Simulating the Earth system response to negative emissions

Abstract. Cumulative emissions budgets and net-zero emission target dates are often used to frame climate negotiations (Frameet al., 2014; Millar et al., 2016; Van Vuuren et al., 2016; Rogelj et al., 2015b; Matthews et al., 2012). However, their utilityfor near-term policy decisions is confounded by an uncertainties in future negative emissions capacity (Fuss et al., 2014; Smith et al., 2016; Larkin et al., 2018; Anderson and Peters, 2016) and in long term Earth System response to climate forcers(Rugenstein et al., 2019; Knutti et al., 2017; Armour, 2017) which may impact the utility of an indefinite carbon budget if peak temperatures occur significantly after net zero emissions are achieved, the likelihood of which in a simple model is conditionalon prior assumptions about the long term dynamics of the Earth System. Here we illustrate that the risks associated with nearterm positive emissions can be framed using a definite cumulative emissions budget with a 2040 time horizon, allowing thenecessity and scope for negative emissions deployment later in the century to be better informed by observed warming overcoming decades.

Download Full-text

The role of prior assumptions in carbon budget calculations

Earth System Dynamics ◽

10.5194/esd-11-563-2020 ◽

2020 ◽

Vol 11 (2) ◽

pp. 563-577

Author(s):

Benjamin Sanderson

Keyword(s):

Carbon Budget ◽

System Response ◽

Earth System ◽

Emissions Reductions ◽

The Earth ◽

Net Zero ◽

Negative Emissions ◽

Near Term

Abstract. Cumulative emissions budgets and net-zero emission target dates are often used to frame climate negotiations (Frame et al., 2014; Millar et al., 2016; Van Vuuren et al., 2016; Rogelj et al., 2015b; Matthews et al., 2012). However, their utility for near-term policy decisions is confounded by uncertainties in future negative emissions capacity (Fuss et al., 2014; Smith et al., 2016; Larkin et al., 2018; Anderson and Peters, 2016), in the role of non-CO2 forcers (MacDougall et al., 2015) and in the long-term Earth system response to forcing (Rugenstein et al., 2019; Knutti et al., 2017; Armour, 2017). Such uncertainties may impact the utility of an absolute carbon budget if peak temperatures occur significantly after net-zero emissions are achieved, the likelihood of which is shown here to be conditional on prior assumptions about the long-term dynamics of the Earth system. In the context of these uncertainties, we show that the necessity and scope for negative emissions deployment later in the century can be conditioned on near-term emissions, providing support for a scenario framework which focuses on emissions reductions rather than absolute budgets (Rogelj et al., 2019b).

Download Full-text

Effects of Earth system feedbacks on the potential mitigation of large-scale tropical forest restoration

Biogeosciences ◽

10.5194/bg-18-2627-2021 ◽

2021 ◽

Vol 18 (8) ◽

pp. 2627-2647

Author(s):

Alexander Koch ◽

Chris Brierley ◽

Simon L. Lewis

Keyword(s):

Land Use ◽

Carbon Cycle ◽

Forest Restoration ◽

Large Scale ◽

Paris Agreement ◽

System Response ◽

Earth System ◽

The Earth ◽

Negative Emissions ◽

The Tropics

Abstract. To achieve the Paris Agreement requires aggressive mitigation strategies alongside negative emission technologies. Recent studies suggest that increasing tree cover can make a substantial contribution to negative emissions, with the tropics being the most suitable region from a biogeophysical perspective. Yet these studies typically do not account for subsequent carbon cycle and climate responses to large-scale land-use change. Here we quantify the maximum potential temperature and CO2 benefits from pantropical forest restoration, including the Earth system response, using a fully coupled, emission-driven Earth system model (HadGEM2-ES). We perform an idealised experiment where all land use in the tropics is stopped and vegetation is allowed to recover, on top of an aggressive mitigation scenario (RCP2.6). We find that tropical restoration of 1529 Mha increases carbon stored in live biomass by 130 Pg C by 2100 CE. Whilst avoiding deforestation and tropical restoration in the tropics removes 42 Pg C compared to RCP2.6, the subsequent reduction in extratropical and ocean carbon uptake means that carbon in the atmosphere only reduces by 18 Pg C by 2100. The resulting small CO2 (9 ppm) benefit does not translate to a detectable reduction in global surface air temperature compared to the control experiment. The greatest carbon benefit is achieved 30–50 years after restoration before the Earth system response adjusts to the new land-use regime and declining fossil fuel use. Comparing our results with previous modelling studies, we identify two model-independent key points: (i) in a world where emission reductions follow the Paris Agreement, restoration is best deployed immediately, and (ii) the global carbon cycle response to reduced emissions limits the efficacy of negative emissions technologies by more than half. We conclude that forest restoration can reduce peak CO2 mid-century, but it can only modestly contribute to negative emissions.

Download Full-text

Earth system feedbacks following large-scale tropical forest restoration

10.5194/bg-2020-432 ◽

2020 ◽

Author(s):

Alexander Koch ◽

Chris Brierley ◽

Simon L. Lewis

Keyword(s):

Land Use ◽

Forest Restoration ◽

Large Scale ◽

Paris Agreement ◽

Climate Feedback ◽

Substantial Contribution ◽

System Response ◽

Earth System ◽

Negative Emissions ◽

The Tropics

Abstract. To achieve the Paris Agreement requires aggressive mitigation strategies alongside negative emission technologies. Recent studies suggest that increasing tree cover can make a substantial contribution to negative emissions, with the tropics being the most suitable region from a biogeophysical perspective. Yet these studies typically do not account for subsequent carbon cycle and climate feedback processes of large-scale land use change. Here we quantify the maximum potential temperature and CO2 benefits from pantropical forest restoration, including earth system feedbacks, using a fully-coupled, emission-driven Earth System Model (HadGEM2-ES). We perform an idealised experiment where all land use in the tropics is stopped and vegetation is allowed to recover, on top of an aggressive mitigation scenario (RCP 2.6). We find that tropical restoration of 1529 Mha increases carbon stored in live biomass by 130 Pg C by 2100 CE. Whilst avoiding deforestation and tropical restoration in the tropics removes 42 Pg C compared to RCP 2.6, feedback processes mean that carbon in the atmosphere only reduces by 18 Pg C by 2100. The resulting, small CO2 (9 ppm) benefit does not translate to a detectable reduction in global surface air temperature compared to the control experiment. The greatest carbon benefit is achieved 30–50 years after restoration before the Earth System response adjusts to the new land-use regime and declining fossil fuel use. We identify three model-independent key points: (i) the carbon benefit of restoration is CO2-scenario dependent, (ii) in a world that follows Paris Agreement emission cuts restoration is best deployed immediately, and (iii) the ocean carbon feedbacks will reduce the efficacy of negative emissions technologies. We conclude that forest restoration can reduce peak CO2 mid-century, but can only be a modest contribution to negative emissions.

Download Full-text

Astronomical forcing and climate : insights from ice core records

10.5194/egusphere-egu2020-22651 ◽

2020 ◽

Author(s):

Valérie Masson-Delmotte

Keyword(s):

Ice Core ◽

Ice Cores ◽

Climate System ◽

System Response ◽

Earth System ◽

Climate Variations ◽

The Earth ◽

Astronomical Forcing ◽

Instrumental Records ◽

Ice Core Records

Ice cores provide a wealth of insights into past changes in climate and atmospheric composition.Obtaining information on past polar temperature changes is important to document climate variations beyond instrumental records, and to test our understanding of past climate variations, including the Earth system response to astronomical forcing.Since the 1960s, major breakthrough in ice core science have delivered a matrix of quantitative Greenland and Antarctic ice core records.Temperature reconstructions from polar ice cores document past polar amplification, and provide quantitative constraints to test climate models.Climate information from the air and ice preserved in deep ice cores has been crucial to unveil the tight coupling between the carbon cycle and climate and the role of past changes in atmospheric greenhouse gas composition in the Earth system response to astronomical forcing.Ice core constraints on past changes in ice sheet topography are also key to characterize the contribution of the Greenland and Antarctic ice sheets to past sea level changes.The construction of a common chronological framework for Greenland and Antarctic ice core records has unveiled the bipolar sequence of events during the glacial-interglacial cycle, and the interplay between abrupt change and the response of the climate system to astronomical forcing.International efforts have started to obtain the oldest ice cores (hopefully back to 1,5 million years) from Antarctica, in order to understand the reasons for the major shifts in the response of the climate system to astronomical forcing at that time, leading to more intense and longer glacial periods.&#160;

Download Full-text

Astronomical forcing and climate : insights from ice core records

10.5194/egusphere-egu21-11488 ◽

2021 ◽

Author(s):

Valérie Masson-Delmotte

Keyword(s):

Ice Core ◽

Ice Cores ◽

Climate System ◽

System Response ◽

Earth System ◽

Climate Variations ◽

The Earth ◽

Astronomical Forcing ◽

Instrumental Records ◽

Ice Core Records

Ice cores provide a wealth of insights into past changes in climate and atmospheric composition.Obtaining information on past polar temperature changes is important to document climate variations beyond instrumental records, and to test our understanding of past climate variations, including the Earth system response to astronomical forcing.Since the 1960s, major breakthrough in ice core science have delivered a matrix of quantitative Greenland and Antarctic ice core records.Temperature reconstructions from polar ice cores document past polar amplification, and provide quantitative constraints to test climate models.Climate information from the air and ice preserved in deep ice cores has been crucial to unveil the tight coupling between the carbon cycle and climate and the role of past changes in atmospheric greenhouse gas composition in the Earth system response to astronomical forcing.Ice core constraints on past changes in ice sheet topography are also key to characterize the contribution of the Greenland and Antarctic ice sheets to past sea level changes.The construction of a common chronological framework for Greenland and Antarctic ice core records has unveiled the bipolar sequence of events during the glacial-interglacial cycle, and the interplay between abrupt change and the response of the climate system to astronomical forcing.International efforts have started to obtain the oldest ice cores (hopefully back to 1,5 million years) from Antarctica, in order to understand the reasons for the major shifts in the response of the climate system to astronomical forcing at that time, leading to more intense and longer glacial periods.&#160;

Download Full-text

Is Climate Change Reversible? CDRMIP simulations of the Earth system response to a massive CO2 increase and decrease (emissions followed by negative emissions).

10.5194/egusphere-egu2020-9435 ◽

2020 ◽

Author(s):

David Keller ◽

Andrew Lenton ◽

Vivian Scott ◽

Naomi Vaughan ◽

Keyword(s):

Climate Change ◽

Local Level ◽

Climate Change Impacts ◽

System Response ◽

Time Lags ◽

Earth System ◽

The Earth ◽

Long Time ◽

Industrial Level

To stabilize long-term climate change at well-below 2&#176;C (ideally below 1.5&#176;C) above pre-industrial levels, large and sustained CO2 emission reductions are needed.&#160; Despite pledges from numerous governments, the world is not on track to achieve the required reductions within the timeframes outlined in the Paris Agreement, and it appears increasingly likely that an overshoot of the 1.5 or 2 &#176;C temperature target will occur.&#160; If this happens, it may be possible to use carbon dioxide removal methods to return atmospheric CO2 concentrations to lower levels or even to reduce the magnitude of the overshoot, with the hope that lower CO2 will rapidly lead to lower temperatures and reverse or limit other climate change impacts.&#160; Here we present a multi-model analysis of how the Earth system and climate respond during the CMIP6 CDRMIP cdr-reversibility experiment, an idealized overshoot scenario, where CO2 increases from a pre-industrial level by 1% yr-1 until it is 4 times the initial value, then decrease again at 1% yr-1 until the pre-industrial level is again reached, at which point CO2 is held constant.&#160; For many modelled quantities climate change appears to eventually be reversible, at least when viewed at the global mean level.&#160; However, at a local level the results suggest some changes may be irreversible, although spatial patterns of change differ considerably between models.&#160; For many variables the response time-scales to the CO2 increase are very different than to the decrease in CO2 with a many properties exhibiting long time lags before responding to decreasing CO2, and much longer again to return to their unperturbed values (if this occurs).

Download Full-text