There are models in the offices where federal climate policy is formulated, such as the EPA’s regulatory shop, the interagency groups that determine emissions targets and compute societal costs, and the budget offices that assess the costs of doing nothing. The models provide analysts with information on the amount of warming that a particular level of atmospheric CO2 will produce, the economic harm that warming produces, and consequently the cost to society of each additional ton of CO2 entering the atmosphere. From the initial commitment made in the Paris Agreement to the renewable energy provisions of the Infrastructure Investment and Jobs Act, these calculations have influenced every climate commitment made by the United States over the past 20 years. The models are predicated on hypotheses regarding the behavior of the ocean. Some of such presumptions are losing their validity.
In policy talks, which typically concentrate on what’s occurring in the atmosphere and on land, the ocean’s function in the climate system is rarely defined with the detail it deserves. The scope of the issue is framed by the fundamental reality that over 90% of the extra heat trapped by greenhouse gas accumulation since industrialization has been absorbed by the ocean. This heat hasn’t only been absorbed passively by the water. It has served as the main buffer between the warmth caused by greenhouse gas forcing and the actual warming of the surface and atmosphere. The overall energy imbalance in the Earth system is significantly understated by the comparatively recorded increase in surface temperature of about 1.2 degrees Celsius from pre-industrial times. The discrepancy has been absorbed by the ocean.
The issue is the increasingly obvious limitations of this buffering capacity. The same physical mechanism that causes a warm soda to go flat more quickly than a cold one is the reason why warmer water holds less dissolved CO2. Surface water absorbs atmospheric carbon dioxide at a slower pace when ocean temperatures rise. Because of the record sea surface temperatures in 2023, which reduced the water’s capacity to absorb carbon, the non-polar seas absorbed around 10% less CO2 than historical models had predicted for that year. The 10 percent shortfall is a significant amount of carbon that remained in the atmosphere instead of being stored in seawater, and if baseline ocean temperatures continue to rise, the circumstances that led to it are probably going to recur and get worse.
An further type of accounting issue is introduced by the AMOC dimension. Ocean heat absorption is treated by standard climate models as a process that disperses heat throughout the ocean system and eventually releases it back to the surface over extended periods of time; this delay buys time but does not result in permanent sequestration. The distribution of the heat is altered when AMOC deteriorates. Recent research indicates that instead of just transferring heat, weaker circulation holds more heat in the ocean system, resulting in a type of warming debt that builds up without being fully accounted for in the models that underpin U.S. emissions objectives. This extra trapped heat, which is lying in ocean layers that will eventually convey their temperature to the surface, represents committed warming that isn’t reflected in the forecasts used by decision-makers.
The direct link between this and U.S. regulatory decision-making is the social cost of carbon. In regulatory cost-benefit analyzes, this amount—currently set by the federal government at about $190 per ton of CO2—determines how climate costs are compared to economic advantages. Infrastructure investment, fuel efficiency requirements, power plant restrictions, and numerous other policy sectors are all impacted. The computation is meant to account for all of the economic harm brought on by an extra ton of carbon dioxide emissions. However, analyzes that include the marine damage categories that are currently underweighted in the standard calculation—lost ecosystem services from degraded ocean ecosystems, coastal infrastructure destruction from accelerated sea level rise, and collapsed fisheries from ocean acidification and warming—indicate that the actual number may be significantly higher. Some estimates suggest that it might nearly quadruple.
The practical implication is that models that assumed ocean buffering would continue at rates the ocean might not be able to sustain were used to calibrate the U.S. net-zero commitments and interim emissions reduction targets, which were pledged as a 50–52 percent reduction from 2005 levels by 2030 under the most recent Nationally Determined Contribution. Achieving the same actual climate outcomes will require faster and more thorough emissions reductions than the current commitments specify if the sink weakens, if AMOC heat retention adds extra warming inertia not included in the baseline projections, and if marine economic damages are accurately priced into the social cost of carbon.
