A Rapid in Time

Musings on navigating the Anthropocene,
by Earth historian Bob Kopp

Mar 5


I’m finding it a bit hard to fit regular blogging into my assistant professor lifestyle, so I’m going to try a different approach: Twitter, backed up with occasional exegesis here. So please follow me

Feb 9

Is natural gas leaky enough to offset its carbon mitigation benefits?

Nature News article on an in-press JGR story be Petron et al.:

Led by researchers at the National Oceanic and Atmospheric Administration (NOAA) and the University of Colorado, Boulder, the study estimates that natural-gas producers in an area known as the Denver-Julesburg Basin are losing about 4% of their gas to the atmosphere — not including additional losses in the pipeline and distribution system. This is more than double the official inventory, but roughly in line with estimates made in 2011 that have been challenged by industry. And because methane is some 25 times more efficient than carbon dioxide at trapping heat in the atmosphere, releases of that magnitude could effectively offset the environmental edge that natural gas is said to enjoy over other fossil fuels.

“If we want natural gas to be the cleanest fossil fuel source, methane emissions have to be reduced,” says Gabrielle Pétron, an atmospheric scientist at NOAA and at the University of Colorado in Boulder, and first author on the study, currently in press at the Journal of Geophysical Research. Emissions will vary depending on the site, but Pétron sees no reason to think that this particular basin is unique. “I think we seriously need to look at natural-gas operations on the national scale.”

Dec 27

Mitigating California’s greenhouse gas emissions

Williams et al. examine the challenges of mitigating emissions of heat-trapping gases in developed economies, via a case study of California’s goal of reducing emissions 80% below 1990 levels by 2050:

Three major energy system transformations were necessary to meet the target (Fig. 2). First, energy efficiency had to improve by at least 1.3% yr−1 over 40 years. Second, electricity supply had to be nearly decarbonized, with 2050 emissions intensity less than 0.025 kg CO2e/kWh [compared to about 1 kg/kWh for traditional coal power generation]. Third, most existing direct fuel uses had to be electrified, with electricity constituting 55% of end-use energy in 2050, compared to 15% today.

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Milankovitch was right!

Peter Huybers statistically evaluates the contribution of the Earth’s ~20 thousand year precession cycle to deglaciations over the last million years, and concludes that both obliquity (how tilted the Earth’s axis is) and precession (the orientation of the tilt) play a role:

Ice sheets tend to collapse in response to unusually large maxima in insolation forcing that result from the coincidence of high obliquity and alignment of perihelion with Northern Hemisphere summer solstice, consistent with the models hypothesized by Milankovitch1 and others5, 6, 7, 8, 9, 10. During these forcing maxima, summer insolation is as much as 40 W m−2 greater at high northern latitudes (Fig. 3b). However, this consistency is not exclusive of all other orbital contributions to deglaciation. For instance, when perihelion aligns with the Northern Hemisphere summer solstice, aphelion occurs during the Southern Hemisphere summer, causing the length of the Southern Hemisphere summer to be longer (Fig. 3b) and, possibly, increasing the escape of CO2 from the Southern Ocean into the atmosphere26, 27, 28, 29. The climate system is thoroughly interconnected across temporal and spatial scales, and, just as neither obliquity nor precession act in isolation, no one region should be expected to exert exclusive influence upon deglaciation.

Synchronizing Antarctic and Northern Hemisphere ice retreat

Weber et al. examine the timing of the retreat of the Antarctic and Northern Hemisphere ice sheets at the end of the Last Glacial Maximum (LGM):

A long-standing hypothesis for ice-sheet synchronization invokes sea-level forcing of Antarctic grounding lines driven by fluctuations of NH ice sheets (41, 42), but until now the chronology of the Antarctic ice sheets has been too limited to evaluate this hypothesis, other than for the deglaciation where existing arguments for a 4- to 5-ky lag relative to the start of deglacial sea-level rise (5, 8, 23) would appear to contradict it. Where dating constraints for onset of the [local Last Glacial Maximum] exist, however, they support a sea-level forcing in placing the associated Antarctic margins at their maximum extent when global sea level was approaching or first reached its LGM lowstand (Fig. 3). In particular, we suggest that NH ice-sheet growth that occurred in response to decreases in insolation and Pacific SSTs (9, 30) caused the global mean sea level to fall, allowing Antarctic ice margins to advance across the continental shelf and reach their maximum extent. At the same time, the reduction in NADW formation (Fig. 3) and attendant heat flux would further contribute to advance of Antarctic marine margins.

The subsequent onset of NH deglaciation ~19 ka in response to boreal summer insolation forcing caused an initial rapid global mean sea-level rise of ~5 to 10 m (Fig. 3) (9, 43, 44). Although this sea-level forcing may explain the contemporaneous retreat of Antarctic grounding lines in the Weddell Sea and, perhaps, Amundsen Sea regions, the lack of a response at other dated Antarctic marine margins appears inconsistent with this hypothesis. This spatial variability in response may reflect different geometries of ice shelves, variations in subshelf pinning points, or differences in sedimentary wedges (size and stiffness) that stabilize grounding lines to a rapid sea-level rise of this magnitude (45). In addition, sea-level calculations indicate that gravitational, deformation, and rotational effects associated with the initial melting of NH ice ~19 ka caused enhanced sea-level rise around the Weddell and Amundsen Seas relative to eustatic, whereas it was equal to or less than eustatic around the Ross Sea and Mac Robertson Land margins (Fig. 4) (SOM). This regional enhancement may have been sufficient to overcome the stabilizing effect provided by sedimentary wedges at grounding lines in the Weddell and Amundsen Seas (45), causing early retreat, whereas grounding lines elsewhere remained immune to the lesser sea-level rise.

Carbon dioxide and Antarctic glaciation

Pagani et al., writing in Science:

The decline in the partial pressure of atmospheric carbon dioxide during the [Eocene-Oligocene] climate event was substantial, but absolute CO2 concentrations depend on the value of εf applied. Collectively, CO2 estimates calculated by using U37K’ and TEX86 SST estimates and a range of εf values indicate that CO2 decreased ~40% from 35.5 to 32.5 million years ago (SOM). Application of reasonable εf values (25 to 28‰) indicates that the partial pressure of atmospheric CO2 fell from 1200 to 1000 ppm to 700 to 600 ppm. Interestingly, the change in CO2 determined from this study, as well as the boron-isotope methodology (11), is consistent with model estimates for a threshold CO2 level required for rapid Antarctic glaciation (8, 29).

We conclude that the available evidence supports a fall in CO2 as a critical condition for global cooling and cryosphere evolution ~34 million years ago. Whether CO2 acted alone to cause the E-O transition or whether a threshold CO2 level in combination with favorable orbital configurations (1) ultimately triggered glaciation cannot be determined from our results. However, during the E-O transition both CO2 decline and enhanced ice albedo account for global temperature changes. Lastly, the long-term permanence of the CO2 decline (10) and the impermanent inorganic carbon isotope shift (1) implicate the role of silicate weathering rates over the influence of short-term organic-carbon burial rates as the primary cause for long-term change in atmospheric carbon dioxide.

Dec 17

On the origin of animals

Erwin et al. review the fossil and molecular record of early animal evolution:

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Dec 6

Don’t count on air capture

House et al., writing in PNAS, suggests that directly removing CO2 from the atmosphere may cost in excess of $1,000/tonne, based on their techno-economic analysis:

Our empirical analyses of operating commercial pro- cesses suggest that the energetic and financial costs of capturing CO2 from the air are likely to have been underestimated. Specifically, our analysis of existing gas separation systems suggests that, unless air capture significantly outperforms these systems, it is likely to require more than 400 kJ of work per mole of CO2, requir- ing it to be powered by CO2-neutral power sources in order to be CO2 negative. We estimate that total system costs of an air capture system will be on the order of $1,000 per tonne of CO2, based on experience with as-built large-scale trace gas removal systems.

They mention biomass combustion with CCS as an alternative approach that, while scale-limited, may be somewhat less costly (probably $150-$400/tonne).

Nov 23

The future of science

Colin Macilwain in Nature:

Those involved in science policy sometimes seem to me to be sleep-walking through the greatest crisis to afflict the West since the Second World War. True, from the point of view of the scientist at the bench, grants continue to flow and results continue to be published. Perhaps this is why wider discourse about science’s role in society has hardly budged an inch….

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