For several thousand years until around 200 years ago, this ‘carbon cycle’ was approximately in balance and steady. Enormous amounts of it are continually exchanged between the atmosphere, land and oceans, as land and marine plants grow, die and decay, and as carbon-rich waters circulate in the ocean. This history of greenhouse gas concentrations has been established by a combination of modern measurements and analysis of ancient air bubbles in polar ice ( Box 2.1). The concentration of CO 2 has increased from 280 parts per million (ppm) before 1800, to 396 ppm in 2013. Photo: © /airspeed Human activities have increased greenhouse gas concentrations in the atmosphereĪtmospheric concentrations of carbon dioxide (CO 2), methane and nitrous oxide began to rise around two hundred years ago, after changing little since the end of the last ice age thousands of years earlier. Are human activities causing climate change? Southern approach to the Sydney Harbour Bridge, NSW.
