Guest Essay by Dr. Gavin Schmidt, NASA GISS
Yesterday, I carried this story: An alarmist prediction so bad, even Gavin Schmidt thinks it is implausible
Today, on Twitter, Karel Haverkorn asked why. To his credit, Dr. Schmidt replied on Twitter in multiple tweets with an essay of bullet points. This marks the first time Dr. Schmidt publishes on WUWT, as well as the first essay here ever composed on Twitter.
I’ve collated his responses below.
Also the PETM (55 My) and Eocene small events. But no evidence under near-current temps. Outside of quaternary range of arctic temps, many fewer constraints…. Pliocene CH4 may well have been higher (but no direct evidence), multiple sources though…
Some more context on Arctic methane release story to follow:
1) Methane is an important part of the anthropogenic radiative forcing over 20thC. Human caused increase from 0.7ppm to 1.8ppm
2) Methane emissions have a direct GHG effect, and they effect atmospheric chemistry and strat water vapour which have additional impacts
3) Direct forcing from anthropogenic methane ~0.5 W/m2, indirect effects add ~0.4 W/m2. (For ref: CO2 forcing is ~1.8W/m2)
4) natural feedbacks involving methane likely to be important in future – via wetland response to T/rain chng, atmos chem &, yes, arctic src
5) monitoring and analysis of atmos conc of CH4 is very important. However, despite dramatic Arctic warming and summer sea ice loss ….. > …. In recent decades, little change has been seen in atmos concentrations at high latitudes.
6) There are large stores of carbon in the Arctic, some stored as hydrates, some potentially convertible to CH4 by anaerobic resporation
7) there’s evidence in deep time records of large, rapid exogenous inputs of carbon into climate system; leading theory relates this to CH4
8) it is therefore not silly or alarmist to think about the possibilities, thresholds and impacts for these kinds of events
9) in more recent past, there have been a number if times when Arctic (not necessarily globe) has been significantly warmer than today.
10) Most recently, Early Holocene, which had significantly less summer sea ice than even 2012. Earlier, Eemian 125kyrs ago was sig warmer
11) At neither of these times is there any evidence for CH4 emissions or concentrations in excess of base pre-industrial conditions.
12) this means that we are not currently near a threshold for dramatic CH4 releases. (Though we may get there)
13) Much of the concern re dramatic changes in Arctic methane come from one off surveys and poorly calibrated remote sensing
16) But we should not take what-if sensitivity experiments as predictions.
Dr. Judith Curry also thinks the “methane time bomb” is implausible.