Yeah, as the question that spawned this thread was more general, it seemed worth while to pick up and answer a question more specifically focused on the fundamental misunderstandings this recently published paper seems to be propagating.

It's really bad. Even the publisher is making pretty misleading comments about the paper, like this tweet from Nature that pretty much implies that the inner core is somehow not rotating at all or rotating in an opposite direction.

I assume this question originated from buzz (and general misunderstanding) around the newly published Yang & Song, 2023 paper. Many are misinterpreting the suggestion made in this paper that there is a slowing or reversal of differential rotation of the inner core to mean instead mean a slowing or reversal of absolute rotation of the inner core. The competition between the EM and gravitational torques are front and center in this paper, so it's an important clarification to make.

> Are either of these processes related to the pressure of the mass above? I always thought that was what caused the heat.

No. The mass above plays a role in the sense that it dictates the rates of heat transfer toward the surface from a given depth, but the fact that it's under pressure does not directly generate heat. There are various depth/pressure/temperature related processes that occur, specifically phase transitions at specific depth/pressure/temperature ranges that do change heat content, e.g., the 660 transition is characterized by an endothermic reaction and thus does contribute to the total heat budget.

> 1. Is radioactive decay triggered by something or does it just happen with those particular elements?

Radioactive decay, and its rate, is an intrinsic property of a given isotope. There is abundant literature and details on radioactive decay, but generally speaking, for a given isotope (e.g., ^(238)U, which is one that is relevant for the question), the "rate" of decay is actually a reflection that there is a fixed probability that any given 238 atom will decay, and given an arbitrarily large group of 238 atoms, we can consider this as a rate of decay.

> Is there something besides the insulating qualities of the crust that keep primordial heat from cooling quicker.

Insulating qualities of the entire planet, i.e., rock is not a great conductor, the relative inefficiency of radiation as a heat transfer mechanism (i.e., how heat is ultimately transferred out of the solid Earth), and the radioactive decay all play an important role. For the last bit, it's not just that radioactive decay is adding heat, but also in doing so, it's effectively slowing down the rate of heat loss. Broadly speaking, the rate of heat transfer is related to the gradient in temperature. Thus a reduced gradient (because the interior stays warmer) means that the rate of heat transfer is less. Things get more complicated as we have to think about convective heat transfer in the outer core and mantle, but broadly this point remains true.

By far the two major sources of heat are primordial heat and radioactive decay. Changes in pressure can cause changes in temperature, but largely static pressure with depth does not generate heat. Tidal heating is not relevant for the Earth (though it's important for other planetary bodies in the solar system).

This seems to ignore the literature stream suggesting that there is a fundamental hemispheric anisotropy of the inner core and that this may relate (in part) to the super-rotation of the inner core (e.g., Dumberry & Mound, 2011, Wazek et al., 2011, Deuss, 2014, Lythgoe et al., 2014, Yu et al., 2017, etc). Is that no longer valid?

I would assume so, but as far as I know on Earth, we tend to find two distinct dip poles roughly equivalent to a north and south pole (i.e., one with a +90 inclination and one with a -90 inclination). You might expect that during a geomagnetic reversal (which is often described as a weakening/temporary collapse of the dipole component and thus a relative strengthening of the higher order field components) you could have more than one pair of dip poles, maybe?

/u/Weed_O_Whirler has covered the relationship between the cardinal directions and the magnetic field of the Earth, but another aspect of your question is basically asking why do we use a coordinate system that (when viewed on a projected map) is a Cartesian coordinate system. At the simplest level, to define a location in an x-y plane, you need two coordinates. You could theoretically define two coordinate axes which are not at right angles to each other, but it would make defining coordinates way more complicated. The simplest solution is to have two orthogonal coordinate axes (and in reality, to have three orthogonal coordinate axes, i.e., elevation) to uniquely define locations. The same logic can be applied to spherical coordinate systems, i.e., why do we define locations with respect to intersections of sets of orthogonal planes with a sphere? Cause it's easier than defining coordinates with a non-orthogonal set of planes and their intersections with a sphere.

It's also worth noting that in most locations, it's actually rare that the sun rises/sets truly due east/west with respect to true north (and the location where the sun set/rises moves because of the inclination of the rotational axis with respect to Earth's orbital plane). Thus, east and west are not defined as such because of the sun, but rather because they specifically are directions along the equator which is orthogonal to the rotational axis. Even if we were on a planet that had a very high inclination (i.e., the orientation of the rotational axis was much closer to being parallel with the orbital plane) it would still make the most sense to define a set of coordinate axes that parallel the equator (and thus are perpendicular to the coordinate axis that parallels the rotational axis).

Getting back to the relation between our coordinate axes and the magnetic field, it would be interesting to consider a hypothetical of early navigation developing on a planet that had a setup like Uranus, which has a rotational axis nearly parallel to the orbital plane but also a magnetic field that is oriented at a high angle with respect to the rotational axis. We define coordinates on Uranus like we do on Earth, i.e., with respect to the rotational axis, but it's hard to know what kind of coordinate system one would develop if you were living on (a habitable rocky planet) that had a similar setup.

To add an extra level to the rabbit hole, there are actually three norths. True north, magnetic dip north, and geomagnetic north. True north has been covered, but magnetic dip north is the location where the magnetic field is oriented vertically (so if you had a magnetic inclinometer, where it would point vertically) whereas geomagnetic north is the intersection of the surface of the Earth with the best fit dipole field. In detail, if the Earth's magnetic field was a true dipole (like a bar magnet) then the north and south dip poles (1) would be antipodal with respect to each other and (2) they would be equivalent to the geomagnetic poles (which are by definition antipodal), but neither of those tend to be true as the location of the dip poles migrate as part of secular variation.

This is covered in one of our FAQs that no one ever reads.

> an event on such a scale could cause sufficient particulate matter to enter the atmosphere that it could create a period of several years of insufficient sunlight reaching the surface of the earth to massively disrupt ecosystems and create a mass extinction event.

So the potential cooling effects of large explosive volcanic eruptions (e.g., events like the 1815 Mount Tambora eruption) are not disputed, but this is not actually relevant for Deccan Traps volcanism or the suggested kill mechanisms related to them. For the Deccan Traps as the cause of the K-Pg extinction, the kill mechanism may have been global warming from pulses of greenhouse gases released by the volcanism (e.g., Tobin et al., 2012) or a combination of this along with ocean acidification and ocean warming (e.g., Keller et al., 2020). I.e., flood basalt eruption and its effects do not equal large caldera eruption and its effects.

> Finally once the crater was found, dated and confirmed it was accepted more or less.

This ignores a pretty active literature stream that has persisted since the impact hypothesis was proposed (and which continues to this day) that questions whether this was the cause (e.g., McLean, 1985, Courtillo & Cisowski, 1987, Pope, 2002, Keller et al., 2004, Fastovsky & Sheehan, 2005, Keller et al., 2020, etc.).

> some claim the impact caused the volcanic eruption, the shock waves converging on the far side of the planet where India would have been at the time

This is generally not what is argued for. What has been suggested is that the impact may have triggered a large pulse of Deccan Traps volcanism, but the timing of the start of Deccan Traps volcanism is demonstrably before the impact (e.g., Renne et al., 2013, Schoene et al., 2014, Renne et al., 2015) but timing of the main eruptive pulse remains controversial, i.e., it may have occurred sufficiently after the impact to be unrelated (e.g., Sprain et al., 2019).

> But less evidence for that

This is debatable, viable kill mechanisms tied to either event are pervasive in the literature (as are people pointing out issues with the alternative kill mechanism(s)). Arguably, the idea that neither the Deccan Traps nor the Chicxulub impact alone would have caused the extinction, but that the occurrence of both in short succession was enough to start the cascade is becoming closer to a consensus view (e.g., Petersen et al., 2016, but also the Renne et al., 2013 and Schoene et al., 2019 papers cited earlier). Similarly, there are suggestions that the K-Pg extinction was relatively protracted, perhaps occurred in pulses, and started before the impact, but with a pulse in extinction linked to the impact (e.g., Tobin, 2017)

The main thrust of this question is covered in an existing FAQ, but the short version is that; (1) Yes, volcanic eruptions can cause global cooling, but (2) many details of the volcanic eruption (e.g., how large is it, where did it occur, what time of year did it occur, where in the ENSO cycle did it occur, etc.) all have substantial impacts on the degree of cooling that will result from a specific eruption, and (3) modelling of the competition between volcanic driven cooling and anthropogenic driven warming suggests given the right details, a large volcanic eruption could pause warming for ~20 years but this would then be followed by ~20 years of accelerated warming (e.g., this paper).

For a deeper dive on all of the above, see the linked FAQ, but it's also worth noting that if we're looking for mitigation strategies for warming, there are probably better ways than hoping for a large volcanic eruption (or other similarly drastic means, like purposefully inducing a nuclear winter). For example, many solar geoengineering proposals, specifically stratopsheric aerosol injection, seek to effectively mimic some of the atmospheric effects of a large volcanic eruption without the eruption.

This is not a direct answer to the part of your question about the degree to which weather systems within a short period of time can be "linked", but with respect to the difference between a given year and an average value, there are two points to consider.

1. Something like the average you mention, i.e., "5 wet days per month" is just that, an average. For most climatological parameters, a reported mean will represent at least 10 years of data (usually more). Importantly, the mean only tells you about the central tendency, but nothing of the variability. If we were dealing with normally distributed data, you could think about something like the standard deviation as a crude metric of variability. So for your given area, if the average wet days for your month of interest was 5 and had a standard deviation of 1, that would broadly suggest 14 wet days is a lower probability event, but if the standard deviation was 5, that would similarly broadly suggest that the precipitation in that month is more variable (assuming you had enough a long enough set of data where you standard deviations were meaningful). Now in reality, while some climatological variables tend to be close to normally distributed (e.g., temperature), precipitation tends to not be well explained by normal distributions. Instead, distributions like the weibull, logistic, exponential, or GEV are better suited for describing precipitation. For these, we might be interested in the "shape" parameter (or equivalent) for a fit to the distribution of rainfall assuming one of these distributions which would give us a sense of the variability and thus how probable a significant deviation from the mean is.
2. In a similar theme, mean climatological parameters will typically represent averages across a variety of cyclical changes in climate, things like ENSO. We are currently experiencing La Niña, so hypothetically, if for your location in this month, it's typically more wet during La Niña (and perhaps drier during El Niño, where the mean reflects somewhere between the two), then this may not be "out of the ordinary" at all.

They're impact craters. They occur, in varying sizes, on pretty much every solid body in the solar system but the number of preserved craters depends on the extent to which the surface of the body in question has been "resurfaced" (largely because cratering rates are much lower than earlier in the history of the solar system). Because of active plate tectonics and surface processes, Earth does not have many preserved impact craters, especially compared to the Moon, Mercury, etc, but there are a few well preserved ones, which are similarly quite circular.

Most craters are circular. There are elliptical craters (e.g., this example from Mercury), but they are relatively rare, accounting for only around 5% of craters (e.g., Bottke et al., 2000). Very specific conditions need to occur for an impact to generate an elliptical, as opposed to circular crater. One of the critical factors is the angle of impact, with shallow angles between the impact trajectory and the surface favoring ellpitical craters, but other conditions, like slow impact velocities, stronger target materials, and larger impactors, modulate what impact angle is able to produce an elliptical crater (e.g., Elbeshausen et al., 2013).

> They claimed the paper shows how radiometric dating is unreliable, because radioisotopes can be leeched or absorbed which would through off the ratio of daughter to parent isotope.

An important aspect here is that not only are these considering relatively rare areas (i.e., areas influenced by radioactive testing and/or natural reactors like Oklo), they are (1) mostly tracing radionuclides that are not used in radiometric dating and (2) more importantly considering migration into and out of material (i.e., minerals) that are not typically used for radiometric dating. That radionuclides are soluble and thus easily mobilized out of oxides like what this paper focuses on is precisely a reason why many of these minerals are not considered suitable for geochronology. Minerals that we actually use regularly for geochronology (e.g., zircon, monazite, apatite, etc.) are used in part because they tend to be relatively resistant to these kind of effects.

Additionally, the underlying premise seems to be that geochronologists just uniformly accept the assumptions of closed-system behavior when in fact we routinely consider, and test for, open-system behavior as a matter of course in most analyses (e.g., Schoene et al., 2013). The degree to which we are concerned about open-system behavior, and potential remedies or corrections, depend on the method. We generally expect that U-Pb dating in zircon will reflect closed-system behavior, but still almost always check via measuring both ^(238)U-^(206)Pb and ^(235)U-^(207)Pb ages whereas we expect the possibility of open-system behavior of something like ^(234)U-^(230)Th dating in carbonates is relatively high and do a variety of things to check whether dates are valid or influenced significantly by open-system behavior.

Tidal heating is not a significant source of geothermal heat on Earth, the two primary sources are radioactive decay and left over heat from planetary accretion. Tidal heating is important on other bodies, e.g., Io, but not on Earth. You are still correct in that both sources of heat on Earth (i.e., heat from radioactive decay and primordial heat) are decreasing on geologic timescales.

Yes, at least in the short term and in the area immediately around (and at the depth) where heat is being extracted. Rocks are not great conductors, so in extracting heat from rocks in a geothermal power plant (usually through cycling a fluid to depth and then back up to the surface), the temperature of the rocks at the target depth in the vicinity of the plant will decrease through time basically because the process by which we extract heat is much more efficient than the process by which the rocks are reheated by conduction or movement of heated brines. This is one of the reasons individual geothermal power plants have lifespans (along with the progressive corrosion of, and precipitation of various solids in, the pipes and pumps involved in the geothermal plant, depending on the type of plant). The lifespan of a plant can be extended if less power is drawn from it (i.e., you can get a lot of power for a short time or less power for a longer time), but eventually the productivity of the plant will still decline through time (e.g., Budisulistyo et al., 2017). Similarly, there are mechanisms to try to replenish heat to slow the degradation of the resource, e.g., one proposal is coupling geothermal and solar heating where you heat fluids at the surface via solar energy and then cycle these hot fluids down to offset some of the heat extracted as part of generating geothermal power (e.g., Wendt & Mines, 2014).

In terms of long-term or large-spread geologic effects of this? Not really. Because rocks are poor conductors, the area of cooling will be relatively localized around the horizons being exploited by the plant and the area will be reheated, but on a timescale that is significantly longer than the typical few decade lifespan of an average geothermal power plant.

Corals only reflect one method for reconstructing past sea level, and while quite accurate recorders, their use is limited to the last few hundred thousand years (e.g., Woodroffe & Webster, 2014). For a complete answer, we must consider the varied array of methods used to reconstruct sea level over the phanerozoic, e.g., sequence stratigraphic techniques, stable isotopic records, etc.

If we're talking about sea level reconstructions further back than we have tide records (i.e., direct measurement of average sea level in multiple places globally) or the modern where we primarily rely on satellite altimetry data (e.g., Strassburg et al., 2014), then there are a wide array of methods used to reconstruct sea level. A non exhaustive list:

1. Dating packages of specific types of coral reefs. Particular species of corals can only live within a very narrow range near sea level. If sea level rises, the existing corals will die (from lack of light) and the colony will move upward building on top of the old corals. If sea level lowers, the existing corals will die (from exposure) and the colony will move downward and build on the flanks of the old corals. If you then date the different packages of corals you have a relative sea level record. To make this an absolute record of sea level, you need to know something about the rate of rock uplift of the area to which the corals are rooted. Dating of packages of corals that are now completely above sea level along tectonically active coast lines, like Papua New Guinea, can be used to construct sea level curves if the rate of rock uplift can be constrained through a variety of other means (e.g., extrapolation of geodetic rates, low temperature thermochronology, etc). There are varieties of studies that estimate portions of the sea level curve through these means (e.g., Cutler et al., 2003, Chappell, 2002).

2. Backstripping sediment records. Basically looking at the stratigraphic records of sedimentary basins and using the record of subsidence (after accounting for sediment compaction and subsidence driven by tectonics) to work out the relative height of different packages which are tied to different sedimentary environments that may be relevant for sea level (e.g., Sahagian et al., 1996, Kominz, 1995, Levy & Christie-Blick, 1991, etc.)

3. Sequence stratigraphy as interpreted from seismic records. This is a methodology really pioneered by the oil industry using large 2D and 3D seismic sections of marine. These rely on identifying packages (sequences) and finding the geometric relations between their boundaries, i.e., onlap, offlap, etc., which provide indications of relative sea level through time (e.g., Christie-Blick, 1991). These techniques have been used to produce large-scale global estimates of sea level (e.g., Vail et al., 1977, Haq et al., 1987, Haq & Al-Qahtani, 2005).

4. Similar to the prior one, sequence stratigraphy applied to continental sedimentary records as opposed to seismic stratigraphy (e.g., Sloss, 1963, Ronov, 1994, Haq & Schutter, 2008 etc.)

5. Proxy records for ocean temperature and ice sheet volume. Specifically, marine oxygen isotope records (i.e., primarily the ratio of two stable isotopes of oxygen, ^(16)O and ^(18)O) preserved in a variety of ways (e.g., in the shells of marine microrganisms) are sensitive to the global volume of ice stored on land because during periods where large ice sheets are building, the ice becomes preferentially enriched in light ^(16)O whereas the oceans become enriched in heavy ^(18)O (e.g., this explainer from NASA). Thus reconstructing this ratio within the ocean as preserved in marine organism shells (where the age of these organisms are known through dating the sediment within which they are deposited) allows us to reconstruct the relative volume of ice and thus the sea level (along with associated data on temperature, etc). There lots of studies building out parts of the global sea level record using multiple independent oxygen isotope records, models, and a variety of other data (e.g., Waelbrock et al., 2002, Siddall et al., 2003, Miller et al., 2011, Grant et al., 2012, Rohling et al., 2014, De Boer et al., 2017, etc.).

6. Recently, new methods have been proposed such as using paleogeographic reconstructions (which themselves represent huge syntheses of stratigraphic, paleomangetic, and paleontological data) to construct sea level curves (e.g., Marcilly et al., 2022).

7. Combinations of portions of above. Many of the above references in fact already combine more than one type of record or proxy to reconstruct a portion of sea level variations. Additionally, there are a variety of efforts to combine as many records as possible to identify disagreements and similarities in these records and to produce as accurate composites as possible (e.g., Miller et al., 2005, Miller et al., 2020).

In summary, if you browse through many of these, you'll see that there are ranges of uncertainty and sets of assumptions for any individual method and the methods do not always agree in detail for specific time periods. However, broadly when comparing the sea level curves derived from different approaches (and using them to refine each other), we find a good amount of coherence and agreement. Thus, while the absolute magnitude of past sea level as estimated from these approaches is likely not completely correct, we have high confidence in the broad patterns and order of magnitude values.

You are incorrect, halosteric (i.e., change in sea level due to salinity related changes in density) sea level change is definitely a thing. It's a smaller component than thermosteric changes, e.g. Durack et al., 2014 estimate the magnitude of halosteric changes to be 1/4 of the magnitude of thermosteric changes, but still significant.

> What is the source of energy that raises the level of sea water through thermal expansion?

The increase in the average temperature of the atmosphere. In short, the ocean absorbs heat from the atmosphere, so if the atmosphere warms up, the ocean will warm up as well. Accounting for the behavior of the ocean as effectively a giant heatsink has been shown to be important for understanding temporal changes in atmospheric temperature (e.g., Kosaka & Xie, 2013).

> Is thermal expansion global?

Yes, but it's not necessarily uniform. We can break out two primary components of sea level rise, mass addition (i.e., melting glaciers and continental ice sheets adding mass to the ocean) and so-called steric components that are changes in volume related to changes in density. Within the steric components, both the salinity and temperature influence this, i.e., colder and saltier water is denser and thus for a given mass takes up less volume. Spatial (and temporal) variations in both temperature and salinity mean that the steric component of sea level rise varies spatially, for example, a relatively high rate of sea level rise (compared to global averages) along the central east coast of the US has been partially attributed to warmer, fresher waters in these regions (e.g., Sallenger et al., 2012).

You are correct, gravity / escape velocity is the primary control and the replies indicating that the presence or absence of a magnetosphere are the primary control reflect a common misconception (e.g., see this post).

This is not correct though. Good counterpoints are Venus, which has no intrinsic magnetic field, only a relatively weak induced one, and yet still has a thick atmosphere or Mercury, which has an intrinsic magnetic field and effectively no atmosphere. This comes up a lot on AskScience and there are numerous threads considering the relative role of gravity, active volcanism, and magnetospheres for keeping planetary atmospheres, e.g., this thread where various posters lay out the details and highlight that gravity / escape velocity is the dominant factor in whether a planetary atmosphere is maintained, this specific comment by one of our panelists addresses this misconception directly.

There is already a pretty detailed discussion of this in our FAQs.