Thread by @anlomedad

@robjlinds @Peters_Glen Did you actually think that thru?
IMO, it's a false hope that plants might decrease CO2 ppm if we only reach true-0. At true-zero, AND under the˚C stress level then, carbon sinks decline further. Like they do today, at merely 1.2C. New sinks🌳will do, too!

https://twitter.com/anlomedad/status/1365502310013616130

@robjlinds @Peters_Glen Look at #AUSfires 2020, emitted 800Mt, IIRC? That was at only 1.2C. The forest grows back slowly, takes up CO2 again, or becomes grassland. True. But for now, it's gone, CO2 ppm has increased, sink decreased – and our˚C goes up.
Much forest land eventually burns or dies in ++˚C!

@robjlinds @Peters_Glen As forests die, as more and more will in ++˚C, decomposing emits CO2e, too. Up goes ppm and up goes˚C from that, if only by a little.

New 🌳do have a short-time advantage in heat/water stress bc roots are shorter than old trees'. But: shallow roots =more 🌳die in storms =++CO2e

@robjlinds @Peters_Glen And respiration vs photosynthesis has a temperature tipping point: over local 24/34˚they become net CO2 source. The forest in Siberia's heatwave in summer 2020 - even the area that didn't burn - was a net CO2 source, just from respiration under heat stress alone. As was EU forest

@robjlinds @Peters_Glen To sum it up:
*at higher than 1.2˚warming, plants and soils more and more often are a CO2 source wrt respiration in heat stress. New trees, too.
*dying from pests/water stress/fires emits CO2e and carbon sinks shrink.
👉Net-zero doesn't work and we can't hope to reduce ppm via🌳

@robjlinds @Peters_Glen We'd have to constantly add to the land carbon sink in order to maintain the level of planned sequestration. That's unrealistic. And by adding to the land carbon sink in form of new trees, we add a, more and more often, net CO2 source under heat stress from respiration alone.
💡

@robjlinds @Peters_Glen @threadreaderapp please unroll

@robjlinds @Peters_Glen @threadreaderapp The general mechanics of carbon sinks under positive CO2 emissions up to now was: 30% land, 20% oceans, 50% atmosphere. Since we still emit CO2 and since land carbon sink declines further from rising temperature, we de facto are decreasing our remaining CO2 budget already!

@robjlinds @Peters_Glen @threadreaderapp And in an assumed “net-zero” regime by, say, 2040 and at 1.5˚C then? Same as today: an ever declining land carbon sink increases the % of how much CO2 stays in the atmo. It wd happen without decline, too. Simply bc general mechanics is, 50% of residual positive makes CO2ppm go up

@robjlinds @Peters_Glen @threadreaderapp The pic shows carbon flux in a zero emission world at CO2 ppm theoretically consistent with well-below-2. It’s not balanced. There’s a slight positive flux into the atmo from Northern land and from oceans even in this uninformed model (used to actually investigate sea level rise)

@robjlinds @Peters_Glen @threadreaderapp source iopscience.iop.org/article/10.108…
oh, and the previous pictures wrt repspiration versus photosynthesis in C3 and C4 plants were from advances.sciencemag.org/content/7/3/ea…

@robjlinds @Peters_Glen @threadreaderapp And that model, setup to investigate sea level rise after a negative emission phase, was uninformed of what happens to C3 and C4 plants and forests.

In general, climate models still don't incorporate that (new) knowledge like this new paper also states nature.com/articles/s4146…

@robjlinds @Peters_Glen @threadreaderapp See? The general mechanics of our carbon sinks don’t support “net zero” hope. Even less so under higher˚C than today. Note: the higher the latitude, the faster the warming. 1 heatwave in boreal forests at 1.5˚- and a year's worth of their carbon sink capacity might be cancelled.

@robjlinds @Peters_Glen @threadreaderapp There’s no way we cd hope to compensate that by adding more and more trees to stay “net zero”. From the looks of it, we’ll have to add more and more trees annually just to stay true zero to keep ˚C the same.

And without mentioning emissions from the already thawing permafrost!