Culturing Science – biology as relevant to us earthly beings

Posts Tagged ‘Climate

Forest canopy height: why do we care?

Wow!  This post was selected as PLoS ONE’s Blog Post of the Month (July 2010) – thanks!  Check out the runners-up and previous winners here.

If you’ve been on the internet at all in the past week, you’ve probably seen these lovely images from NASA, visualizing the height of tree canopies around the world.  They’ve been on science sites along with art ones.  In a sense, that alone is useful: using beautiful visuals to make people think about the world on a larger scale.  But where did these data come from, and what do they really mean?

Why is it important?

The two main sources of anthropogenic influence of atmospheric carbon are (1) the burning of fossil fuels, releasing carbon dioxide into the air and (2) deforestation, removing trees which store large amounts of carbon.  20-50% of carbon in the atmosphere is currently not accounted for in climate models – a huge amount.  Knowing where exactly this carbon is coming from is important for both conservation and making socioeconomic decisions regarding energy use.

Some scientists hypothesize that these unaccounted-for changes in carbon flux are due to poor knowledge of forest stands – both where deforestation has occurred, and where forests are recovering after previous deforestation.  By knowing the size of forest stands, as larger trees can store more carbon, and keeping track of changes in the sizes of these stands, the hope is to have more accurate models for carbon storage and atmospheric carbon.

In the face of climate change, these kind of data are also useful, if updated over time, in seeing how rising temperatures and increased carbon dioxide affect tree growth (and thus carbon sequestration).  Since plants get their carbon from the air, it seems natural that increased carbon dioxide would increase tree growth.  Thus canopy height maps can help us to test this hypothesis.

From where did these data come? Most of the data comes from work by Michael Lefsky et al. in a 2002 paper pubslihed in Global Ecology and Biogeography and a 2005 paper published in Geophysical Research Letters.  In the 2002 paper, Lefsky and his team measured canopy height using LIDAR (Light Detection and Ranging technology), which essentially sends down a laser beam from which distance can be measured based on return time.  LIDAR uses a shorter wavelength than typical radar, making it more sensitive to smaller objects such as particles in the air.  Thus the name, Light Detection and Ranging.  It also has a narrower beam than radar and thus is more specific in its measurements.  In the paper, Lefsky and co. took LIDAR measurements on temperate forest stands and compared them with field measurements of forest height, finding that they matched up very closely.  They also were able to do field measurements on the complexity of forest stands, meaning how much undergrowth lives beneath the canopy, and were able to create an accurate equation for predicting biomass of a forest stand based on LIDAR alone.

The 2005 study was another sort of proof-of-purpose paper, once again showing that LIDAR can predict above-ground biomass.  The authors did not have complexity variables this time, but were still very accurate in their predictions.

For the beautiful figures of canopy height released by NASA, Lefsky combined his own data with that from NASA’s MODIS (Moderate Resolution Imaging Spectroradiometer), which is housed on a satellite and images the earth’s surface every 1-2 days.  Combining the data on terrain from MODIS with his own canopy height data, Lefsky crafted these images over several years.

What does it mean?

Previously, large-scale vegetative modeling was only able to be measured in 1 km swaths of land, done by MODIS alone.  Due to deforestation, there is a great deal of variation in canopy height on a smaller than 1 km scale – and finally we have the tools to create maps of forest canopy and thus help us better track carbon storage at these sites.  The map isn’t perfect – it is a model after all – but it is far more accurate than anything we’ve seen yet.  Expect interpretation from Lefsky and others in the near future.

But this isn’t the end.

As I mentioned earlier, traditional thinking assumes that forest growth will assist in carbon storage through increased growth due to higher carbon dioxide in the atmosphere.  But a recent paper (July 21, 2010) in PLoS ONE by Lucas Silva (“silva” means forest in Latin, lollers), Madhur Anand, and Mark Leithead suggests otherwise.

A major tradeoff for growth in trees (as I’ve discussed elsewhere) is that the opening of leaf stomata (pores) to absorb carbon dioxide also causes water loss through evaporation from these same stomata.  The authors of the PLoS paper looked at tree growth through tree rings and compared it with isotope analysis to measure water loss.  If the trees they studied had increased growth due to increased carbon dioxide, they would also expect more water conservation, as more carbon dioxide would be able to enter the leaves without as much water loss.

Studying 4 species of tree at 4 forest types in Canada, they found a 53% increase in water use efficiency over the last century.  (They looked at both young and old trees to account for varying growth rates and energy use.)  This seems like good news – the trees are absorbing carbon dioxide more readily.  However, they also saw a decline in growth overall.  This suggests that other stresses, such as water, nutrients, and temperature, are limiting their growth despite the ease of access of carbon dioxide.

The next step: can we learn about tree growth from Lefsky’s maps?  Are they accurate enough?  It would be great to measure biogeochemical measures, such as water use efficiency, and compare this to large-scale forest size data.  A girl can dream…

Okay, class: what have we learned?

Lasers are cool!  The LIDAR technology, originally created for studying atmospheric chemistry, reapplied to study canopy heights has allowed us to visualize our forests in a new way.  (And make some beautiful pictures.)  There was a lot of work put into it – and to accurately measure how our forests are changing, increasing work will have to be done to keep the maps updated to create an index of canopy height on our planet.

However, we’ve also learned that we cannot necessarily rely on traditional hypotheses in times of climate change.  While trees have the capacity to remove carbon from the atmosphere and store it, other factors can confound these effects, as we read in the PLoS ONE paper.  While more work certainly needs to be done on this front (using large-scale climate measures for growth instead of dendrochronology, for example), their results are certainly sobering.

So, as usual, we need to do more work!  We need to learn more!  We have to challenge our hypotheses, and challenge new results that support or disprove them.  It’s always easier when you have a mystery to solve: where is all that carbon anyway?

Cohen, W., Harmon, M., Wallin, D., & Fiorella, M. (1996). Two Decades of Carbon Flux from Forests of the Pacific Northwest BioScience, 46 (11) DOI: 10.2307/1312969

Lefsky, M., Cohen, W., Harding, D., Parker, G., Acker, S., & Gower, S. (2002). Lidar remote sensing of above-ground biomass in three biomes Global Ecology and Biogeography, 11 (5), 393-399 DOI: 10.1046/j.1466-822x.2002.00303.x

Lefsky, M., Harding, D., Keller, M., Cohen, W., Carabajal, C., Del Bom Espirito-Santo, F., Hunter, M., & de Oliveira, R. (2005). Estimates of forest canopy height and aboveground biomass using ICESat Geophysical Research Letters, 32 (22) DOI: 10.1029/2005GL023971

Running, S. (1999). A Global Terrestrial Monitoring Network Integrating Tower Fluxes, Flask Sampling, Ecosystem Modeling and EOS Satellite Data Remote Sensing of Environment, 70 (1), 108-127 DOI: 10.1016/S0034-4257(99)00061-9

Silva, L., Anand, M., & Leithead, M. (2010). Recent Widespread Tree Growth Decline Despite Increasing Atmospheric CO2 PLoS ONE, 5 (7) DOI: 10.1371/journal.pone.0011543

Written by Hanner

July 28, 2010 at 8:12 am

DMS(P): the amazing story of a pervasive indicator molecule in the marine food web

This post was chosen as an Editor's Selection for

Dimethylsulfide.  Does that word mean anything to you?  “Why yes,” you organic chemistry nerds may say, “It clearly is a molecule of sulfur with two methyl groups attached.”  That’s as far as I could have gotten – until this past week, when I inundated myself with information on dimethylsulfide (DMS) due to a paper published in Science.  Now I’m enlightened – what a wonderful molecule!  Let me spoil it for you: it is simultaneously a defense mechanism, a chemical cue pervasive throughout the marine food web, and an effector on the earth’s climate.  (See illustration at bottom of post for summary.)  That’s right.  Just a sulfur molecule with two methyl groups attached.  Now let’s back up a bit.

DMS is a sulfur compound that accounts for 50-60% of the total natural reduced sulfur flux to the atmosphere (even more than either volcanoes or vegetation).  While sulfur in the atmosphere can cause acid rain, it is also very important, as it helps form clouds.  In order for water to transition from a gas to liquid in the atmosphere, it needs a small particle in the air to adhere onto, known as a cloud condensation nucleus.  Sulfur oxide, which can be derived from DMS, is one of these particles.  Clouds not only carry our precipitation, but help to reflect sunlight (and thus heat) back into space, affecting our planet’s climate.

After the realization of its importance as a cloud condensation nucleus, scientists began to look for DMS’s planetary source and found that 95% of the atmospheric DMS originates in the oceans – but from where?  As illustrated in my figure, it is actually formed in certain species of phytoplankton and released when cells leak, most often due to herbivory by other organisms.  The phytoplankton itself actually makes a molecule called DMSP (dimethylsuphoniopropionate, if you must know).  When its cell wall begins to break down, stores of DMSP (from an unknown location within the cell) and an enzyme, DMSP-lyase, are released into the surrounding water.  This DMSP-lyase removes the P-group, leaving us with our favorite molecule of the day, DMS.

In 1987, the CLAW hypothesis (named for its authors) was put forward by James Lovelock and a handful of his lackeys to support his infamous Gaia hypothesis which suggests that microorganisms regulate the Earth’s climate to maintain conditions suitable for life.  CLAW hypothesized that levels of DMSP and its P-cleaving enzyme in phytoplankton are regulated by light and temperature, with greater amounts of DMSP produced when it gets warm in order to send more DMS into the atmosphere, deflecting sunlight and reducing global temperatures.  However, like most support for the Gaia hypothesis, this idea requires that phytoplankton act altruistically, releasing the DMS for the good of the planet, which does not make much sense in light of natural selection.  (A good review of this and the above sections can be found in this paper by Rafel Simo.)

A 2001 MEPS paper by Kathryn Van Alstyne and others provides evidence that the release of DMSP and DMSP-lyase by phytoplankton is actually a chemical defense mechanism.  After DMSP-lyase activity, the resulting products are DMS and acrylase or acrylic acid.  The authors found that acrylic acid deterred grazing by two species of sea urchins on algae, and they suggest that this is due to an aversion to acidity by the urchin itself or that the acid irritates its gut microbes.  While the evidence is not directly causative, as the authors showed a reaction to acrylic acid and not to DMSP-produced acrylic acid in vivo, it does suggest (with other evidence) that the important part of this reaction to the phytoplankton is not DMS, but rather its byproduct.

Fig. 1 from Van Alstyne et al. (2001), showing the DMSP-lyase reaction

Smaller grazers, such as isopods, had no reaction to the acrylic acid in that paper.  But a paper published in Science this week (July 16 2010) by Justin Seymour, Rafel Simo, and others looks into the effects of DMSP on the smallest grazers: microbes.  Using “microfluid technology” (see details at end of post), the researchers measured the strength of attraction of 4 different types of microbes (7 species) to varying concentrations of DMSP, DMS, DMSO (dimethylsulfoxide, a DMS and DMSP degradation product), GBT (glycine betaine, a molecule analogous to DMSP in structure and function), and artificial seawater (as a control).

  1. Each of the 3 species of autotrophic plankton reacted differently to DMSP in the water.  The algae Micromonas pusilla showed strong attraction to DMSP, taking it up presumably as a carbon and sulfur source.  The cyanobacterium Synechococcus sp. showed no reaction.  Most strangely, the algae Dunaliella tertiolecta moved very strongly toward DMSP, but not to assimilate it directly; rather, it cleaved the molecule into DMS extracellularly and potentially assimilated that molecule instead.  The authors do not know why this action occurs.
  2. Two species of bacteria, Pseudoalteromonas haloplanktis and Silicibacter sp., each moved toward the DMSP for assimilation as part of their carbon and sulfur requirements.
  3. Two consumers, one a herbivore to eat the algae itself (Oxyrrhis marina) and the other a bacteriovore which strove to eat the bacteria consuming the DMSP, each showed positive chemotaxis and moved toward the DMSP source.

This last part is the most interesting: these two microbe species use the DMSP as a chemical signal that there is food around.  Out of all the molecules that could leak from the burst cell and indicate prey, it is this very molecule, DMSP, that does the trick.

But wait: didn’t the 2001 paper I just wrote about suggest that DMSP is a chemical defense of phytoplankton?  What is it doing drawing in its own predators?  The authors suggest that previous studies have flawed experimental design, releasing far too much bulk DMSP into the environment in contrast to their own “microfluid technology.”  Previous studies, such as a 1997 Nature paper by Gordon Wolfe and a 2003 paper by Suzanne Strom, found that O. marina has a higher tolerance to digesting DMSP and is less repelled than other species.  More work should be done in order to get to the root of this contradiction.

Thus far, we have DMSP attracting bacteria to consume the DMSP itself, drawing in an herbivore to consume the DMSP-producer, and a bacteriovore attracted to the DMSP in order to find its own bacterial prey.  This is not the end of the story, as DMSP is a prey indicator at higher trophic levels as well.  A 2008 Science paper out of UC-Davis and UNC found that planktivorous fish use DMSP as a foraging cue, aggregating near DMSP hotspots.  Furthermore, Gabrielle Nevitt reviewed literature in 2008 on seabirds (Order: Procellariiformes) using DMS as an olfactory cue to identify patches of its own prey, fish and squid, feeding on the zooplankton and its phytoplankton prey.  A similar pattern has also been found in seals and whale sharks.

The evolutionary implications of this are astounding.  It seems as though many molecules released from a leaky phytoplankton cell could be used as indicators of these clusters of consumption.  However, DMSP has something special: sulfur.  We all know that smell, and perhaps it is this stinkiness that has allowed it to become such a pervasive indicator throughout the marine food web.  In her review, Nevitt discusses the evolution of DMS-sensitivity in seabirds, and using phylogenetic trees, shows that only the species that are reared in dark burrows, relying on smell alone to identify food, currently have DMS-sensitivity.  How did this apparent convergent evolution occur?  Is it convergent?  (I have no answers to these questions.)

Are there any implications for climate in these findings, as DMSP is indirectly responsible for increased albedo (sunlight reflection) in our atmosphere?  I doubt that there are any direct consequences that we can enumerate.  As all biogeochemists know, the stuff of the air frequently comes from the stuff we live on and in: soil and water.  This is simply another tie to the way microbes and abiotic stuffs relate to climate-regulating molecules.  The authors of this week’s Science paper note that “microbial behaviors, played out over microscale chemical landscapes, shape planktonic food webs while potentially influencing climate at global scales.”  DMS as a prey cue should create a positive feedback loop, drawing more herbivores to open more cells and leak increasing DMSP, which in turn draws more herbivores.  Some studies show varying ties of DMSP production to species, light, temperature and salinity (review by Stefels et al. here), but it seems to me that the DMS-as-prey-cue and DMS-as-climate-regulator processes are unlinked, so would not work together in any predictable way.

If you take nothing away, take this: sometimes the universe is more connected than we can imagine.

(Sidenote: should I give up strict science and become a science illustrator?)

The idea behind this technique is that microbes inhabit a “dynamic and patchy nutrient landscape” in which nutrient levels vary over micrometer scales.  The microfluidic device is a chamber designed with the “objective of creating a diffusing band of chemoattractant, to simulate an ephemeral, microscale nutrient patch.”  There is a chamber in the center into which fluorescence-labeled chemoattractant is added to the desired concentration, and then input is cut off.  Thus the chemical will dissipate slowly through the closed chamber, trying to imitate the open ocean.  The researchers then add their microorganisms, and measure their distribution and the distribution of the fluorescent chemical.  This allows the researchers to track the intensity and location of the chemical, as well as the behavior of single organisms.

This information is from a paper in Limnology and Oceanography: Methods by J.R. Seymour, T. Ahmed, and R. Stocker entitled “A microfluidic chemotaxis assay to study microbial behavior in diffusing nutrient patches” (2008: 6 (477-488)).  A pdf copy of this paper is available on Roman Stocker’s webpage, here (pdf warning!).

Here’s a picture of the device, thanks to Roman Stocker.  The blue tube is the microbe input, the green tube is chemical input, and the red tube is for drawing out waste.  It sits on top of a microscope.

DeBose, J., Lema, S., & Nevitt, G. (2008). Dimethylsulfoniopropionate as a Foraging Cue for Reef Fishes Science, 319 (5868), 1356-1356 DOI: 10.1126/science.1151109

Nevitt, G. (2008). Sensory ecology on the high seas: the odor world of the procellariiform seabirds Journal of Experimental Biology, 211 (11), 1706-1713 DOI: 10.1242/jeb.015412

Seymour, J., Simo, R., Ahmed, T., & Stocker, R. (2010). Chemoattraction to Dimethylsulfoniopropionate Throughout the Marine Microbial Food Web Science, 329 (5989), 342-345 DOI: 10.1126/science.1188418

Simó, R. (2001). Production of atmospheric sulfur by oceanic plankton: biogeochemical, ecological and evolutionary links Trends in Ecology & Evolution, 16 (6), 287-294 DOI: 10.1016/S0169-5347(01)02152-8

Stefels, J., Steinke, M., Turner, S., Malin, G., & Belviso, S. (2007). Environmental constraints on the production and removal of the climatically active gas dimethylsulphide (DMS) and implications for ecosystem modelling Biogeochemistry, 83 (1-3), 245-275 DOI: 10.1007/s10533-007-9091-5

Van Alstyne, K., Wolfe, G., Freidenburg, T., Neill, A., & Hicken, C. (2001). Activated defense systems in marine macroalgae: evidence for an ecological role for DMSP cleavage Marine Ecology Progress Series, 213, 53-65 DOI: 10.3354/meps213053

G. V. Wolfe, M. Steinke, & G. O. Kirst (1997). Grazing-activated chemical defence in a unicellular marine alga Nature, 387, 894-897

Written by Hanner

July 19, 2010 at 8:19 am

Carl Safina speaks for me, as usual

From science writer and ocean activist Carl Safina’s blog:

Reality: The atmosphere is as thin as shellac on the globe. Where does all the exhaust go? Into that thin, thin layer. We measure carbon dioxide from the exhaust, and find it climbing every year. Physicists have learned that it traps heat. We measure the temperatures worldwide and find them warming. If we’re wrong about all of that, we can look at satellite photos of the polar seas and see how much ice has melted in recent years. Same with almost all the glaciers. In the tropics, reefs have started dying due to excess heat and due to water turning acid from carbon dioxide only since the 1980s. I’ve seen this all with my own eyes.

Stupidity: The idea that the climate scientists in the world are in a conspiracy to lie is just insane. People who think that are either nuts or are being manipulated by the greed of those whose ox will be gored. Even in the days of big tobacco, they never raised such a ridiculous claim against all of science.

Morality: Consider the last energy conversion: Slavery to oil. Slaves are much cheaper. They create a very nice life-style. The economy would collapse without our freedom to have slaves. Obviously, this moral rot is an absurd argument. And this absurdity was vehemently held, and viciously, bloodily defended. Pretty much like today.

Practicality: The world is warming and we’re warming it. That is indisputable. It will have great negative consequences; that’s also indisputable. Denying it won’t change it.

And–no one needs to give up energy. We only need to convert from caveman energy (burning something every time we need energy) to clean energy that powers the whole planet (sun, wind, the energy in the ground, with nuclear as a possible bridge). No one cares whether the energy comes from oil, coal, slaves, sunlight, or wind, as long as the light goes on when you flick the switch and the car goes when you step on the pedal. Except that, wait, we do care if it comes from slaves because that’s immoral. Well, wrecking the future is also immoral. Today’s slaves are our own children whose options are closing because we’re dictating the world they’ll be stuck with.

Patriotism: As China and Germany and Denmark know, far from wrecking the economy, building and exporting the high-tech technology for capturing clean, free-flowing energy and the grids for distributing it will involve tremendous investment and job opportunities.

The United States is falling farther behind in developing these technologies; other countries are positioning to leave us in the dust. The unpatriotic people are not the ones who want the U.S. to lead in developing clean energy. It’s the ones who don’t.

– Carl Safina

I have nothing to add.

Written by Hanner

December 10, 2009 at 2:32 pm

Posted in Uncategorized

Tagged with

Leaf veins as the driving factor in angiosperm evolution (Brodribb and Feild 2009, Ecology Letters)

ResearchBlogging.orgThe news would have us believe that we are living on earth at a time of high carbon dioxide levels.  And it’s true — carbon dioxide levels have been rising since the industrial age, quickly enough to suggest that the effect is human-derived and we may be about to destroy this world for ourselves.

Image by Mark Boucher

But life has existed at much, much higher carbon dioxide levels.  Right now, carbon dioxide is about 400 ppm (parts per million) in the atmosphere; 60 million years ago, it was at 3500 ppm.  There is fossil evidence for a loss of atmospheric carbon dioxide through the Cretaceous period (145-65 million years ago), and scientists have long suspected that the evolution of the angiosperms, or flowering plants,  could provide more evidence.  The word “angiosperm” derives from Greek, meaning “enclosed seed” referring to the flesh surrounding their seeds, i.e. they produce fruit.  The first fossil evidence for angiosperms is pollen from around 130-140 mya (million years ago), and they diversified rapidly in the mid-Cretaceous period, about 100 mya.  As you can see from the below figure, spanning time through the entire Cretaceous period, angiosperms really took off, increasing in number and dominating other plant types.  (Pteridophytes are ferns; Conifers are cone-bearers such as pines; Cycads are something else!)  Their complete take-over is evident at 80 mya.

(Figure from JC McElwain, KJ Willis, and R Lupia's chapter "Cretaceous CO2 decline and the Radiation and Diversification of Angiosperms" from the book A History of Atmospheric CO2 and its effects on Plants, Animals and Ecosystems by TE Cerling and MD Dearing. Page 135.)

Angiosperms have an advantage because they are more productive — that is, they go through photosynthesis more efficiently, allowing them to grow faster and more effectively compete for resources.  There are many aspects of their biology that could affect this rate: greater food and nutrient import by their roots, or greater water flow through the plant, greater nutrient flow, greater ability to absorb carbon dioxide.  The question is: which one of these features was the causal change that allowed angiosperms to take over the world?

Drs. Brodribb and Feild believe they have found the answer: leaf veins, as published in the early edition of Ecology Letters (doi: 10.1111/j.1461-0248.2009.01410.x).  They argue that leaf veins are a proxy for greater photosynthetic capacity, via greater gas exchange capacity. While the veins in a leaf typically carry water, each vein opens to the air through a hole in the leaf (stoma, pl. stomata), through which both water and carbon dioxide enter.  In a world of decreasing carbon dioxide, the leaf would need to keep their stomata open longer or develop more of them in order to absorb enough carbon dioxide from the air to maintain their growth rate.  However, the longer you keep your stomata open, the more water you lose. In order to keep them open to absorb carbon dioxide but also maintain hydration, the authors argue that the plants evolved more complicated leaf venation systems, to deliver water throughout the leaf more efficiently.  Thus we can use the density of these veins to track the evolution of carbon dioxide uptake of angiosperms.

The authors looked at 759 different plant species total: 504 living angiosperms, 89 extinct non-angiosperms, and 166 living non-angiosperms (for comparison).   They modeled the phylogeny (evolutionary tree) of the samples from two well-supported “backbone topologies,” which they consider to be conservative estimates of plant evolution.  They saw a pattern of increased vein density as each of these plants species evolved, but no change in non-angiosperms, demonstrating angiosperm-specific evolution.   The authors then assigned photosynthetic rates to 35 species for which they had evolutionary data using modern leaf samples.  These data showed that, under today’s atmospheric conditions, the plants which evolved later had higher photosynthetic capacities, suggesting  directional evolution towards greater photosynthesis in an atmosphere with lower carbon dioxide.  The non-angiosperm data once again showed no change, demonstrating that they did not go through a similar evolutionary revolution, which perhaps let the angiosperms take over.

Often, in order for a great evolutionary leap, an organism needs some sort of pressure, or hard-times.  For an organism like a plant that consumes carbon dioxide and sunlight, life 100 mya would have been great times — too much carbon dioxide and sun, and a warm climate, removing the necessity for any sort of winter-time adaptations.  Plenty of resources to go around.  Even low-efficiency parts were able to keep the plants alive with ease, so there was no need to evolve more sophisticated photosynthetic machinery.

My question: would a gradual change in carbon dioxide result in the drastic evolutionary changes which Brodribb and Feild have shown?  I looked into the carbon dioxide record for the Cretaceous period and discovered a potential cause for these changes: oceanic anoxic events. Oceanic anoxic events (OAE) were periods of great climatic upheaval, caused by overproduction.  In a hot, carbon dioxide-rich environment, plants went through periods of drastic growth, accumulating organic carbon which then was unable to decompose.  In effect, this sequestered large amounts of carbon, decreasing the amount of carbon dioxide in the atmosphere.

The most severe OAE I know of, and thus a good example,  is the  Azolla Event (Brinkhuis et al. 2006, doi:10.1038/nature04692).  About 50 million years ago, there was a dramatic drop in carbon dioxide levels from 3500 ppm to below 1000 ppm.  (See Pearson 2000 for carbon dioxide reconstructions for 60 million years ago. doi:10.1038/35021000)  In the Arctic Ocean during this time, there were huge blooms of the aquatic fern, Azolla, which absorbed incredible amounts of carbon dioxide from the atmosphere during photosynthesis and sequestered it on the bottom of the ocean when it died and sank.  (Anoxic soil conditions kept the plant matter from decomposing and re-releasing the trapped carbon, instead fossilizing, giving us evidence that this event occurred at all.)  This event was a major cause of the drop in carbon dioxide and temperature during and since the Eocene.

Smaller versions of the Azolla Event occurred throughout the Cretaceous.  These events lasted less than a million years – just a drop of time in the large scale, but the loss of carbon dioxide for a million years can create great evolutionary pressure on organims.  Below is a figure showing three different reconstructions of carbon dioxide levels during the Cretaceous period, the scale in relation to current carbon dioxide concentrations.  The solid line (Berner 1994, American Journal of Science, link) and dashed line (Berner and Kothalva 2001, doi:10.2475/ajs.301.2.182) show similar, downward trends in carbon dioxide levels throughout the Cretaceous.  I’m more interested in the dotted line (Tajika 1999, doi:10.1046/j.1440-1738.1999.00238.x), which takes into account these OAE, evidenced by high-carbon black shale.  During each of the estimated times for these OAE, there is a drastic drop in carbon dioxide levels, after which it rises back up to similar levels as previously, while the total trend is downward.

(Figure from JC McElwain, KJ Willis, and R Lupia's chapter "Cretaceous CO2 decline and the Radiation and Diversification of Angiosperms" from the book A History of Atmospheric CO2 and its effects on Plants, Animals and Ecosystems by TE Cerling and MD Dearing. Page 139)

The estimated times for oceanic anoxic events align well with estimated times for angiosperm diversification and expansion.  The first OAE, around 115 mya, immediately preceded angiosperm diversification, while the second and third precede clear angiosperm dominance, as seen in the first figure.

These oceanic anoxic events, with their severe but temporary drops in atmospheric carbon dioxide, would provide pressure to cause swift evolution in angiosperms toward a higher photosynthetic rate, as Brodribb and Feild showed in their paper.  The alternation during the Cretaceous period between these carbon dioxide droughts and high levels of carbon dioxide from volcanic activity stemmed from tectonic movement from the breakup of Pangea, would give plants cause to evolve these new mechanisms, but also allow them to reap the benefits during times of carbon dioxide wealth and spread rapidly.

To me, Brodribb and Feild’s paper doesn’t suggest that the change in leaf vein density caused the evolution and spread of angiosperms.  This incredible evolution was simply a reaction to the changing carbon dioxide in the environment.  This adaptation allowed them to grow and diversify much more efficiently than non-angiosperms, but I would be surprised if it were the only one.  I’d be interested to know how other carbon dioxide-related mechanisms in plants evolved over time, and if they were also developed to cope with changing atmospheric conditions.

Besides being fascinating, studying evolution in relation to climate change is pertinent right now.  If we can learn more about how plants and other organisms have reacted to changing atmospheric conditions in the past, we could have more information to predict how they will involve in the face of human-induced climate change.  A lot of science can be incredibly forward-thinking, which is why some scientists blow-off detailed studies of evolution.  But biology is all patterns; the more we can learn about its history, the more tools we have to study modern biology and predict the future.

Leaf vein image by Mark Boucher, Flickr

Graph figures from JC McElwain, KJ Willis, and R Lupia’s chapter “Cretaceous CO2 decline and the Radiation and Diversification of Angiosperms” from the book A History of Atmospheric CO2 and its effects on Plants, Animals and Ecosystems by TE Cerling and MD Dearing, available here on Google Books

Brodribb, T., & Feild, T. (2009). Leaf hydraulic evolution led a surge in leaf photosynthetic capacity during early angiosperm diversification Ecology Letters DOI: 10.1111/j.1461-0248.2009.01410.x

Written by Hanner

December 6, 2009 at 3:20 pm

Posted in Journal Article

Tagged with , , ,

Is it possible for us to reduce our carbon emissions? (Garrett 2009, Climatic Change)

ResearchBlogging.orgYou may have heard that we’ve been having a bit of a problem called “global warming” or “climate change.”  The debate is what to do about it — can individuals, day-by-day, affect the amount of greenhouse gases in the atmosphere based on decisions involving diet, waste, and choices of consumption?  What types of alternative energy are the most efficient and effective?  How does industry need to change in order to yield or reduce carbon emissions?  Is this a problem that we can actually solve?

The journal Climatic Change published an article online on November 21 by Timothy Garrett entitled “Are there basic physical constraints on future anthropogenic emissions of carbon dioxide?”  (open access: doi:10.1007/s10584-009-9717-9).  You could easily blow this off as just another doomsday scientist — but the way he structures his argument,  stepping back from the issue and thinking about human civilization in relation to its environment in a new way, makes it stand out.

Garrett creates a new economic model, essentially reducing civilization to production and energy consumption.  The standard model used in the International Panel on Climate Chance (IPCC) Special Report on Emissions Scenarios (SRES) includes the variables p (population) and g (standard of living), which are difficult to predict, causing difficulty in creating reliable models to calculate the climatic state even 50 or 100 years from now.  Garrett argues that these variables are unnecessary, as they are simply responses to energy consumption and efficiency; that we should instead think about civilization as a huge furnace, which needs more energy as productivity increases, but is also inextricably linked to past production.  “The present and future are influenced even by the most distant past, and the past cannot be erased.”  (Waxing philosophical, are we, Garrett?)

Essentially, he boils down the human-planet system to physics.  Carbon dioxide, the output of energy consumption, exits civilization at a constant rate, but accumulates over time.  This tradeoff is represented as the variable η, which is the “rate of return” of energy to a system.  It essentially represents a feedback loop in which the greater the energy consumption and production, the greater is the potential for more consumption and production.  (Remember that carbon emissions are tied to this production and energy consumption.)  What is most important to note is that if η > 0, the system is growing, meaning energy consumption is increasing; when η < 0, it is shrinking; and when η=0, system growth is at a standstill.

It seems obvious that energy consumption would be tied to production in general.  But if energy consumption also is linked to economic growth, then we would have another way to think about how humans, energy and the environment interact.  Garrett based his calculations on the assumption that there is some constant value, λ, which links energy consumption to economic value through the equation:

a = λC,  where a = global primary energy consumption and C = global economic value

If his argument is true, λ has to be constant with time.  To show the existence of this constant, Garrett turned to data for world energy production (and thus consumption) a from the Annual Energy Review (2006) and global economic production P from United Nations data and looked at the whole 36-year timeframe for which he had data.  As you can see from his figure (below), the ratio of a/C for λ stayed constant at around 0.306 exajoules per trillion for the entire period.  (Also note the dramatic increase in η since the industrial revolution.)  (FYI: P is production rate in 1990 dollars/year.)

Garrett does admit that, since he has such a short length of data to work with, this constant could only apply to this 36-year period and not more.  However, I find his evidence sufficient to consider the model further.  As we accumulate more data on energy and economic production, it will be interesting to see if this constant λ is, in fact, constant.

This is a simple concept: that economic growth and increased energy use are linked.  However, what it implies about how to reduce energy use is harder to grasp.  This paper suggests that to bring η below zero and thus lose our forward acceleration of energy use, we have to actually shrink our economy.  For some reason, saying “shrink our consumption” seems doable, but when it is tied to the economic success of countries, developing or stable, it seems like far more of an impossible task.  In this way, Garrett’s paper points out a flaw in current discussions about climate change: we want to reduce emissions, but at the same time keep living our lives the way we do, keeping production high and the economy growing.

The next question is: what if we change our energy to non-carbon-based sources, such as wind or solar power?  For η to equal or be less than zero, we would need to make a switch to non-carbon sources at the same rate as η itself, the rate of return.  The 2005 value for η is 2.1% growth per year (see figure above; According to Garrett, “2.1% of current annual energy production corresponds to an annual addition of approximately 300 GW of new non-carbon emitting power capacity — approximately one new nuclear power plant per day.”

Garrett’s paper seems to present us with an impossible task: up the building of non-carbon-based energy sources while simultaneously downgrading our economy.  (This gets even more complicated with the news that we may be heading towards a uranium shortage, so nuclear power may not be realistic.)  It’s a hefty charge, and one that makes the future seem quite bleak.  However, this work should be taken with a grain of salt.  As much as simplification can be helpful in understanding a system, we cannot just give up.  Other factors can help mitigate our carbon emissions — if we don’t believe this then we’re wasting our time — and work should still continue to figure out those methods.

More than anything, I think that this is a really interesting way to think about humans and civilization on this planet.  When we’re talking and thinking about climate change, it’s easy to play the blame game and assign roles to different parts of the world or society, whether they be developing nations, industrialization and globalization, or rich people in mansions with enormous carbon footprints.  This paper makes the reader step back and realize that it’s not one thing — it’s the entire planet.  One country’s change isn’t going to do it.  While biking to work makes me feel good, we need everyone to bike to work in order to reduce our consumption dramatically enough to reduce emissions, and hopefully get our η in the negative.

Garrett, T. (2009). Are there basic physical constraints on future anthropogenic emissions of carbon dioxide? Climatic Change DOI: 10.1007/s10584-009-9717-9

Written by Hanner

November 30, 2009 at 1:36 pm

Posted in Journal Article

Tagged with ,