Urbanised Territories as a Specific Component of the Global Carbon Cycle
A. Svirejeva-Hopkins, H.-J. Schellnhuber (Januar 2005)
Although Urbanised Territories (UT) produce most part of the anthropogenic emissions, we will only consider the following impacts on the Global Garbon Cycle (GCC):
a) the additional carbon emissions that result from the conversion of natural, surrounding a city land, caused by urbanisation and
b) the change of carbon flows by “urbanised” ecosystems, when the atmospheric carbon is “pumping” through “urbanised” ecosystems into neighboring natural ecosystems along the chain: atmosphere -> vegetation -> dead organic matter, i.e. export flow.
The main task is to estimate the annual regional dynamics of the total carbon balance in UT with respect to the atmosphere from 1980 till 2050.
As a scenario, we use the prognoses of regional urban populations produced by the “hybrid” model (multiregional demographic model + UN regional prognoses). All the estimations of carbon flows are based on two models. In the first model (minimal estimates), a regression equation relating the city area and population, is used, as well as an assumption about a random spatial distribution of cities. In the second model (maximal estimates), the so-called Γ-model is used, based on the assumption that the distribution of populated areas with respect to population density is a Γ-distribution with a non-random spatial distribution of cities. The urbanised area is sub-divided into „green“ (parks, etc.), built-up and informal settlements (favelas) areas.
The regional and world dynamics of carbon emission and export, and the annual total carbon balance are calculated. Qualitatively, both models give similar results, but there are some quantitative differences. In the first model, the world annual emissions as a result of land conversion will attain a maximum of 205 MtC between ca. 2020-2030. Emissions will then slowly decrease, so that by the year 2050, they will equal ca. 150 MtC. The maximum contributions to world emissions are given by China and the Asia and Pacific regions. In the second model, the world annual emissions increase from 1.12 GtC per year in 1980 up to 1.25 GtC per year in 2005, after which it will begin to decrease, such that by the year 2050, emissions will have decreased to 623 MtC. If we compare the emission maximum, 1.25 GtC per year, with the annual emission caused by the process of deforestation, 1.36GtC per year in 1980, then we can say that the role of UT is of a comparable magnitude to the role of deforestation.
Regarding the world dynamics of the annual export of carbon by UT, we observe its monotonous, almost linear growth by almost three times, from 24 MtC in 1980 up to 66 MtC in 2050 in the first model, and from 249 MtC up to 505 MtC in the second one. In the latter case, the transport power of UT is therefore comparable to the amount of carbon transported by rivers into the Ocean (196-537 MtC per year).
By estimating the total balance we find that “urbanistic” land conversion shifts the total balance towards a “sink” state. This is most distinctly seen in the Asia and Pacific, China and Africa regions at the initial stage of their “urbanistic” evolution, while UT of the Economies in Transition, and all Highly Industrialised regions play the role of carbon sink (because of the dominance of export flows). The Arabian and Latin America and the Caribbean regions are carbon sources.
According to the first model, the world UT are functioning as a source with power increasing up to its maximal value, 160 MtC, in the year 2020. Fortunately, urbanisation is inhibited in the interval 2020-2030, and by the middle of this century, the growth of urbanised areas would almost stop. Hence, the total emission of natural carbon at that stage will stabilise at the level of the 1980s (80MtC per year).
As estimated by the second model, the total balance, being almost constant until 2000, then starts to decrease at an almost constant rate. If its maximal value in 2000 was 905 MtC, then by 2050 this value has fallen to 118 MtC. By extrapolating this dynamics into the future, we can say that by the end of the XXI century, the total carbon balance will be equal to zero, and may even become negative. This means that the world UT are evolving from a “source” state, when at the beginning of this century they emit annually about 1 GtC, to a “neutralistic” state, when the exchange flows are fully balanced and therefore can be excluded from consideration in the GCC. In the second model, when the balance becomes negative, the system begins to take up carbon from the atmosphere, i.e., to become a “sink”. However, it is necessary to note that the formation of “sink” in urbanised territory is accompanied by the appearance of “sources” in other locations.
