A light dusting or something more thorough?

Thursday 5th March 2015

I’ve never really been a fan of The Daily Telegraph or The Sunday Telegraph newspapers. The right-leaning editorial biases and conservative views on many issues grate with me, and one particular contributor’s opinion pieces and blogs on climate change topics (let’s just call him J.D.) make me want to reach for a large bottle of whiskey with the same initials.

But credit where credit’s due. An online article in The Telegraph from 4 March 2015 has a fascinating short video clip that illustrates the transport of dust from the Sahara to the Amazon on the other side of the Atlantic Ocean: http://www.telegraph.co.uk/news/science/11450232/Incredible-video-explains-how-the-Sahara-fertilises-the-Amazon-rainforest.html#disqus_thread

A static screen shot captures the essence of the video, which is based on data collected by NASA’s Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite from 2007 through 2013.

The path of the dust clouds blown from the Sahara to the Amazon basin (highlighted in green) and other parts of South and Central America

The path of the dust clouds blown from the Sahara to the Amazon basin (highlighted in green) and other parts of South and Central America

The full set of findings have been published online by Yu and colleagues in Geophysical Research Letters (accepted article, DOI: 10.1002/2015GL063040) but the video and accompanying short article bring some of the key facts and figures more prominently into the public domain. About 182 million tonnes of dust is blown off the western edge of the Sahara each year. Some of this dust falls from, or is rained out of, the atmosphere into the Atlantic. But on average 132 million tonnes makes it to the eastern coast of South America, whereupon the Amazon receives its average yearly share of about 27.7 million tonnes. The Amazon dust contains about 22 thousands tonnes of phosphorous, an essential nutrient that acts as a fertilizer for the rainforests. Most of the phosphorous comes from the Bodélé Depression in Chad, where an ancient, now desiccated, lake bed is exposed to vigorous wind erosion. This amount of phosphorous is roughly the same amount that is washed away from the Amazon basin by rain and floods every year, suggesting that the African dust plays a key role in preventing phosphorus depletion from the Amazon over decades to centuries.

As The Telegraph article states, the findings ‘show how one of the planet’s driest places is helping sustain one of its most fertile’. There’s obviously much more to it than that, for as the last line of the article hints, the findings are ‘part of a bigger research effort to understand the role of dust in the environment and on local and global climate’. In more expansive, technical terms, dust is a key, but still poorly understood, component of global biogeochemical cycles and the climate system, including possible influences on hurricane generation and suppression. There’s also the debate about whether a future ‘greening of the Sahara’ would necessarily be a good thing. If the current global warming trend continues, then with warmer seas and more intense heating of the Sahara, there’s a chance that more and more rain will penetrate further and further into the heart of the Sahara, perhaps enabling the return of lush wetlands, rivers and lakes. Such features have characterized the Sahara many times in the past, most recently up until about 5-6 thousand years ago.

So why is this a bad thing? First, when the Sahara is green and wet, dust transport off North Africa and across the Atlantic is much more limited, so phosphorous shortages may limit the productivity of the Amazon. Second, there have been some suggestions that when the Sahara is better watered, the Amazon may become much drier. In this complex, teleconnected world, a Saharan expansion of wetlands, rivers and lakes might coincide with widespread desiccation and burning off the Amazon rainforest. This would have widespread implications not only for biodiversity but also for the oxidation and loss of substantial amount of organic carbon to the atmosphere. Along with the rampant human contributions of greenhouse gases to the atmosphere, this is something that the world can ill afford.

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3 thoughts on “A light dusting or something more thorough?

  1. “… there is also mounting evidence to suggest that when the Sahara is better watered, the Amazon may become much drier. ” What evidence is that, Stephen? ( – excuse my ignorance!)

    • Rats! A blog that actually gets read and commented on by sharp-eyed academics. The basic idea came to my attention via Fred Pearce (see the link below for an extract from one of his books that touches on the issue of Saharan and Amazon changes):
      http://www.independent.co.uk/environment/why-deserts-will-inherit-the-earth-481091.html

      This is hardly peer-reviewed stuff so what about in the literature? From home, given the paywalls, it turns out to be rather hard to find something to verify the statement. The review of African hydrological tropical changes by Gasse (2000, QSR, v19) hints at some of the Holocene links, as do more recent papers. I’ll send these by email. There is also plenty to suggest that recent Amazon drought is linked with warmer Atlantic SSTs and changes in the position of ITCZ. For example, see another online newspaper article that refers to published research:
      http://www.theguardian.com/environment/2011/feb/03/tree-deaths-amazon-climate

      So warmer Atlantic = more Sahelian/Saharan rainfall but less Amazonian rainfall?

      On reflection, ‘mounting evidence’ is not a good choice of words. I’m going to change these to something more cautious ….

      • My conceptual model is that warmer equatorial Atlantic (e.g. from diminished heat transport polewards when ocean conveyor stalls because of high latitude meltwater input pulse) reduces the ocean-continent temperature+pressure gradient (which drives the monsoon), so the summer rains penetrate less into Africa, equals drought in the Sahel. I know there is a counter argument to this, but I’m damned if I can remember what it is! Dei Huws (SOS Bangor) could remind me.
        Thanks for the refs. But this weekend is all about writing a workpackage to interrogate a subglacial lacustrine sedimentary archive in Greenland. So forgive me if leave this discussion, for now at least.

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