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