Green Metrics, Clean Metrics

Carbon Footprints of Nations

Here is an important tool to help us understand the uneven distribution of GHG emissions by consumption category and country: The Carbon Footprint of Nations website from the Norwegian University of Science and Technology, based on a global trade-linked analysis using the input-output method.

Here is how global emissions break down by purpose:

• 72% of global emissions are related to household consumption
• 10% of emissions are from government consumption
• 18% of emissions are attributed to investments

And here is the break down by consumption category:

• Food accounts for 20% of GHG emissions
• Operation and maintenance of residences contribute 19%
• Transportation is 17%

The IPCC estimates that agriculture contributes 13.5% of global emissions, based on land area used for agriculture, numbers of livestock and other details (but not including land use change). The 20% share of food is based on an input-output analysis and presumably includes processing, packaging and other added value beyond agriculture -- a very useful estimate if accurate.

Kumar Venkat

Tesco's organic carbon footprints

An analysis performed for Tesco, the UK retain chain, showed last year that organic potatoes and tomatoes have the same/similar carbon footprint as conventionally grown varieties, without including any soil carbon sequestration. This supports some of the results I have discussed previously that suggest that organics don't have a clear advantage unless soil carbon effects are included. And including those effects does require a proper time frame to be considered along with a recognition that soil carbon will reach an equilibrium value under a constant management method.

Kumar Venkat

Do organics have a lower carbon footprint?

There is not a strong correlation between organic food production methods and lower carbon footprints. Our analysis shows that some organic crop products grown in California -- including alfalfa, broccoli, some apples, some wine and raisin grapes, blueberries, and walnuts -- generate somewhat higher cradle-to-farmgate GHG emissions than comparable conventional products grown in the same region. We've also seen that other organics -- such as strawberries and some wine grapes -- have lower emissions.

What are the key reasons for this? Organics require many of the same inputs as conventional production -- such as irrigation, fuel, electricity and transportation -- and sometimes they use more of one or the other of these inputs depending on the geographical location and management practices. They do save the emissions generated from the production of synthetic fertilizers and pesticides, but their on-farm energy use from fossil sources -- including gasoline, diesel and electricity -- is comparable to conventional production and sometimes higher. Organics have slightly lower nitrous oxide emissions from nitrogen applied to the soil, but in the same order of magnitude as conventionals based on the IPCC model. It is likely that much of the compost/manure used in organic production is sourced locally (for example, from within a 200 mile radius), but local delivery of these materials typically does not use the most efficient transport (single-unit trucks as opposed to the semi-trailers used for long-haul transport) and transporting large volumes of these inputs adds further to the emissions. In some cases, there is an additional impact from the periodic planting of cover crops. Finally, some organic systems have a significantly lower yield than conventional systems and may also have less optimal economies of scale.

What about carbon sequestered and/or released from the soil? When land use and management practices have been unchanged for many years, it is widely accepted that the soil carbon content approaches a spatially-averaged stable equilibrium value specific to the local conditions. Unless the management practices have changed recently (for example, within the IPCC default transition period of 20 years between equilibrium values) on a given cropland, the soil carbon content must be considered to be unchanged on average. In such an equilibrium state, any new carbon added to the soil through organic amendments is balanced by the carbon dioxide released through oxidation of soil organic matter -- although these are biological processes and therefore not instantaneous. So, organics generally do not get credit for building soil carbon except in the transition years following a switch from conventional to organic production. The results cited here are all based on the assumption that the agricultural systems are in steady state and the soil carbon is at equilibrium.

What about meat production? Any differences in the emissions generated to produce organic and conventional feedstuffs (hay, grain, etc.) will be reflected in the overall carbon footprints of conventional and organic meats. But meat production is impacted significantly by processes that are independent of feed production: manure management and enteric fermentation (the latter for ruminant animals). So, two meat production systems that use similar management practices and similar feed recipes -- except for some minor differences in how and where the feedstuffs are produced -- will likely not show much difference in their carbon footprints.

What about meat and milk from grass-fed animals? I bring this up because some organic systems include a fairly significant grass-fed component. Ruminants that get most of their feed from forage and hay produce significantly more methane from enteric fermentation than animals that are mostly grain-fed. For a given amount of dry matter or gross energy consumed, the methane emissions can vary by a factor of 2-4 depending on the types of feed consumed. Plus, manure dropped on an open pasture generates more direct nitrous oxide compared to many managed systems. On the positive side, grass-fed animals do not consume grains produced and transported using fossil-fuel based inputs. On balance, grass-fed animal products from ruminants are likely to have higher carbon footprints compared to products from conventionally housed/fed animals (this is without considering any gains in soil carbon).

A recent article in Time magazine claims that grass-fed animals have lower net emissions – even with the higher methane emissions – because grazing animals help the soil sequester carbon by working manure and decaying organic matter into the soil. The article does not cite any literature sources. While there may well be a benefit during a transition period, it is unlikely that the soil carbon can accumulate indefinitely without reaching equilibrium.

See my next post for part 2 of this discussion.

Kumar Venkat

Transitional soil carbon sequestration in agriculture

In my last post, I looked at organic production with the assumption that the soil carbon was at a stable equilibrium and therefore did not influence the product carbon footprint. This, of course, raises the question of how the soil carbon changes in the transitional years (i.e., the first 20 years) after switching from conventional to organic production. There are potentially a number of uncertainties and unknowns involved in estimating this, so much so that the PAS 2050 carbon footprint standard specifically excludes this from consideration -- and virtually all product carbon footprint assessments have assumed that agricultural systems are in some steady state condition. However, the question remains, so I'll use IPCC's land-use change model to provide a simple analytical example.

Let us start with some assumptions. Let us choose the climate zone to be "warm temperate" and the precipitation regime to be "dry" (which approximate portions of California). We also need to know the soil type: LAC (low-activity clay) soil is a reasonable choice, because its default carbon content is somewhere between HAC and sandy soils.

If some undeveloped land in this region was converted and used in conventional production (assume full till and very low levels of organic carbon added) for 40 years, and then if it was switched to organic production (assume reduced till and high levels of organic carbon added) for the next 40 years, then according to the IPCC model the soil carbon profile (in tonnes of carbon per hectare) would look like this over the 80-year period:
 

 

Note that I am using IPCC's default 20-year periods here for soil carbon reaching equilibrium values under each landuse/management method and the model is linear. During the transition to organic production (years 41 through 60), the model predicts that an equivalent of 638 kg of CO2 would be added to the soil annually per acre of land. I independently derived an average annual sequestration equivalent of 593 kg of CO2 for cultivated land converted to organic farming based on data from a new report from the UK Soil Association, which confirms that the IPCC model prediction is in the right ball park.

Now, take an example of an actual organic product grown in California: Broccoli, with an annual yield of 6500 kg per acre. During the transition years (41-60), this would reduce the carbon footprint of each kg of broccoli by about 25% if we credit the entire carbon sequestration in each transitional year to that year's production. 

Soil carbon sequestration clearly reduces the carbon footprint of organics during the transitional years (if we assume that the management practices will continue indefinitely and keep the carbon in the soil), but not so once the system reaches steady state. This means that we'll need to classify organic systems as transitional and steady-state on a case-by-case basis before we can decide whether they get credit for carbon sequestration in any given year.

One of the remaining issues is that not all organic production occurs in transitional systems. A literature survey in the UK Soil Association report cited above includes examples of both transitional (< 20 years after conversion to organic) and steady-state (> 20 years after conversion to organic) systems. Another issue: Some studies (see Smith et al 2007) suggest that the majority of the soil carbon sequestration occurs within the first 10 years after management change, in which case many commercial organic production systems may already be in (or close to) steady state.

I do think there is a more robust way to model the sequestration effect by using a 100-year assessment period and a sufficiently long production period. Although standards don't provide guidance on this issue as of now, the obvious analogy is the way that long-term carbon storage in biomass, wood and concrete is modeled (PAS 2050, Annex C). Applying a 50-year production period to our example, the average reduction in product carbon footprint over that period is about 9%. If the carbon is released from the soil after 50 years because of a future land-use change, the effective reduction goes down to 4%.  With a 100-year production period, the reduction is about 4.5%. If our starting time for modeling is in the middle of the transitional period, then the footprint reductions diminish further.

While the aggregate effect of additional soil carbon sequestration is important and has been identified as a tool for climate change mitigation, its effect on product carbon footprint appears to be quite modest over a sufficiently long production period.

Kumar Venkat