The temperature of Europe during the Holocene reconstructed from pollen data

We present the first area-average time series reconstructions of warmest month, coldest month and mean annual surface air temperatures across Europe during the last 12,000 years. These series are based on quantitative pollen climate reconstructions from over 500 pollen sites assimilated using an innovative four-dimensional gridding procedure. This approach combines three-dimensional spatial gridding with a fourth dimension represented by time, allowing data from irregular time series to be ‘focussed’ onto a regular time step. We provide six regional reconstructed temperature time series as well as summary time series for the whole of Europe. The results suggest major spatial and seasonal differences in Holocene temperature trends within a remarkably balanced regional and annual energy budget. The traditional mid-Holocene thermal maximum is observed only over Northern Europe and principally during the summer. This warming was balanced by a mid-Holocene cooling over Southern Europe, whilst Central Europe occupied an intermediary position. Changes in annual mean temperatures for Europe as a whole suggest an almost linear increase in thermal budget up to 7800 BP, followed by stable conditions for the remainder of the Holocene. This early Holocene warming and later equilibrium has been mainly modulated by increasing winter temperatures in the west, which have continued to rise at a progressively decreasing rate up to the present day.

Heat and drought 2003 in Europe: a climate synthesis

Martine Rebetez

Heat and drought were extreme in summer 2003 in Europe. Climatic data show that most extreme were maximum air temperatures in June and August; maps of these two months show a striking similarity in geographical range. Over wide regions, monthly mean temperatures were more than 4 ◦C above the long term mean values. The heat lasted from May to September in large parts of western and central Europe. The lack of precipitation was most severe in summer 2003 but precipitation was generally below normal from February 2003 until June 2004 (with the exception of two months). Sunshine duration was above normal, particularly in regions which usually have rainy and cloudy episodes in summer, and relative air humidity was below normal during the whole year of 2003. The drought experienced by the vegetation in summer 2003 was exacerbated by the length of the period with scarce precipitation and humidity, by the heat during the summer and by long sunshine duration.

2006

The role of methane and biogenic volatile organic compound sources in late glacial and Holocene fluctuations of atmospheric methane concentrations

Jed Oliver Kaplan

Recent analyses of ice core methane concentrations suggested that methane emissions from wetlands were the primary driver for prehistoric changes in atmospheric methane. However, these interpretations conflict as to the location of wetlands, magnitude of emissions, and the environmental controls on methane oxidation. The flux of other reactive trace gases to the atmosphere also controls apparent atmospheric methane concentrations because these compounds compete for the hydroxyl radical (OH), which is the primary atmospheric sink for methane. In a series of linked biosphere-atmosphere chemistry-climate modeling experiments, we simulate the methane and biogenic volatile organic compound emissions from the terrestrial biosphere from the Last Glacial Maximum (LGM) to the present. Using a state-of-the-art chemistry-climate model, we simulate the atmospheric concentrations of methane, OH, and other reactive trace gas species. Over the past 21,000 years, methane emissions from wetlands increased slightly to the end of the Pleistocene but then decreased again, reaching levels at the preindustrial Holocene that were similar to the LGM. Global wetland area decreased by 14% from LGM to the preindustrial time. Emissions of biogenic volatile organic compounds (BVOCs), however, nearly doubled over the same period of time. Atmospheric OH burdens and methane concentrations were affected by this major change in BVOC emissions, with methane lifetimes increasing by more than 2 years from LGM to the present. We simulate a change in methane concentration of ∼385 ppb, accounting for 88% of the ∼440 ppb increase in methane concentrations observed in ice cores. Thus glacial-interglacial changes in atmospheric methane concentrations would have been modulated by BVOC emissions. In addition, the increase in atmospheric methane concentrations since the mid-Holocene is partly caused in our results by the increases in anthropogenic methane emissions over this period. While the interplay between BVOC and wetland methane emissions since the LGM cannot explain the entire record of ice core methane concentrations, consideration of BVOC source dynamics is central to understanding ice core methane. Rapid changes in atmospheric methane concentrations, also observed in ice cores, require further study.

2006

Geophysical Applications of Vegetation Modeling

Jed Oliver Kaplan

This thesis describes the development and selected applications of a global vegetation model, BIOME4. The model is applied to problems in high-latitude vegetation distribution and climate, trace gas production, and isotope biogeochemistry. It demonstrates how a modeling approach, based on principles of plant physiology and ecology, can be applied to interdisciplinary problems that cannot be adequately addressed by direct observations or experiments. The work is relevant to understanding the potential effects of climate change on the terrestrial biosphere and the feedbacks between the biosphere and climate. BIOME4 simulates the distribution of 15 high-latitude biomes, including five tundra vegetation types, for the present day using observed climate, and the LGM, mid-Holocene, and a “greenhouse” scenario for 2100 using the output of GCMs. In the LGM simulations, the high-latitudes show a marked increase in the area of graminoid and forb tundra, which is also the predominant feature in the paleodata. This vegetation has no widespread modern analog; it was favored by the cold, dry climate, and supported large mammoth populations. Mid-Holocene simulations indicate a modest, asymmetrical northward advance of the Arctic treeline compared to present, with greatest extension in central Siberia (up to 300 km), and little to no change in the Western Hemisphere. This result is in good agreement with pollen and megafossil data from the same period. Differential warming of the continents in response to increased high-latitude solar radiation is hypothesized to account for the asymmetry. Vegetation changes in the 2100 projection, which assumes a continued exponential increase in atmospheric GHG concentrations, are more radical than those simulated for the mid-Holocene. The year-round forcing due to GHGs increases both summertime and annual temperatures in the high latitudes by up to double the mid-Holocene anomaly. However the potential treeline advances and biome shifts in our simulation are unlikely to be realized within 100 years, because of the time required for migration and establishment of new vegetation types. Potential natural wetland area for the present day was simulated by BIOME4 as 11.0x106km2. This value is higher than other estimates but includes small (

Lund University2001

Geophysical Applications of Vegetation Modeling

Jed Oliver Kaplan

Lund University2001

The role of methane and biogenic volatile organic compound sources in late glacial and Holocene fluctuations of atmospheric methane concentrations

Jed Oliver Kaplan

2006