aerosols and land surface geoengineering

[Colin Forrest]

More on the above

Terpene emissions are reckoned to protect the trees from pathogens and browsers, and also increase with temperature, as does isoprene. They are reduced carbon species and react with OH radicals to produce secondary aerosols, which become more polar as they are oxidised, and they do form clouds, in suitable circumstances. "There is substantial evidence to prove that terpenoids are one of the main precursors for aerosol formation in several sites (Tunved et al., 2006; Barriera et al., 2021)" from  Undetected biogenic volatile organic compounds from Norway spruce drive total ozone reactivity measurements, Thomas et al 2023.

Terpenes are made from a number of isoprene (C5H8) units, Land vegetation is reckoned to emit about 500 Tg per year isoprene, 100 Tg of terpenes, 100 Tg of methanol, and minor amounts of other NMVOCs. Grasses, wetlands, crops and trees (some species within some families of some, mostly broadleaved, trees ) produce isoprene. The poplar family are known to be high isoprene emitters and have been well studied.

 Can we breed or genetically engineer trees to produce more aerosols ? Some work on selection on genetics has been done, for example; 

 " Genotypic differences in the responses to warming treatment were also observed. alpha-Pinene ( a terpene) emission, which has been suggested to protect plants from elevated temperature, increased from genotype 5.2 only. Isoprene emission from genotype 2.2 decreased, whereas genotype 5.2 was able to retain high isoprene emission level also under elevated temperature." and "  We consider aspen genotype 5.2 to have better potential for adaptation to increasing temperature because of thicker photosynthetic active palisade layer and higher isoprene and alpha-pinene emission levels compared to genotype 2.2" from  Emissions of volatile organic compounds and leaf structural characteristics of European aspen (Populus tremula) grown under elevated ozone and temperature, Hartikainen et al , 2009.

Isoprene interacts with nitrogen oxides from air pollution and increases harmful low level tropospheric ozone levels, So you would plant aspen genotype 2.2, a low emitter, around and downwind of urban centres, and genotype 5.2, a high emitter, in remote regions, where increased cloud cover would help protect the trees from increased temperature and direct solar radiation locally, and contribute to global cooling.

However these positives would have to be balanced against the negative, which that for every Tg of extra isoprene added to the atmosphere, roughly a Tg of methane which would otherwise have been oxidised will remain there, adding extra warming, and increasing the lifetime of all the other methane molecules which are emitted after that.

On a geoengineering scale, I don't know enough about tropical ecosystems to comment, but the Arctic is greening, and the european aspen mentioned above is an ideal pioneer tree species. Replacing conifers, tundra or wetland with a mixed deciduous woodland ( maybe 20% N-fixers, 50% high isoprene emission aspen, 30% others for maximum isoprene emission) would : 

  * increase the land surface albedo by ~0.02

  * create local clouds above the forests, protecting the trees and increasing the overall albedo by another, possibly significant, amount.

 * reduce the amount of HO radicals available to oxidise methane.

Once the trees reach canopy closure and a leaf area index (LAI) of 2, their albedo won't increase much, but isoprene and terpene emissions will increase with temperature and the productivity, i.e. Gross primary production (GPP),

 Planting trees will generally reduce the water table, which will :

  *  Lower the water table and reduce methane emissions

  * increase CO2 emissions from microbial decomposition of oxygenated soil

  * create oxygenated organic soil and increase methane consumption by soil microbes 

  * increase carbon storage in wood

The soil temperature under woodland is cooler than in the open, due to direct shading and the higher evapotranspiration rate of the trees ( which means more solar energy in used to evaporate water and less in increasing the temperature). This reduces the activity and emissions of  all the soil micro-organisms, (for a change from low ground cover to woodland).

In the UK the overall net greenhouse gas balance of a conifer plantation planted on typical moorland soil is 6 tons of CO2 (eq) per hectare per year, after subtracting around 1 ton CO2 from soil emissions. There are only minor methane comings and goings on these well drained sites. These CO2 capture rates are higher than in other temperate research sites and wouldn't be obtainable at higher latitudes, but would still be positive. CO2 soil emissions from planting and establishment can be high, perhaps taking the first 10 years of tree growth to compensate.

 

Replacing boreal coniferous woodland, moors, marshes, poor grassland or tundra with deciduous woodland cover would reduce CH4 emissions, increase CO2 capture, and increase albedo. Once the effects are integrated over 50 years or more, I wouldn't be suprised if it produced more cooling than SAI, once a full life cycle analysis (LCA) was done on both technologies. However we don't have 50 years.

Regards,  Colin Forrest