Asymmetric dynamical ocean responses in warming icehouse and cooling greenhouse climates

Abstract

Warm periods in Earth's history tend to cool more slowly than cool periods warm. Here we explore initial differences in how the global ocean takes up and gives up heat and carbon in forced rapid warming and cooling climate scenarios. We force an intermediate-complexity earth system model using two atmospheric CO 2 scenarios. A ramp-up (1% per year increase in atmospheric CO 2 for 150 years) starts from an average global CO 2 concentration of 285 ppm to represent warming of an icehouse climate. A ramp-down (1% per year decrease in atmospheric CO 2 for 150 years) starts from an average global CO 2 concentration of 1257 ppm to represent cooling of a greenhouse climate. Atmospheric CO 2 is then held constant in each simulation and the model is integrated an additional 350 years. The ramp-down simulation shows a weaker response of surface air temperature to changes in radiative forcing relative to the ramp-up scenario. This weaker response is due to a relatively large and fast release of heat from the ocean to the atmosphere. This asymmetry in heat exchange in cooling and warming scenarios exists mainly because of differences in the response of the ocean circulation to forcing. In the ramp-up, increasing stratification and weakening of meridional overturning circulation slows ocean heat and carbon uptake. In the ramp-down, cooling accelerates meridional overturning and deepens vertical mixing, accelerating the release of heat and carbon stored at depth. Though idealized, our experiments offer insight into differences in ocean dynamics in icehouse and greenhouse climate transitions.

A Critical Examination of the Role of Marine Snow and Zooplankton Fecal Pellets in Removing Ocean Surface Microplastic

“…For this study we use the University of Victoria Earth System Climate Model (UVic ESCM;Weaver et al, 2001;Eby et al, 2009;Keller et al, 2012). This particular version includes small phytoplankton and calcifiers (Kvale et al, 2015a), which were deactivated for the simulations as in Kvale et al (2015bKvale et al ( , 2018Kvale et al ( , 2019 to keep the model simple. The UVic ESCM used here therefore contains mixed phytoplankton and diazotroph phytoplankton categories, and a single zooplankton type.…”

Phytoplankton calcifiers control nitrate cycling and the pace of transition in warming icehouse and cooling greenhouse climates

“…The CAL model RD displays a slight difference in physical response relative to the other two ( Figure 1) due to a significantly longer spin-up (100 KY as opposed to 20 KY), in which internal drift resulted in small differences in initial state. In the RU, global ocean ventilation shoals and maximum northern hemisphere overturning declines from about 20 Sv at year 1 to about 11 Sv at integration year 400 (Kvale et al, 2018). Global average surface air temperature increases by more than 6.8°C, and zonally averaged upper ocean temperatures increase by as much as 8.6°C by year 500 (Kvale et al, 2018).…”

“…We explore this control in two sets of idealized experiments-a rapid icehouse to greenhouse transition, and a rapid greenhouse to icehouse transition. The physical responses of the ocean circulation to the atmospheric CO 2 forcing are described in Kvale et al (2018).…”

“…In the RU, global ocean ventilation shoals and maximum northern hemisphere overturning declines from about 20 Sv at year 1 to about 11 Sv at integration year 400 (Kvale et al, 2018). Global average surface air temperature increases by more than 6.8°C, and zonally averaged upper ocean temperatures increase by as much as 8.6°C by year 500 (Kvale et al, 2018). Antarctic Bottom Water formation collapses (Kvale et al, 2018).…”

Author reply to Reviewer comments

“…Similarly counterintuitive anthropogenic carbon uptake responses to changing global circulation have been identified by . Their, and our, findings highlight the complexities of physical and biogeochemical changes that go beyond simple predictions of the global carbon and nutrient storage response to changing remineralization depths (e.g., Kwon et al, 2009;Meyer et al, 2016;Landolfi et al, 2017).…”