Archive for emissions

Air-sea gas exchange estimates biased by multi-day surface trapping

Posted by mmaheigan 
· Tuesday, August 20th, 2019 

Routine measurements of air-sea gas exchange assume a homogeneous gas concentration across the upper few meters of the ocean. But is this assumption valid? A recent study in Biogeosciences revealed substantial systematic gradients of nitrous oxide (N2O) in the top few meters of the Peruvian upwelling regime. These gradients lead to a 30% overestimate of integrated N2O emissions across the entire region, with local emissions overestimated by as much as 800%.

Figure caption: Air-sea gas exchange estimates can be biased by gas concentration gradients within the upper few meters of the ocean; in particular, surface trapping over several days’ duration can generate substantial gradients.

The N2O gradients off Peru form during multi-day events of surface trapping, in which near-surface stratification dampens turbulent mixing. Until now, surface trapping was assumed to be a diurnal (driven by solar warming) process without memory, whereby only weak gradients would form during the hours of trapping and then dissipate. It is likely that multi-day surface trapping occurs in other ocean regions as well. The total impact on emission estimates of different greenhouse gases is yet to be quantified, but given the findings in the Peruvian upwelling system, could be significant globally.

Authors:
Tim Fischer, Annette Kock, Damian L. Arévalo-Martínez, Marcus Dengler, Peter Brandt, Hermann W. Bange (GEOMAR)

Suddenly shallow: A new aragonite saturation horizon will soon emerge in the Southern Ocean

Posted by mmaheigan 
· Monday, May 27th, 2019 

Earth System Models (ESMs) project that by the end of this century, the aragonite saturation horizon (the boundary between shallower, saturated waters and deeper, undersaturated waters that are corrosive to aragonitic shells) will shoal all the way to the surface in the Southern Ocean, yet the temporal evolution of the horizon has not been studied in much detail. Rather than a gradual shoaling, a new shallow aragonite saturation horizon emerges suddenly across many locations in the Southern Ocean between now and the end of the century (Figure 1, left), as detailed in a new study published in Nature Climate Change.

Figure 1: Maximum step-change in the depth of the aragonite saturation horizon (left), timing of the step-change (center), and cause of the change (right). Xs on the time axis (center) indicate when the shallow horizon emerges in each ensemble member. (click image to enlarge)

 

The emergence of the shallow aragonite saturation horizon is apparent in each member of an ensemble of climate projections from an ESM, but the step change occurs during different years (Figure 1, center). The shoaling is driven by the gradual accumulation of anthropogenic CO2 in the Southern Ocean thermocline, where the carbonate ion concentration exhibits a local minimum and approaches undersaturation (Figure 1, right).

The abrupt shoaling of the Southern Ocean aragonite saturation horizon occurs under both business-as-usual and emission-stabilizing scenarios, indicating an inevitable and sudden decrease in the volume of suitable habitat for aragonitic organisms such as shelled pteropods, foraminifers, cold-water corals, sea urchins, molluscs, and coralline algae. Widespread reductions in these habitats may have far-reaching consequences for fisheries, economies, and livelihoods.

Authors:
Gabriela Negrete-García (Scripps Institution of Oceanography)
Nicole Lovenduski (University of Colorado Boulder)

 

See also OCB2019 plenary session: Carbon cycle feedbacks from the seafloor (Wednesday, June 26, 2019)

Sensitivity of future ocean acidification to carbon-climate feedbacks

Posted by mmaheigan 
· Thursday, May 10th, 2018 

There are vast unknowns about the future oceans, from what species or habitats may be most under threat to the continuity of earth system processes that maintain global climate. Modeling can be used to predict future states and explore the impacts of climate change, but several key uncertainties such as carbon-climate feedbacks hamper our predictive power.

Authors of a recent study in Biogeosciences (Matear and Lenton 2018) used a global earth system model to explore the effects of carbon-climate feedbacks on future ocean acidification. Ocean acidification can have wide-ranging impacts on keystone species from reef-building corals to pteropods, a major food web species in the Southern Ocean. The study included four representative scenarios (from IPCC) comparing concentration pathway simulations to emission pathway simulations (RCP2.6, RCP 4.5, RCP6, RCP8.5) to determine carbon-climate feedbacks. The high emission scenarios (RCP8.5 and RCP6) showed surface water undersaturation a decade or more earlier than expected. Surprisingly, the medium (RCP4.5) scenario carbon-climate feedbacks showed the greatest acidification response, doubling the extent of undersaturation and subsequently halving the area that could sustain coral reefs by 2100. The low emissions scenario also showed significant declines in saturation state.

Surface ocean aragonite saturation state for the 2090s for RCP2.6 and RCP 8.5 concentration and emission pathways. The contour line delineates a saturation state of 3 (coral reef threshold), the white line a saturation state of 1, when aragonite becomes unstable and corals dissolve.

The extra atmospheric CO2 from the carbon-climate feedback resulted in accelerated ocean acidification in all emission scenarios. These feedbacks may also affect global warming and deoxygenation. This is particularly important, given that many policymakers are aiming for low emission commitments, but may still be severely underestimating the extent and timing of ocean acidification. There is a great need to improve our ability to predict carbon-climate feedbacks so we do not underestimate projected ocean acidification and its impacts on both sensitive ecosystems and the human communities that rely on them for food, coastal protection and other ecosystem services.

Authors:
Richard Matear (CSIRO Oceans and Atmosphere, Australia)
Andrew Lenton (Antarctic Climate and Ecosystems CRC, Australia)

Volcanic carbon dioxide drove ancient global warming event

Posted by mmaheigan 
· Thursday, March 29th, 2018 

A study recently published in Nature suggests that an extreme global warming event 56 million years ago known as the Palaeocene-Eocene Thermal Maximum (PETM) was driven by massive CO2 emissions from volcanoes during the formation of the North Atlantic Ocean. Using a combination of new geochemical measurements and novel global climate modelling, the study revealed that atmospheric CO2 more than doubled in less than 25,000 years during the PETM.

The PETM lasted ~150,000 years and is the most rapid and extreme natural global warming event of the last 66 million years. During the PETM, global temperatures increased by at least 5°C, comparable to temperatures projected in the next century and beyond. While it has long been suggested that the PETM event was caused by the injection of carbon into the ocean and atmosphere, the source and total amount of carbon, as well as the underlying mechanism have thus far remained elusive. The PETM roughly coincided with the formation of massive flood basalts resulting from of a series of eruptions that occurred as Greenland and North America started separating from Europe, thereby creating the North Atlantic Ocean. What was missing is evidence linking the volcanic activity to the carbon release and warming that marks the PETM.

To identify the source of carbon, the authors measured changes in the balance of isotopes of the element boron in ancient sediment-bound marine fossils called foraminifera to generate a new record of ocean pH throughout the PETM. Ocean pH tells us about the amount of carbon absorbed by ancient seawater, but we can get even more information by also considering changes in the isotopes of carbon, which provide information about the carbon source. When forced with these ocean pH and carbon isotope data, a numerical global climate model implicates large-scale volcanism associated with the opening of the North Atlantic as the primary driver of the PETM.

 

North Atlantic microfossil-derived isotope records from extinct planktonic foraminiferal species M. subbotinae relative to the onset of the PETM carbon isotope excursion (CIE). The negative trend in carbon isotope composition (A) during the carbon emission phase is accompanied by decreasing pH (decreasing δ11B, panel B) and increasing temperature (decreasing δ18O, panel C). Panels D and E zoom in on the PETM CIE, showing microfossil δ13C (D) and δ11B-based pH (E) reconstructions. Also included in E are data from Penman et al. (2014) on their original age model, with recalculated (lab-based) pH values.

 

These new results suggest that the PETM was associated with a total input of >12,000 petagrams of carbon from a predominantly volcanic source. This is a vast amount of carbon—30 times larger than all of the fossil fuels burned to date and equivalent to all current conventional and unconventional fossil fuel reserves. In the following Earth System Model simulations, it resulted in the concentration of atmospheric CO2 increasing from ~850 parts per million to >2000 ppm. The Earth’s mantle contains more than enough carbon to explain this dramatic rise, and it would have been released as magma poured from volcanic rifts at the Earth’s surface.

How the ancient Earth system responded to this carbon injection at the PETM can tell us a great deal about how it might respond in the future to man-made climate change. Earth’s warming at the PETM was about what we would expect given the CO2 emitted and what we know about the sensitivity of the climate system based on Intergovernmental Panel on Climate Change (IPCC) reports. However, the rate of carbon addition during the PETM was about twenty times slower than today’s human-made carbon emissions.

In the model outputs, carbon cycle feedbacks such as methane release from gas hydrates—once the favoured explanation of the PETM—did not play a major role in driving the event. Additionally, one unexpected result was that enhanced organic matter burial was important in ultimately drawing down the released carbon out of the atmosphere and ocean and thereby accelerating the recovery of the Earth system.

 

Authors:
Marcus Gutjahr (National Oceanography Centre Southamption, GEOMAR)
Andy Ridgwell (Bristol University, University of California Riverside)
Philip F. Sexton (The Open University, UK)
Eleni Anagnostou (National Oceanography Centre Southamption)
Paul N. Pearson (Cardiff University)
Heiko Pälike (University of Bremen)
Richard D. Norris (Scripps Institution of Oceanography)
Ellen Thomas (Yale University, Wesleyan University)
Gavin L. Foster (National Oceanography Centre Southamption)

 

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