The Earth is getting warmer at a faster rate than ever, in which oceans play a critical role. 93% of the net energy are absorbed by global ocean surface in the form of the Ocean Heat Uptake (OHU), which is the key factor modulating the rate of global warming.

Since 1850, the heat absorbed by oceans is increasing with stronger global warming. The Southern Ocean (surrounding the Antarctic continent, south of 30°S ) plays a dominant role in global OHU while the North Atlantic takes a small share. This phenomenon has been largely due to the concurrent increase in GHGs and aerosols during past centuries and related to changes in deep ocean circulations, in which strong thermohaline circulation in the Atlantic--the Atlantic Meridional Overturning Circulation (AMOC) plays a key role.

Future emissions of GHGs and aerosols in the 21st century will be different from those in the historical period (since 1850), according to a new study published in Science Advances on Nov 6, 2020. The article revealed the effects of aerosols and GHGs on regional OHU under different future warming scenarios. Based on the target of the 2015 Paris Agreement that limits global warming of 1.5°C or 2°C above pre-industrial levels by 2100, the new study first found the hemispheric asymmetry of OHU and its reasons under a low-emission scenario.

The study, conducted by a research team from Institute of Atmospheric Physics (IAP) at the Chinese Academy of Sciences, cooperating with researchers from University of California Riverside (UCR), found that during the 21st century, the projected OHU shows collectively positive trends in the North Atlantic and Southern Ocean under the high-emission scenario. While under the low-emission scenario, as compared to the negative long-term trend of Southern Ocean OHU, North Atlantic OHU has a positive long-term trend, which makes the North Atlantic the focus of global OHU in the future. The opposite OHU trends in the Southern Ocean can be attributed partially to distinct GHG trajectories under the two scenarios. While the common positive OHU trends in the North Atlantic in the two scenarios are due to the total effects from anthropogenic aerosols and GHGs. They also presented that projected decline in anthropogenic aerosols induces a weakening of the AMOC and divergence of meridional ocean heat transport, which leads to enhanced OHU in the North Atlantic.

"Previous studies have found that comparing to the historical period, from the middle to high emission scenarios, the North Atlantic OHU takes more and more percentages in global OHU while the Southern Ocean OHU takes less and less share even though the Southern Ocean still contributes most to global OHU. However, the regional OHU change under the low-emission scenario is not well understood." said the leading author MA Xiaofan, a Ph.D. candidate from IAP. "We discover an obvious hemispheric asymmetry in OHU under the low-emission scenario. In the long term, North Atlantic OHU keeps increasing and will become the main region of OHU. Our finding reveals the importance of aerosol effects and AMOC influences, which are helpful to the attribution of climate events." 

 Cutting emissions makes the North Atlantic the focus of ocean heat uptake under global warming

"To sustain the balance of Earth's ecosystem, human societies have been taking measures to slow down the global warming. Our work shows the scientific significance of the low-warming target. It also helps us to better understand and project the climate change under low-emission scenarios", said the corresponding author Prof. HUANG Gang from IAP.

HUANG also noted that due to the limits of observations, these results rely on the climate model simulations. He looks forward to carrying out further studies with the development of ocean observation systems and datasets.

Reference:

X. Ma, W. Liu, R. J. Allen, G. Huang, X. Li, Dependence of regional ocean heat uptake on anthropogenic warming scenarios. Sci. Adv. 6, eabc0303 (2020). DOI:10.1126/sciadv.abc0303. https://advances.sciencemag.org/content/6/45/eabc0303