While the ocean’s large-scale overturning circulation is thought to have been significantly different under the climatic conditions of the Last Glacial Maximum (LGM), the exact nature of the glacial circulation and its implications for global carbon cycling continue to be debated. Here we use a global array of ocean–atmosphere radiocarbon disequilibrium estimates to demonstrate a ∼689±53 14C-yr increase in the average residence time of carbon in the deep ocean at the LGM. A predominantly southern-sourced abyssal overturning limb that was more isolated from its shallower northern counterparts is interpreted to have extended from the Southern Ocean, producing a widespread radiocarbon age maximum at mid-depths and depriving the deep ocean of a fast escape route for accumulating respired carbon. While the exact magnitude of the resulting carbon cycle impacts remains to be confirmed, the radiocarbon data suggest an increase in the efficiency of the biological carbon pump that could have accounted for as much as half of the glacial–interglacial CO2 change.
Abstract. Reduced-complexity climate models (RCMs) are critical in the policy and decision making space, and are directly used within multiple Intergovernmental Panel on Climate Change (IPCC) reports to complement the results of more comprehensive Earth system models. To date, evaluation of RCMs has been limited to a few independent studies. Here we introduce a systematic evaluation of RCMs in the form of the Reduced Complexity Model Intercomparison Project (RCMIP). We expect RCMIP will extend over multiple phases, with Phase 1 being the first. In Phase 1, we focus on the RCMs' global-mean temperature responses, comparing them to observations, exploring the extent to which they emulate more complex models and considering how the relationship between temperature and cumulative emissions of CO2 varies across the RCMs. Our work uses experiments which mirror those found in the Coupled Model Intercomparison Project (CMIP), which focuses on complex Earth system and atmosphere–ocean general circulation models. Using both scenario-based and idealised experiments, we examine RCMs' global-mean temperature response under a range of forcings. We find that the RCMs can all reproduce the approximately 1 ∘C of warming since pre-industrial times, with varying representations of natural variability, volcanic eruptions and aerosols. We also find that RCMs can emulate the global-mean temperature response of CMIP models to within a root-mean-square error of 0.2 ∘C over a range of experiments. Furthermore, we find that, for the Representative Concentration Pathway (RCP) and Shared Socioeconomic Pathway (SSP)-based scenario pairs that share the same IPCC Fifth Assessment Report (AR5)-consistent stratospheric-adjusted radiative forcing, the RCMs indicate higher effective radiative forcings for the SSP-based scenarios and correspondingly higher temperatures when run with the same climate settings. In our idealised setup of RCMs with a climate sensitivity of 3 ∘C, the difference for the ssp585–rcp85 pair by 2100 is around 0.23∘C(±0.12 ∘C) due to a difference in effective radiative forcings between the two scenarios. Phase 1 demonstrates the utility of RCMIP's open-source infrastructure, paving the way for further phases of RCMIP to build on the research presented here and deepen our understanding of RCMs.
Climate model experiments reveal that transient global warming is nearly proportional to cumulative carbon emissions on multi-decadal to centennial timescales 1-5 . However, it is not quantitatively understood how this near linear dependence between warming and cumulative carbon emissions arises in transient climate simulations 6,7 . Here, we present a theoretically-derived equation of the dependence of global warming on cumulative carbon emissions over time. For an atmosphere-ocean system, our analysis identifies a surface warming response to cumulative carbon emissions of 1.5±0.7 K for every 1,000 Pg of carbon emitted. This surface warming response is reduced by typically 10 to 20% by the end of the century and beyond. The climate response remains nearly constant on multidecadal to centennial timescales as a result of partially-opposing effects of oceanic uptake of heat and carbon 8 . The resulting warming then becomes proportional to cumulative carbon emissions after many centuries, as noted earlier 9 . When we incorporate estimates of terrestrial carbon uptake 10 , the surface warming response is reduced to 1.1±0.5 K for every 1000 Pg of carbon emitted, but this modification is unlikely to significantly affect how the climate response changes over time. We suggest that our theoretical framework may be used to diagnose the global warming response in climate models and mechanistically understand the differences between their projections.Warming of the Earth's surface, ΔT(t), depends on the increase in radiative forcing, R(t), from atmospheric CO 2 minus the net heat flux into the Earth System, N(t), 11where ΔT is the change in global mean surface temperature relative to the pre-industrial era, λ is the equilibrium climate feedback parameter [(W m -2 ) K -1 ] or equivalently λ -1 is the climate sensitivity 12 , R(t) and N(t) are positive when downward and in W m -2 . The net heat flux, N(t), is dominated by ocean heat uptake, since over 90% of N(t) passes into the ocean interior 13 . ε is the non-dimensional ocean heat uptake efficacy 11 , accounting for how ocean heat uptake may be more effective than radiative forcing in altering ΔT. The 2 radiative forcing, R(t), taken to be at the top of the troposphere, is directly linked to atmospheric CO 2 via a logarithmic relationship, 14where CO 2 is measured as a mixing ratio (ppmv), a=5.35 Wm -2 is a CO 2 radiative-forcing coefficient and Δln CO 2 (t) represents ln CO 2 (t) -ln CO 2 (t 0 ) where t 0 is the preindustrial. An increase in cumulative carbon emissions naturally leads to a longterm increase in atmospheric CO 2 , radiative forcing and surface warming, which might be augmented by further warming from non-CO 2 greenhouse gases or partly opposed by cooling from aerosols 6,7,15 . Focussing on the dominant effect of cumulative carbon emissions on global warming 15 , the relationship between the logarithmic change in atmospheric CO 2 , Δln CO 2 (t), and cumulative carbon emissions over time must be found.The rise in ln CO 2 from cumulative emissions is affected ...
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