Can plants grow under extreme (>1%) carbon dioxide concentrations?

Can plants grow under extreme (>1%) carbon dioxide concentrations?

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Related to but distinct from this question due to the concentrations involved.

While small carbon dioxide concentration increases can positively impact plant growth under some circumstances and at the same time negatively affect it under other circumstances, these changes are relatively small (doubling CO2 from 0.04% to 0.08%).

Is there any research on plants grown at extremely high (>1%) carbon dioxide concentrations in the air, assuming Earth atomspheric pressure? For the purpose of this question, assume that the plants are supplied with pH modifiers such as limestone in the soil to prevent soil acidification.

Yes they can, but their normal growth is somewhat impaired. A study by Bugbee and collaborators showed that while the yield of rice and wheat increases with CO2 up to about 0.1% CO2, yield decreases sharply as CO2 climbs from 0.1% to 0.25%. There is a smaller loss in yield as CO2 is further increased from 0.25% to 2%.

One interesting thing to note about the study is that the above results turn somewhat on how "yield" is defined. Bugbee defines yield in terms of the mass of the parts of the plant that people can actually eat. They make a note in the study that the vegetative growth of the plants (the green leafy parts humans can't eat) was not affected by the jump from optimal to super-optimal CO2 levels.

Ask the Experts: Does Rising CO2 Benefit Plants?

Climate change skeptics have an arsenal of arguments for why humans need not cut their carbon emissions. Some assert rising CO2 levels benefit plants, so global warming is not as bad as scientists proclaim. &ldquoA higher concentration of carbon dioxide in our atmosphere would aid photosynthesis, which in turn contributes to increased plant growth,&rdquo Rep. Lamar Smith (R&ndashTexas) wrote in an op-ed last year. &ldquoThis correlates to a greater volume of food production and better quality food.&rdquo Scientists and others calling for emission cuts are being hysterical, he contends.

So is it true rising atmospheric CO2 will help plants, including food crops? Scientific American asked several experts to talk about the science behind this question.

There is a kernel of truth in this argument, experts say, based on what scientists call the CO2 fertilization effect. &ldquoCO2 is essential for photosynthesis,&rdquo says Richard Norby, a corporate research fellow in the Environmental Sciences Division and Climate Change Science Institute of Oak Ridge National Laboratory. &ldquoIf you isolate a leaf [in a laboratory] and you increase the level of CO2, photosynthesis will increase. That&rsquos well established.&rdquo But Norby notes the results scientists produce in labs are generally not what happens in the vastly more complex world outside many other factors are involved in plant growth in untended forests, fields and other ecosystems. For example, &ldquonitrogen is often in short enough supply that it&rsquos the primary controller of how much biomass is produced&rdquo in an ecosystem, he says. &ldquoIf nitrogen is limited, the benefit of the CO2 increase is limited&hellip. You can&rsquot just look at CO2, because the overall context really matters.&rdquo

Scientists have observed the CO2 fertilization effect in natural ecosystems, including in a series of trials conducted over the past couple decades in outdoor forest plots. In those experiments artificially doubling CO2 from pre-industrial levels increased trees&rsquo productivity by around 23 percent, according to Norby, who was involved in the trials. For one of the experiments, however, that effect significantly diminished over time due to a nitrogen limitation. That suggests &ldquowe cannot assume the CO2 fertilization effect will persist indefinitely,&rdquo Norby says.

In addition to ignoring the long-term outlook, he says, many skeptics also fail to mention the potentially most harmful outcome of rising atmospheric CO2 on vegetation: climate change itself. Its negative consequences&mdashsuch as drought and heat stress&mdashwould likely overwhelm any direct benefits that rising CO2 might offer plant life. &ldquoIt&rsquos not appropriate to look at the CO2 fertilization effect in isolation,&rdquo he says. &ldquoYou can have positive and negative things going at once, and it&rsquos the net balance that matters.&rdquo So although there is a basic truth to skeptics&rsquo claim, he says, &ldquowhat&rsquos missing from that argument is that it&rsquos not the whole picture.&rdquo

Scientists have also looked specifically at the effects of rising CO2 on agricultural plants and found a fertilization effect. &ldquoFor a lot of crops, [more CO2] is like having extra material in the atmosphere that they can use to grow,&rdquo says Frances Moore, an assistant professor of environmental science and policy at the University of California, Davis. She and other experts note there is an exception for certain types of plants such as corn, which access CO2 for photosynthesis in a unique way. But for most of the other plants humans eat&mdashincluding wheat, rice and soybeans&mdash&ldquohaving higher CO2 will help them directly,&rdquo Moore says. Doubling CO2 from pre-industrial levels, she adds, does boost the productivity of crops like wheat by some 11.5 percent and of those such as corn by around 8.4 percent.

A lack of nitrogen or other nutrients does not affect agricultural plants as much as wild ones, thanks to fertilizer. Still, research shows plants &ldquoget some benefits early on from higher CO2, but that [benefit] starts to saturate&rdquo after the gas reaches a certain level, Moore says&mdashadding, &ldquoThe more CO2 you have, the less and less benefit you get.&rdquo And while rising carbon dioxide might seem like a boon for agriculture, Moore also emphasizes any potential positive effects cannot be considered in isolation, and will likely be outweighed by many drawbacks. &ldquoEven with the benefit of CO2 fertilization, when you start getting up to 1 to 2 degrees of warming, you see negative effects,&rdquo she says. &ldquoThere are a lot of different pathways by which temperature can negatively affect crop yield: soil moisture deficit [or] heat directly damaging the plants and interfering with their reproductive process.&rdquo On top of all that, Moore points out increased CO2 also benefits weeds that compete with farm plants.

Rising CO2&rsquos effect on crops could also harm human health. &ldquoWe know unequivocally that when you grow food at elevated CO2 levels in fields, it becomes less nutritious,&rdquo notes Samuel Myers, principal research scientist in environmental health at Harvard University. &ldquo[Food crops] lose significant amounts of iron and zinc&mdashand grains [also] lose protein.&rdquo Myers and other researchers have found atmospheric CO2 levels predicted for mid-century&mdasharound 550 parts per million&mdashcould make food crops lose enough of those key nutrients to cause a protein deficiency in an estimated 150 million people and a zinc deficit in an additional 150 million to 200 million. (Both of those figures are in addition to the number of people who already have such a shortfall.) A total of 1.4 billion women of child-bearing age and young children who live in countries with a high prevalence of anemia would lose more than 3.8 percent of their dietary iron at such CO2 levels, according to Meyers.

Researchers do not yet know why higher atmospheric CO2 alters crops&rsquo nutritional content. But, Myers says, &ldquothe bottom line is, we know that rising CO2 reduces the concentration of critical nutrients around the world,&rdquo adding that these kinds of nutritional deficiencies are already significant public health threats, and will only worsen as CO2 levels go up. &ldquoThe problem with [the skeptics&rsquo] argument is that it&rsquos as if you can cherry-pick the CO2 fertilization effect from the overall effect of adding carbon dioxide to the atmosphere,&rdquo Myers says. But that is not how the world&mdashor its climate&mdashworks.

Plant carbon metabolism and climate change: elevated CO 2 and temperature impacts on photosynthesis, photorespiration and respiration

Contents Summary 32 I. The importance of plant carbon metabolism for climate change 32 II. Rising atmospheric CO2 and carbon metabolism 33 III. Rising temperatures and carbon metabolism 37 IV. Thermal acclimation responses of carbon metabolic processes can be best understood when studied together 38 V. Will elevated CO2 offset warming-induced changes in carbon metabolism? 40 VI. No plant is an island: water and nutrient limitations define plant responses to climate drivers 41 VII. Conclusions 42 Acknowledgements 42 References 42 Appendix A1 48 SUMMARY: Plant carbon metabolism is impacted by rising CO2 concentrations and temperatures, but also feeds back onto the climate system to help determine the trajectory of future climate change. Here we review how photosynthesis, photorespiration and respiration are affected by increasing atmospheric CO2 concentrations and climate warming, both separately and in combination. We also compile data from the literature on plants grown at multiple temperatures, focusing on net CO2 assimilation rates and leaf dark respiration rates measured at the growth temperature (Agrowth and Rgrowth , respectively). Our analyses show that the ratio of Agrowth to Rgrowth is generally homeostatic across a wide range of species and growth temperatures, and that species that have reduced Agrowth at higher growth temperatures also tend to have reduced Rgrowth , while species that show stimulations in Agrowth under warming tend to have higher Rgrowth in the hotter environment. These results highlight the need to study these physiological processes together to better predict how vegetation carbon metabolism will respond to climate change.

Keywords: acclimation drought nitrogen stomatal conductance warming water use efficiency.

Effects of different elevated CO2 concentrations on chlorophyll contents, gas exchange, water use efficiency, and PSII activity on C3 and C4 cereal crops in a closed artificial ecosystem

Although terrestrial CO2 concentrations [CO2] are not expected to reach 1000 μmol mol(-1) (or ppm) for many decades, CO2 levels in closed systems such as growth chambers and greenhouses can easily exceed this concentration. CO2 levels in life support systems (LSS) in space can exceed 10,000 ppm (1 %). In order to understand how photosynthesis in C4 plants may respond to elevated CO2, it is necessary to determine if leaves of closed artificial ecosystem grown plants have a fully developed C4 photosynthetic apparatus, and whether or not photosynthesis in these leaves is more responsive to elevated [CO2] than leaves of C3 plants. To address this issue, we evaluated the response of gas exchange, water use efficiency, and photosynthetic efficiency of PSII by soybean (Glycine max (L.) Merr., 'Heihe35') of a typical C3 plant and maize (Zea mays L., 'Susheng') of C4 plant under four CO2 concentrations (500, 1000, 3000, and 5000 ppm), which were grown under controlled environmental conditions of Lunar Palace 1. The results showed that photosynthetic pigment by the C3 plants of soybean was more sensitive to elevated [CO2] below 3000 ppm than the C4 plants of maize. Elevated [CO2] to 1000 ppm induced a higher initial photosynthetic rate, while super-elevated [CO2] appeared to negate such initial growth promotion for C3 plants. The C4 plant had the highest ETR, φPSII, and qP under 500-3000 ppm [CO2], but then decreased substantially at 5000 ppm [CO2] for both species. Therefore, photosynthetic down-regulation and a decrease in photosynthetic electron transport occurred by both species in response to super-elevated [CO2] at 3000 and 5000 ppm. Accordingly, plants can be selected for and adapt to the efficient use of elevated CO2 concentration in LSS.

Keywords: C3 and C4 plants Chlorophyll fluorescence Elevated [CO2] Gas exchange Life support systems Water use efficiency.

Drought impact study shows new issues for plants and carbon dioxide

Credit: CC0 Public Domain

Extreme drought's impact on plants will become more dominant under future climate change, as noted in a paper out today in the journal Nature Climate Change. Analysis shows that not only will droughts become more frequent under future climates, but more of those events will be extreme, adding to the reduction of plant production essential to human and animal populations.

"Even though plants can, in many cases, benefit from increased levels of carbon dioxide that are predicted for the future atmosphere, the impact of severe drought on destroying these plants will be extreme, especially in the Amazon, South Africa, Mediterranean, Australia, and southwest USA," said lead study author Chonggang Xu of Los Alamos National Laboratory. Future drought events are typically associated with low humidity, low precipitation, high temperature, and changes in carbon released from fire disturbances.

The frequency of extreme droughts (defined by low plant-accessible soil water) per year is predicted to increase by a factor of

3.8 under a high greenhouse-gas emission scenario and by a factor of

3.1 under an intermediate greenhouse-gas emission scenario during 2075-2099, compared to the historical period of 1850-1999.

Drought is already the most widespread factor affecting plant production via direct physiological impacts such as water limitation and heat stress. But indirectly it also can have devastating effect, through increased frequency and intensity of disturbances such as fire and insect outbreaks that release large amounts of carbon back into the atmosphere.

Plants fix carbon dioxide into an ecosystem through photosynthesis, and this process plays a key role in the net carbon balance of the terrestrial biosphere that contributed to its regulation of atmospheric carbon dioxide. And even though higher carbon dioxide concentrations in future decades can help increase plant production, the combination of low soil water availability, heat stress, and disturbances associated with droughts could negate the benefits of such fertilization.

"Future plant production under elevated carbon dioxide levels remains highly uncertain despite our knowledge on carbon dioxide fertilization effects on plant productivity," Xu said.

The research team analyzed the outputs from 13 Earth System Models (ESMs) and the results show that due to a dramatic increase in the frequency of extreme droughts, the magnitude of globally-averaged reductions in plant production will be nearly tripled by the last quarter of this century relative to that of the study's historical period (1850-1999).

For plants living through mild or moderate droughts, the situation is not as dire. The problem is that more of the droughts that come will be the extreme ones. "Our analysis indicates a high risk of increasing impacts of extreme droughts on the global carbon cycle with atmospheric warming," Xu said, "At the same time though, this drought risk will be potentially mitigated by positive anomalies of plant production associated with favorable environmental conditions."

Carbon dioxide and water use in forests

Plants are expected to respond to rising levels of atmospheric carbon dioxide by using water more efficiently. Direct evidence of this has been obtained from forests, but the size of the effect will prompt debate. See Letter p.324

In a study published on page 324 of this issue, Keenan et al. 1 report that the efficiency with which forests use water has increased over the past 20 years, and conclude that this is a consequence of rises in the concentration of atmospheric carbon dioxide. Footnote 1 The findings call for a reassessment of models of the terrestrial carbon cycle.

The concentration of CO2 in the atmosphere is rising at an unprecedented rate. In May this year, it reached 400 parts per million, 43% above the pre-industrial concentration of 280 p.p.m. (ref. 2). Much of this increase has occurred in recent decades, with the rate of increase over the past 20 years being 5% per decade 2 . This drastic upsurge in atmospheric CO2 should have stimulated plant productivity worldwide, because we know from experiments that rising CO2 concentrations increase the rate of photosynthesis and reduce water use in plants 3 . Such effects are fundamental to our current understanding of the carbon cycle — for example, most terrestrial carbon-cycle models explain the current land sink for carbon by assuming that rising CO2 levels have enhanced plant productivity 4 .

However, detecting the effects of rising CO2 concentrations on terrestrial vegetation outside controlled experiments has proven remarkably difficult, provoking numerous debates about whether such effects are really occurring 5,6,7,8,9,10 . There are few high-quality, long-term records of plant productivity and water use that can be used to test for such effects. The main types of data come from plot surveys, tree-ring records, satellite images, aerial photographs and measurements of stream flow. Each of these is an indirect measurement and has a relatively coarse time resolution. Even where trends in these data have been detected, it has been extremely difficult to attribute them to rising CO2 levels, because simultaneous changes in many confounding factors — such as rainfall, temperature, land use and fire frequency — have occurred 11 .

Keenan et al. bring a new source of data to bear on this problem. The eddy-covariance technique, developed in the 1980s to quantify the exchange of gases between the atmosphere and land areas, has revolutionized plant-ecosystem science because it continuously monitors the functioning of whole ecosystems on an hourly timescale 12 . Using instruments mounted above a vegetation canopy, eddy covariance can be used to measure the carbon uptake and water use of whole ecosystems on a spatial scale of up to one square kilometre. Over the past 20 years, eddy-covariance towers have been established worldwide in a broad range of ecosystems, and high-quality long-term data sets are now becoming available 12 .

The authors used these data to analyse long-term changes in ecosystem-scale water-use efficiency. Plants lose water through evaporation whenever they open their stomata to admit CO2 for photosynthesis, and water-use efficiency is a measurement of the rate at which the plant exchanges water for carbon. This measurement is expected to be a good indicator of the effects of rising CO2 concentrations on vegetation — because rising CO2 levels both increase carbon uptake by plants and reduce plant water use, the effects of higher levels of CO2 on water-use efficiency should be larger and more consistent than the effects on carbon gain or water use alone.

Keenan and colleagues report that the water-use efficiency of forest canopies in the Northern Hemisphere (Fig. 1) over the past two decades shows a remarkable upward trend. The trend was consistent, with no decreases at any of the 21 forest sites examined. The rates of increase across all sites were large (averaging about 3% per annum) and highly statistically significant.

Keenan et al. 1 analysed the flux of water vapour and carbon dioxide above forests in the Northern Hemisphere, such as at Willow Creek, California (pictured).

To prove that this rise was caused by increasing levels of atmospheric CO2, Keenan et al. examined a suite of potential confounding factors. They found that, across all 21 sites, there were no trends in meteorological variables (precipitation, wind speed, temperature and humidity) or structural attributes of canopies (leaf area, leaf-nitrogen content and canopy roughness) that could explain the observed rise in water-use efficiency. The authors conclude that the trend was most consistent with a strong fertilization effect of increased CO2.

This observation-based finding will probably stimulate considerable debate and further research because, although the reported trend is convincing, the magnitude of the trend is much larger than would be predicted from our existing knowledge of plant responses to CO2. Decades of controlled experiments 13,14,15 have consistently found that the intercellular CO2 concentration (Ci) in photosynthesizing tissue is proportional to the atmospheric CO2 concentration (Ca) — that is, Ci/Ca is constant. The trend identified by Keenan et al., however, implies that intercellular CO2 has remained constant, and so Ci/Ca has strongly decreased with increasing concentrations of atmospheric CO2.

To put it another way, controlled experiments 16,17 have found that the effect of increased atmospheric CO2 levels on water-use efficiency is roughly proportional to the increase in atmospheric CO2. By contrast, according to our calculations, the increase in water-use efficiency found by Keenan and co-workers in the eddy-covariance data is approximately six times larger than the corresponding increase in atmospheric CO2. Consequently, the authors show that current models of the terrestrial carbon cycle do not capture the magnitude of the trend obtained from eddy covariance. This is to be expected, because the models were developed from, and so are consistent with, data from controlled experiments 17 .

Keenan and colleagues' study thus provides an intriguing challenge to our understanding of ecosystem functioning — there is a significant increase in water-use efficiency that we cannot currently explain. The implication is either that plants are markedly more responsive to rising CO2 levels than we previously thought, or that there are other, unknown factors behind the observed trend in the eddy-covariance data. Our view is that it is unlikely that the effect of CO2 on water-use efficiency is being underestimated by the magnitude that Keenan et al. suggest, because the response of water-use efficiency to CO2 is so predictable in controlled experiments 16,17 . Nevertheless, the authors have ruled out most other potential drivers for the trend. More research is clearly needed to understand these findings, including long-term studies with complementary observational data streams, such as measurements of changes in plant biomass and water use by whole ecosystems.

Carbon dioxide feeds plants, but are Earth's plants getting full?

Credit: lowpower225 / shutterstock

Plants do a lot of work for us, producing the air we breathe, the food we eat, and even some of our medicine. But when it comes to removing carbon dioxide in the atmosphere, we may have been overestimating their ability.

Photosynthesis acts as the lungs of our planet—plants use light and carbon dioxide (CO2) to make the sugars they need to grow, releasing oxygen in the process. When atmospheric CO2 concentrations increase, as they have been thanks to humans burning fossil fuels, one might think that plants are enjoying a smorgasbord of food for unlimited growth. But a new study published in Science shows this excess of riches is not as effective as previously thought.

Since CO2 is the main source of food for plants, increasing levels of it directly stimulate the photosynthetic rate of most plants. This boost in photosynthesis, known as the "CO2 fertilization effect," enhances growth in many of earth's plant species, with the effects seen most clearly in crops and young trees, and less so in mature forests.

The amount of CO2 used by photosynthesis and stored in vegetation and soils has grown over the past 50 years, and now absorbs at least a quarter of human emissions in an average year. We've been assuming that this benefit will continue to increase as CO2 concentrations rise, but data collected over a 33-year period show us that might not be true.

Fertilization is in decline

Estimating the size of the global CO2-fertilization effect accurately is no easy task. We have to understand what limits photosynthesis from one region to another, and at every scale from molecules within a leaf through to whole ecosystems.

The big research team behind the new Science study used a combination of data from satellites and on-the-ground observations and models of the carbon cycle. Using this powerful toolkit, they found that the fertilization effect declined across much of the globe from 1982 to 2015—a trend that correlates well with observed changes in nutrient concentrations and available soil water.

FLUXNET towers around the world measure the exchanges of carbon dioxide, water vapor, and energy between the biosphere and atmosphere. Credit: Caitlin Moore, Author provided

In many ways, the combination of these different tools helps to paint a more complete picture of how the world's ecosystems are photosynthesising. The researchers used a collection of long-term measurements from flux towers like the one pictured below which continuously monitor the CO2 and water used by plants and are dotted across earth's biomes and provide the best means of measuring photosynthesis at the ecosystem scale.

Flux towers are limited in their measurement range (1 km or so) – but the data these towers collect helps verify the satellite estimates of how much photosynthesis is going on. With satellites and flux towers now providing records since the 1990s (and earlier in some cases), scientists are able to assess long-term trends in global photosynthesis. These can then be compared to "models"—the computer-based simulations predicting plant–environment interactions—as the researchers did in this recent study.

What might the models be missing?

Researchers in the latest study found that the decrease in CO2 fertilization was related to the availability of nutrients and water, which the computer simulations might not be accounting for properly. We know that nutrients such as nitrogen and phosphorus are declining) in some areas—which may be unaccounted for. Plants can also acclimate, or change how they grow, when the environment changes.

Just like we can spend less on groceries when food is plentiful, plants invest less nitrogen in photosynthesis when they are grown at high CO2. When this happens, CO2 fertilization is less effective than before. Because some plants have a stronger response than others, the response can be difficult to account for in computer simulations.

For many years, some people have assumed that carbon fertilization will mitigate climate change by slowing the rate at which CO2 is increasing in the atmosphere. Although the effect is built into the models used to predict future climates, the argument has become widely misinterpreted by those who believe the world is overreacting to climate change.

But if the new study is right, and we have indeed been overestimating the amount of carbon that plants will pull from the atmosphere in the future, even our most cautious climate projections have likely been optimistic.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

More Photosynthesis Doesn’t Mean More Food

Yes, we now get far more food from each acre of farmland than we did a century ago. But extra carbon dioxide only accounts for a small fraction of the increase.

“A 30 percent increase in photosynthesis does not translate into a 30 percent increase in strawberries off the land,” said Dr. Campbell.

While photosynthesis does pull carbon dioxide out of the atmosphere, much of that gas goes right back into the air. The reason: At night, the chemical reactions in plants essentially run backward. In a process known as respiration, plants pump out carbon dioxide instead of pulling it in.

“Part of the story is that photosynthesis is going up, and part of the story is that so is respiration,” said Dr. Campbell.

While the increase in photosynthesis is greater than that of respiration, the ultimate benefit to crops has been small — and it doesn’t explain our modern agricultural revolution.

“The driving factor has to be the fertilizers, the seed varieties, the irrigation,” Dr. Campbell said.

Reevaluating global goals

The 1.5- and 2-degree temperature targets are currently the centerpiece of international climate mitigation efforts&mdashthe ultimate goal that world leaders are striving to meet under the Paris climate agreement. But some experts are concerned that these targets may not encompass all the risks associated with climate change. And some say they may not be promoting the level of global climate action originally hoped for.

As a result, some experts are now advocating for additional climate goals that tackle the issue in different ways.

Baker and his co-authors suggest adopting a concrete cap on atmospheric carbon dioxide concentrations&mdasha goal to keep carbon dioxide levels from passing a certain threshold.

A CO2 target would be a new step for world leaders, but it&rsquos not necessarily a new idea. Some climate activists have argued for similar strategies. The climate advocacy organization, co-founded by prominent activist Bill McKibben, was established around the idea that world leaders should work to get atmospheric carbon dioxide concentrations below 350 parts per million.

For Baker and colleagues, the idea is to use a CO2 target to avoid bad outcomes if the planet&rsquos climate sensitivity ends up being on the lower side after all.

&ldquoIf you have a temperature target, and you have a low climate sensitivity, that sounds great because it means you can emit more carbon dioxide into the atmosphere,&rdquo Baker said.

But even if it turned out that a 1.5- or 2-degree goal could be achieved with greater emissions than previously thought&mdasha larger &ldquocarbon budget,&rdquo in other words&mdashthe new research shows that the extra emissions could still be damaging on their own. Adding a carbon dioxide limit on top of the temperature target would address this issue.

It could also help address some recent criticisms of the &ldquocarbon budget,&rdquo which some experts say may not be as useful for global climate policy as once thought.

A temperature target is only useful if policymakers know how to achieve it. As a result, numerous studies in the last decade have attempted to determine the amount of carbon the world can emit without exceeding certain temperatures. This value would inform policymakers about how quickly they need to reduce greenhouse gas emissions in the coming years.

The problem is that the budgets don&rsquot always agree with one another. A wide variety of assumptions can influence a carbon budget estimate&mdasheverything from how much warming the Earth has already experienced to the role of non-CO2 greenhouse gases, like methane&mdashand scientists are still debating many of them.

As a result, different research groups have produced different estimates over the years. In the last six months, for instance, several studies have suggested that the carbon budget is significantly larger than previously thought (Climatewire, June 19).

Recently, some experts have suggested that these uncertainties may confuse policymakers, or allow them to argue there&rsquos still time to reduce emissions later&mdashespecially if recent estimates seem to suggest that there&rsquos more leeway than before (Climatewire, May 22).

A CO2 goal, in addition to a temperature goal, could address some of those issues, says Glen Peters, from the Centre for International Climate Research, who has previously questioned the carbon budget&rsquos usefulness in international climate policy.

An unmovable target just for carbon dioxide would take &ldquoa big part of the uncertainty out&rdquo regarding the planet&rsquos climate sensitivity, he said in an email. It would also eliminate the need to deal with other uncertainties that affect carbon budget calculations, such as how much the planet has already warmed or when the baseline &ldquopreindustrial era&rdquo actually started.

But it could also create new problems, he cautioned. There&rsquos some uncertainty about exactly how emissions translate into carbon dioxide concentrations in the atmosphere. Scientists are still trying figure out how much carbon dioxide is soaked up by the land and the ocean in the planet&rsquos natural carbon cycle. And setting a carbon dioxide cap also doesn&rsquot address the role of other greenhouse gases, like methane.

&ldquoSo, a carbon budget for CO2 concentrations would solve some problems but create others,&rdquo Peters noted.

Oliver Geden of the German Institute for International and Security Affairs, another scientist critical of the carbon budget&rsquos policy use, also noted that setting a CO2 cap isn&rsquot the same thing as developing a concrete timeline for reducing greenhouse gas emissions. Ideally, the target would lead to the timeline, but as he pointed out, merely setting a goal &ldquodoes not tell a single government what to do.&rdquo

So both temperature targets and CO2 targets have their limitations. Baker and his co-authors recommend adopting both&mdashkeeping the temperature target and adding a CO2 limit on top of it. Peters says that would &ldquomake sense&rdquo as a way to at least narrow down the uncertainties, even if it doesn&rsquot solve all the problems.

Thomas Stocker, a climate scientist at the University of Bern in Switzerland, also suggests that multiple targets may be the way to go. He previously co-authored research suggesting that a temperature threshold doesn&rsquot capture all the risks associated with climate change. Adopting a set of specific, individual climate goals&mdashfor instance, limits on the amount of global and regional warming, ocean acidification, agricultural losses or sea-level rise&mdashwould require greater reductions in carbon emissions than those needed to meet a global average temperature goal alone, the research suggests.

Setting a carbon dioxide limit would be &ldquoanother complication in the political process,&rdquo he acknowledged. But if it were paired with existing temperature targets, it could be another step in the right direction.

&ldquoWe all realize that whilst the 1.5- and 2-degree Celsius limits in the Paris agreement are politically easily communicable and understandable goals, for the protection of the climate system as a whole they are not sufficient,&rdquo he told E&E News. &ldquoIn other words, one has to really look at what generates impacts, and the various components of the climate system&mdashand to what extent are we willing to accept the level of impacts that a given climate change is affecting.&rdquo

Clearing the air: The hidden wonders of indoor plants

Plants located in your home or office are beneficial to your health in more ways than you might think. Credit: Miss Monk

It may come as a surprise but air pollution levels indoors are almost always higher than outside, even in busy city centres. Even more surprising is that indoor plants have the ability to mitigate high levels of most airborne contaminants.

Ventilation systems constantly "refresh" buildings with air from outdoors after a filtration process that removes some large particles, such as pollen, from it. Once inside, this air is augmented by a large range of indoor-sourced pollutants.

Two of the most significant of these are volatile organic compounds (VOCs) and carbon dioxide.

Volatile organic compounds are petrochemical vapours that are "outgassed" or continuously liberated from building materials, such as paint and carpet, as well as furnishings, plastics and electronic equipment. In high concentrations, many of these agents are acutely toxic and carcinogenic. Some even disrupt the endocrine systems of animals.

Up to 900 different compounds have been detected in some buildings. The most commonly found ones include benzene, ethylbenzene, toluene and xylene.

While the concentration of volatile organic compounds in modern buildings is generally quite low, there's growing evidence that continued chronic exposure to even low levels of these chemicals may result in the condition known as sick building syndrome.

Sufferers of this syndrome experience acute or sub-acute discomfort and health effects that appear to be linked to the duration of time spent in a building. Typical symptoms range from drowsiness, physical irritability, difficulty concentrating, fatigue and nausea.

These symptoms can be severe enough to greatly diminish a person's ability to work effectively. Their direct cause is usually unknown to the sufferer, but they're relieved soon after leaving the building.

The other major indoor pollutant, carbon dioxide (CO2), is produced by human respiration. High levels of CO2 (above 800 to 1,000 parts per million) cause rooms to feel "stuffy". But sick building syndrome-like symptoms can occur at much lower concentrations than this.

When CO2 levels are above 1,000 ppm, building occupants can become quite unwell. But this level is uncommon in modern buildings thanks to efficient mechanical ventilation systems.

Indoor plants help break down hundreds of volatile organic compounds found in modern buildings. Credit: bfishadow

The ability of plants to improve indoor air quality was recognised in the 1980s, when NASA researched growing plants on space stations. Results indicated the surprising removal of previously high volatile organic compound concentrations in their model spacecraft.

Initial research focused on the plant species themselves. But the consistency of the air-cleaning capacity among distantly related species suggested that it was not necessarily a property of particular plants.

Then, around the year 2000, Australian researchers determined that virtually all of the volatile organic compound-removing ability of potted plants resided in the pot. It was the normal bacteria of the potting mix that took up the volatile organic compounds.

But the plants are not superfluous: experiments where the plants were removed leaving only the potting mix showed a gradual loss of performance over a few weeks.

The plants supply the soil bacteria with key nutrients that sustain their viability and health.

More recent experimentation has monitored the background concentrations of volatile organic compounds in offices with and without plants over some weeks. These findings indicate that even three potted plants in an average-sized office will reduce airborne volatile organic compounds to an extremely low level.

Plant-mediated CO2 removal has received less research attention, primarily because this pollutant is well controlled by modern air conditioning systems. But field trials have shown that between three and six medium-sized plants in a non-air conditioned building can reduce CO2 concentrations by a quarter.

The question now is whether we should be using air conditioning for ventilation purposes at all when indoor plants can do the work for us at a greatly reduced cost. But a lot more work is needed before we get the complete picture of the potential of plants to deal with indoor CO2.

The growing push for more sustainable buildings should give this field impetus.

Indoor plants at work

Environmental psychologists have long proposed that indoor plants can improve workplace performance and satisfaction.

Having a plant in the office has positive outcomes, including an improved emotional state, reduced negative mood states, reduced distraction, increased creativity, and improved task-performance.

Many studies have related these effects to the idea of biophilia, which suggests being near a plant returns us, in some small way, to our evolutionary beginnings in the prehistoric forest.

Far from being just another form of interior decoration, plants are important for maintaining the habitability of the indoor environment, where most of us spend the great majority of our lives.

We know plants have always maintained air quality and kept us happy and productive. Dramatically rising energy costs and a growing emphasis on sustainability should make us consider the role plants will play in the indoors of the future.

This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives).