Tuesday, June 09, 2015

On the Margins

Recently, there has been considerable interest in a paper showing that rainfall in the Sahel has increased likely due to sea surface temperature trends in the North Atlantic as compared to the oceans in general.  Now Eli is not here to tell you about that paper by Alessandra Giannini, she has done it herself,



and so have others, but a related one that seemingly has snuck in under the radar, "The dominant role of semi-arid ecosystems in the trend and variability of the land CO2 sink", by a large group of authors, including Michael Raupach to whom the paper is dedicated as he died before publication. The paper appeared in Science, 11 May 2015.

The authors show that while the carbon sink is dominated by forests, especially tropical forests, the annual variability (hi Victor) in the carbon sinks is dominated by arid lands such as the Sahel which makes sense, because in wet years, the semi-arid lands will bloom, in dry ones, not much.

The figure to the right requires some explanation.  The y axes are percent contribution to the biosphere's net carbon flux from each of the land types listed at the bottom.  It is the difference between the global primary production and the respiration.  LPJ-GUESS is a biogeochemical vegetation model and TRENDY is a model intercomparison project, go read the paper for details.  IAV is the interannual variation.

Semi-arid lands like the Sahel dominate both the trend and the interannual variation, which makes sense, they are on the margin and small differences in weather make a large difference in the amount of vegetation.

The paper points out that in 2011 there was a large increase in the carbon sink, which is attributed to a very wet year in Australia.

As to the future, a large increase in the carbon sink appears when it is cool and wet in the semi-arid regions, a large decrease when it is hot and dry.  In general the level of precipitation is more important than the temperature.

Taking this into account bunnies should go back and look at Giannini's video.  If the Sahel is wetter, then there will be more primary production of vegetation, but if it is hotter, there will be more respiration and decay.  That means more food, a very good thing, but not as much natural carbon sequestration as if it were cooler

9 comments:

Russell Seitz said...

In my meager sub-Saharan experience, when the sahel blooms, it as much reflects several involuntary years of fields lying involuntarily fallow as the arrival of the rains.

Aaron said...

In North Africa, herds are wealth, and the Sahel is massively over grazed - by cattle in the south and sheep in the north.

A group at MIT did a paper on the issue back in the 1970s - they fenced off (and guarded) areas and in a couple of years the fenced off areas returned to grassland while the surrounding areas remained desert.

Florifulgurator said...

Overgrazing is a weapon of mass destruction on par with the plow. It was an amplifying factor in the agricultural collapse of Syria. Other examples: Iceland/ Viking Greenland, Chinese Loess Plateau. And then there's virtual overgrazing, like growing soya in deforested Amazon to feed cattle in Texas...

KAP said...

Overgrazing as a cause for desertification has been given a bad rap. For a completely different view, and very convincing results, see this TED talk by Allan Savory:

https://www.youtube.com/watch?v=vpTHi7O66pI

Dan L. said...

Alan Savory's notions that intensive trampling and grazing can save grasslands are not supported by peer reviewed studies. To put it politely, his views are those of an outlier.

Wiki:
"A 2014 review in the International Journal of Biodiversity criticized Savory's methods and assertions, finding little peer-review support for many of his more contentious assertions. The authors concluded that: 'Ecologically, the application of HM [holistic management] principles of trampling and intensive foraging are as detrimental to plants, soils, water storage, and plant productivity as are conventional grazing systems. Contrary to claims made that HM will reverse climate change, the scientific evidence is that global greenhouse gas emissions are vastly larger than the capacity of worldwide grasslands and deserts to store the carbon emitted each year.' "

EliRabett said...


What happens in the Sahel stays in the Sahel, but the point is that even if rainfall increases the net carbon sequestration will not be large if temperature rises. If the damn goats eat the grass even less.

David B. Benson said...

I attribute it to the expansion of the Hadley cell.

BBD said...

What the Hadley Cell giveth, it also taketh away...

llewelly said...

I would like to know how much carbon is likely to be emitted by the forests of the pacific northwest as they dry out.