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Geothermal ocean warming discussion thread

by Judith Curry

“The atmosphere bias of climate science makes it impossible for them to see geological forces and therefore, impossible for them to understand the earth’s climate.” – Thongchai

When conducting the literature survey for my report on sea level rise [link; see section 4.2], I became intrigued by under-ocean heat sources.

“Wunsch (2018) identified lower bounds on uncertainties in ocean temperature trends for the period 1994-2013. The trend in integrated ocean temperature was estimated by Wunsch to be 0.011 ± 0.001 oC/decade (note: this rate of warming is much less than the surface warming, owing to the large volume of ocean water). This corresponds to a 20- year average ocean heating rate of 0.48 ±0.1 W/m2 of which 0.1 W/m2 arises from the geothermal forcing. I have rarely seen geothermal forcing (e.g. underwater volcanoes) mentioned as a source of ocean warming – the numbers cited by Wunsch reflect nearly a 20% contribution by geothermal forcing to overall global ocean warming over the past two decades.”

Makes me wonder how much of the TOA radiative energy imbalance calculated from ocean heat content reflects seafloor geothermal heat fluxes?

Climate modelers are beginning to pay attention to seafloor geothermal fluxes.  The first such study that I’ve spotted is Adcroft et al. (2012), using a uniform geothermal heat flux of 50 mW/m2 through the sea floor. They found substantial changes in deep circulation to this heat flux.

The GFDL ESM2 Global Coupled Climate-Carbon Earth System Model (2012) [link] states that it incorporates ocean geothermal heat flux following Adcroft et al.  I don’t know if this is what the current  (CMIP6) version of ESM2 uses.

The most interesting analysis that I’ve spotted on this is Downes et al. (2016) The transient response of Southern Ocean Circulation to Geothermal Heating in a Global Climate Model [link]

Abstract. Model and observational studies have concluded that geothermal heating significantly alters the global overturning circulation and the properties of the widely distributed Antarctic Bottom Water. Here two distinct geothermal heat flux datasets are tested under different experimental designs in a fully coupled model that mimics the control run of a typical Coupled Model Intercomparison Project (CMIP) climate model. The regional analysis herein reveals that bottom temperature and transport changes, due to the inclusion of geothermal heating, are propagated throughout the water column, most prominently in the Southern Ocean, with the background density structure and major circulation pathways acting as drivers of these changes. While geothermal heating enhances Southern Ocean abyssal overturning circulation by 20%–50%, upwelling of warmer deep waters and cooling of upper ocean waters within the Antarctic Circumpolar Current (ACC) region decrease its transport by 3–5 Sv (1 Sv = 106 m3 s−1). The transient responses in regional bottom temperature increases exceed 0.1°C. The large-scale features that are shown to transport anomalies far from their geothermal source all exist in the Southern Ocean. Such features include steeply sloping isopycnals, weak abyssal stratification, voluminous southward flowing deep waters and exported bottom waters, the ACC, and the polar gyres. Recently the Southern Ocean has been identified as a prime region for deep ocean warming; geothermal heating should be included in climate models to ensure accurate representation of these abyssal temperature changes.

This is by no means an exhaustive literature survey on incorporation of seafloor geothermal heat flux into ocean models, but I suspect that the GFDL model is the most advanced one in this regard.

The motivation for this particular thread is an email that I received today, and also some tweets I spotted.

The Miocene

Why the Miocene?  This blurb from the current AGU Call for Abstracts provides a good summary:

“The Miocene (23 to 5.3 mya) is a crucial, dynamical interval in Earth’s history that provides unparalleled insights into the functioning of greenhouse climates.  At times during the Miocene, Antarctic ice volume was half modern, the Arctic Ocean was ice-free in winter, and extratropical temperatures nearly as warm as in the Eocene. This is an enigma, because the continental configurations and ocean circulation were much closer to modern than in the Paleogene, and atmospheric pCO2 was in the 300-600 ppm range.  Taken at face value, this implies either a system highly sensitive to greenhouse gas forcing or the presence of still unexplained forcings and feedbacks.”

A blog post by Thongchai suggests that the mid Miocene warming is caused by solid Earth dynamics [link].

“The general consensus in the bibliography below seems to be that the Mid Miocene warming event is best explained in terms of deep ocean circulation or the so called “oceanographic control of Miocene climate“. Many of these authors who are still in paleo climate research now tend to soft pedal these anomalies and discrepancies in public discourse to present the Mid Miocene warming in terms of the CO2 greenhouse effect although their new improved assessment appears to contradict what they had written twenty or more years ago. In many of the works below, particularly the later papers, it appears that the authors are struggling to relate grossly anomalous situations to the greenhouse effect of atmospheric CO2.”

The list of references is interesting; this is a provocative hypothesis that has been inadequately investigated.

Modern climate

With regards to the impact of geothermal ocean warming, Ron Clutz has a good post summarizing the published literature on this. Some excerpts:

“Little attention is paid to geothermal heat fluxes warming the ocean from below, mostly because of limited observations and weak understanding about the timing and extent of eruptions.”

“There appear to be three major issues around heating of the ocean from below through the seafloor:

1.  Is geothermal energy powerful enough to make a difference upon the vast ocean heat capacity?
2.  If so, Is geothermal energy variable enough to create temperature differentials?
3.  Most of the ocean floor is unexplored, so how much can we generalize from the few places we  have studied?”

“Without geothermal heat fluxes, the temperatures of the abyssal ocean would be up to 0.5 C lower than observed, deep stratification would be reinforced by about 25%, and the strength of the abyssal circulation would decrease by between 25% and 50%, substantially altering the ability of the deep ocean to transport and store not only heat but also carbon and other climatically important tracers . It has been hypothesised that interactions between the ocean circulation and geothermal heating are responsible for abrupt climatic changes during the last glacial cycle.”

“Geothermal heating contributes to an overall warming of bottom waters by about 0.4◦C, decreasing the stability of the water column and enhancing the formation rates of North Atlantic Deep Water and Antarctic Bottom Water by 1.5 Sv (10% ) and 3 Sv (33% ), respectively. Increased influx of Antarctic Bottom Water leads to a radiocarbon enrichment of Pacific Ocean waters, increasing ∆14C values in the deep North Pacific from -269◦/◦◦when geothermal heating is ignored in the model, to -242◦/◦◦when geothermal heating is included. A stronger and deeper Atlantic meridional overturning cell causes warming of the North Atlantic deep western boundary current by up to 1.5◦C,”

Lots of interesting material and references in Ron’s blog post.

A series of papers on mid-ocean spreading zone seismic activity and global temperatures have been published by Arthur Viterito [link]. As per personal communication with AV, the seismic data he used is from IRIS Wilber 3 [link].  Note the jump in the late 1990’s.

JC reflections

Our understanding of the link between sea floor geothermal heat flux and climate seems to be in its infancy

There seems to be a sufficient number of publications and observational evidence that lend credence to a link; the issue is the magnitude of the effect.  Dismissing such an effect as unimportant given our current state of understanding is misguided.

Since this is a topic that I haven’t spent a lot of time investigating, I look forward to insights and references from the comments.

 

 

Source: 
Judith Curry

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