The storage was in the soil, not the plants - as I recall. I assume higher NPP and drier soils are contributing.
A On Sep 17, 2012 4:05 AM, "RAU greg" <gh...@sbcglobal.net> wrote: > "Geoengineering allows natural carbon sinks to enjoy all the benefits of > high CO2without the associated drawbacks of high temperatures, and these > sinks become stronger as a result. From looking at the different sinks, we > found that the sequestration was due almost entirely to the land, rather > than the ocean." > > Has meso-scale experimentation with elevated CO2 in plant communities > shown greater net storage of carbon under elevated CO2? > e.g.: > > http://www.nature.com/scitable/knowledge/library/effects-of-rising-atmospheric-concentrations-of-carbon-13254108 > http://en.wikipedia.org/wiki/Free-air_concentration_enrichment > > While some increases in primary production are found, soil respiration > also is seen to increase: > http://naldc.nal.usda.gov/download/40769/PDF > so the net effect on air CO2 could/should be zero, especially if plants > are not carbon limited, which would seem the usual case. But in a very > brief search I see no discussion of this. > > Anyway, is there empirical evidence that land sinks increase under high > CO2 at constant T? > > -Greg > > > ------------------------------ > *From:* Andrew Lockley <andrew.lock...@gmail.com> > *To:* geoengineering <geoengineering@googlegroups.com> > *Sent:* Sun, September 16, 2012 3:51:19 PM > *Subject:* [geo] Modelling Geoengineering, Part II | ClimateSight > > Poster's note: Fascinating and very readable blog post by UVic modellers. > Only available here as it won't be published. > > A > > http://climatesight.org/2012/09/16/modelling-geoengineering-part-ii/ > > ClimateSight > > Climate science and the public > > Menu > > Modelling Geoengineering, Part II > > Near the end of my summer at the UVic Climate Lab, all the scientists > seemed to go on vacation at the same time and us summer students were left > to our own devices. I was instructed to teach Jeremy, Andrew Weaver’s other > summer student, how to use the UVic climate model – he had been working > with weather station data for most of the summer, but was interested in > Earth system modelling too.Jeremy caught on quickly to the basics of > configuration and I/O, and after only a day or two, we wanted to do > something more exciting than the standard test simulations. Remembering > an old post I wrote, I dug up this paper (open access) by Damon Matthews > and Ken Caldeira, which modelled geoengineering by reducing incoming solar > radiation uniformly across the globe. We decided to replicate their method > on the newest version of the UVic ESCM, using the four RCP scenarios in > place of the old A2 scenario. We only took CO2 forcing into account, > though: other greenhouse gases would have been easy enough to add in, but > sulphate aerosols are spatially heterogeneous and would complicate the > algorithm substantially.Since we were interested in the carbon cycle > response to geoengineering, we wanted to prescribe CO2emissions, rather > than concentrations. However, the RCP scenarios prescribe concentrations, > so we had to run the model with each concentration trajectory and find the > equivalent emissions timeseries. Since the UVic model includes a reasonably > complete carbon cycle, it can “diagnose” emissions by calculating the > change in atmospheric carbon, subtracting contributions from land and ocean > CO2 fluxes, and assigning the residual to anthropogenic sources.After a few > failed attempts to represent geoengineering without editing the model code > (e.g., altering the volcanic forcing input file), we realized it was > unavoidable. Model development is always a bit of a headache, but it makes > you feel like a superhero when everything falls into place. The job was > fairly small – just a few lines that culminated in equation 1 from the > original paper – but it still took several hours to puzzle through the > necessary variable names and header files! Essentially, every timestep the > model calculates the forcing from CO2 and reduces incoming solar radiation > to offset that, taking changing planetary albedo into account. When we were > confident that the code was working correctly, we ran all four RCPs from > 2006-2300 with geoengineering turned on. The results were interesting (see > below for further discussion) but we had one burning question: what would > happen if geoengineering were suddenly turned off?By this time, having > completed several thousand years of model simulations, we realized that we > were getting a bit carried away. But nobody else had models in the queue – > again, they were all on vacation – so our simulations were running three > times faster than normal. Using restart files (written every 100 years) as > our starting point, we turned off geoengineering instantaneously for RCPs > 6.0 and 8.5, after 100 years as well as 200 years. > > Results > > Similarly to previous experiments, our representation of geoengineering > still led to sizable regional climate changes. Although average global > temperatures fell down to preindustrial levels, the poles remained warmer > than preindustrial while the tropics were cooler:Also, nearly everywhere on > the globe became drier than in preindustrial times. Subtropical areas were > particularly hard-hit. I suspect that some of the drying over the Amazon > and the Congo is due to deforestation since preindustrial times, > though:Jeremy also made some plots of key one-dimensional variables for > RCP8.5, showing the results of no geoengineering (i.e. the regular RCP – > yellow), geoengineering for the entire simulation (red), and geoengineering > turned off in 2106 (green) or 2206 (blue):It only took about 20 years for > average global temperature to fall back to preindustrial levels. Changes in > solar radiation definitely work quickly. Unfortunately, changes in the > other direction work quickly too: shutting off geoengineering overnight led > to rates of warming up to 5 C / decade, as the climate system finally > reacted to all the extra CO2. To put that in perspective, we’re currently > warming around 0.2 C / decade, which far surpasses historical climate > changes like the Ice Ages.Sea level rise (due to thermal expansion only – > the ice sheet component of the model isn’t yet fully implemented) is > directly related to temperature, but changes extremely slowly. When > geoengineering is turned off, the reversals in sea level trajectory look > more like linear offsets from the regular RCP.Sea ice area, in contrast, > reacts quite quickly to changes in temperature. Note that this data gives > annual averages, rather than annual minimums, so we can’t tell when the > Arctic Ocean first becomes ice-free. Also, note that sea ice area is > declining ever so slightly even withgeoengineering – this is because the > poles are still warming a little bit, while the tropics cool.Things get > really interesting when you look at the carbon cycle. Geoengineering > actually reducedatmospheric CO2 concentrations compared to the regular RCP. > This was expected, due to the dual nature of carbon cycle feedbacks. > Geoengineering allows natural carbon sinks to enjoy all the benefits of > high CO2without the associated drawbacks of high temperatures, and these > sinks become stronger as a result. From looking at the different sinks, we > found that the sequestration was due almost entirely to the land, rather > than the ocean:In this graph, positive values mean that the land is a net > carbon sink (absorbing CO2), while negative values mean it is a net carbon > source (releasing CO2). Note the large negative spikes when geoengineering > is turned off: the land, adjusting to the sudden warming, spits out much of > the carbon that it had previously absorbed.Within the land component, we > found that the strengthening carbon sink was due almost entirely to soil > carbon, rather than vegetation:This graph shows total carbon content, > rather than fluxes – think of it as the integral of the previous graph, but > discounting vegetation carbon.Finally, the lower atmospheric CO2 led to > lower dissolved CO2 in the ocean, and alleviated ocean acidification very > slightly. Again, this benefit quickly went away when geoengineering was > turned off. > > Conclusions > > Is geoengineering worth it? I don’t know. I can certainly imagine > scenarios in which it’s the lesser of two evils, and find it plausible > (even probable) that we will reach such a scenario within my lifetime. But > it’s not something to undertake lightly. As I’ve said before, desperate > governments are likely to use geoengineering whether or not it’s safe, so > we should do as much research as possible ahead of time to find the safest > form of implementation.The modelling of geoengineering is in its infancy, > and I have a few ideas for improvement. In particular, I think it would be > interesting to use a complex atmospheric chemistry component to allow for > spatial variation in the forcing reduction through sulphate aerosols: > increase the aerosol optical depth over one source country, for example, > and let it disperse over time. I’d also like to try modelling different > kinds of geoengineering – sulphate aerosols as well as mirrors in space and > iron fertilization of the ocean.Jeremy and I didn’t research anything that > others haven’t, so this project isn’t original enough for publication, but > it was a fun way to stretch our brains. It was also a good topic for a > post, and hopefully others will learn something from our experiments.Above > all, leave over-eager summer students alone at your own risk. They just > might get into something like this. > > ABOUT > > Kate is a young climate scientist from the Canadian Prairies. She became > interested in climate science as a teenager, and increasingly began to > notice the discrepancies between scientific and public knowledge on climate > change. She started writing this blog at age sixteen, simply to keep > herself sane, but she hopes she'll be able to spread accurate information > far and wide while she does so. > > -- > You received this message because you are subscribed to the Google Groups > "geoengineering" group. > To post to this group, send email to geoengineering@googlegroups.com. > To unsubscribe from this group, send email to > geoengineering+unsubscr...@googlegroups.com. > For more options, visit this group at > http://groups.google.com/group/geoengineering?hl=en. > -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to geoengineering@googlegroups.com. To unsubscribe from this group, send email to geoengineering+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en.
