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Volume 9, issue 2 | Copyright

Special issue: The Earth system at a global warming of 1.5°C and 2.0°C

Earth Syst. Dynam., 9, 817-828, 2018
https://doi.org/10.5194/esd-9-817-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Jun 2018

Research article | 13 Jun 2018

Climate, ocean circulation, and sea level changes under stabilization and overshoot pathways to 1.5 K warming

Jaime B. Palter1, Thomas L. Frölicher2,3, David Paynter4, and Jasmin G. John4 Jaime B. Palter et al.
  • 1University of Rhode Island, Graduate School of Oceanography, Narragansett, RI, USA
  • 2Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland
  • 3Oeschger Centre for Climate Change Research, University of Bern, Bern, Switzerland
  • 4National Oceanic and Atmospheric Administration, Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA

Abstract. The Paris Agreement has initiated a scientific debate on the role that carbon removal – or net negative emissions – might play in achieving less than 1.5K of global mean surface warming by 2100. Here, we probe the sensitivity of a comprehensive Earth system model (GFDL-ESM2M) to three different atmospheric CO2 concentration pathways, two of which arrive at 1.5K of warming in 2100 by very different pathways. We run five ensemble members of each of these simulations: (1) a standard Representative Concentration Pathway (RCP4.5) scenario, which produces 2K of surface warming by 2100 in our model; (2) a stabilization pathway in which atmospheric CO2 concentration never exceeds 440ppm and the global mean temperature rise is approximately 1.5K by 2100; and (3) an overshoot pathway that passes through 2K of warming at mid-century, before ramping down atmospheric CO2 concentrations, as if using carbon removal, to end at 1.5K of warming at 2100. Although the global mean surface temperature change in response to the overshoot pathway is similar to the stabilization pathway in 2100, this similarity belies several important differences in other climate metrics, such as warming over land masses, the strength of the Atlantic Meridional Overturning Circulation (AMOC), ocean acidification, sea ice coverage, and the global mean sea level change and its regional expressions. In 2100, the overshoot ensemble shows a greater global steric sea level rise and weaker AMOC mass transport than in the stabilization scenario, with both of these metrics close to the ensemble mean of RCP4.5. There is strong ocean surface cooling in the North Atlantic Ocean and Southern Ocean in response to overshoot forcing due to perturbations in the ocean circulation. Thus, overshoot forcing in this model reduces the rate of sea ice loss in the Labrador, Nordic, Ross, and Weddell seas relative to the stabilized pathway, suggesting a negative radiative feedback in response to the early rapid warming. Finally, the ocean perturbation in response to warming leads to strong pathway dependence of sea level rise in northern North American cities, with overshoot forcing producing up to 10cm of additional sea level rise by 2100 relative to stabilization forcing.

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Limiting global warming to 1.5 °C may require carbon removal from the atmosphere. We explore how the climate system differs when we achieve the 1.5 °C limit by rapid emissions reductions versus when we overshoot this limit, hitting 2 °C at mid-century before removing CO2 from the atmosphere. We show that sea level, ocean acidification, regional warming, and ocean circulation are very different under the overshoot pathway at 2100, despite hitting the 1.5 °C target for surface warming.
Limiting global warming to 1.5 °C may require carbon removal from the atmosphere. We explore how...
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