A Model‐Based Investigation of Terrestrial Plant Carbon Uptake Response to Four Radiation Modification Approaches - Duan - 2020 - Journal of Geophysical Research: Atmospheres - Wiley Online Library

[Andrew Lockley]

Poster's note: this has the opposite sign to other work on the subject eg https://keith.seas.harvard.edu/publications/solar-geoengineering-reduces-atmospheric-carbon-burden

https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019JD031883

Journal of Geophysical Research: AtmospheresVolume 125, Issue 9

Research Article

A Model‐Based Investigation of Terrestrial Plant Carbon Uptake Response to Four Radiation Modification Approaches

Lei Duan Long Cao Govindasamy Bala Ken Caldeira

First published:04 April 2020

https://doi.org/10.1029/2019JD031883

Abstract

A number of radiation modification approaches have been proposed to counteract anthropogenic warming by intentionally altering Earth's shortwave or longwave fluxes. While several previous studies have examined the climate effect of different radiation modification approaches, only a few have investigated the carbon cycle response. Our study examines the response of plant carbon uptake to four radiation modification approaches that are used to offset the global mean warming caused by a doubling of atmospheric CO2. Using the National Center for Atmospheric Research Community Earth System Model, we performed simulations that represent four idealized radiation modification options: solar constant reduction, sulfate aerosol increase (SAI), marine cloud brightening, and cirrus cloud thinning (CCT). Relative to the high CO2 state, all these approaches reduce gross primary production (GPP) and net primary production (NPP). In high latitudes, decrease in GPP is mainly due to the reduced plant growing season length, and in low latitudes, decrease in GPP is mainly caused by the enhanced nitrogen limitation due to surface cooling. The simulated GPP for sunlit leaves decreases for all approaches. Decrease in sunlit GPP is the largest for SAI which substantially decreases direct sunlight, and the smallest for CCT, which increases direct sunlight that reaches the land surface. GPP for shaded leaves increases in SAI associated with a substantial increase in surface diffuse sunlight, and decreases in all other cases. The combined effects of CO2 increase and radiation modification result in increases in primary production, indicating the dominant role of the CO2 fertilization effect on plant carbon uptake.

Plain Language Summary

A number of radiation modification approaches have been proposed to intentionally alter Earth's radiation balance to counteract anthropogenic warming. However, only a few studies have analyzed the potential impact of these approaches on the terrestrial plant carbon cycle. Here, we simulate four idealized radiation modification approaches, which include direct reduction of incoming solar radiation, increase in stratospheric sulfate aerosols concentration, enhancement of marine low cloud albedo, and decrease in high‐level cirrus cloud cover, and analyze changes in plant photosynthesis and respiration. The first three approaches cool the earth by reducing incoming solar radiation, and the last approach allows more outgoing thermal radiation. These approaches are designed to offset the global mean warming caused by doubled atmospheric CO2. Compared to the high CO2 world, all approaches will limit plant growth due to induced surface cooling in high latitudes and will lead to reduced nitrogen supply in low latitudes, leading to an overall reduction in the plant carbon uptake over land. Different approaches also produce different changes in surface direct and diffuse sunlight, which has important implications for plant photosynthesis. Relative to the unperturbed climate, the combined effects of enhanced CO2 and radiation modifications leads to an increase in plants' primary production.

[Govindasamy Bala]

Andrew,

This is no contradiction between the Keith et al's commentary and this paper. Keith et al.'s paper is about stocks and this JGR paper is about the rate of flow of carbon between the atmosphere and the land biosphere (flux). The stocks and fluxes can behave very differently. The cooling caused by SRM reduces the rate of fluxing of carbon between the atmosphere and plants but overall it helps to build the carbon stocks in biomass and soils and hence reduce the atmospheric CO2. 

Another good example for stocks and fluxes behaving very differently is the change in precipitation (flux) and atmospheric water vapor (stock) under global warming. It is well established now that precipitation increases at the rate of 2-3% per deg warming while water vapor increases at the rate of about 7% per deg warming. 

Best,

Bala

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[Andrew Lockley]

If the incoming flux decreases, the stock will reduce. To counter this, the outgoing flux must decrease by as much, or more. What is this corresponding decrease in the outward flux?

Is it that decomposition of leaf litter, etc. is slowed by cooler and drier conditions? 

In situations where plants don't remain to decomposition (agro forestry), what will be the effect? Your results imply a loss of NPP. 

Andrew 

[Govindasamy Bala]

Andrew,

You are absolutely right that "In situations where plants don't remain to decomposition (agro forestry), there will be a loss of NPP"

Stock changes between two time periods are basically the integral of the net fluxes between the two time periods. In a warming scenario, there is net outward flux (and stocks decline) because the integrated respiratory fluxes more than the integrated in flux of NPP. In SRM scenario, integrated net flux is positive because the integrated respiratory fluxes are smaller than integrated in flux. 

Best,

Bala

[Andrew Lockley]

So, to confirm:

Are you saying that SRM effect on the carbon cycle still appears to be the net removal of Atmospheric CO2?

 If so, SRM can legitimately be used as a CDR technique. It may therefore be eligible for Carbon credits, as per this paper.

https://journals.sagepub.com/doi/abs/10.1177/1461452916630082

[Stephen Salter]

Hi All

I have not yet found my way round the paywall but wondered if the paper takes account of a reduction of solar input over the sea affecting conditions for crops over land.

Stephen

Emeritus Professor of Engineering Design. School of Engineering, University of Edinburgh, Mayfield Road, Edinburgh EH9 3DW, Scotland S.Sa...@ed.ac.uk, Tel +44 (0)131 662 1180 WWW.homepages.ed.ac.uk/shs, YouTube Jamie Taylor Power for Change

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[Govindasamy Bala]

Andrew,

"Are you saying that SRM effect on the carbon cycle still appears to be the net removal of Atmospheric CO2?"

Yes, that is what the models say since this first 2008 PNAS paper by Matthews and Ken on this topic which showed that CO2 levels would be lower in SRM scenarios. This work finds that CO2 is reduced from 900 ppm to about 800 ppm in the atmosphere by 2100 in the A2 scenario. Not a lot as CO2 forcing goes up only  logarithmically with CO2 concentration

 

https://www.pnas.org/content/104/24/9949  

There would be of course large uncertainties but I think the qualitative result would not change across models. I would not go that far to say it is a CDR technique. I would rather say it is a secondary benefit or a co-benefit. 

Bala

[Andrew Lockley]

The reason that the CDR aspect is significant is that there is already a way to monetise this, through voluntary carbon offsets. This was first suggested by Sargoni and I https://www.researchgate.net/publication/284534197_Environment_Policy_Solar_Radiation_Management_and_the_voluntary_carbon_market

There's no such scheme available to monetise radiative forcing 

Andrew 

[Govindasamy Bala]

Andrew,

Technically, carbon and radiative forcing are equivalent to each other. There are standard formulas to go from carbon to radiative forcing. 

Bala

[Andrew Lockley]

That is indeed correct, but there is no accepted approach to financialise temporary radiative forcing. The effect on the carbon cycle would give a way to create a business model for SRM operations - as described in the papers I've sent. 

Andrew 

[Govindasamy Bala]

With so much uncertainty surrounding this small indirect carbon cycle effects of SRM, I would not bother about monetizing calculations at this time. 

Bala

[Andrew Lockley]

I would recommend that you consider the numbers on this, before forming a firm view. To order of magnitude, 1t S delivered is $1000 (mileage may vary). If 1Mt a year is roughly enough to offset the RF of global warming, then about 10pc of that is a CO2 effect. That's about 0.1 millionth of global warming per t of S, on an annual basis - according to your figures. Assuming we sustain the intervention for a century, that's $100k for maintenance of that 1t, for a century - again offsetting 0.1 millionth. 

Offsets go for about $3/t https://www.energysage.com/other-clean-options/carbon-offsets/costs-and-benefits-carbon-offsets/

There's about 1Tt of CO2 to offset - ie $3T, using the offset price. 0.1 millionths of that is $300k

So your $100k costs gives you a $300k return. 

Not bad, unless (as usual) I've fluffed my 4th grade math. 

Andrew 

[Renaud de RICHTER]

Dear Andrew and Bala,

All your discussion you had during this post is very difficult to understand.

Bala and the other authors of the article cited in the subject found with their model that a 2 X CO2 by the end of century will enhance gross primary production (GPP) and net primary production (NPP).

See Table 1: "Changes in key climate and carbon variables over land for the 2 × CO2 case relative to 1 × CO2 case, and radiation modification cases relative to the 2 × CO2 case".

•  under 2 X CO2 => GPP (+33.1 GTC/yr) and NPP (+7.9 GTC/yr) mainly by the fertilizing effect of atmospheric CO2.
  
•  meanwhile SRM / SAI will decrease GPP (-14.7 GTC/yr) and NPP (-2.1 GTC/yr) mainly by cooling effect.
  

In the abstract it is written "Relative to the high CO2 state, all these approaches reduce gross primary production (GPP) and net primary production (NPP)."

So it looks like the opposite to your statement..." SRM effect on the carbon cycle still appears to be the net removal of Atmospheric CO2...".

Where am I wrong?

Thanks!

Bw,

Renaud

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[Andrew Lockley]

If I understand correctly, SRM reduces decomposition more than NPP - presumably because cooler, drier soils are less biologically active.

Andrew 

[Renaud de RICHTER]

Yes indeed, but the model used maybe (I hope) took already this factor in consideration, did it?

[Govindasamy Bala]

Renaud,

You are not wrong.

The SRM world has two forcings; one from increased CO2 and the other from sunlight reduction. The sunlight reduction alone offsets the warming from CO2 and causes a slight reduction in NPP. However, the CO2-fertilization effect (and the associated very large (??) increase in NPP) is mostly not offset by the sunlight reduction. The CO2-fertilization effect (and ocean acidification) are the carbon cycle effects of CO2 increase that are mostly not offset by SRM. 

Hence, relative to the control climate, NPP is larger in the SRM world but NPP is slightly less relative to the warmer climate. Yes, what you comparing the SRM world with is extremely important in interpreting these results. Hope this helps. 

Bala

[Govindasamy Bala]

Andrew,

In the case of CDR like DAC, one can immediately know much carbon is extracted and pricing is easy.  In the case of carbon stocks increase due to SRM, attribution of the stock increase to SRM would be almost an impossible task in the real world. 

Bala

[Stephen Salter]

Dear Bala

 . . . .  However countries facing expensive damage from hurricanes and typhoons could measure surface temperatures  in surrounding seas and pay to have them reduced to more acceptable values.  I understand the 26.5 C is nice. Rough calculations appear to give extremely attractive returns on investment at least according to my assumptions.  I can send the equations to you and anyone else who  would like them and would be grateful for any more accurate than my own.

Stephen

Emeritus Professor of Engineering Design. School of Engineering, University of Edinburgh, Mayfield Road, Edinburgh EH9 3DW, Scotland S.Sa...@ed.ac.uk, Tel +44 (0)131 662 1180 WWW.homepages.ed.ac.uk/shs, YouTube Jamie Taylor Power for Change

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[Andrew Lockley]

What is the margin of error? 

[Govindasamy Bala]

Dear Stephen,

Here too, the attribution would be a big challenge. In this case the challenge would be because of the presence of large internal variability in the climate system, particularly on regional scales. 

Bala

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[Stephen Salter]

Dear Bala

I agree that day-to-day and even month-to-month there will be large variations on the cooling input.  But the process would begin at the end of a typhoon season. The ocean is an excellent integrator and the fleet controllers would adjust the number of spray vessels through the year to get close to the target temperature at the start of the next typhoon season.

Stephen

Emeritus Professor of Engineering Design. School of Engineering, University of Edinburgh, Mayfield Road, Edinburgh EH9 3DW, Scotland S.Sa...@ed.ac.uk, Tel +44 (0)131 662 1180 WWW.homepages.ed.ac.uk/shs, YouTube Jamie Taylor Power for Change