Juegos del lenguaje como mecanismo de transgresión literaria

Deforestation in the tropics has been responsible for the most of the global phytomass loss during the twentieth century. This satellite image from 2010 (acquired by the Moderate Reso-lution Imaging Spectroradiometer on NASA ’ s Terra satellite) shows the extent of forest clear-ing in the state of Rond ô nia in western Brazil. The image can be downloaded at http://

earthobservatory.nasa.gov/Features/WorldOfChange/images/amazon/amazon_deforestation _2010214_lrg.jpg .

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grasslands that are lightly grazed by well-managed numbers of domestic animals (alpine meadows with cattle and sheep) or natural forests where harvests remove only annually renewable fruits, seeds, or nuts (such as the collection of Brazil nuts in an old-growth tropical rain forest) or are limited to a few valuable animals (such as the trapping of small mammals for their pelts in Canada ’ s boreal forest): the primary production of these areas remains virtually (or largely) intact.

In between these two extremes is a wide continuum of human interventions.

Many urban and most suburban residential areas have retained some of their site ’ s potential primary productivity thanks to lawns, parks, and street trees: these new anthropogenic ecosystems are highly fragmented and have a low biodiversity and low productivity when compared with their natural predecessors, but on a small scale they might approach or even surpass the performance of the ecosystems they replaced. Selective logging (including its extreme mode, which uses helicopters to remove large tree trunks from steep slopes) takes only some targeted trees, preserves much of the site ’ s productivity, and does not change its regenerative potential, while harvesting timber by forest clear-cutting is analogous in its destructive impact to clearing land for fi eld crops.

This most extensive of all anthropogenic land conversions is a unique hybrid of destruction and high productivity. The cultivation of annual or perennial crops is usually predicated on a near total elimination of a climax natural ecosystem, and most of the fi elds have lower primary productivities than the plants they replaced.

But in many cases the difference in productivity is not that large, and good agro-nomic practices (multicropping with rotations that include high-yielding leguminous cover crops) may actually result in higher yields. The conversion of a short-grass Canadian prairie to an alfalfa fi eld may have a minimal impact on overall primary productivity, and it will maintain such important ecosystem services as protecting soil against erosion, retaining moisture, and adding bacterially fi xed nitrogen.

Similarly, conversions of tropical forests to rubber or cocoa plantations maintain a semblance of an arboreal ecosystem (indeed, the FAO classifi es these plantations in a forest category), support a relatively rich complement of heterotrophs, and continue to provide protection against excessive soil erosion. But in both cases, a crucial difference remains: before conversion, the primary productivity of a natural ecosystem was entirely available for consumption by a variety of wild heterotrophs, and carbon and nutrients in the unconsumed organic matter were recycled (largely in situ) through bacterial and fungal metabolism.

The different consequences of these interventions and a continuum of invasive practices that makes it impossible to defi ne clear intervention categories are the best

arguments against any simplistic aggregation of areas “ affected, ” “ modifi ed, ” “ trans-formed, ” or “ impacted ” by human actions. And these categorical complications are not the only challenge in assessing the aggregate consequences of anthropogenic changes: whereas quantifying their large-scale extent has become easier thanks to modern remote sensing techniques, major uncertainties remain. The global monitor-ing of these changes became possible only with the launchmonitor-ing of the Earth observa-tion satellites and with the gradually improving resoluobserva-tion and multispectral images produced by their sensors (the sequence was discussed in the fi rst chapter). At the same time, those who produce global maps and data sets of changing land use do not do enough to stress many of the inherent limitations of the data, and hence the untutored users of these products have unrealistic opinions about their accuracy, reliability, and comparability.

What these limitations mean is perhaps best illustrated by taking a closer look at the global satellite monitoring of cropland. Most of today ’ s gridded global land-use databases have a resolution of 5 arc-minutes, encompassing an area of about 9.2 × 9.2 km, or roughly 8,500 ha of farmland. Even on Canadian prairies with their large holdings averaging 400 – 500 ha, that resolution would aggregate all crops planted on about 20 different farms into a single data point. In most parts of Asia, such a resolution would homogenize hundreds of different farms planted to scores of different crops — and unlike on the Canadian prairies, with their single crop per year, nearly all of those farms would harvest at least two crops a year, many fi elds would be triple-cropped, and suburban vegetable fi elds would produce four to six crops every year. Obviously, a 5-minute resolution is good enough to identify large-scale patterns of land use but not to provide accurate assessments of specifi c plant composition or productivity, especially in multicropped areas.

Data series for the more recent presatellite eras must be assembled from the best available national statistics or periodic land-use mappings. These sources use non-uniform defi nitions of land-cover and land-use categories, their reliability varies from excellent to dubious, and while they may offer enough quantitative pegs to make reasonably good continent-wide estimates for nineteenth-century Europe or North America, they have little or no information for pre-1900 Africa and large parts of Asia. But even such questionable, fragmentary. and widely spaced numbers are largely absent for the Middle Ages and entirely unavailable for antiquity: land-cover reconstructions for those eras must rely heavily on assumptions based on anecdotal written evidence and, where available, on painstaking paleoecological reconstructions.

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Twenty Millennia of Phytomass Change

Given the lasting uncertainties in quantifying the aboveground phytomass in today ’ s ecosystems (leaving aside even greater errors in estimating the belowground biomass and its productivity), it may seem audacious to model global phytomass stores going back 20,000 years, to the last glacial maximum (LGM), or even “ just ” to about 10,000 years ago, to the preagricultural era of the early Holocene, or 5,000 years ago, to the mid-Holocene, the time of the fi rst complex civilizations. The inherent uncertainties of such attempts are obvious, but the uncertain totals should be able to convey the magnitude of change between the LGM, when only small bands of dispersed hunters (totaling no more than few hundred thousand people) roamed the Earth, whose northern latitudes were covered by massive continental glaciers, and the era of maximum phytomass storage during the preagricultural period, when forests occupied large areas of Eurasia and North America.

Different studies have used global climate models and paleoecological, palyno-logical, pedopalyno-logical, and sedimentological evidence to reconstruct past vegetation cover and carbon storage, usually expressing the latter as a combination of vegeta-tion and soil carbon, with some studies including and others excluding the post-LGM peatland growth. Not surprisingly, there have been some extreme fi ndings.

After reconstructing past ecosystem distributions, Adams and Faure (1998) con-cluded that the total terrestrial carbon storage (plants, soils, and peat) 18,000 years before the present was only 931 Gt, compared to a total of 2,130 Gt C for the late twentieth century — the total arrived at by adding 560 Gt C in plants (Olson, Watts, and Allison 1983), 1,115 Gt C in nonpeat soils (Post et al. 1982), and 461 Gt C in peatlands (Gorham 1991) — a nearly 2.3-fold increase, for a net gain of about 1,200 Gt.

In contrast, Prentice and Fung (1990) claimed that (excluding peat) there was no signifi cant increase in overall terrestrial carbon storage, putting the overall LGM-to-present carbon gain at 0 ± 50 Gt C: this would mean that the terrestrial biosphere was no (or not a major) carbon sink during the period of rapid deglaciation. But Prentice et al. (1993) presented a new model that showed the late twentieth-century terrestrial carbon storage to be at least 300 Gt C and as much as 700 Gt C higher than during the LGM. Moreover, seven other studies published by 1998 converged to a very similar range, indicating a gain of about 30% in the overall terrestrial carbon storage (in absolute terms, mostly 300 – 700 Gt C) from the time of the LGM to the late twentieth century (Peng, Guiot, and Van Campo 1998).

Two more studies came out in 1999, the fi rst one echoing the just noted consensus and concluding that the total terrestrial carbon addition since the LGM amounted to 550 – 680 Gt (Beerling 1999), the other one ending up with LGM totals ranging from 710 Gt C less to 70 Gt C more than the present level (Fran ç ois et al. 1999).

And two studies published in 2002 (for some reason, these reconstructions have not been pursued after that date) came up with a very similar result: total terrestrial carbon storage during the LGM was 821 Gt lower (Kaplan et al. 2002) or at least 828 Gt C (and as much 1,106 Gt C) lower (Otto et al. 2002) than at the beginning of the twenty-fi rst century. After reviewing most of these studies (whose simple mean is about 650 Gt C), Maslin and Thomas (2003) concluded that the difference between estimates centering on 500 Gt C and those averaging around 1,000 Gt C can be explained by taking into account isotopically light emissions of CH 4 from gas hydrates.

For the mid-Holocene (6,000 years before the present) we have two sets of cal-culations: Fran ç ois et al. (1999) offered an inconclusive range of as much 132 Gt C less and 92 Gt C more than at present, while Beerling ’ s (1999) two models ended up just 103 Gt C apart, with the lower total at 750 Gt of plant carbon and with 1,363 Gt C in vegetation and soils. As already noted in chapter 1, a reconstruction of natural ecosystems based on 106 major plant formations found that the maximum preagricultural standing phytomass was about 2,400 Gt, or around 1,200 Gt C (Bazilevich, Rodin, and Rozov 1971), while a reconstruction by Adams et al. (1990) found that the Earth ’ s potential vegetation could store 924 Gt C.

By 2010 there were are at least eight explicit values for the preindustrial phyto-mass carbon (usually assumed to apply to the year 1700), ranging from 610 to 1,090 Gt C, with the most realistic range between approximately 620 and 910 Gt C (K ö hler and Fischer 2004). The latest reconstruction of preindustrial terrestrial phytomass storage used an unprecedented approach as it tested a global carbon model against nearly 1,500 surface pollen spectra from sample sites in Africa and Eurasia. That approach yielded a total plant storage of 907 Gt C, with the largest share (about 20%) in tropical rain forest, fi tting into the previously established most likely range (Wu et al. 2009).

Pongratz et al. (2009) combined a global climate model and a closed carbon cycle model with previous reconstructions of anthropogenic land-use changes (Pon-gratz et al. 2008) to estimate that between 800 and 1850 carbon emissions (direct from destroyed vegetation and indirect due to lowered NPP) amounted to 53 Gt, and that another 108 Gt C were added between 1850 and 2000. The net effect of these land-use changes (after accounting for the biosphere ’ s carbon sinks) was

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96 Gt C, or roughly a net loss of 200 Gt of phytomass during the second millen-nium of the Common Era. Other estimates of more recent losses attributable to anthropogenic land use range from 136 Gt C for the years 1850 – 1998 (Bolin et al.

2001) to 200 Gt C for the years 1800 – 2000 (House, Prentice, and Le Qu é r é 2002).

Subtracting these estimates from about 900 Gt C of preindustrial terrestrial phy-tomass would leave us with anywhere between 600 and 800 Gt C by the late twentieth century.

In contrast to these values, the estimate of terrestrial phytomass stores for the year 1950 offered by Whittaker and Likens (1975) seems to be too high at 1,837 Gt or about 920 Gt C, while Post, King, and Wullschleger (1997) modeled the change in terrestrial carbon storage during the twentieth century and concluded that it increased from 750 Gt C in 1910 to 780 Gt C in 1990. But the fi rst assessment report from the Intergovernmental Panel on Climate Change chose 550 Gt C as the most likely value for the 1980s (IPCC 1990), and the third assessment listed two global totals, a lower one of just 466 Gt C, from the German Advisory Council on Global Change (WBGU 1998), and a higher one of 654 Gt C, from Saugier, Roy and Mooney (2001), for the twentieth century ’ s end. Of course, the compared values for the LGM, the mid-Holocene, the preindustrial era, 1950, and the late twentieth century come from studies that used a variety of approaches and assumptions and whose only commonality is a signifi cant margin of error.

This makes it impossible to make any clear quantitative conclusions, but the trend appears to be indisputable and the relative magnitudes seem plausible. If we assume that during the LGM the total continental carbon stores were about 500 Gt C lower than at present and that at least 35% – 40% of the LGM terrestrial carbon total was in plants, the peak glacial vegetation stored 175 – 200 Gt C less than did the ter-restrial biosphere around the year 2000, or close to 500 Gt C; K ö hler and Fischer (2004) put the most realistic range at 350 – 640 Gt C. By the mid-Holocene that total could have more than doubled to more than 1,000 Gt C, and human activities subsequently reduced it to no more than 900 Gt C by the onset of the industrial era and, most likely, to less than 700 Gt C by the year 2000.

The general sequence is undoubtedly correct: a reduced phytomass during the LGM, with a substantial gain during the Holocene (doubling does not seem exces-sive, as the total area of tropical rain forest had roughly tripled between 18,000 and 5,000 years before present and that of cool temperature forests expanded more than 30-fold [Adams and Faure 1998]), followed by millennia of gradual decline as a result of the extension of cropland and wood harvests, and then by accelerated deforestation losses since the mid-twentieth century. What lies ahead is uncertain

because our understanding of NPP in a warmer world with higher atmospheric CO 2 levels is a mixture of confi dent conclusions and unsatisfactory speculations.

Thanks to our knowledge of the biophysical and biochemical processes that govern photosynthesis, we can make some useful predictions regarding the short-term plant response. Indisputably, the current partial pressure of the atmospheric CO 2 (at about 390 ppm, or nearly 0.04% by volume) is considerably below the concentration needed to saturate the photosynthesis of C 3 plants, the dominant species in both natural and managed ecosystems: species-specifi c responses show saturation at levels that are twice to three times the current tropospheric concen-tration. Moreover, C 4 species, whose optimum productivity is above 30 ° C, would benefi t from warmer temperatures. This means not only enhanced but also more effi cient photosynthesis operating (thanks to reduced stomatal conductance) with higher water-use effi ciency.

Models of global NPP based on satellite observations indicated that climate change has already eased the CO 2 and temperature constraints on plant productivity and that between 1982 and 1999, the annual rate rose by 6%, or about 3.4 Gt C (Nemani et al. 2003). But during that time global photosynthesis was also boosted owing to volcanic aerosols that were emitted all the way to the stratosphere by the Mount Pinatubo eruption in 1991: that effect resulted from the fact that plant cano-pies use diffuse radiation more effi ciently than they use direct-beam radiation (Gu et al. 2003). On the other hand, a pronounced European heat wave during the summer of 2003 depressed GPP by nearly a third and turned the continent tempo-rarily into a anomalously signifi cant (about 500 Mt C) net source of carbon (Ciais et al. 2005). And the best estimates for the decade between 2000 and 2009 indicate that the record-breaking average temperatures accompanied by extensive droughts had a global effect on terrestrial NPP, reducing it by as much as 2 Gt C/year (in 2005) and depressing it, on the average, by 0.55 Gt C/year (Zhao and Running 2010).

Should extreme heat events become more common as the average global tem-peratures rise, then both trends would have a signifi cant cumulative impact on biospheric carbon storage. Long-term responses that would require acclimation and shifts of vegetation boundaries are extremely diffi cult to quantify because of dynamic links among NPP, radiation, temperature, and precipitation (both in terms of averages and in terms of seasonal and monthly fl uctuations), atmospheric CO 2 levels, and nutrient availability. A single modeling exercise suffi ces to illustrate these uncertainties. Schaphof et al. (2006) applied different climate change scenarios based on fi ve global circulation models — with the global temperature averages in

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2071 – 2100 ranging from 3.7 ° C to 6.2 ° C above the 1971 – 2000 mean, and with the increases in annual continental precipitation between 6.5% and 13.8% — to the Lund-Potsdam-Jena global vegetation model and found the following large uncertainties.

Even the overall response of the biosphere was unclear, with changes ranging from the loss of 106 Gt C to the additional storage of 201 Gt C, with three out of fi ve climate models indicating that the biosphere will be a net source of carbon (particularly thanks to boreal forests), one yielding a neutral outcome, and one implying a carbon sink, and with the response of the tropical rain forests of South America and Central Africa being most uncertain. Moreover, the relative agreement between different models was less for individual seasons than for the annual mean.

This implies not only a large relative error — assuming 650 Gt C of plant carbon in the year 2000, the range would be +30% to – 16% — but an absolute uncertainty (about 300 Gt C) nearly as large as the most likely total of plant carbon decline that the biosphere has experienced since the mid-Holocene.

Land for Food Production

No other human activity has been responsible for such a large-scale transformation of terrestrial ecosystems as food production. All kinds of natural ecosystems have been converted to permanent fi elds; this category of land use is still commonly labeled arable land, although in some countries substantial shares of it are now managed with reduced tillage or no-till cultivation. This land claimed no less than 1.380 Gha in 2010, or nearly 11% of all continental surfaces. The addition of permanent plantations (growing fruits, tea, coffee, cacao, palms) extends the total of about 1.53 Gha of agricultural land, or about 12% of the earth ’ s ice-free surface (FAO 2011f). For comparison, the total based on combining various agricultural inventory data satellite-derived land coverage (at 5-minute or roughly 10 km resolution) came up with a nearly identical median value of 1.5 Gha and a 90% confi dence range of 1.22 – 1.71 Gha in the year 2000 (Ramankutty et al.

2008).

These totals refer to the existing stock of farmland, not to the area of all crops that are actually planted in a calendar- or a crop-year; in national terms that aggre-gate can be both a bit lower and substantially higher. This is because in many countries some arable land is always fallowed, while variable shares of farmland are planted to more than one crop a year. Consequently, multicropping ratios — expressing the number of crops harvested regionwide or nationwide per unit of

arable land every calendar year — range from less than 1 (in Canada or on the U.S.

northern Great Plains, where some fi elds are fallowed and the rest are planted only once a year) to about 1.2 (China ’ s current national average, refl ecting northern single-cropping and southern multicropping) to more than 2 (in the most produc-tive coastal and interior provinces of South China, where triple-cropping is not uncommon).

The total numbers of domestic animals that relied solely or largely on grazing eventually became too large to be supported by seasonal or year-round productivity of natural grasslands. As a result, the conversion of natural ecosystems to heavily grazed pastures has affected not only many grassy ecosystems but has been a major

The total numbers of domestic animals that relied solely or largely on grazing eventually became too large to be supported by seasonal or year-round productivity of natural grasslands. As a result, the conversion of natural ecosystems to heavily grazed pastures has affected not only many grassy ecosystems but has been a major