Thread by @airscottdenning

🧵Thread on the "Failing Amazon Carbon Sink" paper by Gatti et al last week.

Nutshell: Amazon has been a carbon sink for decades; now it's switching to a carbon source.

Gatti et al tracked CO2 & CO in AIR ITSELF to measure regional CO2 balance. 1/n
go.nature.com/3hBndYi

Dave Keeling invented a precise IR gas analyzer in the 1950s. He installed one in Hawaii & one at South Pole.

By 1960 he'd shown that (1) CO2 is rising; (2) rate of increase is only ~50% of fossil fuel combustion; and (3) there's a seasonal cycle driven by NH vegetation.

2/n

By 1975, oceanographers had established that ocean uptake could only account for about ½ of the missing ½ of fossil fuel CO2. (they did this by tracking radiocarbon from 1960s nuke tests). They asserted that vegetation on land must be taking up the other ½ of the missing ½.

3//n

Ecologists thought the oceanographers were crazy because "everybody knew" land was a net *source* (not sink) of CO2. Great drama for a decade. Ice cores settled the matter in 1985. Oceanographers were right. Land ecosystems are a net sink of ~25% of fossil fuel emissions.

4/n

We were SHOCKED! SHOCKED! that land was a carbon sink. Of course plants eat CO2 for a living (photosynthesis), but all things must die. Dead plants are eaten by bacteria & turned back into CO2.

Land sink implies Growth > Death over many decades at planetary scale!

5/n

Four hypotheses to explain sustained Growth > Death:
1) CO2 fertilization (plants overeat)
2) N fertilization (ag & air pollution)
3) Regrowing forests (ag abandonment)
4) Boreal warming (longer growing season, more nutrients)

1 is good
2,3,&4 will soon saturate or reverse.

6/n

Tropical forests are the most productive ecosystems on Earth. Plot surveys (more on this later) and eddy flux suggest strong tropical sinks, but (1) CO2 fertilization must dominate there because
2) no big N deposition source;
3) no major ag abandonment ;
4) all-year growth

7/n

Forest inventory plots are just as hard as they sound:

Travel 1000s of km into forest. Define 100x100m plot. Measure every tree with a tape. Go back & do it again year after year!

100s of plots in the Amazon. Most find declining net carbon sinks.

@ForestPlots @ymalhi

8/n

Mapping changes with satellites is important too, but ridiculously hard over tropical forests because -- RAIN FOREST! -- nearly always obscured by clouds.

New instruments will help, but very short time series means change detection will take many years from now.

9/n

What @LucianaGatti5 & colleagues did -> amazing & heroic!

Built a state-of-science trace carbon gas lab at INPE (Brazilian space agency) outside Sao Paulo.

Fitted "suitcases" w/ flasks & autosamplers. Shipped to remote airports. Strapped into passenger seats of planes.

10/n

Little "air taxi" planes flew to 4500 m from remote airports 2x/month for 9 years.

Flasks automatically filled at different altitudes. Suitcases shipped back to lab for analysis.

Almost 600 vertical profiles of CO2, CO, CH4, 13C, 18O, COS. Amazing!

11/n

Back trajectory analysis from profiles shows regional area of influence footprints upwind. Other sampling on remote islands & coastal towers define background "pre-Amazon" trace gas concentrations.

Simple mass balance model to estimate effect of contact w/forest on CO2.

12/n

Drawdown/enhancement of CO2 per km of air travel in Trade Winds -> kgC/hectare over 1000s of km.

Similar analysis of combustion gases (CO) and remote sensing allows attribution to growth, decay, & fire.

Very different results by region & season!

13/n

Western Amazon: Trade Winds carry water vapor toward Andes. Wet wet wet beyond your sodden imagination! Almost never dries out. Close to carbon balance w/growth ~ death.

Eastern Amazon: wet season Oct-Apr, dry season May-Sep. Lots of deforestation/degradation/drought/fire.

14/n

Dry season in Eastern Amazon is warming 3x as fast as global mean! Impacted by shift in Bowen Ratio (less veg, less transpiration, more sensible heat).

THIS REGION IS NOW A NET SOURCE OF CO2!

Death > Growth in SE Amazon, even before fire emissions.

Bad Bad Bad!

15/16

Unlike previous studies, Gatti et al (2021) results are regionally representative of a wildly heterogeneous continent because they measured air instead of trees or cloudy pixels.

Great article. Read it here:
nature.com/articles/s4158…

16/16