This report is part of a series of 21 Synthesis and Assessments (SAP) aimed at providing current assessments of climate change science to inform public debate, policy, and operational decisions. These reports are also intended to help develop future program research priorities. The guiding vision is to provide the Nation and the global community with the science-based knowledge needed to manage the risks and capture the opportunities associated with climate and related environmental changes. This SAP assesses abrupt climate change events where key aspects of the climate system change faster than the responsible forces would suggest and/or faster than society can respond to those changes. Illustrations.

Methane (CH4) is a potent greenhouse gas that is often found in a solid, hydrate clathrate form in marine sediments along continental margins and will often escape from the seafloor and rise through the water column as bubbles. The estimated marine methane hydrate inventory is over 600 times greater than the current atmospheric concentration so the fate of this ebullitive methane flux is of great interest. Traditional methods of measuring this flux such as acoustic imaging, optical sensors, and modeling suffer from limited information regarding the bubbles’ composition. Studies that attempt to constrain CH4 bubble composition suffer from low spatiotemporal resolution and adaptability. The current study presents the design, development and deployment of a novel, in situ bubble sampling system, the Bubble Delivery System (BDS), to quantify gas chemical composition in the water column. The BDS was deployed at the Cascadia Margin – a region well known for its active CH4 bubble seeps – where 95 samples were collected from McArthur Ridge, Hydrate Ridge, Heceta Deep and Heceta Shallow over the course of seven remotely operated vehicle dives. By combining this approach with the use of an underwater mass spectrometer, in situ analysis of these samples indicated that the bubbles contained between 84.6 to 100% CH4 and exhibited a high level of variability both spatially and temporally. Bubbles emitted from Heceta Deep exhibited anomalously elevated levels of carbon dioxide compared to the other sites.

The dynamics of methane transport in lake sediments control the release of methane into the water column above, and the portion that reaches the atmosphere may contribute significantly to the greenhouse effect. The observed dynamics are poorly understood. In particular, variations in the hydrostatic load on the sediments, from both water level and barometric pressure, appear to trigger free gas bubbling (ebullition). We develop a model of methane bubble ow through the sediments, forced by changes in hydrostatic load. The mechanistic, numerical model is tuned to and compared against ebullition data from Upper Mystic Lake, MA, and the predictions match the daily temporal character of the observed gas releases. We conclude that the combination of plastic gas cavity deformation and ow through "breathing" gas conduits explains methane venting from lake sediments. This research lays the groundwork for integrated modeling of methane transport in the sediment and water column to constrain the atmospheric flux from methane-generating lakes.

One of Springer’s Major Reference Works, this book gives the reader a truly global perspective. It is the first major reference work in its field. Paleoclimate topics covered in the encyclopedia give the reader the capability to place the observations of recent global warming in the context of longer-term natural climate fluctuations. Significant elements of the encyclopedia include recent developments in paleoclimate modeling, paleo-ocean circulation, as well as the influence of geological processes and biological feedbacks on global climate change. The encyclopedia gives the reader an entry point into the literature on these and many other groundbreaking topics.

Methane bubbling from lakes contributes significantly to atmospheric methane levels, and methane is second only to carbon dioxide in global warming potential. Microorganisms in aquatic sediments produce methane while consuming organic matter, and the majority of this methane is released via bubbling. Bubbles dissolve as they rise, and the fraction of original methane that dissolves versus escapes to the atmosphere is strongly influenced by bubble size. While bubble sizes are critical to methane fate, traditional methods of measuring bubbles sizes in situ are resource intensive (i.e. sonar or video cameras). In this work we design, build, and deploy a fleet of novel optical bubble size sensors capable of measuring methane bubbles in situ for long periods of time. Data from our field campaign on Upper Mystic Lake, MA illuminate spatial differences in bubble size distributions and provide an estimate of the contribution from methane bubble dissolution to dissolved methane accumulation. These results improve our understanding of processes governing the emission of this important greenhouse gas. In addition to transporting gas, bubbles effectively transport particles in water columns. This process has been used extensively in industry since the 1900s to separate chemicals of interest from bulk solutions. While bubbles also transport particulate matter in marine systems, to date very little work has focused on the possibility that methane bubbles transport particles in freshwater systems. We use laboratory and field experiments on Upper Mystic Lake to show that bubbles can transport arsenic-containing sediment particles to the surface of the lake from depths exceeding 15 m. While we estimate that arsenic transport is insignificant at the relatively modest methane bubbling levels in Upper Mystic Lake, other water bodies experience an order of magnitude more ebullition and bubbling may therefore constitute a significant contaminant flux in these systems. Furthermore, bubbles may also transport organisms (or pathogens) from the sediment to the water surface.