The ability of the climate system to recover from changes to the composition of the atmosphere (particularly increasing CO2 concentrations) is complicated by the fact that the removal of CO2 from the atmosphere involves several processes that occur across different timescales. Mixing with surface ocean waters and reacting with dissolved carbonate ions occurs across 10–100 years, transport and mixing throughout the deep ocean between 100 and 1000 years, the reaction of CO2 with deep-sea carbonate sediments across 100–10,000 years, and long-term neutralisation by weathering of carbonate and silicate minerals on the continents from 10,000 to 1 million years (Zeebe & Zachos 2013).
As part of an intercomparison project of Earth System Models of Intermediate Complexity, Zickfeld et al. (2013) investigated the extent to which climate change is reversible on human timescales. Using idealised scenarios and 4 Representative Concentration Pathways for GHGs and aerosols for the year 3000, they simulated the change in the concentration of atmospheric CO2, surface air temperature and sea level if CO2 emissions ceased at 3000 or underwent a linear decrease to pre-industrial concentrations across 100 or 1000 years.
In the case of negative CO2 emissions after 3000, it was left to the natural carbon sinks to absorb excess CO2, so that atmospheric CO2 concentrations declined slowly, and climate change was largely irreversible on centennial to millennial timescales. In the cases of ramping down CO2 levels to pre-industrial concentrations between 100 and 1000 years, surface air temperature and sea level change exhibited a substantial timelag relative to atmospheric CO2 because of the large thermal inertia of the ocean. Even 900 years after CO2 was restored to pre-industrial levels, surface air temperature and sea level were considerably higher than under pre-industrial conditions. When atmospheric CO2 was slowly returned to pre-industrial levels (taking 1000 years), surface air temperature decreased more slowly, and sea level continued to rise for several centuries before starting to fall. The decline of CO2 to pre-industrial levels, taking between 100 and 1000 years, required large negative emissions (i.e. net removal of CO2 from the atmosphere).
This work suggests that the climate system has little resilience, and that it is very difficult to return to pre-industrial levels from a given level of warming on timescales appropriate to human activities, even after complete elimination of emissions. The modelling suggested that significant negative emissions have the potential to reverse global warming, but whether CO2 capture technology is feasible at the necessary scale is debatable.
The ability of the oceans to absorb CO2 and heat, and thus limit the rate and immediate extent of changes in climate, can be seen as the climate system displaying resilience to the changes in the composition of the atmosphere. In recent decades, the oceans have taken up approximately 25 per cent of the annual anthropogenic CO2 emissions to the atmosphere from major sources (Le Quéré et al. 2009). However, the capacity of the oceans to absorb CO2 appears to be limited, because the continued absorption of CO2 results in the oceans gradually becoming more acidic (Caldeira & Wickett 2003), with a reduction in the ocean’s carbonate mineral saturation. Laboratory studies indicate that a further decrease in seawater pH of 0.2–0.3 pH units (becoming more acidic) would inhibit or slow calcification in many marine organisms, such as corals, foraminifera and some calcareous plankton, thus potentially affecting the entire marine food chain (Zeebe & Zachos 2013).
Evidence of past climate resilience may be gleaned from the climate palaeorecord. Zeebe et al. (2016) have determined that the rate of carbon released to the atmosphere during the Palaeocene–Eocene Thermal Maximum era (PETM; approximately 56 million years ago, which is considered to be the geological period with the highest carbon release rates of the past 66 million years) was 0.6–1.1 billion tonnes (petagrams; Pg) of carbon per year (Pg C/y), which is 10 times lower than the current rate (10 Pg C/y). The PETM was marked by a rapid onset of 6 °C global warming, followed by a gradual recovery across the next 150,000 years (Zachos et al. 2001). The onset was accompanied by intense dissolution of carbonate sediments throughout the deep sea and acidification of surface waters (Zachos et al. 2005), resulting in changes to biodiversity and abundancy in communities of marine calcifiers (e.g. Tremolada & Bralower 2004). Although many species ultimately survived, the community perturbations persisted for tens of thousands of years, recovering only as carbon levels abated and the planet cooled. Zeebe and Zachos (2013) suggest that the present and future rate of climate change and ocean acidification are faster than any species can adapt to, and could lead to widespread future extinctions in marine and terrestrial environments that will exceed those during the PETM. The fossil record indicates that recovery of biotic diversity after mass extinctions generally takes several million years.