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

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      The Paris Climate Agreement has initiated a scientific debate on the role that carbon removal &amp;ndash; or net negative emissions &amp;ndash; might play in achieving less than 1.5&amp;thinsp;K of global mean surface warming by 2100. Here, we probe the sensitivity of a comprehensive Earth System Model to three different atmospheric CO<sub>2</sub> concentration pathways, two of which arrive at 1.5&amp;thinsp;K 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 2&amp;thinsp;K of surface warming by 2100 in our model; 2) a <q>stabilization</q> pathway in which atmospheric CO<sub>2</sub> concentration never exceeds 440&amp;thinsp;ppm and the global mean temperature rise is approximately 1.5&amp;thinsp;K by 2100; and 3) an <q>overshoot</q> pathway that passes through 2&amp;thinsp;K of warming at mid-century, before ramping down atmospheric CO<sub>2</sub> concentrations, as if using carbon removal, to end at 1.5&amp;thinsp;K 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, sea ice coverage, and the global mean sea level change and its regional expressions. In 2100, the <q>overshoot</q> ensemble is closer to RCP4.5 than the ‘stabilization’ ensemble with respect to global steric sea level rise and the strength of the Atlantic Meridional Overturning Circulation (AMOC). There is strong ocean surface cooling in the North Atlantic 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 10&amp;thinsp;cm of additional sea level rise by 2100 relative to stabilization forcing.

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      Earth System Dynamics Discussions
      Earth Syst. Dynam. Discuss.
      Copernicus GmbH
      November 27 2017
      : 1-19
      © 2017


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