Recently a comment published in Nature made the case for a catastrophic release of the methane (a very potent GHG) in the Arctic which in turn could cause sudden warming and cause massive economic damage in the trillions of dollars.
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
14) thus potential for Arctic CH4 to have threshold behaviour is real, but very lg scenario used in Nature comment is not realistic
15) We should be monitoring the Arctic better than we are, and we should be alert for ‘surprises’ in the greenhouse.
16) But we should not take what-if sensitivity experiments as predictions.