How big is the human carbon debt?
This post begins from the following premise: All combustion of stored carbon (i.e. fossil fuel) incurs a planetary debt.
The debt is incurred on two levels. The first is the climate change caused by emitting long-lived CO2 into the atmosphere. The second is the destructive impact human activity has on the biosphere of the planet - activity enabled by fossil fuel energy, that includes such things as aquifer draw-down, habitat destruction leading to extinctions, soil fertility depletion, and the over-fishing and eutrophication of rivers, lakes and oceans. This debt draws on stocks of carbon laid down in the past, and is payable in full by our descendants.
It would be interesting to have some sort of idea how big this debt has become over the years. The debt can be quantified by comparing the amount of carbon we emit by burning fossil fuels to the carbon we emit by breathing. The result is a rough order of magnitude estimate of our planetary debt in person-years.
According to CDIAC we have burned about 383 billion tonnes of carbon (GtC) since 1750.
An average person emits 850 grams of CO2 per day. That represents 185 grams of carbon per day, or about 68 kg of carbon per person-year. The 383 billion tonnes of carbon we have released from fossil fuels represents a planetary debt equivalent to 5.7 trillion person-years, where each person-year is just one human being burning nothing but a little biomass.
As a way of judging the scale of this number, it's estimated that about 108 billion people have lived on the planet since Homo sapiens appeared on the scene. If each of them had a life expectancy of 45 years, that would give a total of 4.8 trillion person-years. Our carbon debt since 1750 handily exceeds the carbon emitted by every person who has ever lived.
What does this debt of 5.7 trillion person-years imply?
One way to look at it is that we are at least (5.7/4.8) = 120% into overshoot.
Another way to look at it is that if all 7 billion people on earth were to die suddenly later this afternoon, it would take the planet at least (5700/7)= 800 years of human-free operation to come back into balance. Of course this estimate of 800 years is the bare minimum, since the replenishment of aquifers and the geological sequestration atmospheric carbon will take much longer than that.
We are currently adding 140 billion person-years to that debt every year through our continued consumption of fossil fuels. That means we're adding at least 20 years of additional debt to that 800-year repayment time, every year.
Two factors will interfere with us paying down this debt even if we could find some way to do so.
First and foremost is the very short time remaining until climate destabilization makes life difficult for us - my estimate is that we have 30 to 40 years until that happens.
Second, even if we replace all fossil fuel consumption with wind, solar and water power today, we would still be left with the existing debt to pay down. As long as we keep damaging the biosphere further through our actions (the second level of the debt I referred to above), even if we stopped emitting carbon entirely the debt wiould keep growing rather than shrinking. There really is no way to pay down any of the debt while maintaining any semblance of our current activity levels.
Is it any wonder that more and more people are realizing that there is no way out of the corner we've painted ourselves and the rest of the biosphere into?
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