What is ocean acidification? (a really brief summary)

Again, as part of my teaching assistant job, I’m going to try to do a little overview of ocean acidification (OA).

Nuts and bolts

Yes, I’m going to talk about chemistry.  Wait!  Don’t run away!  It won’t be too scary at all.  Let’s start with the sea surface, where CO2 is exchanged between the atmosphere and the ocean.  It’s constantly trying to reach equilibrium which means that if that if CO2 levels in the atmosphere are high, then CO2 gets “pulled” into the ocean.

When CO2 dissolves into the sea water, it can take on one of four forms:  dissolved (aqueous) carbon dioxide (CO2 (aq)), carbonic acid (H2CO3), bicarbonate (HCO3), and carbonate (CO32−).

carbon species in water

What determines the relative concentrations of these different forms of carbon, you might ask?  Turns out it is strongly dependent on the temperature and the alkalinity of the water.  Have a look at this diagram, also known as a “Bjerrum plot”:

An aside: You may be wondering why carbonic acid doesn’t show up on the Bjerrum plot. That’s because it occurs in extremely low concentrations and is frequently lumped in with the CO2.

Looks kinda wacky.  Let’s go through it a step at a time.  The x-axis is indicating the pH, which is a measure of how acidic (or basic) the solution is.  In other words, it’s a measure of the concentration of hydrogen ions (H^+) hanging around in the water (that’s what the H in pH stands for). It’s worth noting here that pH is, by definition, a logarithmic scale.  If you move down one number on the pH scale, you’re decreasing by a factor of 10.  If you move up one number, you’re increasing by a factor of 10.  In case you’re curious, here’s the equation:

pH = -log([H^+])

So: the pH is the negative log of the hydrogen ion concentration, [H+]. A pH of 7 is neutral (fresh stream water would have a pH of about 7).  If it’s higher, it’s basic, and if it’s lower it’s acidic.

I know I said I wouldn’t use too much chemistry, but for those who might be interested, here’s what’s going on:

CO2 (aq) + H2O \leftrightarrow H2CO3 \leftrightarrow HCO3 + H+ \leftrightarrow CO32− + 2 H+.

As more CO2 is dissolved into the ocean, more hydrogen ions are released, causing that vertical blue line in the Bjerrum plot to move to the left. More hydrogen ions means decreased pH, aka increased acidity. Which is what people are talking about when they say “ocean acidification”.  And when that happens, the equilibrium ratios of the different chemical species of carbon shifts too – carbon dioxide increases, bicarbonate doesn’t change much, and carbonate decreases fairly quickly with even small changes in pH.

Magnitude of OA

How much has the pH of the oceans changed since the industrial revolution?  It has decreased by approximately 0.1 pH units.  Maybe it sounds small, but remember that pH is a logarithmic scale – so it’s actually a 30% increase in hydrogen ion concentration.

Impacts of ocean acidification

Calcium carbonate shell dissolution

I’ll start off with this:  it’s complicated.  And no one can really say exactly what is going to happen.  But we are able to ascertain certain things.  There have been, and continue to be, many studies focusing on exactly what the impacts of OA will be.  This is often done by taking certain organisms that are believed to be susceptible to acidification, and conducting controlled experiments where they are exposed to increased acidity.

The animals that are likely to be affected are those with shells or plates made of calcium carbonate (CaCO3).  These shells and plates are formed when dissolved ions in seawater precipitate to form CaCO3. In order for those shells to remain intact (ie. not dissolve!) the surrounding seawater needs to be saturated with respect to CO32−.

carbonate saturation

Detrimental effects have been shown for many organisms, for example: shellfish, foraminifera, coccolithophores, pteropods (e.g. [1],[2],[3]). It’s well worth noting that in certain cases, and for certain species, studies have shown surprising effects in the opposite direction (e.g. [4],[5]).  Like I said:  It’s complicated!

There’s a lot more you can learn about ocean acidification, but it’s more than I want to cover here.  As always, if any of my (7 or so) readers finds errors in this or other posts: let me know so I can fix them!


[1] Orr, James C., et al. “Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms.” Nature 437.7059 (2005): 681-686.
[2] Beaufort, L., et al. “Sensitivity of coccolithophores to carbonate chemistry and ocean acidification.” Nature 476.7358 (2011): 80-83.
[3] Kuffner, Ilsa B., et al. “Decreased abundance of crustose coralline algae due to ocean acidification.” Nature Geoscience 1.2 (2007): 114-117.
[4] Ocean Acidification: A risky shell game (WHOI Oceanus) https://www.whoi.edu/oceanus/viewArticle.do?id=52990
[5] Range, P., et al. “Seawater acidification by CO< sub> 2 in a coastal lagoon environment: Effects on life history traits of juvenile mussels< i> Mytilus galloprovincialis.” Journal of Experimental Marine Biology and Ecology 424 (2012): 89-98.

T.A. time, version 2

In our department, one of the requirements for graduate students is that they TA for at least one quarter.  I’ve done my one quarter, and since then I’ve been hoping for a chance to give it another shot.  Not only is it fun, but getting a wide variety of experiences in grad school can really help down the road, both in getting a job, and in doing well at that job.

As of this quarter, I’m TA-ing again.  My favorite part? I get to help out with some of the online course stuff – which means I get to geek out on the background technical and web stuff, and (best of all) I get to make drawings to go with lecture topics! The idea is to try to make the class fun and interesting for first-year, non-science majors who might just be taking this course as a filler.

[The course is already offered as a regular lecture-based class, and the website for it is here: https://catalyst.uw.edu/workspace/mrasmuss/18105/105205. ]

I have to admit, I’m a bit nervous about it.  So I’ve been breaking out the old paper and pen (or, um, the tablet and stylus…) to try to get back into the swing of things.  Anyone who knows me knows that it’s been absolutely forever since I made a cartoon.  Here’s my first stab at it.  It’s a bit random and isn’t particularly related to any one lecture topic.  It actually stems from a conversation I had with Dax a little while ago… (gotta start somewhere)

correlation vs causation

COSEE Broader Impacts workshop

At our advisor’s suggestion, our lab group attended the COSEE Broader Impacts workshop yesterday.  I’m glad I went – it was really interesting.  Reviewers at NSF or NOAA or Sea Grant consider a couple of main things when examining a proposal.  Well, obviously the intellectual merit of the proposal is of the utmost importance, but it’s not also important to demonstrate the broader impacts of the proposed research as well.  There were a variety of different sessions in the workshop covering topics like teaching and outreach, communicating with policy-makers, the use of visual data, social media tools, and diversity in science and research.  (and more!)

I’m really interested in teaching and outreach (although I have almost no experience in either, if we’re being honest…).  Philip Bell, from the UW College of Education, and also Directory of the Institute for Science and Math Education, gave an interesting talk – a lot about policy and standards stuff that I was unfamiliar with, but he also highlighted the vast amount of literature available on science education that is available for free on the National Academies Press website.  Very cool!  I can’t wait to download some onto my iPad.  He passed several of the books around the room during his talk, and I thought there were a lot of interesting things that I’d want to delve into more deeply.  There are consensus studies that compile the results of hundreds of studies, and also practitioner’s guides.

Here’s a one of the practitioners guides that got passed around:

I’m curious about what kids are learning, how science is taught at K-12 levels.  Apparently new studies show that young children (even kindergarten age) have far more advanced reasoning skills than what was previously thought.

One other talk that I found to be really interesting and useful was the one given by Rick Keil that was specifically about Broader Impacts, and on recognizing broader impacts that your lab group is already doing.  It was surprising to see it like that, there were several things that I hadn’t considered before.   Here is Rick’s overview on the COSEE website.

Puget Sound forearc basin

I had another good question from my Ocean 200 kids on Monday that I had to check for them.  It was a question about the relationship between the glacial and tectonic processes in the Puget Sound area.  Here’s what I gave them:

Around 20,000 years ago there was a glacier between the Cascades and the Olympics. It advanced and retreated periodically for a long time, before finally leaving for good about 13,000 years ago. Puget sound is actually in the forearc basin of the Cascadia subduction zone (not the backarc basin). The glacier did not cause this depression, rather, it filled the basin that was already there (ie. filling in a topographic low).

As it sat there, it pushed down all the land in the area, resulting in a relative rise in sea level (relative to the subsiding land). When the glacier retreated, the land slowly started to rise up again to reach isostatic equilibrium. This is called post-glacial rebound. It’s still happening today, but very, very slowly.

I hope this helps, and doesn’t have any outrageous flaws in it.  Also:  the sketch was done entirely on the iPad 🙂

Forearc basins and Greek yogurt

On Thursday we’ll have a full lecture on strongly non-linear inverse problems.  I’m just starting to warm up to the weakly non-linear inverse problems.  One big local minimum is good enough for me!  It was a tough class.  We went through our entire homework set from about a month ago where we did a series of linear algebra proofs.  It was tedious, but good to get an explanation of the underlying implications.  And I guess it was kind of neat to show that the solution found using singular value decomposition is identical to the solution found using eigenvalue decomposition.  🙂

Whales and mid-terms

I love my Ocean 200 quiz section.  It’s a small section, but they are all smarty pants and they keep me on my toes.  I got to teach them all about tides today – that part went well.  I love talking about tides, and they were into it.  At the end of class I handed back the mid-terms.  Turns out, I can’t really answer biology-type questions.  (sigh).

Mixed semi-diurnal tides (and wine)

Finally – tomorrow I get to talk about something unrelated to biology in my quiz section 🙂  Tides!  My favorite!  Okay, I’m kind of getting to like the zooplankton, but really, I’d much rather teach about the moon and the sun and the Bay of Fundy.  And glaciers carving out Puget Sound.

Tides, wine, and my fave royal.

What the bjerrum??

Drawing-time study break!

...but I am le tired.

So I tried to do this homework problem where we calculate the horizontal forces contributing to ridge push.  Just a simple little calculation based on the differences in pressure calculated at some compensation depth beneath the ridge and at some distance away from the ridge.  No problem.  Imagine my surprise when I discovered that instead of the plates being pushed apart at the ridge, they are in fact being pushed together.  Which breaks several laws of physics, and doesn’t match observations either.  Brilliant!

Now, back to practicing my presentation.  Look out fin whales!  You will be counted!  (To some statistically quantified uncertainty level!)