Carbon cycle conundrums

Abstract

What will control future rates of climate change? The carbon cycle is the largest contributor to anthropogenic climate change, yet despite decades of research (1), significant mysteries about its behavior remain. Global analyses show that the Earth system absorbs approximately half of anthropogenic fossil fuel emissions. This uptake is partitioned between absorption by the oceans and storage in terrestrial ecosystems. Uptake by the Earth system reduces the climate effects of emitted CO2 to approximately half of what would occur without sinks. Models tend to project sinks, particularly terrestrial sinks, into the future based on assumptions about the behavior of mechanisms derived from small plot or laboratory studies. The observational record is now long enough and rich enough to provide strong constraints on both the human and the biogeochemical behavior of the carbon cycle. In this issue of PNAS, Canadell et al. (2) report an increase in the rate of increase in CO2 in the atmosphere (henceforth called the “growth rate”). They analyze the historical carbon record and industrial and biogeochemical causes to changes in its behavior over time. Industrial growth is responsible for ≈65% of the change in the growth rate, 17% is caused by increasing carbon intensity (fossil fuel use/gross world product), and 18% is caused by reduced sinks in the Earth system. The reduction over time of the efficiency of the sinks is of great concern because it implies a weakening in the ability of the Earth system to mitigate the effects of fossil fuel emissions and a potential positive feedback that may strengthen in the future. A progressive weakening of Earth system sinks has long been of concern (3), but until recently, insufficient information has existed to diagnose the behavior of the carbon sinks over time. The fraction of fossil fuel-derived CO2 remaining in …