The effect of anthropogenic climate change in the ocean is challenging to project because atmosphere-ocean general circulation models (AOGCMs) respond differently to forcing. This study focuses on changes in the Atlantic Meridional Overturning Circulation (AMOC), ocean heat content (
This study investigates the formation mechanism of the ocean surface warming pattern in response to a doubling CO2with a focus on the role of ocean heat uptake (or ocean surface heat flux change, Δ
- NSF-PAR ID:
- 10364239
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 35
- Issue:
- 6
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- p. 1885-1899
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The 2015 Paris Agreement proposed targets to limit global-mean surface temperature (GMST) rise well below 2°C relative to preindustrial level by 2100, requiring a cease in the radiative forcing (RF) increase in the near future. In response to changing RF, the deep ocean responds slowly (ocean slow response), in contrast to the fast ocean mixed layer adjustment. The role of the ocean slow response under low warming targets is investigated using representative concentration pathway (RCP) 2.6 simulations from phase 5 of the Coupled Model Intercomparison Project. In RCP2.6, the deep ocean continues to warm while RF decreases after reaching a peak. The deep ocean warming helps to shape the trajectories of GMST and fuels persistent thermosteric sea level rise. A diagnostic method is used to decompose further changes after the RF peak into a slow warming component under constant peak RF and a cooling component due to the decreasing RF. Specifically, the slow warming component amounts to 0.2°C (0.6°C) by 2100 (2300), raising the hurdle for achieving the low warming targets. When RF declines, the deep ocean warming takes place in all basins but is the most pronounced in the Southern Ocean and Atlantic Ocean where surface heat uptake is the largest. The climatology and change of meridional overturning circulation are both important for the deep ocean warming. To keep the GMST rise at a low level, substantial decrease in RF is required to offset the warming effect from the ocean slow response.more » « less
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Abstract We investigate the dependence of radiative feedback on the pattern of sea‐surface temperature (SST) change in 14 Atmospheric General Circulation Models (AGCMs) forced with observed variations in SST and sea‐ice over the historical record from 1871 to near‐present. We find that over 1871–1980, the Earth warmed with feedbacks largely consistent and strongly correlated with long‐term climate sensitivity feedbacks (diagnosed from corresponding atmosphere‐ocean GCM
abrupt‐4xCO2 simulations). Post 1980, however, the Earth warmed with unusual trends in tropical Pacific SSTs (enhanced warming in the west, cooling in the east) and cooling in the Southern Ocean that drove climate feedback to be uncorrelated with—and indicating much lower climate sensitivity than—that expected for long‐term CO2increase. We show that these conclusions are not strongly dependent on the Atmospheric Model Intercomparison Project (AMIP) II SST data set used to force the AGCMs, though the magnitude of feedback post 1980 is generally smaller in nine AGCMs forced with alternative HadISST1 SST boundary conditions. We quantify a “pattern effect” (defined as the difference between historical and long‐term CO2feedback) equal to 0.48 ± 0.47 [5%–95%] W m−2 K−1for the time‐period 1871–2010 when the AGCMs are forced with HadISST1 SSTs, or 0.70 ± 0.47 [5%–95%] W m−2 K−1when forced with AMIP II SSTs. Assessed changes in the Earth's historical energy budget agree with the AGCM feedback estimates. Furthermore satellite observations of changes in top‐of‐atmosphere radiative fluxes since 1985 suggest that the pattern effect was particularly strong over recent decades but may be waning post 2014. -
Keynote points • Thermal expansion from a warming ocean and land ice melt are the main causes of the accelerating global rise in the mean sea level. • Global warming is also affecting many circulation systems. The Atlantic meridional overturning circulation has already weakened and will most likely continue to do so in the future. The impacts of ocean circulation changes include a regional rise in sea levels, changes in the nutrient distribution and carbon uptake of the ocean and feedbacks with the atmosphere, such as altering the distribution of precipitation. • More than 90 per cent of the heat from global warming is stored in the global ocean. Oceans have exhibited robust warming since the 1950s from the surface to a depth of 2,000 m. The proportion of ocean heat content has more than doubled since the 1990s compared with long-term trends. Ocean warming can be seen in most of the global ocean, with a few regions exhibiting long-term cooling. • The ocean shows a marked pattern of salinity changes in multidecadal observations, with surface and subsurface patterns providing clear evidence of a water cycle amplification over the ocean. That is manifested in enhanced salinities in the near-surface, high-salinity subtropical regions and freshening in the low-salinity regions such as the West Pacific Warm Pool and the poles. • An increase in atmospheric CO2 levels, and a subsequent increase in carbon in the oceans, has changed the chemistry of the oceans to include changes to pH and aragonite saturation. A more carbon-enriched marine environment, especially when coupled with other environmental stressors, has been demonstrated through field studies and experiments to have negative impacts on a wide range of organisms, in particular those that form calcium carbonate shells, and alter biodiversity and ecosystem structure. • Decades of oxygen observations allow for robust trend analyses. Long-term measurements have shown decreases in dissolved oxygen concentrations for most ocean regions and the expansion of oxygen-depleted zones. A temperature-driven solubility decrease is responsible for most near-surface oxygen loss, though oxygen decrease is not limited to the upper ocean and is present throughout the water column in many areas. • Total sea ice extent has been declining rapidly in the Arctic, but trends are insignificant in the Antarctic. In the Arctic, the summer trends are most striking in the Pacific sector of the Arctic Ocean, while, in the Antarctic, the summer trends show increases in the Weddell Sea and decreases in the West Antarctic sector of the Southern Ocean. Variations in sea ice extent result from changes in wind and ocean currents.more » « less
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Abstract Dramatic increases in air temperature and precipitation are occurring in the High Arctic (>70°N), yet few studies have characterized the long‐term responses of High Arctic ecosystems to the interactive effects of experimental warming and increased rain. Beginning in 2003, we applied a factorial summer warming and wetting experiment to a polar semidesert in northwest Greenland. In summer 2018, we assessed several metrics of ecosystem structure and function, including plant cover, greenness, ecosystem CO2exchange, aboveground (leaf, stem) and belowground (litter, root, soil) carbon (C) and nitrogen (N) concentrations (%) and pools, as well as leaf and soil stable isotopes (δ13C and δ15N). Wetting induced the most pronounced changes in ecosystem structure, accelerating the expansion of
Salix arctica cover by 370% and increasing aboveground C, N, and biomass pools by 94%–101% and root C, N, and biomass pools by 60%–122%, increases which coincided with enhanced net ecosystem CO2uptake. Further, wetting combined with warming enhanced plot‐level greenness, whereas in isolation neither wetting nor warming had an effect. At the plant level, the effects of warming and wetting differed among species and included warming‐linked decreases in leaf N and δ15N inS. arctica , whereas leaf N and δ15N inDryas integrifolia did not respond to the climate treatments. Finally, neither plant‐ nor plot‐level C and N allocation patterns nor soil C, N, δ13C, or δ15N concentrations changed in response to our manipulations, indicating that these ecosystem metrics may resist climate change, even in the longer term. In sum, our results highlight the importance of summer precipitation in regulating ecosystem structure and function in arid parts of the High Arctic, but they do not completely refute previous findings of resistance in some High Arctic ecosystem properties to climate change.
