Plants produce biogenic aerosols and provide a negative feedback to warming climate

Another study showing a negative feedback to a warming climate. Needless to say such a negative feeback finds no place in climate models. (The lead author points out that this previous statement is erroneus). Needless to say I doubt if such negative feedback is included in all climate models.

 Paasonen, P., et. al. 2013. Evidence for negative climate feedback: warming increases aerosol number concentrationsNature Geoscience doi: 10.1038/NGEO1800

The International Institute for Applied Systems Analysis (IIASA) reports:

© Veronika Markova |

As temperatures warm, plants release gases that help form clouds and cool the atmosphere, according to research from IIASA and the University of Helsinki. 

The new study, published in Nature Geoscience, identified a negative feedback loop in which higher temperatures lead to an increase in concentrations of natural aerosols that have a cooling effect on the atmosphere. “Plants, by reacting to changes in temperature, also moderate these changes,” says IIASA and University of Helsinki researcher Pauli Paasonen, who led the study. 

Scientists had known that some aerosols – particles that float in the atmosphere – cool the climate as they reflect sunlight and form cloud droplets, which reflect sunlight efficiently. Aerosol particles come from many sources, including human emissions. But the effect of so-called biogenic aerosol – particulate matter that originates from plants – had been less well understood. Plants release gases that, after atmospheric oxidation, tend to stick to aerosol particles, growing them into the larger-sized particles that reflect sunlight and also serve as the basis for cloud droplets. The new study showed that as temperatures warm and plants consequently release more of these gases, the concentrations of particles active in cloud formation increase. 

“Everyone knows the scent of the forest,” says Ari Asmi, University of Helsinki researcher who also worked on the study. “That scent is made up of these gases.” While previous research had predicted the feedback effect, until now nobody had been able to prove its existence except for case studies limited to single sites and short time periods. The new study showed that the effect occurs over the long-term in continental size scales. 

The effect of enhanced plant gas emissions on climate is small on a global scale – only countering approximately 1 percent of climate warming, the study suggested. “This does not save us from climate warming,” says Paasonen. However, he says, “Aerosol effects on climate are one of the main uncertainties in climate models. Understanding this mechanism could help us reduce those uncertainties and make the models better.”  

The study also showed that the effect was much larger on a regional scale, counteracting possibly up to 30% of warming in more rural, forested areas where anthropogenic emissions of aerosols were much lower in comparison to the natural aerosols. That means that especially in places like Finland, Siberia, and Canada this feedback loop may reduce warming substantially. ….. 

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3 Responses to “Plants produce biogenic aerosols and provide a negative feedback to warming climate”

  1. ktwop Says:

    The lead author has sent this email:
    I noticed you have given a space for our research in your blog That is great!

    However, I wonder what is your second own sentence based on, “Needless to say such a negative feeback finds no place in climate models.”? My experience so far seems to be the opposite. If you have some information on this, could you please share that with me.

    I would hope to have your response.

    Pauli Paasonen

    Pauli Paasonen, Ph.D.

    Division of Atmospheric Sciences
    Department of Physics
    University of Helsinki

    Currently at:
    IIASA (International Institute for Applied Systems Analyses)
    Schlossplatz 1
    A-2361 Laxenburg, Austria

  2. ktwop Says:

    Dear Dr.Paasonen,

    Thank you for your mail.
    I am not a “climate expert” but merely an interested observer. But I am a former user of mathematical models in engineering where real things had to get built and had to work.

    That there are negative feedbacks emanating from forests has long been known and yet is still unknown.
    For example this in addition to your paper.

    Click to access Bonan_2008.pdf

    Those depending upon carbon dioxide concentration get some attention but effects of temperature are not usually considered I think.

    IPCC AR4 Chapter 9 focuses on the effects of deforestation and forestation for mitigation effects but pays little attention to the temperature sensitivities.

    Click to access ar4-wg3-chapter9.pdf

    The feedbacks and sensitivities of global temperature (whatever that might be defined to be) are not very well known or quantified.

    It is common in engineering that many parameters in a complex model have to use empirical data. But confirmation bias is the one thing that is not allowed. Unfortunately – and in my opinion – even those climate mathematical models which pay token regard to the existence of negative forcings tend – because they are not known – to disregard or minimise such forcings. The kind of confirmation bias built into the models I have looked at would never be allowed in any engineering project where something real has to be built.
    It is good that your experience is that these are now being considered. That can only improve these far from perfect models of a chaotic system where “we don’t know what we don’t know”.

    The IPCC process is badly flawed and in my opinion confirmation bias has replaced scientific rigor In general the effects of clouds as a “cooling” agent are grossly underestimated (and of course solar effects in general and in cloud formation in particular are virtually excluded). The effect of aerosols and particulates are also underestimated – but again that is just my opinion.

    Hence my comment.

  3. ktwop Says:

    Dr. Paasonen points out in an email that climate models have considered his work — ktwop

    Thank you for your answer. The reason why I wrote my previous email is that in my knowledge at least some IPCC models have the feedback described in our study already inbuilt, since they have temperature dependent biogenic volatile organic compound emissions, which eventually grow particles to cloud condensation nuclei -sizes.

    Now that our results are public, the model results can be compared to those. I would not be sure that the models underestimate the aerosol – cloud feedbacks. Because of very badly understood and possibly positive further indirect aerosol climate effects (in addition to cloud albedo and cloud lifetime effects) it can be that the negative forcing by clouds is overestimated. But that is an area for which we just do not have tools to estimate. And, these are now my personal opinions.

    However, the above discussion is out of the main point:
    Saying that “Needless to say such a negative feeback finds no place in climate models.” leaves no room for being an opinion of the author or being any way uncertain, it says that the feedback is in no means included in the models, end of the story. I find this erroneous.

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