The mystery of the shifting tropical rain belt

from (credit: Pacific Worlds)

This week’s science+comics interview is brought to you by Yen-Ting Hwang, a UW Atmospheric Sciences student in the final year of her PhD. Ting’s work is focused on large-scale climate dynamics.

The Sahel region in Africa is a semi-arid boundary along the southern edge of the Sahara desert. The people living in this region are balanced on the very edge of habitable terrain, and rely on a short annual rainy season for their crops to grow. The slightest shift in annual climate can result in devastating crop failures and ultimately widespread famine. During the second half of the twentieth century, the region was hit by year after year of drought.

For her final thesis chapter, Ting has tried to understand why that drought might have happened when it did. The life-sustaining annual rainfall in the region occurs as part of a seasonally migrating tropical rainfall pattern. Scientists call this the ITCZ, or the inter-tropical convergence zone, and it’s a rain band that circles the globe like a belt. Solar radiation near the equator is far stronger than at the poles, and when the moist air near the equator is heated, it rises. The rising air eventually reaches an altitude where it’s cooled, and the moisture that was carried aloft condenses to form clouds and rain.

This air is transported poleward at high altitude until it reaches about 30 degrees latitude either north or south, at which point it sinks down toward the earth’s surface. The air then moves back toward the equator, picking up moisture along the way. This cycle is called Hadley Cell circulation.


By piecing together climate observations, scientists know that the drought in the Sahel region between about 1950-1990 was related to a very slight southward shift of the tropical rain belt. This type of shift happens when the temperature difference between the northern and southern hemispheres changes. If the northern hemisphere is cooler, the northern Hadley cell will strengthen as it tries to pull warm air up from the south, and the tropical rainfall band will shift southward.

To answer this question, Ting compared results from twenty different IPCC models (IPCC = Intergovernmental Panel on Climate Change). She looked at a variety of different possible factors that could result in an uneven heating or cooling of the planet between the northern and southern hemispheres – things like clouds and ocean circulation. The factor that showed the strongest correlation was surprising: it was the aerosols!

Aerosols are basically tiny particles that are suspended in the air. Naturally occurring aerosols are always floating around our atmosphere – dust from deserts, smoke from forest fires, and even sea salt. But there are also anthropogenic aerosols – the ones that occur as a result of pollution. There are different types, but Ting found that the strong correlation was with sulfate aerosols, which are white in color. These aerosols have two main effects:

a) They reflect sunlight, which means less sunlight reaches the earth’s surface.
b) Aerosols in the air tend to cause clouds to persist for longer than they would other wise – you might imagine droplets of water staying aloft when they have a handy bit of sulfate to stick to.

Overall, both of these lead to a cooling effect.

Aerosols don’t stick around in the air for long though, and they don’t travel far from their source. It’s a bit counterintuitive to imagine that the smog from an American or European factory could be affecting tropical precipitation. But the slightest shift in the temperature balance between the northern and southern hemispheres is all it takes. Here’s a cartoon showing the basics of how this works:


While the Sahel region was being decimated by drought in the twentieth century, industrialization in developed countries was on the rise.   Since most of the industrialization happening at the time was in the northern hemisphere, that hemisphere was cooler than it otherwise would have been.  And model results indicate that this change was enough to shift the tropical rain band southward.  By the 1980s and 1990s, air pollution was widely recognized as being harmful, and various environmental regulations were put into place.  And, lo and behold: that’s about the same time that the ITCZ shifted back to its previous position.

More recently, Ting has been looking at how we can use climate models to predict how the tropical rainfall band might shift or change given different climate scenarios that are tested by IPCC models.  Of course, the effect of aerosols is only one piece of the puzzle.  The northern hemisphere cooling that Ting was investigating was overlaid on an overall warming trend.  The northern hemisphere was actually warming between 1950-1990, just not as quickly as the southern hemisphere.  In fact, recent studies indicate that the northern hemisphere appears to be warming more quickly, which may have significant consequences for tropical precipitation patterns.

To learn more about Ting and her lab group, check out these links:

Press releases about some of their other work:

Also, this paper is in press, so you can read all the details soon!

Hwang, Y.-T., Frierson, D. M. W., and S. M. Kang. Anthropogenic sulfate aerosol and the southward shift of tropical precipitation in the late 20th century. in press, Geophysical Research Letters

Reading and writing and super-mussels

Don’t tell me how to interpret your results!

super musselsThe other day, I stumbled on a climate change skeptic website while doing research for my ocean acidification post.  This ‘skeptic’ article included a summary of a peer reviewed article by a prominent scientist on the effect of an acidic ocean environment on mussels near deep sea hydrothermal vents.  The summary included this description of the results of that paper:

[…] there is ample reason to believe that even the worst case atmospheric CO2-induced acidification scenario that can possibly be conceived would not prove a major detriment to most calcifying sea life. Consequently, what will likely happen in the real world should be no problem at all […]

Well, that’s amazing!  If that was the only summary I read, I might be led to believe a few things:

  1. There is rock-solid evidence indicating that ocean acidification, though it may be happening, is totally not a big deal.  Dude, look at those mussels!  They are doing great!
  2. The mussels adapted to environments even more acidic than the most dire projections by acidification “alarmists”, and
  3. the scientist who did that work is well-known and well-respected, and is clearly a climate change skeptic also.

I was curious, so I looked up the original paper to learn more [1]. This is the first sentence of the abstract by Tunnicliffe et al. (2009):

Increasing atmospheric carbon dioxide levels are causing ocean acidification, compromising the ability of some marine organisms to build and maintain support structures as the equilibrium state of inorganic carbon moves away from calcium carbonate.

Seems pretty reasonable to me.  Carbon dioxide is increasing, the ocean is becoming more acidic, and marine organisms are having a hard time because of it.  So I read on:

Few marine organisms tolerate conditions where ocean pH falls significantly below today’s value of about 8.1 and aragonite and calcite saturation values below 1.

So far they don’t seem very “skeptical” of ocean acidification.  Reading further:

We identify four-decade-old mussels, but suggest that the mussels can survive for so long only if their protective shell covering remains intact: crabs that could expose the underlying calcium carbonate to dissolution are absent from this setting.  The mussels’ ability to precipitate shells in such low-pH conditions is remarkable.  Nevertheless, the vulnerability of molluscs to predators is likely to increase in a future ocean with low pH.

Woah.  It actually seems like she’s found this remarkable organism who lives in a unique environment that can survive massively acidic conditions.  But she is NOT extrapolating these results to other types of calcifying organisms.  And in fact, even these tough mussels do in fact suffer from the increased acidity – they have far thinner shells than nearby mussels living in less acidic waters, which leaves them more vulnerable to predators.  If you read on in the paper, you learn that part of the reason these mussels have such incredible survival rates in these conditions is due to a lack of predators:  typical predators in nearby (less-acidic) waters are crabs, but the crabs can’t survive in the very low pH environment.

All this got me thinking about the differences that exist between authors’ intended message and how their work is subsequently portrayed and disseminated to the public.  What was Dr. Tunnicliffe trying to communicate?

I see a couple of different problems:  1)  The language used in peer reviewed literature is technical and often uses a lot of field-specific jargon, and 2) Access to peer-reviewed scientific articles is limited (by cost, mainly).

Easy on the jargon!

Was the “climate skeptic” summary accurate?  Was my summary accurate?  Were we both wrong?

I thought Dr. Tunnicliffe’s article was really well-written, clear, and informative, and even so – look how easy it was to subjectively interpret and share her results!

A lot of the time, scientists write peer-reviewed articles for each other.  That’s not to say they’re intentionally being exclusive – it just makes sense to try to aim their writing to other scientists who will use the work as a jumping-off point.  And, as a reader, if you’re not sufficiently familiar with a particular field, it can be difficult to glean the relevant information and to parse the jargon, particularly when you don’t have several hours to critically read it.

So what’s a scientist to do?  I really would like to know. It’s important to have rigorous and peer-reviewed documentation of your research so that others in the same field can build on or question your findings. That’s basically what makes science tick.  But what if authors could also write summaries intended for non-scientists (or scientists in other fields, for that matter) to accompany their more technical papers.  Like Cliff’s Notes of their own work.  So my blog readers could look up the Tunnicliffe paper and easily see for themselves whether my simpleminded take on it was total hooey.  (it might very well be…)

If a scientist publishes a peer-reviewed article and no one reads it, is it really science?

Of course, I think I’d be remiss not to mention that I am one of those privileged few who have almost unrestricted access to any peer-reviewed literature I care to get my grubby hands one – while I’m a student at a large American university, I can log into my school’s library website and look at anything I want instantly.  And if I can’t get it instantly in PDF format, I can ask the library to order it in for me, free of charge.  So without even getting to the fact that scientific papers are dense and difficult to synthesize, they’re simply off-limits to almost everyone unless they’re willing to fork over the cash.  For example, the Tunnicliffe paper on the Nature Geoscience website costs $32.  Whew.

And we wonder why there’s a mistrust of the scientific community:  we write as cryptically as we can (hey – in scientists’ defense – getting all those technical details into a reasonable number of pages for publication is not easy, people), and then we publish in peer-reviewed journals that charge people $30-40 for the PDF.  Ugh.

But!  The peer-review system is important in maintaining a certain standard and ensuring the credibility of published results.  I don’t know how to change that.  So:  until we figure out how to make peer-reviewed articles more open, maybe some “published-article Cliff’s Notes” or “cheat sheets” wouldn’t hurt.  (I guess now that I’ve said it, it would be shameful of me NOT to do it for my own paper.  At least I’ve only got the one.)

I’d love to have feedback!  It would help me figure out if anyone is thinking the same thing or if I’m totally out to lunch on this one.


More on open access:

Some interesting thoughts on the future of the scientific journal industry (think social media-style):


[1]  Tunnicliffe, Verena, et al. “Survival of mussels in extremely acidic waters on a submarine volcano.Nature Geoscience 2.5 (2009): 344-348.

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)
[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.