Joint Seminar: Pre-industrial is not pristine

The basic question of Global Change is how humans have altered

atmospheric composition and climate. The industrial revolution is

usually taken as point of reference to determine changes with respect to

the 'pristine baseline'. But human societies have influenced the land

surface, terrestrial trace gas emissions and atmospheric chemical

composition ever since the taming of fire -although it was not until the

mid-Holocene that advances in agriculture and increasing use of coastal

margin productivity went hand in hand with the establishment of sizeable

civilizations. The associated deforestation was negligible for the

atmospheric CO_2 concentration but effects of land clearance on

emissions of non-CO_2 trace gases could well have been noteworthy since

atmospherically reactive chemical species are not well mixed (and thus

of importance to regional chemistry even when changes are not detectable

globally). Frequently they are (unlike, e.g., CO_2 or CH_4 ) not

preserved in ice cores. To assess their pre-anthropogenic baseline

therefore requires modelling that is process-based to be applicable over

environmental conditions encountered over centuries to millennia and

that accounts for human population growth atop of climate effects.


We apply a recently developed model that couples a process-based leaf

isoprene and monoterpene emission algorithm to the dynamic global

vegetation model framework LPJ GUESS. Our model analysis differs in

essential aspects from empirical algorithms used in previous work.

Importantly, the incomplete coupling of assimilation to isoprene

metabolism that underlies the observed CO_2 inhibition of its production

is accounted for. Deforestation associated with the spread and change of

agricultural practices is estimated for the last 6000 years based on the

work by Olofsson and Hickler [2007], and includes estimates of human

biomass burning.


The effects of early human settlements on simulated changes in regional

emission patterns are considerable with, for instance, isoprene

emissions in Europe declining visibly already 3000 years ago. By 1750,

emissions were approximately 30% below the potential natural vegetation

levels, and they continued to decline over the last 200 years as the

CO_2 inhibition of leaf isoprene production became visible. In some

regions increasing human activity resulted in an altered isoprene to

monoterpene ratio. The changes in the regional terrestrial BVOC signals

are also visible in the global totals. The associated fire related

emission patterns are complex and depend on the type of agriculture

(slash and burn /vs/. permanent). Our results point to a discernible

human influence on atmospheric chemistry that dates back much further

than the second half of the 18^th century and that must be considered in

atmospheric chemistry simulations.




13:30 h


Bundesstr. 53, room 022/023
Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg


Almut Arneth, GeoBiosphere Science Centre, Lund University, Sweden


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