This is what happens when I have an insomnia problem. I wrote most of this in the small hours of this morning. Note that I've put this together from several sources, in a branch of science I'm only passingly familiar with, between about 2 and 4 in the morning, so I may have made mistakes. I think I've got most of them. Correct or challenge me if you spot any.
I want to keep plants in conditions that they can sequester carbon (and produce food) for as much time as possible. This means keeping them in suitable temperature ranges.
When plants grow, they need a number of factors to be in the right range. Among others:
Light – enough of it and at the right wavelengths to break molecular bonds. If there isn't enough of it, or if it's at the wrong wavelengths, it won't happen. At some point, depending on the plant and other conditions, there is so much light that the plant can't do any more with it. We call this saturation. Other factors can saturate as well.
Temperature – photosynthesis slows down to a point not really worth bothering with much below 10C. Only slightly above 30C the proteins involved in photosynthesis degrade and photosynthesis stops. The efficiency curve tails off sharply to zero above this temperature. Obviously, under outdoor conditions excess heat will be associated with light saturation.
Carbon Dioxide – modern plants evolved in conditions when CO2 had an atmospheric concentration around 280 parts per million by volume (ppmv). If other conditions can be controlled, in some but not all species more photosynthesis will take place at higher concentrations, to about 1000ppmv. Current atmospheric concentrations are around 400ppmv, and rising frighteningly fast. At present, concentrations much above or below 400ppmv are unlikely outside the controlled conditions of a greenhouse. Note that, for a whole range of reasons, more is not necessarily better, especially if other variables can't be controlled. Indeed, in uncontrolled conditions, outside a greenhouse, more is often a problem, because plants today have not evolved in high CO2 conditions. We need to be aware that more CO2 may encourage pest outbreaks. Also, higher CO2 concentrations reduce nutritional quality in some foods, but there is not much we can do about that except sequester as much of it as we can. Here I'm trying to work out how to better control more variables.
Water – again, enough, but not too much. This varies wildly across the plant kingdom, from aquatic species that need to live in water to some cacti and succulent species that can go for months or years without rainfall. With climate change plants face two problems. One is increased evaporation and transpiration (to a point they can control the latter under higher temperatures, but only to a point), and the other is an increased frequency of severe flooding events. Plants therefore need conditions in which as much water is stored in soils for as long as possible without being waterlogged. We can also provide irrigation ponds.
This post is an exercise in two things:
1) keeping plants in that ideal range, and preferably towards the optimum, for as long as possible and
2) bouncing as much of the light and heat energy they do not use or do not want back into space, where it can't do any more damage. Obviously, reducing atmospheric levels of carbon dioxide and methane is part of the equation, and this is part of that process.
These are not independent values. In general, we can do both – most of the time. To a point, this is an exercise in function stacking with caveats.
So, we use light to break carbon dioxide bonds in order to produce sugars. Photosynthesis is, however, a dismally inefficient process. Most of the energy gets converted to heat or reflected back out into space. With global warming an existential threat, the more of that energy we can reflect back into space, the better.
So, we know the best ways of preventing quite so much carbon dioxide and methane from being released into or sticking around in the atmosphere: plant food forests, and raise fewer livestock animals that will just convert all those hard-won sugars and carbohydrates back into carbon dioxide and methane, because science.
Okay, what about reflecting more of it back into space? As noted, above about 30C photosynthesis rates collapse. For much of the world, not only will greater albedo help the planet, it will help plants too – but only within limits. In some conditions, if too much light is reflected, plants won't be able to photosynthesise as effectively.
As this image shows, savanna habitats (habitats with maybe 40% canopy coverage, or what we call forest gardens), are at the high end for productive habitats in relation to albedo: forest gardens reflect more light back into space than about half of all crops and most meadows (i.e. grazing land, where livestock also convert a lot of plant sugars straight back into CO2 and, worse, methane, thus undoing much of the good work of forest gardeners). In terms of albedo, the forest gardeners have the edge against the ranchers, and against a lot of the monoculturalists (who are pretty poor at locking carbon up in soils) and beat the aquaculture people hands down. Incidentally, pond soils are poor at holding carbon.
"By Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany (Own work) [CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons"
In terms of water, however, remember that evaporation will help lower ambient temperatures, and plants need water to survive. This is not a campaign against ponds, but ponds covered with vegetation are better than open water. Rainwater needs to be slowed down, so more of it soaks into soils and be captured in ponds (without stripping soil by erosion in the process).
In general terms, afforestation in temperate and tropical zones tends to cool the planet (albedo is only one factor). Savanna habitats would seem to act more so.
This also means that photosynthesis can continue even when it would otherwise be too hot for the process, locking up even more carbon. It also has implications for slowing evaporation of water from soils. Greater reflectivity seems to enable plants to hold on to water better.
Note that bare soil has lower albedo than vegetated soil, unless the soil is dry. If you expect the soil to dry out anyway, you can keep it uncovered. If you can keep it moist, then keep it vegetated! Vegetated soil sequesters carbon, but you can't sequester carbon without water.
So, one thing we can be doing is using (or breeding) plants with waxier, and/or hairy leaves, which reflect more sunlight. We can also breed for variegated leaves. For breeders, caution will be needed in terms of ensuring that there is no reduction in chloroplast activity in sunlight conditions where this is not saturated.
Leaf angle counts too. Our savannah habitats are largely tree covered, but what about the rest of the canopy? It turns out that erect leaves (like most grasses and the allium family) reflect more light than horizontal leaves. This suggests that planting more alliums, for example, in the herb layer may help keep temperatures down. This distribution varies between varieties, and plant breeding may allow us to improve on this. Hopefully people with more knowledge than I have in this field are working on how to identify the traits we need to be looking for.
We need to be careful, however: plants with more of a horizontal leaf profile are associated with greater drought resistance.
Keeping plants healthy is also crucial, but this is just good practice. A healthy plant produces a better crop.
So, how might this affect yields? One obvious problem is that the more light that is reflected the less light is used for photosynthesis. That said, greater reflectance is also associated with more chlorophyll. Both waxy and hairy leaves are associated with higher yields (at least in some cases, and we'd need to identify which) and greater pest resistance. More importantly, the cooler growing-period environment should keep yields up.
Hi Neil, I do touch on this in the book. Albedo impacts are much stronger at higher latitudes and minimized in the tropics. As you get to higher latitudes (like Northern Canada, Scandinavia, Siberia, perhaps Tierra del Fuego) planting evergreen trees has a net warming impact because their albedo outweighs the carbon they sequester. This is because a dark green tree is so much darker than a white snowy field. In mid-latitudes, like temperate North America, evergreen trees lose something like 10 to 40% of their impact due to albedo. these impacts are much less for deciduous trees. Fortunately we have very few evergreen crop trees for temperate and boreal climates, pine nuts are really the only ones I can think of. But deciduous trees are still worse than grassland or annual cropland in these climates. Perennial grains, when developed, will be a really valuable strategy for high – latitude climates. This is true both in terms of their albedo and their cold hardiness. I think this is an area to think quite a bit more about, we're really lacking a lot of the data we would need in order to. Really understand and develop a platform for albedo – friendly agriculture at high latitudes. Nice to see you thinking about this though!