Some additional comments on your most recent comment that "priority should be given to crops which can limit GHG emissions". I agree but think this should be qualified in various ways.
1. Emissions from agriculture come mostly from use of fuel-burning machinery, and secondarily from deforestation (in slash and burn agriculture). Now, concerning machinery, there is a worldwide increase in the use of no-tillage or limited-tillage systems by commercial farmers (e.g. in the US, Brasil or Argentina), which greatly reduce the use of machinery and thus of fuel, and besides keep the land under green cover most of the time, and sequester more carbon when crop residues are mixed with topsoil before planting again on the bed created by the previous crop. Now this is mostly practiced by modern and relatively big farmers in high and middle income countries, hardly by subsistence peasants (at least not on in massive numbers). Such systems require know how, machinery and other inputs not within the reach of the poorest farmers.
2. Plants and trees absorb carbon from the atmosphere while they are growing (and emit it while they are rotting on the ground). A stable forest neither absorbs nor emits carbon in net terms.
3. There is evidence that the planet is getting greener, in the sense that green cover and vegetagtion Net Primary Production is increasing worldwide in spite of deforestation. This is partly due to the very presence of higher concentrations of CO2 in the atmosphere. In this respect there is a new paper by Liu and others (2015, see ref below) verifying that the planet is getting greener. This agrees with other recent studies such as those by Donohue et al (2013) and Hemming et al (2013) also referenced below. A review of Liu's paper can be found in an article published by Reuters: http://www.trust.org/item/20150330150036-h00zp/. F
Recent satellite-based studies of global green cover focused on semi-arid regions confirms Liu's conclusion by showing a general increase in green cover linked to increased CO2 concentrations in the atmosphere (Donohue et al 2013). A simulation with the Hadley Centre climate model for equilibrium doubling of preindustrial concentration of CO2 found an expected net global increase of 57% in Net Primary Production of plants at the world scale (Hemming et al 2013). This expected increase results from a gross increase of 75% in photosynthetic capacity and a 21% decrease due to plant responses to changes in the climate. It tends to support the idea that CO2 effects may be larger than climate effects (e.g. temperature and rainfall) to determine the impact of anthropogenic climate change on vegetation of all kinds (natural or cultivated). Dr Josep Carbonell, head of the World Carbon Project, sums it all up by stating: "CO2 is bad for the climate, but good for plants".
REFERENCES
Donohue, Randall; Michael L.Roderick, Tim R.McVicar, & Graham D.Farquhar (2013). Impact of CO2 fertilization on maximum foliage cover across the globe’s warm, arid environments. Geophysical Research Letters 40:1-5.
Hemming, Deborah; Richard Betts & Matthew Collins (2013). Sensitivity and uncertainty of modelled terrestrial net primary productivity to doubled CO2 and associated climate change for a relatively large perturbed physics ensemble. Agricultural and Forest Meteorology 170:79–88. http://www.sciencedirect.com/science/article/pii/S0168192311003157.
Liu, Y.Y., A.I.J.M. van Dijk, R.A.M. de Jeu, J.G. Canadell, M.F. McCabe, J.P. Evans and G. Wang (2015), Recent reversal in loss of global terrestrial biomass, Nature Climate Change, doi: 10.1038/nclimate2581.
Dear Florence,
Some additional comments on your most recent comment that "priority should be given to crops which can limit GHG emissions". I agree but think this should be qualified in various ways.
1. Emissions from agriculture come mostly from use of fuel-burning machinery, and secondarily from deforestation (in slash and burn agriculture). Now, concerning machinery, there is a worldwide increase in the use of no-tillage or limited-tillage systems by commercial farmers (e.g. in the US, Brasil or Argentina), which greatly reduce the use of machinery and thus of fuel, and besides keep the land under green cover most of the time, and sequester more carbon when crop residues are mixed with topsoil before planting again on the bed created by the previous crop. Now this is mostly practiced by modern and relatively big farmers in high and middle income countries, hardly by subsistence peasants (at least not on in massive numbers). Such systems require know how, machinery and other inputs not within the reach of the poorest farmers.
2. Plants and trees absorb carbon from the atmosphere while they are growing (and emit it while they are rotting on the ground). A stable forest neither absorbs nor emits carbon in net terms.
3. There is evidence that the planet is getting greener, in the sense that green cover and vegetagtion Net Primary Production is increasing worldwide in spite of deforestation. This is partly due to the very presence of higher concentrations of CO2 in the atmosphere. In this respect there is a new paper by Liu and others (2015, see ref below) verifying that the planet is getting greener. This agrees with other recent studies such as those by Donohue et al (2013) and Hemming et al (2013) also referenced below. A review of Liu's paper can be found in an article published by Reuters: http://www.trust.org/item/20150330150036-h00zp/. F
Recent satellite-based studies of global green cover focused on semi-arid regions confirms Liu's conclusion by showing a general increase in green cover linked to increased CO2 concentrations in the atmosphere (Donohue et al 2013). A simulation with the Hadley Centre climate model for equilibrium doubling of preindustrial concentration of CO2 found an expected net global increase of 57% in Net Primary Production of plants at the world scale (Hemming et al 2013). This expected increase results from a gross increase of 75% in photosynthetic capacity and a 21% decrease due to plant responses to changes in the climate. It tends to support the idea that CO2 effects may be larger than climate effects (e.g. temperature and rainfall) to determine the impact of anthropogenic climate change on vegetation of all kinds (natural or cultivated). Dr Josep Carbonell, head of the World Carbon Project, sums it all up by stating: "CO2 is bad for the climate, but good for plants".
REFERENCES
Donohue, Randall; Michael L.Roderick, Tim R.McVicar, & Graham D.Farquhar (2013). Impact of CO2 fertilization on maximum foliage cover across the globe’s warm, arid environments. Geophysical Research Letters 40:1-5.
Hemming, Deborah; Richard Betts & Matthew Collins (2013). Sensitivity and uncertainty of modelled terrestrial net primary productivity to doubled CO2 and associated climate change for a relatively large perturbed physics ensemble. Agricultural and Forest Meteorology 170:79–88. http://www.sciencedirect.com/science/article/pii/S0168192311003157.
Liu, Y.Y., A.I.J.M. van Dijk, R.A.M. de Jeu, J.G. Canadell, M.F. McCabe, J.P. Evans and G. Wang (2015), Recent reversal in loss of global terrestrial biomass, Nature Climate Change, doi: 10.1038/nclimate2581.