Quadrupling Of Atmospheric CO2 Research Articles

The Fast Response of the Tropical Circulation to CO2 Forcing

Atmosphere-only CMIP5 idealized climate experiments with quadrupling of atmospheric CO2 are analyzed to understand the fast response of the tropical overturning circulation to this forcing and the main mechanism of this response. A new metric for the circulation, based on pressure velocity in the subsidence regions, is defined, taking advantage of the dynamical stability of these regions and their reduced sensitivity to the GCM’s cloud and precipitation parameterization schemes. This definition permits us to decompose the circulation change into a sum of relative changes in subsidence area, static stability, and heating rate. A comparative analysis of aqua- and Earth-like planet experiments reveals the effect of the land–sea contrast on the total change in circulation. On average, under the influence of CO2 increase without surface warming, the atmosphere radiatively cools less, and this drives the 3%–4% slowdown of the tropical circulation. Even in an Earth-like planet setup, the circulation weakening is dominated by the radiatively driven changes in the subsidence regions over the oceans. However, the land–sea differential heating contributes to the vertical pattern of the circulation weakening by driving the vertical expansion of the tropics. It is further found that the surface warming would, independently of the CO2 effect, lead to up to a 12% slowdown in circulation, dominated by the enhancement of the static stability in the upper troposphere. The two mechanisms identified above combine in the coupled experiment with abrupt quadrupling, causing a circulation slowdown (focused in the upper troposphere) of up to 18%. Here, the independent effect of CO2 has a considerable impact only at time scales less than one year, being overtaken quickly by the impact of surface warming.

Precipitation Shifts over Western North America as a Result of Declining Arctic Sea Ice Cover: The Coupled System Response

Abstract Changes in Arctic sea ice cover have the potential to impact midlatitude climate. A previous sensitivity study utilizing the National Center for Atmospheric Research’s (NCAR) atmospheric general circulation model [AGCM; Community Climate Model, version 3 (CCM3)] to explore climate sensitivity to declining Arctic sea ice cover suggested that, as Arctic sea ice cover is reduced, precipitation patterns over western North America will shift toward dryer conditions in southwestern North America and wetter conditions in northwestern North America. Here, three complementary lines of research validate and explore the robustness of this possible climate change impact: 1) repetition of the previous sensitivity study (specified constant Arctic sea ice cover and atmospheric CO2) with an updated version of the NCAR AGCM [third Community Atmosphere Model (CAM3)], 2) investigation of the climate response to dynamically reduced Arctic sea ice cover (driven by a quadrupling of atmospheric CO2) in the coupled NCAR Community Climate System Model (CCSMv3), and 3) analysis of similar results from six other coupled climate system models. Results from the CAM3 sensitivity study are similar to those from the original study with declining Arctic sea ice cover driving up to 25% less mean annual precipitation (MAP) over southwestern North America and up to an 8% increase in MAP over northwestern North America. The seven coupled models also reproduce this same general pattern. At the time of CO2 quadrupling, Arctic sea ice cover is reduced (up to 90% in boreal winter) and MAP over southwestern North America decreases by up to 30% while MAP in northwestern North America increases by up to 40%. These results represent a significant shift in the precipitation pattern over western North America and support the findings of the original sensitivity study in suggesting that, as future reductions in Arctic sea ice cover take place, there will be a substantial impact on water resources in western North America.

Response of thermohaline circulation and thermal structure to removal of ice sheets and high atmospheric CO2 concentration

The thermohaline circulation (THC) response to ice sheet removal and quadrupling of atmospheric CO2 in a coupled model and the equilibrium thermal structure are examined. After THC weakening, diffusion of heat and salt to the northern North Atlantic at deep layers increases the temperature and salinity there, in response to CO2‐quadrupling. Resulting convective instability induces the exchange of warmer, saltier water in deep layers and cooler, fresher water near the surface. This contributes to a gradual increase in the THC intensity, culminating in its complete and rapid recovery due to positive haline feedback overcoming negative thermal feedback on the THC. Removal of ice sheets prolongs the overall recovery and strengthens the final THC due to precipitation changes over the northern North Atlantic and Labrador Sea. Bottom water and high‐latitude sea‐surface temperatures are higher without ice sheets, leading to a smaller meridional temperature gradient as indicated by Cenozoic reconstructions.

Consequences of global change for oceans: A review

The possible effects of global climate change on the oceans are described through a review of the results produced by GCM's that explicitly incorporate the dynamics of the interior of world oceans. Changes at asymptotic equilibrium influence the whole water column, but equilibrium in the deep sea is reached after several thousands years. The transient response of these models after 25 years following the onset of the perturbation (doubling or quadrupling of atmospheric CO2) affects the upper layer of the oceans (<1000 m) producing an increase in temperature between 2–4 °C. Models with realistic geography, as compared with simplified ones with N-S symmetry, produce warming near the north pole but a small cooling close to the antarctic continent. The main impacts of the predicted changes upon marine ecosystems are identified within several possible scenarios. Special mention is made of the expansion/contraction of pelagic habitats, oceanic island habitats, ocean wide distributional changes and the dynamical effects upon bioproduction.