The non linear nature of forcing is related more to
positive feedbacks and changes that are still being studied, such
as cyclic changes in moisture content and regional dispersion, the methane cycles in the ocean or the potential of methane clathrate / hydrate release, and of course the race to feed more people on a planet which will inevitably add more nitrous oxide to the atmosphere and create more dead zones in the oceans, droughts, floods, fires, dogs and cats living
together, mass hysteria....
Proxy evidence suggests a strongly
positive NAO and persistent La Nina mode of ENSO over the period ~ 1050 — 1400CE, possibly operating
together as a self - reinforcing
positive feedback loop (Trouet et al. 2009).