Carbon lags and shifting the blame

Published today in the journal Environmental Research Letters is a paper by Carnegie ecologists Julia Pongratz and Ken Caldeira which shows that carbon dioxide released into the atmosphere before the industrial revolution continues to have a effect on climate today. Pre-industrial emissions from land use changes are said to be responsible for around nine per cent of the increase in global average temperature since that time.

Pongratz and Caldeira’s study is interesting, not only for its illustration of latency in the carbon-temperature relationship, but also for the sociological and political implications. These the authors hesitantly acknowledge, if only to preempt the hijacking of their work for political ends.

Having modelled pre-industrial carbon emissions on a global scale, Pongratz and Caldeira calculated the influence on emissions of the five-fold population increase that occurred between 850 and 1850 CE. This millennial growth was dominated by south and east Asia, with China and India accounting for half, culminating in a world population of a billion by 1850.

The researchers estimate that 20–40% of China and India’s history of carbon dioxide emissions is made up of pre-industrial emissions related to population growth, and their model shows that the emissions continue to have an effect on climate today. As for the causes of the pre-industrial emissions, land use changes are primary.

Excess carbon dioxide uptake by vegetation occurs at a very slow pace, as does the release of carbon through decay and the use of wood products. Land clearance for farming, on the other hand, results in a rapid release of carbon dioxide into the atmosphere through burning of vegetation.

The science aside, politics comes into it owing to the need to distribute the burden of greenhouse gas reductions in a world where emissions are rising rapidly and contributing to potentially devastating anthropogenic climate change.

Should states in existence today be held responsible for carbon emissions that occurred at a time when their effect was unknown? Even if they should, Pongratz and Caldeira show that accounting for pre-industrial emissions shifts the attribution of anthropogenic global warming from industrialised to less-industrialised countries by only 2–3%. That is a very small number, but much political hot air could be expended over it.

Says Caldeira…

“Accounting systems are not natural facts, but human inventions. Once an accounting system is defined, it becomes a matter of scientific investigation to determine what numbers should go in the ledger, but broader questions of who is responsible for what and who owes what to whom are judgments that lie outside the scope of science.”

This is a critically important point, but I fear that Caldeira’s words will be lost in the political verbiage and misrepresentation that could result from the publication of the scientific study. Still, though, the numbers should speak for themselves.

Further reading

Julia Pongratz & Ken Caldeira, “Attribution of atmospheric CO2 and temperature increases to regions: importance of preindustrial land use change”, Env. Res. Lett. 7, 034001 (2012)