- There is a high level of uncertainty regarding how natural carbon sinks will respond to human-induced environmental changes, including climate change.
- Natural carbon sinks may absorb less carbon in the future than we are expecting now.
- If natural sinks are indeed weaker than expected, then warming will be stronger than expected in IPCC scenarios. Hence, even more ambitious efforts on emissions reduction will be required.
The remaining carbon budget for staying within the Paris Agreement target range for global warming substantially depends on the future contribution of natural carbon sinks on land and in the ocean. Despite rising CO2 emissions, a relatively constant fraction of about 44% (on average) of these emissions have remained in the atmosphere over the past 50 years. That means that the natural sinks on land and in the ocean have increased their carbon uptake along with the increases in atmospheric CO2.
However, recent data suggests that the increasing land sink trend may have slowed down. This could just be due to natural variability, but it could also be signalling the start of a “saturation point” in the Earth system. The relative importance of these two factors is highly uncertain. The latter would imply that rising temperatures, altered rainfall patterns and weather extremes, along with other human-induced disturbances, are reducing the buffering capacity of the land sinks, or could even lead to their destabilisation. There is sparse but strong evidence indicating that some land sinks are changing faster than expected. For example, most models do not reproduce the increasing sensitivity of the carbon cycle to tropical droughts, witnessed in the past decades. In boreal forests, drought-induced increase in tree mortality corresponds to a decrease in carbon sink capacity. Events of elevated tree mortality following heat and drought are now being observed worldwide, in places where this had not been expected earlier.
The increase in the ocean sink stalled in the 1990s, but has recovered since. The main driver for the increasing sink is the rising CO2 content in the atmosphere, pushing anthropogenic carbon into the ocean. But the natural processes of the carbon cycle can modify the sinks, increasing or decreasing carbon storage beyond what is expected solely from CO2 content in the atmosphere. For example, the warming of the ocean tends to push natural CO2 out of the ocean, reducing net CO2 uptake. In the Southern Ocean, changes in wind patterns have exposed more carbon-rich deep waters to the air-sea interface, inducing a loss of natural carbon. The Arctic Ocean, on the other hand, is currently the only large-scale region where the carbon sink is strengthening, as melting sea ice exposes more ocean surface, which can then absorb more CO2.
Overall, the uncertainties regarding natural carbon sinks are concerning. Scientists and policymakers need to be alert to a potential problem: plans to avoid exceeding the Paris Agreement limits on global warming rely on current model projections of sink capacity. If the models are overestimating the sinks, then the true carbon budget might actually be even smaller, and current pathways for achieving net-zero emissions would be insufficient to meet the targets. To reduce uncertainties and avoid over-reliance on natural carbon sinks (or solutions based on them, see In Focus box and Insight 3), we need reliable and robust quantifications of the sinks.
A reality check on nature-based solutions (NbS)
Nature-based solutions rely on land and ocean carbon sinks, but over-relying on the future strength of these carbon sinks is a risky strategy, as explained above.
- In the case of land sinks, the problem is well-illustrated with the example of fire: while it is a key driver of change that will increase in the future, fire processes (along with other natural disturbances) are not fully incorporated in current assessments of the future carbon sequestration potential of forests.
- Similarly, the carbon absorption gains of re/afforestation in some regions would be largely offset by counteracting albedo effects, especially in the short term. Yet forest conservation and re/afforestation are prominent in the NDCs of many countries, constituting virtually all currently deployed CDR (see Insight 3).
- For the ocean, an area of great uncertainty is the accounting of the carbon flows between coastal ecosystems, sea shelves and beyond. The rapidly rising frequency and intensity of marine heatwaves, as well as extremes in oxygen loss and acidification, has the potential to also impact the ocean carbon sink, but this is not well understood.
- While acknowledging the importance of NbS in maintaining the integrity of natural carbon sinks (the protection of ecosystems that comes through well-conceived and implemented NbS), for the reasons outlined in this section, mitigation plans should not over-rely on them.
Implications & Recommendations
Mitigation strategies must not over-rely on natural carbon sinks. Given the growing scientific concern about the future of natural carbon sinks, it is essential to strengthen emissions reductions to account for this uncertainty.
To inform future planning and decision-making it is important to:
- Coordinate and mobilise sustainable funding for a fit-for-purpose ocean and land carbon observation system.
- Perform comprehensive vulnerability assessments of carbon sinks (especially those on land) as part of robust MRV systems for CDR (see Insight 3), in order to make solutions effective and permanent.