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Australian deserts are controlling global CO2 levels?

Does the world owe Australia bezillions of dollars in carbon credits? With years of La Nina rainfall on arid outback Australia, “we” (or rather the citizen plants of Australia) have apparently been sucking down the CO2 at a phenomenal rate: “almost 60 per cent of carbon uptake attributed to Australian ecosystems.” But, sigh, call me unconvinced. I think what this paper demonstrates is that consensus and simulations are not worth much, and that we don’t know where global CO2 is going.

And anyway, the Australian outback vegetation explosion is ephemeral. While there may have been a lot of  carbon sucked out of the atmosphere in 2011, as noted in the paper, it quickly gets oxidized and goes back into the atmosphere over the next year or two when the grasses and flowers die. So maybe hold off on those bezillions of dollars of carbon credits.

Global carbon markets turnover $50-180 billion a year with the aim of changing global carbon dioxide levels. (Yes, that’s what they say these markets are for.) But the brutal truth is that we are still guessing where exactly our carbon emissions end up. The consensus was that probably tropical forests were doing  extra global sucking and were the mystery sink. A few weeks ago it was Indian wetlands. Today, this new study suggests it is really the arid lands of Australia. There goes the consensus on land sinks of CO2.

Apparently the sinks are not doing too badly at keeping up with the sources. Those annual rises and falls in both don’t seem to have a lot to do with human civilization.

Figure 1a | Interannual variability of NEE and FPAR anomalies. a, Annual NEE, where positive values represent carbon uptake, blue is LPJ, red is MACCII, and the residual land sink is in grey. The standard deviations are 60.58 PgCyr21 for LPJ, 60.4 PgCyr21 for the inversion, and 60.8 PgCyr21 for the residual (see Methods).

Anyone think its a bad idea to launch global markets based on a ubiquitous molecule produced and used by every bit of Life On Earth, which has a cycle we haven’t figured out yet. Anyone? No mention of this in Nature.

Daniel Metcalf, Nature :

Of the roughly 10 billion tonnes of carbon emitted each year from human activity, only around half remains in the atmosphere, with the rest being absorbed by the oceans and by plants on land. This CO2 sink has been growing steadily, but the situation could change as shifts in climate and human land use intensify.

…the land sinks seems to be highly sensitive to variations in temperature and rainfall over yearly timescales.

Where was that land sink? Don’t tell me the “simulations” were wrong?

“…large-scale assessments of carbon budgets have indicated that the culprit is somewhere on land, and simulations went one step further by pinpointing tropical forests as the prime suspect. So far, so simple, until 2011 — when scientists estimated an extraordinarily high value for the land sink which seemed to be linked not to tropical forests as the prime suspect, but to semi arid ecosystems in the Southern Hemisphere.

Their results challenge the current consensus about what regulates atmospheric CO2 from year to year…

I think most of this paper is based on simulations and calculations, but satellites do show 6% more green in arid Australia.

Methods: We use multiple data sources, including carbon accounting methods, carbon-cycle
model simulations, and satellite-based vegetation products to investigate the magnitude
and mechanisms driving variability in the terrestrial carbon sink.

I’ve put in this graph because it has nice colors and shows that all the action is happening in Australia. FPAR means “fraction of photosynthetic active radiation”.

Seasonal AVHRR FPAR anomalies (z score) for the year 2011. Figure 2 extended data

These graphs have some of the most impenetrable captions I’ve seen.

Caption: The z score is calculated relative to the long-term seasonal mean and standard deviation of FPAR (1982–2011); see legend to Fig. 1c. The seasons DJF, MAM, JJA and SON are defined by the first letter of each month. [So lets look at Figure 1: Interannual variability of NEE and FPAR anomalies. c, AVHRR FPAR anomalies for the southern (S) and northern (N) hemispheres with respect to the 1982–2011 long-term average where the seasonal anomalies were calculated as the z score for each season (s) and each grid cell (i,j) for each year (y); Equation not reproduced here.]

ABSTRACT (Poulter et al 2014)

The land and ocean act as a sink for fossil-fuel emissions, thereby slowing the rise of atmospheric carbon dioxide concentrations1. Although the uptake of carbon by oceanic and terrestrial processes has kept pace with accelerating carbon dioxide emissions until now, atmospheric carbon dioxide concentrations exhibit a large variability on interannual timescales2, considered to be driven primarily by terrestrial ecosystem processes dominated by tropical rainforests3. We use a terrestrial biogeochemical model, atmospheric carbon dioxide inversion and global carbon budget accounting methods to investigate the evolution of the terrestrial carbon sink over the past 30 years, with a focus on the underlying mechanisms responsible for the exceptionally large land carbon sink reported in 2011 (ref. 2). Here we show that our three terrestrial carbon sink estimates are in good agreement and support the finding of a 2011 record land carbon sink. Surprisingly, we find that the global carbon sink anomaly was driven by growth of semi-arid vegetation in the Southern Hemisphere, with almost 60 per cent of carbon uptake attributed to Australian ecosystems, where prevalent La Niña conditions caused up to six consecutive seasons of increased precipitation. In addition, since 1981, a six per cent expansion of vegetation cover over Australia was associated with a fourfold increase in the sensitivity of continental net carbon uptake to precipitation. Our findings suggest that the higher turnover rates of carbon pools in semi-arid biomes are an increasingly important driver of global carbon cycle inter-annual variability and that tropical rainforests may become less relevant drivers in the future. More research is needed to identify to what extent the carbon stocks accumulated during wet years are vulnerable to rapid decomposition or loss through fire in subsequent years.

For the carbon accounting nerds…


We use multiple data sources, including carbon accounting methods, carbon-cycle model simulations, and satellite-based vegetation products to investigate the magnitude and mechanisms driving variability in the terrestrial carbon sink. NPP (the total photosynthesis minus plant autotrophic respiration losses) is simulated by the LPJDGVMand also estimated independently with the MODISNPP algorithm,MOD17A3. The balance between carbon uptake from NPP and losses from soil respiration and disturbance (NEE) is quantified from the Global Carbon Project, the LPJ DGVM, and the MACC-II atmospheric inversion system (http://www. copernicus-atmosphere.eu/).NEP(the balance between gross carbon inputs from photosynthesis and losses from ecosystem respiration, excluding disturbance) is estimated from upscaled gridded flux tower observations.Optical and passive microwave satellite  are employed to assess vegetation greenness trends (AVHRR FPAR3g) and vegetation structure or vegetation optical depth (VOD). Monthly and seasonal precipitation fluctuation is quantified fromTRMM3B43v7 (http://mirador.gsfc.nasa.gov) and NCEP-DOE Reanalysis II  (http://www.esrl.noaa.gov), and the Climatic Research Unit (CRU) TS3.21 (http://www.cru.uea.ac.uk/). Regional summaries of the global gridded data followed boundaries from the eleven land regions specified in the TRANSCOM atmospheric inversion experiment. We further differentiate North and South Africa to distinguish between wet and semi-arid climateswith the ratio of precipitation to potential evaporation set to 0.7. Historical (1860–2005) simulations of net biome production, equivalent to NEE, from the Fifth Coupled Model Intercomparison Project (CMIP5) are merged with the Representative Concentration Pathway 8.5 (RCP8.5) to create temporal composites spanning 1860–2099 for 15 Earth system models.

h/t The HockeySchtick

Thanks to dissident-robert for help.


Metcalf, D. (2014) Climate science: A sink down under, Nature, doi:10.1038/nature13341

Poulter, B. et al. (2014) Contribution of semi-arid ecosystems to interannual variability of the global carbon cycle Nature http://dx.doi.org/10.1038/nature13376 (2014).

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