July 24, 2017 5.18am EDT
Tropical rainforests are among the biggest contributors to the global greening boom. AAP Image/Dave Hunt
Rising carbon dioxide is making the world’s plants more water-wise
July 24, 2017 5.18am EDT
Pep Canadell, Francis Chiew, Lei Cheng, Lu Zhang, and Yingping Wang
Land plants are absorbing 17% more carbon dioxide from the atmosphere now than 30 years ago, our research published today shows. Equally extraordinarily, our study also shows that the vegetation is hardly using any extra water to do it, suggesting that global change is causing the world’s plants to grow in a more water-efficient way.
Water is the most precious resource needed for plants to grow, and our research suggests that vegetation is becoming much better at using it in a world in which CO₂ levels continue to rise.
The ratio of carbon uptake to water loss by ecosystems is what we call “water use efficiency”, and it is one of the most important variables when studying these ecosystems.
Our confirmation of a global trend of increasing water use efficiency is a rare piece of good news when it comes to the consequences of global environmental change. It will strengthen plants’ vital role as global carbon sinks, improve food production, and might boost water availability for the well-being of society and the natural world.
Yet more efficient water use by the world’s plants will not solve our current or future water scarcity problems.
Changes in global terrestrial uptake of carbon dioxide, water use efficiency and ecosystem evapotranspiration during 1982-2011.
Boosting carbon uptake
Plants growing in today’s higher-CO₂ conditions can take up more carbon – the so-called CO₂ fertilisation effect. This is the main reason why the terrestrial biosphere has taken up 17% more carbon over the past 30 years.
The enhanced carbon uptake is consistent with the global greening trend observed by satellites, and the growing global land carbon sink which removes about one-third of all CO₂ emissions generated by human activities.
Increasing carbon uptake typically comes at a cost. To let CO₂ in, plants have to open up pores called stomata in their leaves, which in turn allows water to sneak out. Plants thus need to strike a balance between taking up carbon to build new leaves, stems and roots, while minimising water loss in the process. This has led to sophisticated adaptations that has allowed many plant species to conquer a range of arid environments.
One such adaptation is to close the stomata slightly to allow CO₂ to enter with less water getting out. Under increasing atmospheric CO₂, the overall result is that CO₂ uptake increases while water consumption does not. This is exactly what we have found on a global scale in our new study. In fact, we found that rising CO₂ levels are causing the world’s plants to become more water-wise, almost everywhere, whether in dry places or wet ones.
Growth hotspots
We used a combination of plot-scale water flux and atmospheric measurements, and satellite observations of leaf properties, to develop and test a new water use efficiency model. The model enables us to scale up from leaf water use efficiency anywhere in the world to the entire globe.
We found that across the globe, boreal and tropical forests are particularly good at increasing ecosystem water use efficiency and uptake of CO₂. That is due in large part to the CO₂ fertilisation effect and the increase in the total amount of leaf surface area.
Importantly, both types of forests are critical in limiting the rise in atmospheric CO₂ levels. Intact tropical forest removes more atmospheric CO₂ than any other type of forest, and the boreal forests of the planet’s far north hold vast amounts of carbon particularly in their organic soils.
Meanwhile, for the semi-arid ecosystems of the world, increased water savings are a big deal. We found that Australian ecosystems, for example, are increasing their carbon uptake, especially in the northern savannas. This trend may not have been possible without an increase in ecosystem water use efficiency.
Previous studies have also shown how increased water efficiency is greening semi-arid regions and may have contributed to an increase in carbon capture in semi-arid ecosystems in Australia, Africa and South America.
Pleased to see a paper quantifying a benefit of CO2, if somewhat grudgingly. The first reaction of CAGW scientists was to tell us that the greening was another ill wind brought on by fossil fuels.
I have pointed out (unnecessarily, of course, on this site) in response that the greening (also, and maybe more dramatic,in the oceans ) is exponential and endothermic (reversing my thought that CO2 was not a force in cooling). I suggested that this should cause a flattening of the CO2 growth and wondered if some sharp reader might be able to quantify the attendant sink and amount of cooling (to an engineer’s mind everything that moves and changes is an engine).
This paper doesn’t do this exactly. They quote that 1/3 of fuel emissions are sequestered as if this is a constant – it isn’t – this may be the only thing identified in climate that is accelerating! With phytoplankton you also have to add on the calcium carbonate extra that sequesters CO2 in hard parts (think White Cliff’s of Dover, a Cretaceous formation made entirely of the coccolithosphore shells of an ancient sequestration).
Max Planck’s ‘other law’ that science advances one funeral at a time would identify the authors of this piece as young ones. They are bold but a little deferential, the next ones will answer my questions without fear.
“Plant Reproduction under elevated CO2 coditions: a meta-analysis of reports on 79 crop and wild species”, by Jablonski, Wang & Curtis is available on-line as free full html. Lots of nuances are known about elevated CO2 & good categorical data readily available in this report for anyone wishing to know more than generalizations about CO2 influence.
Sorry haven’t time now to summarize highlights for those following this thread, other than to say that although CO2 enrichment affects our different crops & also dstinct parts of any one crop differently these kind of plants apparently respond more than wild plants do.
Edit: spell “… conditions ….” in report title .
This is “bears **** in the woods” obvious. I learned that CO2 levels affect plant water stress at University – probably even before that as it is such a basic tenet of plant metabolism. It is such a no-brainer that it rarely gets mentioned, despite stomatal number being considered as a proxy for CO2 levels in geological time series.
[Stomata are the pores on plant laves which allow for gas exchange into the leaf. They are very sensitive to water levels, closing when the plant is water stressed to conserve moisture, but this means no more CO2 uptake and is a lovely example of the trade-off between CO2 fixation and water use in plants. In high-CO2 environments, there are fewer stomata as the plant does not need as many to get CO2 – and is therefore much less sensitive to water stress.]
Do we have Chuck Da Mod to thank for the concise explanation of stomata and their role in moisture conservation? If so thank you muchly!!! If it is Rob same to you! I have tried explaining this off the cuff and got bogged down in terms and concepts that are unfamiliar to most people. I’ll be crib noting this baby.
All Rob, He just used brackets in the same style as moderators.
Many thanks to you both, then. And you are doing a great job keeping things chugging along whilst Anthony is walkabout!
Rob – July 31, 2017 at 1:45 pm
Using stomatal numbers obtained from fossilized biomass to determine atmospheric CO2 ppm levels in geological time series proxies ……. is equivalent to having actual thermometer readings of the average summertime near-surface air temperatures for a given geological time series proxy.
The leaf foliage stomata numbers and/or sizes are determined by the available quantity of atmospheric CO2 ppm at the time the leaf growth occurs in early Springtime. The leaf producing plants are not “psychic”, they cannot “predict” what the atmospheric CO2 ppm quantities will be for the next growing season …… any more than they can predict what the availability of water will be for the remainder of the growing season.
Anyway, I’m pleased to know that there are a few more “biology” knowledgeable people posting commentary that discredits some of the “junk science” being mimicked by the CAGW believing crowd.
Look on evolution. Life deposited huge mass of CO2 into calcium carbonate, so plants are very hungry. Equilibrium of CO2 binding to ribulóse-1,5-bisphosphate is about at 80 ppm and is determined by thermodynamic. It means that evolution cannot change it. But C4 plants evolved CO2 pump to bypass the problem and C4 maize will starve out most other plants in close space. Life may end due to CO2 starvation. Again, the evolution found way how to return carbon to circulation.
I’m pretyy sure this has been known for awhile — that within bounds, plants grow more efficiently with less water as CO2 levels increase.
When CO2 increase causes flora to become more water conservative and growth efficient, how does it effect their oxygen production? We know the Earth is becoming greener by observations because of the increase of Carbon Dioxide, are Oxygen levels increasing or decreasing in the atmosphere?
Very good question, one I am sure greens will not want answered or talked about. Kinda hard to convince people that more oxygen would be bad.
When I looked it up, most information says that Oxygen levels are decreasing and scientists don’t know why. Really? I have an example that one of the several mathematicians on here can hash out… Back in the 1970’s the USA government – to fight Smog from fossil fuels – required manufacturers to convert Carbon Monoxide to Carbon Dioxide. Now nearly every industry in the USA was required to end Smog of Carbon Monoxide and convert it to Carbon Dioxide. Other countries have taken to doing this too. Now the air is cleaner in the countries that did this, while simultaneously reducing Carbon Monoxide Smog they have increased Carbon Dioxide while using more Oxygen in the process. That all combustion-able sources deplete Oxygen from the air and produce Carbon Monoxide and Dioxide, this doubling of the Oxygen in the conversion to reduce the Monoxide to form Dioxide depletes the free Oxygen in the atmosphere at a faster pace. So while the planet has become greener in the last several decades – faster than the pre-industrial age was doing – all the greening is not keeping up the Oxygen required to burn combustion-ables and the exponentially increasing requirements of the oxygen dependent Fauna. But, Earth is in no way nearing a point that Oxygen Levels are depleting to harmful levels from what I’ve been studying lately.
Got to admit this is the first I have heard of a drop in oxygen levels. You’d think Al Gore:TheGoreacle would jump all over this and declare it a plot by evil industrialists to undermine the efforts to save the planet undertaken by the valiant ecowarriorsblahblahblahblah. He could get all red faced about it and do that goofy thing with his eyebrows that he seems to think is so impressive! Seriously, though, I would think any drop in oxygen levels would be something that would draw more interest. Curious.
True…giving ammunition to idiots amd fools with fully automatic weapons… Some of what I’ve read about was in Forbes and other known publications. Because Oxygen is still above 21% and we’re only declining fractions of a percent over decades is not enough to bring the alamist any attention to it, because there’s no scare tactics they can sell.