Shocked and Persuaded

So there is a common expression that is getting even more common and more over used called “Back Of The Envelope” which basically is an expression that describes calculations that are very course spatially or vertically OR rough enough that they can be done on the back of an envelope/napkin.

I decided it might be fun to take this concept and apply it to the commonly held belief that man emits 45 Gigatons (1 Gigaton = 1 Billion Tons) per annum. This has been cited in many a popular and scientific paper. Hope you enjoy! I would just note that below there are a bunch of numbers and all assumptions came from some of my Ph.D. work and not out of thin air. It is interesting that my BOTEs while not exactly the same as the IPCC’s independent estimate of 45 Petagrams (1 Pg = 1 Gt) are similar. The issue going forward will be identifying the black-boxes that remain in these equations. (NOTE: All local ecosystem units are in Grams of Carbon per meter squared per year and this is a standard unit of measurement for ecosystem fluxes).

Introduction

Over the course of the past 260 years 10.5 and 17.0 Million km² of forest and savannah lands, respectively, have been cleared for a variety of purposes. During that same period of time 15.0 and 34.0 M km² of land were converted to crops and grazing land. The additional 21.5 M km² converted to the latter two agricultural purposes most likely came from expansion into native grasslands, drainage of wetlands and swamps (i.e., Southeast Asia), and irrigation technologies allowing for production of heat tolerant crops on desert and scrublands. In order to gain an estimate of how this land-use volatility has influenced the cumulative flow of carbon we used the aforementioned values in concert with ecosystem productivity values drawn from Auch and Ross [1-4].

Methods

Values extracted from Auch and Ross were in grams of Carbon per meter squared per year (g C m^-2 yr^-1) they were converted to Petagrams in the following manner:

1 g C m^-2 yr^-1×1,000 = g C km²

(g C km²×km² in question) × 260 years = Total grams since 1750

Total grams since 1750 / (1×10¹²) = Petagrams

OR

Total grams since 1750 / (1×10⁶) = Tons

This conversions were calculated across all land-use types for Net (NPP) and Gross Primary Productivity (GPP), with the former simply GPP minus annual litterfall. In addition we estimated annual leaf litter decay for each land-use type using the aforementioned publications. For the sake of simplicity we converted only NPP to total and annual CO₂ (ppm) assuming [5]:

Petagrams/2.13 = ppm CO₂

We assumed that the 21.5 M km² of excess land accreted to crops and grazing came from conversion of arid, desert, or grasslands with a average NPP and GPP rates of 581 and 1,120 g C m^-2 yr^-1.

Results

The clearing of terrestrial ecosystems has decreased the terrasphere’s Carbon Capture and Storage (CCS) capability by 17.4 ppm annually in the last 260 years. Coincidentally the Net CO₂ Abatement is approximately equal to the capacity lost due to forest, savannah, and desert/grassland lost. This has been partially offset by the ability of crops and grazed lands to “capture” 5,000 Pg (9.0 ppm annually) of carbon during this same period. The Net Effect of long-term terrestrial ecosystem displacement and agricultural proliferation has been a decrease of: 1) 4,622 Pg NPP, 2) 1,175 Pg of litter decay/soil carbon creation, 3) 9,000 Pg GPP, and 4) the ratio of long-term structural biomass production (i.e., NPP) to biomass turnover increased 1.07, which is to say soil carbon pools are being drawn down at a rate 121% in excess of what they otherwise would have been beneath native systems. This last statement speaks to a kind of chronic and broad carbon – as well as nitrogen and phosphorus – displacement due to land-use change.

In the last 260 years the CO₂ yield curve (the gap between forests as CO₂ sinks and anthropogenic sources) inverted with current anthropogenic sources meeting and exceeding native ecosystem buffering capacity by 0.49 ppm annually. Using rough estimates of aquatic productivity the “abatement” capability of the earth’s ecosystems increases by 7.3 ppm annually (i.e. 17.5 ppm). The true anthropogenic CO₂ footprint is:

1749 to 2009: 17.5 ppm + 0.49 ppm = 17.96 ppm

17.96 × 2.13 = 38.3 Petagrams

2001 to 2009: 17.5 ppm + 1.93 ppm = 19.41 ppm

19.41 × 2.13 = 41.3 Petagrams

2004 to 2009: 17.5 ppm + 2.95 ppm = 20.43 ppm

20.43 × 2.13 = 43.5 Petagrams

This last figure is slightly less than the often cited 45Pg of anthropogenic CO₂ of Barnes et al [6]

The aforementioned inversion gap increased by 265% in the 2^nd half of the 20^th century and 400% in the first decade of the 21^st century.