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

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.

Wetlands at the Last Glacial Maximum: Distribution and methane emissions

Jed Oliver Kaplan

The global distribution of potential wetlands and their methane (CH4) emissions at the present-day and the Last Glacial Maximum (LGM) are estimated using a GCM simulation of LGM climate, a vegetation model, and simple algorithms for determining wetland area based on topography and soil moisture, and CH4 emissions based on ecosystem carbon turnover in wet soils. LGM wetland area was 15% larger than present, but CH4 emissions were 24% less. Extensive wetlands were simulated on the exposed continental shelves. The soil CH4 sink was simulated as 14 Tg now but

2002

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

Flooding of the continental shelves as a contributor to deglacial CH4 rise

Jed Oliver Kaplan

The transition from the last glacial and beginning of BollingAllerod and Pre-Boreal periods in particular is marked by rapid increases in atmospheric methane (CH4) concentrations. The CH4 concentrations reached during these intervals, similar to 650750 ppb, is twice that at the last glacial maximum and is not exceeded until the onset of industrialization at the end of the Holocene. Periods of rapid sea-level rise as the Last Glacial Maximum ice sheets retreated and associated with melt-water pulses appear to coincide with the onset of elevated concentrations of CH4, suggestive of a potential causative link. Here we identify and outline a mechanism involving the flooding of the continental shelves that were exposed and vegetated during the glacial sea-level low stand and that can help account for some of these observations. Specifically, we hypothesize that waterlogging (and later, flooding) of large tracts of forest and savanna in the Tropics and Subtropics during the deglacial transition and early Holocene would have resulted in rapid anaerobic decomposition of standing biomass and emission of methane to the atmosphere. This novel mechanism, akin to the consequences of filling new hydroelectric reservoirs, provides a mechanistic explanation for the apparent synchronicity between rate of sea-level rise and occurrence of elevated concentrations of ice core CH4. However, shelf flooding and the creation of transient wetlands are unlikely to explain more than similar to 60?ppb of the increase in atmospheric CH4 during the deglacial transition, requiring additional mechanisms to explain the bulk of the glacial to interglacial increase. Similarly, this mechanism has the potential also to play some role in the rapid changes in atmospheric methane associated with the DansgaardOeschger cycles. Copyright (c) 2012 John Wiley & Sons, Ltd.

Wiley-Blackwell2012

Geophysical Applications of Vegetation Modeling

Jed Oliver Kaplan

Lund University2001