Guest “Surfing the Radcliffe Wave” by David Middleton,

Approximately 34 million years ago (mya), during the Early Oligocene Epoch, the Earth entered its current “icehouse” climate mode, with ice sheets covering Antarctica. After warming up again towards the end of the Oligocene, Earth’s climate began a long cooling trend, punctuated by a brief warm period during the Late to Middle Miocene Epoch.

The Middle Miocene Climatic Optimum (MMCO) was an anomalous warm interval from ~17 to ~14 mya. The incorrect assumption that CO₂ is the climate change “control knob” has led to many attempts to link the MMCO to the CO₂ and other greenhouse gas emissions related to the Columbia River Basalt Group (CRBG) eruptions. The CBRG eruptions do appear to be coeval with the MMCO…

Flood basalts, the largest volcanic events in Earth history, are thought to drive global environmental change because they can emit large volumes of CO₂ and SO₂ over short geologic time scales. Eruption of the Columbia River Basalt Group (CRBG) has been linked to elevated atmospheric CO₂ and global warming during the mid-Miocene climate optimum (MMCO) ~16 million years (Ma) ago. However, a causative relationship between volcanism and warming remains speculative, as the timing and tempo of CRBG eruptions is not well known. We use U-Pb geochronology on zircon-bearing volcanic ash beds intercalated within the basalt stratigraphy to build a high-resolution CRBG eruption record. Our data set shows that more than 95% of the CRBG erupted between 16.7 and 15.9 Ma, twice as fast as previous estimates. By suggesting a recalibration of the geomagnetic polarity time scale, these data indicate that the onset of flood volcanism is nearly contemporaneous with that of the MMCO.

Kashbohm & Schoene (2018)

These flood basalt eruptions certainly would have emitted a lot of CO₂, other volcanic gasses and sulfate aerosols. However the CO2 outgassing was probably only a fraction of what would have been required to explain the MMCO.

Armstrong McKay et al., 2014 estimated that the main phase of the CRBG eruptions, along with “cryptic degassing” of country rock, etc., emitted 4,090 to 5,670 billion tons of carbon over a 900,000 period. This only works out to 5-6 million tonnes of carbon per year… That’s an order of magnitude less than a rounding error. Our current 10 billion tonnes per year is only equivalent to 3% of the total annual sources in the Earth’s carbon budget. Self et al., 2005 found that CO₂ emissions from flood basalt eruptions were insignificant relative to the mass of CO₂ in the atmosphere and unlikely to have played a significant role in past episodes of “global warming.” Although they did note that the sulfur gas emissions and sulfate aerosols may truly have been unprecedented. However, these would have had a cooling effect on the climate.

Furthermore, there is little evidence of significantly elevated atmospheric CO₂ associated with the CRBG.

Neither the MMCO nor the subsequent cooling, leading the growth of the East Antarctic Ice Sheet (EAIS), appear to be driven by changes in atmospheric CO₂.

There is no evidence for either high pCO₂ during the late early Miocene climatic optimum or a sharp pCO₂ decrease associated with EAIS growth.

Pagani et al., 1999

While the cause of the MMCO remains somewhat of a mystery (probably tectonically-driven changes in oceanic circulation), the cause of the subsequent cooling (AKA Middle Miocene Climate Transition or MMCT) may have now been identified.

Our Solar System Crossed ‘Radcliffe Wave’ during Miocene Epoch, Astronomers Say

Feb 25, 2025 by Natali Anderson

As our Solar System orbits the Milky Way, it encounters various environments, including dense regions of the interstellar medium. These encounters can expose parts of the Solar System to the interstellar medium, while also increasing the flow of interstellar dust into the Solar System and Earth’s atmosphere. The discovery of new Galactic structures, such as the 9,000-light-year-long Radcliffe wave, raises the question of whether the Sun has encountered any of them. According to new research, the Solar System’s trajectory intersected the Radcliffe wave in the Orion star-forming region between 15 and 12 million years ago (Miocene epoch). Notably, this period coincides with the Middle Miocene climate transition on Earth, providing an interdisciplinary link with paleoclimatology.

[…]

The Radcliffe wave is a narrow sinusoidal gas structure, which comprises many known star-forming cloud complexes, such as CMa, Orion, Taurus, Perseus, Cepheus, North America Nebula, and Cygnus.

This gas structure, with an estimated mass of 3 million solar masses, appears to coherently oscillate like a traveling wave and it is thought to be part of the Milky Way’s spiral structure.

“Imagine it like a ship sailing through varying conditions at sea,” said Dr. Efrem Maconi, a doctoral student at the University of Vienna.

“Our Sun encountered a region of higher gas density as it passed through the Radcliffe wave in the Orion constellation.”

[…]

Sci.News

The paper, Maconi et al., 2025, was recently published in the open access journal Astronomy & Astrophysics. The authors were very careful to not draw sweeping conclusions, however the correlation between our solar system’s Radcliffe Wave transit and the MMCT is very interesting.

Here are the Miocene temperature and CO2 reconstructions plotted together:

The Radcliffe Wave transit certainly appears to be more likely to have influenced the MMCT than a decline in atmospheric CO₂. The notion of celestial climate drivers is not new. The concept is similar to that proposed by Nir Shaviv and Jan Veizer in their 2003 GSA Today paper.

Atmospheric levels of CO₂ are commonly assumed to be the main driver of global climate. Independently, empirical evidence suggests that the galactic cosmic ray flux (CRF) is linked to climate variability. Both drivers are presently discussed in the context of daily to millennial variations. To the extent that they actually exist, they should also operate over geological time scales. Here we analyse the reconstructed low-latitude sea surface temperature over the Phanerozoic (past 545 Myr), and compare it with the variable CRF reaching the Earth and with the reconstructed partial pressure of atmospheric CO₂ (pCO₂). We find that at least 66% of the variance in the reconstructed temperature trend can be attributed to CRF variations arising from solar system passages through the spiral arms of the galaxy, an observation that enables us to estimate the CRF/temperature relationship. Assuming that the entire residual variance in temperature is due solely to the CO₂ greenhouse effect, or that one of the reconstructed Phanerozoic pCO₂ trends is validated, we can place an upper limit to the long-term “equilibrium” warming effect of CO₂, one which is potentially lower than that based on general circulation models (GCMs).

Shaviv & Veizer, 2003

After incorporating their “celestial driver” model, Shaviv & Veizer estimated that the maximum equilibrium climate sensitivity (ECS) was 1.9 ºC per doubling of atmospheric CO₂, with a most likely value of 0.5 ºC per doubling. Unknown celestial climate drivers may be the reason that ECS estimates derived from paleoclimatology data are nearly twice that of those derived from contemporaneous instrumental data.

The Radcliffe Wave was only recently discovered (2019), even though it is relatively close (~400 light years) to the current position of our solar system. It was an “unknown unknown” potential climate change driver before the publication of Maconi et al., 2025.

How many other “unknown unknowns” are out there?

References

Armstrong McKay, David, Toby Tyrrell, Paul A. Wilson, & Gavin Foster. (2014). “Estimating the impact of the cryptic degassing of Large Igneous Provinces: A mid-Miocene case-study”. Earth and Planetary Science Letters. 403. 254–262. 10.1016/j.epsl.2014.06.040. Special thanks to David Armstrong McKay for kindly sending me a copy of his paper.

Kasbohm, Jennifer, and Blair Schoene. “Rapid Eruption of the Columbia River Flood Basalt and Correlation with the Mid-Miocene Climate Optimum.” Science Advances, American Association for the Advancement of Science, 1 Sept. 2018, advances.sciencemag.org/content/4/9/eaat8223.

Maconi, E., J. Alves, C. Swiggum, S. Ratzenböck, J. Großschedl, P. Köhler, N. Miret-Roig, S. Meingast, R. Konietzka, C. Zucker, A. Goodman, M. Lombardi, G. Knorr, G. Lohmann, J. C. Forbes, A. Burkert and M. Opher. “The Solar System’s passage through the Radcliffe wave during the middle Miocene”. A&A, 694 (2025) A167 DOI: https://ift.tt/fZu8Hgm

Pagani, Mark, Michael Arthur & Katherine Freeman. (1999). “Miocene evolution of atmospheric carbon dioxide”. Paleoceanography. 14. 273-292. 10.1029/1999PA900006.

Royer, D.L., et al. 2006.  “Tertiary Paleobotanical Atmospheric CO₂ Reconstruction. IGBP PAGES/World Data Center for Paleoclimatology”  Data Contribution Series # 2006-021. NOAA/NCDC Paleoclimatology Program, Boulder CO, USA.

Self, Stephen & Thordarson, Thorvaldur & Widdowson, Mike. (2005). “Gas Fluxes from Flood Basalt Eruptions”. Elements. 1. 10.2113/gselements.1.5.283.

Shaviv, N.J. and Veizer, J. (2003) “Celestial Driver of Phanerozoic Climate?”. GSA Today, 4-10.
https://doi.org/10.1130/1052-5173(2003)013<0004:CDOPC>2.0.CO;2

Tripati, A.K., C.D. Roberts, and R.A. Eagle. 2009.  “Coupling of CO₂ and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years”.  Science, Vol. 326, pp. 1394 1397, 4 December 2009.  DOI: 10.1126/science.1178296

Zachos, J. C., Pagani, M., Sloan, L. C., Thomas, E. & Billups, K. “Trends, rhythms, and aberrations in global climate 65 Ma to present”. Science 292, 686–-693 (2001).