Ulla Kokfelt - Academia.edu (original) (raw)

Papers by Ulla Kokfelt

"Abstract. The major objectives of this paper are: (1) to review the pros and cons of the scenar... more "Abstract. The major objectives of this paper are: (1) to
review the pros and cons of the scenarios of past anthropogenic
land cover change (ALCC) developed during the last
ten years, (2) to discuss issues related to pollen-based reconstruction
of the past land-cover and introduce a new method,
REVEALS (Regional Estimates of VEgetation Abundance
from Large Sites), to infer long-term records of past landcover
from pollen data, (3) to present a new project (LANDCLIM:
LAND cover – CLIMate interactions in NW Europe
during the Holocene) currently underway, and show preliminary
results of REVEALS reconstructions of the regional
land-cover in the Czech Republic for five selected time windows
of the Holocene, and (4) to discuss the implications and
future directions in climate and vegetation/land-cover modeling,
and in the assessment of the effects of human-induced
changes in land-cover on the regional climate through altered
feedbacks. The existing ALCC scenarios show large
discrepancies between them, and few cover time periods
older than AD 800. When these scenarios are used to assess
the impact of human land-use on climate, contrasting
results are obtained. It emphasizes the need for methods
such as the REVEALS model-based land-cover reconstructions.
They might help to fine-tune descriptions of past landcover
and lead to a better understanding of how long-term
changes in ALCC might have influenced climate. The REVEALS
model is demonstrated to provide better estimates
of the regional vegetation/land-cover changes than the traditional
use of pollen percentages. This will achieve a robust
assessment of land cover at regional- to continental-spatial
scale throughout the Holocene. We present maps of REVEALS
estimates for the percentage cover of 10 plant functional
types (PFTs) at 200 BP and 6000 BP, and of the two
open-land PFTs “grassland” and “agricultural land” at five
time-windows from 6000 BP to recent time. The LANDCLIM
results are expected to provide crucial data to reassess
ALCC estimates for a better understanding of the land
suface-atmosphere interactions."

Journal of Paleolimnology, 2010

Journal of Geophysical Research, 2010

1] Climate change and thawing of permafrost will likely result in increased decomposition of terr... more 1] Climate change and thawing of permafrost will likely result in increased decomposition of terrestrial organic carbon and subsequent carbon emissions to the atmosphere from terrestrial and aquatic systems. The quantitative importance of mineralization of terrestrial organic carbon in lakes in relation to terrestrial carbon fluxes is poorly understood and a serious drawback for the understanding of carbon budgets. We studied a subarctic lake in an area of discontinuous permafrost to assess the quantitative importance of lake carbon emission for the catchment carbon balance. Estimates of net ecosystem production and stable carbon-isotope composition of dissolved organic carbon in the lake water suggest substantial input and respiration of terrestrial organic carbon in the lake. The lake was a net source of CO 2 and CH 4 to the atmosphere at ice breakup in spring and during the whole ice-free period. The carbon emission from the lake was similar in magnitude to the terrestrial net release of carbon to the atmosphere. The results indicate that lakes are important sources of catchment carbon emission, potentially increasing the positive feedback from permafrost thawing on global warming. Citation: Karlsson, J., T. R. Christensen, P. Crill, J. Förster, D. Hammarlund, M. Jackowicz-Korczynski, U. Kokfelt, C. Roehm, and P. Rosén (2010), Quantifying the relative importance of lake emissions in the carbon budget of a subarctic catchment,

Global Change Biology, 2009

Recent accelerated decay of discontinuous permafrost at the Stordalen Mire in northern Sweden has... more Recent accelerated decay of discontinuous permafrost at the Stordalen Mire in northern Sweden has been attributed to increased temperature and snow depth, and has caused expansion of wet minerotrophic areas leading to significant changes in carbon cycling in the mire. In order to track these changes through time and evaluate potential forcing mechanisms, this paper analyses a peat succession and a lake sediment sequence from within the mire, providing a record for the last 100 years, and compares these with monitored climate and active layer thickness data. The peat core was analysed for testate amoebae to reconstruct changes in peatland surface moisture conditions and water table fluctuations. The lake sediment core was analysed by near infrared spectroscopy to infer changes in the total organic carbon (TOC) concentration of the lake-water, and changes in d 13 C and C, N and d 15 N to track changes in the dissolved inorganic carbon (DIC) pool and the influence of diagenetic effects on sediment organic matter, respectively. Results showed that major shifts towards increased peat surface moisture and TOC concentration of the lake-water occurred around 1980, one to two decades earlier than a temperature driven increase in active layer thickness. Comparison with monitored temperature and precipitation from a nearby climate station indicates that this change in peat surface moisture is related to June-September (JJAS) precipitation and that the increase in lakewater TOC concentration reflects an increase in total annual precipitation. A significant depletion in 13 C of sediment organic matter in the early 1980s probably reflects the effect of a single or a few consecutive years with anomalously high summer precipitation, resulting in elevated DIC content of the lake water, predominantly originating from increased export and subsequent respiration of organic carbon from the mire. Based on these results, it was not possible to link proxy data obtained on peat and lake-sediment records directly to permafrost decay. Instead our data indicate that increased precipitation and anomalously high rainfall during summers had a significant impact on the mire and the adjacent lake ecosystem. We therefore propose that effects of increased precipitation should be considered when evaluating potential forcing mechanisms of recent changes in carbon cycling in the subarctic.

Geophysical Research Letters, 2010

Environmental Science & Technology, 2010

Can a climate-triggered export of old contaminants from the soil alter the lead (Pb) contaminant ... more Can a climate-triggered export of old contaminants from the soil alter the lead (Pb) contaminant burden of subarctic lakes? To address this question, we reconstructed the pollution history of three high latitude lakes situated in a region where a recent climatic shift has occurred. Dated sediment records were used as archives of past Pb inputs to the lakes, where the difference in the 206 Pb/ 207 Pb ratio between atmospheric contaminants ( 206 Pb/ 207 Pb ratio <1.16) and geogenic Pb in the catchment soil ( 206 Pb/ 207 Pb ratio >1.22) were used to trace fluxes of Pb contaminants. Lead contaminants were found in sediments deposited since Roman times. A significant export of Pb from the soil contaminant pool is indicated in two of the lakes surrounded by near-shore permafrost soils. Here, levels of Pb contaminants and 206 Pb/ 207 Pb ratios of sediments deposited after the 1970s appear not to have been strongly affected by the g90% reduction in atmospheric deposition rates and increasing 206 Pb/ 207 Pb ratios of atmospheric Pb since the 1990s. We concluded that soil processes stimulated by the ongoing climate change at high latitudes might work counteractive to efforts to reduce contaminant levels in subarctic lakes.

Climate of the Past, 2010

Published by Copernicus Publications on behalf of the European Geosciences Union.

Climate change impacts on sub-arctic palsa mires and greenhouse gas feedbacks

The Holocene, 2014

Here, we present results from the most comprehensive compilation of Holocene peat soil properties... more Here, we present results from the most comprehensive compilation of Holocene peat soil properties with associated carbon and nitrogen accumulation rates for northern peatlands. Our database consists of 268 peat cores from 215 sites located north of 45°N. It encompasses regions within which peat carbon data have only recently become available, such as the West Siberia Lowlands, the Hudson Bay Lowlands, Kamchatka in Far East Russia, and the Tibetan Plateau. For all northern peatlands, carbon content in organic matter was estimated at 42 ± 3% (standard deviation) for Sphagnum peat, 51 ± 2% for non-Sphagnum peat, and at 49 ± 2% overall. Dry bulk density averaged 0.12 ± 0.07 g/cm 3 , organic matter bulk density averaged 0.11 ± 0.05 g/cm 3 , and total carbon content in peat averaged 47 ± 6%. In general, large differences were found between Sphagnum and non-Sphagnum peat types in terms of peat properties. Time-weighted peat carbon accumulation rates averaged 23 ± 2 (standard error of mean) g C/m 2 /yr during the Holocene on the basis of 151 peat cores from 127 sites, with the highest rates of carbon accumulation (25-28 g C/m 2 /yr) recorded during the early Holocene when the climate was