The signing of the Paris Agreement in December 2015 signaled the world’s renewed focus on limiting global temperature rise to below 2 degrees Celsius, with a goal to lessen the adverse impacts of climate change. However, one of these impacts, sea-level rise, is already occurring and will continue long after emissions and temperatures stabilize. In other words, policies and decisions made now will set sea-level rise on a course to higher or lower levels. To better assess these effects, a recent paper published in Nature Communications examined the implications of the Paris Agreement’s goals on global sea levels up until the year 2300.
If we are to achieve the 2 degree Celsius goal of the Paris agreement, global greenhouse gas (GHG) emissions must peak and subsequently decline in the near future. This decline would coincide with the removal of emissions already in the atmosphere, through natural sinks, carbon capture and storage technologies, or both; ultimately leading to global net-zero GHG emissions sometime between 2050 and 2100. Most previous studies examining sea-level rise under different climate change scenarios only looked forward to 2100, and though a few extended farther into the future, none had yet to consider the implications of meeting the aims of the Paris Agreement.
The goal of this study was to fill this gap and assess the legacy of the Paris Agreement on sea level rise beyond the 21st century, author Alexander Nauels told GlacierHub. Another important motivation for the study was to investigate the effect of delayed climate mitigation action on future sea-level rise, he added.
Sea-level rise due to climate change is driven by several elements, including the thermal expansion of the oceans as they warm, the retreat of mountain glaciers, and the mass loss of ice sheets in Antarctica and Greenland. These elements react on different timescales to increasing temperatures ranging from hundreds (shallow water thermal expansion and glaciers) to thousands (major ice sheets) of years. Thus, emissions today will lock in future sea-level rise well into the future.
To explore the relationship between the provisions of the Paris Agreement and sea-level rise, the study utilized a carbon cycle and climate model composite, together with a sea-level model. These models were driven by fossil fuel and industry emission scenarios that meet the Paris Agreement’s goal of limiting temperature rise to 2° C. These scenarios resemble the IPCC’s Representative Concentration Pathways (RCP) 2.6 scenario where emissions peak by 2020 and then decline thereafter. The emissions in these scenarios were limited to fossil fuels and industry because as Nauels states they are, “…by far the most important emission share when it comes to global decarbonistion.”
The scenarios chosen met either the net-zero GHG emissions goal of the Paris Agreement, seeing a gradual temperature decline over time due to GHG removal by carbon sinks, or a net-zero CO2 goal that would only limit temperature rise to 2° C. Why the two different scenario groups? Joeri Rogelj, another author of the study, told GlacierHub that they wanted to be able to distinguish between scenarios that only stabilize warming, partially meeting the Paris Agreement’s targets (net-zero CO2) and ones that fully comply with the Paris Agreement’s targets (net-zero GHG). This distinction enabled the authors to analyze the effect that delayed or insufficient mitigation action would have on sea-level rise.
There was a stark difference between the more stringent requirements of the Paris Agreement, slowly decreasing temperature through carbon sinks and action that would only stop temperature rise at 2° C. Under net-zero GHG scenarios, median sea-level rise was 73-123 cm, while under net-zero CO2 scenarios the median rise was a much higher level at 116-164 cm. Sea-level rise also continues through 2300 in all scenarios, emphasizing the need for immediate mitigation action, although, the rate begins to slow soon after emissions peak at 0.06-0.7 cm and 0.33-0.49 cm per year for the net-zero GHG and net-zero CO2 scenarios, respectively. Ominously, under net-zero CO2 scenarios, results showed that the possibility of sea-level rise of up to 5 m by 2300 was within the 90% confidence interval.
What happens if humanity only stabilizes temperatures instead of meeting the goals of the Paris Agreement? When the authors compared the net-zero GHG and net-zero CO2 scenario groups, they found that median sea-level rise was 40 cm higher for the net-zero CO2 scenario. Another relevant factor for 2300 sea-level rise is the timing of the emissions peak. If the peak in global emissions is delayed by five years, an additional 20 cm of rise was found to occur in 2300 and when based on the 95th percentile the rise is an additional 1 m.
There is a good chance that global temperatures will increase by more than 1.5° C at least temporarily, with a 2017 study putting the chances of staying below a higher threshold of 2° C at 5%. The authors assessed this possible ‘temperature overshoot’ and found for every 10-year period where temperature rise is greater than 1.5° C a 4 cm increase in median sea-levels is expected. Overall, if global temperatures top 1.5° C no scenario showed median sea-level rise less than 1.2 m by 2300.
Lastly, the authors examined the connections between sea-level rise and the Paris Agreement’s Nationally Determined Contributions (NDC), the emission reduction goals of individual countries. If implemented in full, the NDCs would lead to a median sea-level rise between 1.45 and 1.64 meters under the net-zero CO2 scenarios and a median sea-level between 1.05 and 1.23 meters under the net-zero GHG scenarios. 95th percentile estimates for the NDCs were even more dramatic, with net-zero CO2 and net-zero GHG sea-level rises between 4.1 to 4.8 m and 2.3 to 3 m respectively.
Further research is needed to develop more precise estimates of sea-level rise into the future, according to Rogelj. He proposes several concrete steps inculding better continuous observations and improved model development for Antarctic ice sheet instabilities and Greenland ice discharge, both of which contributed the most to this study’s uncertainty ranges.
The findings of this study point to continued sea-level rise up until 2300, even if global GHG emissions reach net-zero levels. However, the authors note that high-end scenarios “can be halved through early and stringent emission reductions,” highlighting the urgent need for fast action on climate change from individuals all the way up to the world’s biggest countries.