Taking the long view, we see that today’s concentrations of CO2
are the highest they’ve been for at least 800,000 years. How do we know this? Long-term trends in CO2
concentrations can be measured at high resolution using preserved air samples from ice cores. In fact, since the beginning of the pre-industrial era in 1850, humans have emitted approximately 2,400 gigatonnes of heat-trapping carbon dioxide into the atmosphere, predominantly from the use of fossil fuels and land use change. About 55 % of this total has been absorbed by land and ocean sinks, with the remainder accumulating in ever-higher concentrations in the global atmosphere (Friedlingstein et al., 2019).
To dramatically reduce future societal harm from the catastrophic consequences of global temperature rises, our emissions need to not only stabilize but also decrease significantly. Decarbonization is the first step on the path to climate stabilization — one that is as big a challenge as it is necessary. However, even if the world achieved a stabilization in CO2
emissions, this would not translate into the same for atmospheric concentrations. This is because CO2
accumulates in the atmosphere based on what we call a ‘residence time’. Residence time is the time required for emitted CO2
to be removed from the atmosphere through natural processes in Earth’s carbon cycle. If we stopped emitting CO2
today, it would take several hundred year
s before the majority of human emissions were removed from the atmosphere through natural processes.
Therefore, the science tells us approaches that actively remove carbon from the atmosphere will need to be deployed at a large scale, this century.
Further reading: Carbonfuture article
on unleashing the potential of carbon removal through trust and Chapter 12.3
of the IPCC's April 2022 report