Philip Goodwin

Ocean‐atmosphere partitioning of anthropogenic carbon dioxide on multimillennial timescales

[1] A theory for the ocean-atmosphere partitioning of anthropogenic carbon dioxide on centennial timescales is presented. The partial pressure of atmospheric CO2 (PCO2 )i s related to the external CO2 input (DSC) at air-sea equilibrium by: PCO2 = 280 ppm exp(DSC/[IA + IO/R]), where IA, IO, and R are the pre-industrial values of the atmospheric CO2 inventory, the oceanic dissolved inorganic carbon inventory, and the Revelle buffer factor of seawater, respectively. This analytical expression is tested with two- and three-box ocean models, as well as for a version of the Massachusetts Institute of Technology general circulation model (MIT GCM) with a constant circulation field, and found to be valid by at least 10% accuracy for emissions lower than 4500 GtC. This relationship provides the stable level that PCO2 reaches for a given emission size, until atmospheric carbon is reduced on weathering timescales. On the basis of the MIT GCM, future carbon emissions must be restricted to a total of 700 GtC to achieve PCO2 stabilization at present-day transient levels.

Analytical relationships between atmospheric carbon dioxide, carbon emissions, and ocean processes

[1] Carbon perturbations leading to an increase in atmospheric CO2 are partly offset by the carbon uptake by the oceans and the rest of the climate system. Atmospheric CO2 approaches a new equilibrium state, reached after ocean invasion ceases after typically 1000 years, given by PCO2 = P0exp(dIc/IB), where P0 and PCO2 are the initial and final partial pressures of atmospheric CO2, dIc is a CO2 perturbation, and IB is the buffered carbon inventory of the air-sea system. The perturbation, dIc, includes carbon emissions and changes in the terrestrial reservoir, as well as ocean changes in the surface carbon disequilibrium and fallout of organic soft tissue material. Changes in marine calcium carbonate, dICaCO3, lead to a more complex relationship with atmospheric

How warming and steric sea level rise relate to cumulative carbon emissions

Surface warming and steric sea level rise over the global ocean nearly linearly increase with cumulative carbon emissions for an atmosphere-ocean equilibrium, reached many centuries after emissions cease. Surface warming increases with cumulative emissions with a proportionality factor, ΔTsurface:2×CO2/(IB ln 2), ranging from 0.8 to 1.9 K (1000 PgC)−1 for surface air temperature, depending on the climate sensitivity ΔTsurface:2×CO2 and the buffered carbon inventory IB. Steric sea level rise similarly increases with cumulative emissions and depends on the climate sensitivity of the bulk ocean, ranging from 0.4 K to 2.7 K; a factor 0.4 ± 0.2 smaller than that for surface temperature based on diagnostics of two Earth System models. The implied steric sea level rise ranges from 0.7 m to 5 m for a cumulative emission of 5000 PgC, approached perhaps 500 years or more after emissions cease.

Pathways to 1.5 °C and 2 °C warming based on observational and geological constraints

To restrict global warming to below the agreed targets requires limiting carbon emissions, the principal driver of anthropogenic warming. However, there is significant uncertainty in projecting the amount of carbon that can be emitted, in part due to the limited number of Earth system model simulations and their discrepancies with present-day observations. Here we demonstrate a novel approach to reduce the uncertainty of climate projections; using theory and geological evidence we generate a very large ensemble (3 × 104) of projections that closely match records for nine key climate metrics, which include warming and ocean heat content. Our analysis narrows the uncertainty in surface-warming projections and reduces the range in equilibrium climate sensitivity. We find that a warming target of 1.5 °C above the pre-industrial level requires the total emitted carbon from the start of year 2017 to be less than 195–205 PgC (in over 66% of the simulations), whereas a warming target of 2 °C is only likely if the emitted carbon remains less than 395–455 PgC. At the current emission rates, these warming targets are reached in 17–18 years and 35–41 years, respectively, so that there is a limited window to develop a more carbon-efficient future.A 1.5 °C climate target implies total emissions of carbon from the start of 2017 must fall below 195 to 205 PgC, according to an observationally constrained very large ensemble of simulations with an efficient Earth system model.

A framework to understand the transient climate response to emissions

Global surface warming projections have been empirically connected to carbon emissions via a climate index defined as the transient climate response to emissions (TCRE), revealing that surface warming is nearly proportional to carbon emissions. Here, we provide a theoretical framework to understand the TCRE including the effects of all radiative forcing in terms of the product of three terms: the dependence of surface warming on radiative forcing, the fractional radiative forcing contribution from atmospheric CO2 and the dependence of radiative forcing from atmospheric CO2 on cumulative carbon emissions. This framework is used to interpret the climate response over the next century for two Earth System Models of differing complexity, both containing a representation of the carbon cycle: an Earth System Model of Intermediate Complexity, configured as an idealised coupled atmosphere and ocean, and an Earth System Model, based on an atmosphere–ocean general circulation model and including non-CO2 radiative forcing and a land carbon cycle. Both Earth System Models simulate only a slight decrease in the TCRE over 2005–2100. This limited change in the TCRE is due to the ocean and terrestrial system acting to sequester both heat and carbon: carbon uptake acts to decrease the dependence of radiative forcing from CO2 on carbon emissions, which is partly compensated by changes in ocean heat uptake acting to increase the dependence of surface warming on radiative forcing. On decadal timescales, there are larger changes in the TCRE due to changes in ocean heat uptake and changes in non-CO2 radiative forcing, as represented by decadal changes in the dependences of surface warming on radiative forcing and the fractional radiative forcing contribution from atmospheric CO2. Our framework may be used to interpret the response of different climate models and used to provide traceability between climate models of differing complexity.

Observational constraints on the causes of Holocene CO2 change

The mechanisms that controlled past atmospheric CO2 levels are not directly measurable, hence many proxy data sources are combined when reconstructing past carbon cycling. The accuracy of Holocene modeling reconstructions is checked by seeking consistency between data-based observables and their numerically simulated counterparts. A new framework is presented to evaluate which combinations of observables can best constrain carbon cycle mechanisms with the minimum of uncertainty. We show that when previous studies have combined ocean temperatures, ocean [CO32?], and the ?13C of atmospheric CO2 as observables, uncertainties in the data sources are amplified by over 2 orders of magnitude when reconstructing the mechanisms responsible for CO2 increase. However, incorporating mean ?13C of ocean DIC since 8000 years ago as an additional data source reduces the uncertainties by more than a factor of 5, making this observable a priority for future research. Our analysis indicates that the 20 ppm increase in CO2 between 8000 years BP and preindustrial was caused by significant CaCO3 precipitation and a reduction in the ocean soft tissue pump. Meanwhile, an increase in terrestrial carbon storage opposed the CO2 increase. The methods presented here are useful for investigating a range of paleoclimate events.

Radiocarbon constraints on the glacial ocean circulation and its impact on atmospheric CO2

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.

A new approach to projecting 21st century sea-level changes and extremes

Future increases in flooding potential around the world’s coastlines from extreme sea level events is heavily dependent on projections of future Global Mean Sea Level (GMSL) rise. Yet the two main approaches for projecting 21st century GMSL rise – i.e., process-based versus semi-empirical – give inconsistent results. Here, a novel hybrid approach to GMSL projection, containing a process-based thermosteric contribution and a semi-empirical ice-melt contribution, is embedded within a conceptual Earth System Model (ESM). The ESM is run 10 million times with random perturbations to multiple parameters, and future projections are made only from the simulations that are historically consistent. The projections from our hybrid approach are found to be consistent with the dominant process-based GMSL projections from the Climate Model Intercomparison Project phase 5 (CMIP5) ensemble, in that our future ensemble-mean projections lie within ±2 cm of CMIP5 for the end of the 21st century when CMIP5 simulated histories are used to constrain our approach. However, when observations are used to provide the historic constraints for our hybrid approach, we find higher ice-melt sensitivity and additional ensemble-mean GMSL rise of around 13 to 16 cm by the end of the century. We assess the impact of this additional GMSL rise, projected from observation-consistency, on the increase in frequency of extreme sea level events for 220 coastal tide-gauge sites. Accounting for regional effects, we infer a 1.5 to 8 times increase in the frequency of extreme sea-level events for our higher GMSL projections relative to CMIP5.

Stabilization of global temperature at 1.5°C and 2.0°C: implications for coastal areas

The effectiveness of stringent climate stabilization scenarios for coastal areas in terms of reduction of impacts/adaptation needs and wider policy implications has received little attention. Here we use the Warming Acidification and Sea Level Projector Earth systems model to calculate large ensembles of global sea-level rise (SLR) and ocean pH projections to 2300 for 1.5°C and 2.0°C stabilization scenarios, and a reference unmitigated RCP8.5 scenario. The potential consequences of these projections are then considered for global coastal flooding, small islands, deltas, coastal cities and coastal ecology. Under both stabilization scenarios, global mean ocean pH (and temperature) stabilize within a century. This implies significant ecosystem impacts are avoided, but detailed quantification is lacking, reflecting scientific uncertainty. By contrast, SLR is only slowed and continues to 2300 (and beyond). Hence, while coastal impacts due to SLR are reduced significantly by climate stabilization, especially after 2100, potential impacts continue to grow for centuries. SLR in 2300 under both stabilization scenarios exceeds unmitigated SLR in 2100. Therefore, adaptation remains essential in densely populated and economically important coastal areas under climate stabilization. Given the multiple adaptation steps that this will require, an adaptation pathways approach has merits for coastal areas. This article is part of the theme issue ‘The Paris Agreement: understanding the physical and social challenges for a warming world of 1.5°C above pre-industrial levels’.

Comparing Climate Sensitivity, Past and Present

Climate sensitivity represents the global mean temperature change caused by changes in the radiative balance of climate; it is studied for both present/future (actuo) and past (paleo) climate variations, with the former based on instrumental records and/or various types of model simulations. Paleo-estimates are often considered informative for assessments of actuo-climate change caused by anthropogenic greenhouse forcing, but this utility remains debated because of concerns about the impacts of uncertainties, assumptions, and incomplete knowledge about controlling mechanisms in the dynamic climate system, with its multiple interacting feedbacks and their potential dependence on the climate background state. This is exacerbated by the need to assess actuo- and paleoclimate sensitivity over different timescales, with different drivers, and with different (data and/or model) limitations. Here, we visualize these impacts with idealized representations that graphically illustrate the nature of time-dependent actuo- and paleoclimate sensitivity estimates, evaluating the strengths, weaknesses, agreements, and differences of the two approaches. We also highlight priorities for future research to improve the use of paleo-estimates in evaluations of current climate change.