; 1.5 mm per year of sea level rise is from added mass to the oceans. The rest is from expansion. Not only did the two methods agree, but they agreed region by region. They showed, for instance, that the South Atlantic and the South Indian/Atlantic Oceans are rising very rapidly. The North Pacific, South Pacific, and Indian Oceans are rising modestly. The southern Pacific is falling modestly and the North Atlantic is basically constant.

Next, they calculated the relative sea level rise for waters from the surface down to different depths (300, 1000, 2000, 3000, 4000, and 6000 m) to determine which layers make the largest contributions to sea level rise. The authors report that the deeper we go into the ocean, the less heating has occurred (this is expected and well known). Interestingly, they find that every water layer, even the deepest waters, have contributed some to sea level rise. They also report that the sea level rise contribution from the layers 300-2000 meters is much more than previously reported.

Dr. Johnson summarized their results,

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Comments 1 to 3:

  1. The title of this study by Perky et al. is "Relative contributions of ocean mass and deep steric changes to sea level rise between 1993 and 2013." The two methods used to calculate the SLR due to mass increase give the same results but for different (overlapping) periods. The Abstract suggests identical results stating:-. "The global mean trend of ocean mass addition is 1.5 (±0.4) mm yr-1 for 1996–2006 from the residual method and the same for 2003–2013 from the GRACE method." And the post says this agreement extends to regional measurements.

    Yet there has surely been an acceleration in global ice loss. In recent years GRACE is showing something like 750Gt ice loss per year from Greenland, Antarctica & other glaciers which would equal  2mm yr-1 So I'm not sure it is entirely correct to say the two methods yield a complete agreement given we should perhaps be expecting a difference for the two periods as ice loss has accelerated.

    Of course, the other ingredient in changing ocean mass is changes in land water storage (recently put at +0.77mm yr-1 SLR by Pokhrel et al (2012) averaged  for 1961-2003) which might be recently providing a compensating deceleration.

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  2. Needs proofreading— "reason" & "region" are swapped in a couple of instances...

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  3. Good post, John

    There is a third possible contribution to sea level rise: the increase in human extraction of groundwater, which then makes its way to the oceans.

    Do the authors mention this at all?

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