Early detection of anthropogenic climate change signals in the ocean interior

Abstract

Robust detection of anthropogenic climate change is crucial to: (i) improve our understanding of Earth system responses to external forcing, (ii) reduce uncertainty in future climate projections, and (iii) develop efficient mitigation and adaptation plans. Here, we use Earth system model projections to establish the detection timescales of anthropogenic signals in the global ocean through analyzing temperature, salinity, oxygen, and pH evolution from surface to 2000 m depths. For most variables, anthropogenic changes emerge earlier in the interior ocean than at the surface, due to the lower background variability at depth. Acidification is detectable earliest, followed by warming and oxygen changes in the subsurface tropical Atlantic. Temperature and salinity changes in the subsurface tropical and subtropical North Atlantic are shown to be early indicators for a slowdown of the Atlantic Meridional Overturning Circulation. Even under mitigated scenarios, inner ocean anthropogenic signals are projected to emerge within the next few decades. This is because they originate from existing surface changes that are now propagating into the interior. In addition to the tropical Atlantic, our study calls for establishment of long-term interior monitoring systems in the Southern Ocean and North Atlantic in order to elucidate how spatially heterogeneous anthropogenic signals propagate into the interior and impact marine ecosystems and biogeochemistry.

What goes in must come out: the oceanic outgassing of anthropogenic carbon

“…the convergence of Ekman transport at the surface of the subtropical gyres and the formation and equatorward transport of mode and intermediate waters at sub-surface. The convergence of subsurface C ant into the subtropical gyres is also consistent with the early emergence of subsurface ocean acidification signals in these regions projected across an ensemble of CMIP6 ESMs (Tjiputra et al 2023). The surface warming pattern simulated in NorESM2-LM model over the 21st century is consistent with that identified in other ESMs (IPCC 2021).…”

“…The SSP5-8.5 is on the high end of the radiative forcing with the carbon emissions increasing until 2080 after which they start to slowly decline to reach zero in 2250. The simulations outputs are available online (Tjiputra et al 2023).…”

More Frequent Abrupt Marine Environmental Changes Expected

“…The $$\left[{\text{CO}}_{3}{2}^{-}\right]$$ distribution indicates that near surface depth levels are strongly affected when atmospheric [CO 2 ] starts to steeply rise and when also over short time a lot of anthropogenic carbon accumulates in the upper water column and cannot get mixed down quickly enough into deeper layers (Figure 2e). The rising trend in abrupt $$\left[{{\text{CO}}_{3}}^{2-}\right]$$ occurrences at 1,000 and 2,000 m after 2050 is consistent with the progressing invasion of anthropogenic carbon into deeper layers and the emergence of ocean acidification signals in these depths, especially in the ventilation regions (Tjiputra et al., 2023).…”

Machine learning reveals regime shifts in future ocean carbon dioxide fluxes inter-annual variability

“…Although estimates of present-day ocean carbon uptake rate and its long-term trends agree reasonably well using either tool, discrepancies in the spatio-temporal variability of the air-sea CO 2 flux (CO 2 flux, hereafter) remain, and these biases even appear to increase over time 5 . However, our ability to accurately quantify the magnitude of the ocean carbon sink and the variability of the CO 2 flux across multiple timescales is crucial to project the future evolution of the Earth's climate and to improve our ability to robustly detect long-term anthropogenic climate change 6,7 .…”