As the global atmospheric concentration of CO2 nudges 400 parts per million, I reflect on some of the changes in understanding and policy since I started working in this area, in 1995.
At that time our grasp of the global carbon cycle was quite uncertain. We knew CO2 in the atmosphere was at 360 ppm (about 2750 billion tonnes CO2). We had a good idea that we were adding about 22 billion tonnes CO2 per year from fossil fuels and maybe 6 or 7 billion tonnes from deforestation.
By subtracting increases of carbon in the atmosphere from the emissions it was known that only half the CO2 emitted each year remained in the atmosphere, the balance being taken up by various “sinks”.
But there was considerable uncertainty about the locations of various “sinks” of CO2.
We now understand more about the strength and locations of these sinks (oceans, forests and grasslands) but uncertainties remain about their vulnerability to environmental change.
One thing seems clear. Despite the annual round of climate negotiations the rate of carbon accumulation in the atmosphere is increasing, and seem likely to continue to increase at a rate beyond many of the scenarios envisaged back in 1995.
Forests remain an important sink of CO2, taking up around one quarter of what is emitted by our use of fossil fuels each year (now up to 31 billion tonnes per year) – much like a benevolent uncle paying part of one’s debts. But despite progress in some countries on tackling deforestation, there are warning signs that forest health is declining. The benevolent uncle may be unable to help much longer and some forests could tip from becoming sinks to sources.