Hi all, we ( @junglecave, @sclem2525, & I) have a new paper out entitled "Methane, Monsoons, and Modulation of Millennial-scale Climate” in @theAGU’s #GeophysicalResearchLetters!

Check it out here: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL087613

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The fundamental question driving our study is:

"Does orbital forcing *modulate* millennial-scale climate variability?"

We define 'modulation' following signal processing lit., where a modulator impacts a carrier signal & results in amplitude or frequency modulation. (2/10)

In other words: can the background state of Earth’s orbital configuration impact the strength and frequency of "abrupt", millennial-scale #climatechange events such as Heinrich or Dansgaard-Oeschger events, which involve the ocean and cryosphere? (3/10)

To address this, amongst other records, we focus on two long, well-dated reconstructions: atmospheric methane & the composite stalagmite δ¹⁸O record from eastern China -- commonly interpreted as a proxy for "Asian monsoon intensity". (4/10)

Why these two records? Well, they are state-of-the-art in terms of dating+resolution, & span multiple Milankovitch cycles! But apart from that, they have been previously linked to precession cycles & both have been hypothesized to be *modulated* by orbital cycles! (5/10)

More monsoon rains are thought to increase wetland area extent & hence, atm. methane. However, whereas Brook et al. posit millennial-scale methane activity with insolation, Cheng et al. suggest the opposite relationship with monsoons! What gives? We were curious. (6/10)

Our analysis finds an unambiguous signature of #orbital precession in the magnitude of millennial-scale #methane variability. However, we could find no evidence of primary orbital frequencies directly modulating the millennial component of Chinese #stalagmite δ¹⁸O! (7/10)

In fact, millennial-scale variance in composite Chinese stalagmite δ¹⁸O shared a *lot more* commonalities in its modulating signatures with #Antarctic ice-core δD than atmospheric CH₄ (which comes from the same archive as the latter!! - what!) (8/10)

What does this mean? Well, read the paper to get more juicy deets! As a teaser: we posit that extratropical CH₄ sources might be more important than we think for abrupt changes. Also, #Milankovitch cycles do not pace millennial-scale Chinese stalagmite δ¹⁸O variability! (9/10)

Finally, we find an increasing trend (!!) in the amplitude of millennial-scale climate change since ~430 ka ("the Mid-Brunhes Event"). We do not observe this trend in CH₄ (& as we know, insolation doesn't trend over these timescales).

Read more: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020GL087613

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