The implications of carbon dioxide and methane exchange for the heavy mitigation RCP2.6 scenario under two metrics

Greenhouse gas emissions associated with Representative Concentration Pathway RCP2.6 could limit global warming to around or below a 2 °C increase since pre-industrial times. However this scenario implies very large and rapid reductions in both carbon dioxide (CO2) and non-CO2 emissions, and suggests a need to understand available flexibility between how different greenhouse gases might be abated. There is a growing interest in developing a greater understanding of the particular role of shorter lived non-CO2 gases as abatement options. We address this here through a sensitivity study of different methane (CH4) emissions pathways to year 2100 and beyond, by including exchanges with CO2 emissions, and with a focus on related climate and economic advantages and disadvantages. Metrics exist that characterise gas equivalence in terms of climate change effect per tonne emitted. We analyse the implications of CO2 and CH4 emission exchanges under two commonly considered metrics: the 100-yr Global Warming Potential (GWP-100) and Global Temperature Potential (GTP-100). This is whilst keeping CO2-equivalent emissions pathways fixed, based on the standard set of emissions usually associated with RCP2.6. An idealised situation of anthropogenic CH4 emissions being reduced to zero across a period of two decades and with the implementation of such cuts starting almost immediately gives lower warming than for standard RCP2.6 emissions during the 21st and 22nd Century. This is despite exchanging for higher CO2 emissions. Introducing Marginal Abatement Cost (MAC) curves provides an economic assessment of alternative gas reduction strategies. Whilst simpler than utilising full Integrated Assessment Models (IAMs), MAC curves are more transparent for illustrative modelling. The GWP-100 metric places a relatively high value on climate change prevented for methane emission reduction, as compared to an equivalent mass of CO2 reduction. This in combination with the strong non-linearity in MAC curves (moving quickly from relatively cheap removal to emissions difficult to cut at any cost) causes little change under cost minimisation from standard RCP2.6 emissions. This reflects the original development of RCP2.6 standard emissions from similar minimisation. With gas exchange under GTP-100, however, we find much less methane is abated, resulting in higher temperatures, whilst costs are slightly lower. Our results also highlight the point at which greater methane mitigation would become beneficial from both a climate and economic aspect. If by 2030 removal of all methane were to become possible at an average cost less than $1000 per tonne of CH4, then this would be the cheapest option, for GWP-100 metric and our CO2 MAC curve. Critically this would increase the possibility of constraining warming to two degrees

Emission targets for avoiding dangerous climate change

A number of recent studies have found a strong link between peak global warming due to anthropogenic carbon dioxide and cumulative carbon emissions from the start of the industrial revolution. This thesis builds on this work by using a simple climate model to apply the concept of cumulative emissions to emission floors, by comparing cumulative emissions with other types of emissions target, and by extending the work to apply to noncarbon dioxide (CO2) greenhouse gases and short-lived climate forcers (SLCFs). Though peak global warming correlates well with cumulative carbon emissions, the link to emissions over shorter periods or in the years 2020 or 2050 is shown to be weaker. It is also shown that the introduction of emissions floors does not reduce the importance of cumulative emissions, but may make some warming targets unachievable. For pathways that give a most likely warming up to about 4°C, cumulative emissions from pre-industrial times to year 2200 correlate strongly with most likely resultant peak warming in the simple model used, regardless of the type of emissions floor used. The maximum rate of CO2- induced warming is not determined by cumulative emissions but is shown to be limited by the peak rate of CO2 emissions. A simple model of non-CO2 greenhouse gases is also developed and used to investigate SLCFs. It is shown that emissions of SLCFs today have little impact on peak warming, and that delaying near-term reductions in SLCFs would not have a significant impact on peak warming. Only once CO2 emissions are falling do SLCF emissions have a significant impact on peak warming. A global climate policy framework is presented as an example of how the work in this thesis could be used in policy. Future work is also discussed, particularly verification of these results in a more complex model

Emission targets for avoiding dangerous climate change

A number of recent studies have found a strong link between peak global warming due to anthropogenic carbon dioxide and cumulative carbon emissions from the start of the industrial revolution. This thesis builds on this work by using a simple climate model to apply the concept of cumulative emissions to emission floors, by comparing cumulative emissions with other types of emissions target, and by extending the work to apply to noncarbon dioxide (CO2) greenhouse gases and short-lived climate forcers (SLCFs). Though peak global warming correlates well with cumulative carbon emissions, the link to emissions over shorter periods or in the years 2020 or 2050 is shown to be weaker. It is also shown that the introduction of emissions floors does not reduce the importance of cumulative emissions, but may make some warming targets unachievable. For pathways that give a most likely warming up to about 4°C, cumulative emissions from pre-industrial times to year 2200 correlate strongly with most likely resultant peak warming in the simple model used, regardless of the type of emissions floor used. The maximum rate of CO2- induced warming is not determined by cumulative emissions but is shown to be limited by the peak rate of CO2 emissions. A simple model of non-CO2 greenhouse gases is also developed and used to investigate SLCFs. It is shown that emissions of SLCFs today have little impact on peak warming, and that delaying near-term reductions in SLCFs would not have a significant impact on peak warming. Only once CO2 emissions are falling do SLCF emissions have a significant impact on peak warming. A global climate policy framework is presented as an example of how the work in this thesis could be used in policy. Future work is also discussed, particularly verification of these results in a more complex model.</p

The link between a global 2 °C warming threshold and emissions in years 2020, 2050 and beyond

In the Copenhagen Accord, nations agreed on the need to limit global warming to two degrees to avoid potentially dangerous climate change, while in policy circles negotiations have placed a particular emphasis on emissions in years 2020 and 2050. We investigate the link between the probability of global warming remaining below two degrees (above pre-industrial levels) right through to year 2500 and what this implies for emissions in years 2020 and 2050, and any long-term emissions floor. This is achieved by mapping out the consequences of alternative emissions trajectories, all in a probabilistic framework and with results placed in a simple-to-use set of graphics. The options available for carbon dioxide-equivalent (CO2e) emissions in years 2020 and 2050 are narrow if society wishes to stay, with a chance of more likely than not, below the 2 °C target. Since cumulative emissions of long-lived greenhouse gases, and particularly CO2, are a key determinant of peak warming, the consequence of being near the top of emissions in the allowable range for 2020 is reduced flexibility in emissions in 2050 and higher required rates of societal decarbonization. Alternatively, higher 2020 emissions can be considered as reducing the probability of limiting warming to 2 °C. We find that the level of the long-term emissions floor has a strong influence on allowed 2020 and 2050 emissions for two degrees of global warming at a given probability. We place our analysis in the context of emissions pledges for year 2020 made at the end of and since the 2009 COP15 negotiations in Copenhagen

Emission targets for avoiding dangerous climate change

A number of recent studies have found a strong link between peak global warming due to anthropogenic carbon dioxide and cumulative carbon emissions from the start of the industrial revolution. This thesis builds on this work by using a simple climate model to apply the concept of cumulative emissions to emission floors, by comparing cumulative emissions with other types of emissions target, and by extending the work to apply to noncarbon dioxide (CO2) greenhouse gases and short-lived climate forcers (SLCFs). Though peak global warming correlates well with cumulative carbon emissions, the link to emissions over shorter periods or in the years 2020 or 2050 is shown to be weaker. It is also shown that the introduction of emissions floors does not reduce the importance of cumulative emissions, but may make some warming targets unachievable. For pathways that give a most likely warming up to about 4&deg;C, cumulative emissions from pre-industrial times to year 2200 correlate strongly with most likely resultant peak warming in the simple model used, regardless of the type of emissions floor used. The maximum rate of CO2- induced warming is not determined by cumulative emissions but is shown to be limited by the peak rate of CO2 emissions. A simple model of non-CO2 greenhouse gases is also developed and used to investigate SLCFs. It is shown that emissions of SLCFs today have little impact on peak warming, and that delaying near-term reductions in SLCFs would not have a significant impact on peak warming. Only once CO2 emissions are falling do SLCF emissions have a significant impact on peak warming. A global climate policy framework is presented as an example of how the work in this thesis could be used in policy. Future work is also discussed, particularly verification of these results in a more complex model.This thesis is not currently available on ORA

Climate system properties determining the social cost of carbon

The choice of an appropriate scientific target to guide global mitigation efforts is complicated by uncertainties in the temperature response to greenhouse gas emissions. Much climate policy discourse has been based on the equilibrium global mean temperature increase following a concentration stabilization scenario. This is determined by the equilibrium climate sensitivity (ECS) which, in many studies, shows persistent, fat-tailed uncertainty. However, for many purposes, the equilibrium response is less relevant than the transient response. Here, we show that one prominent policy variable, the social cost of carbon (SCC), is generally better constrained by the transient climate response (TCR) than by the ECS. Simple analytic expressions show the SCC to be directly proportional to the TCR under idealized assumptions when the rate at which we discount future damage equals 2.8%. Using ensemble simulations of a simple climate model we find that knowing the true value of the TCR can reduce the relative uncertainty in the SCC substantially more, up to a factor of 3, than knowing the ECS under typical discounting assumptions. We conclude that the TCR, which is better constrained by observations, less subject to fat-tailed uncertainty and more directly related to the SCC, is generally preferable to the ECS as a single proxy for the climate response in SCC calculations

Cumulative carbon emissions, emissions floors and short-term rates of warming: implications for policy

A number of recent studies have found a strong link between peak human-induced global warming and cumulative carbon emissions from the start of the industrial revolution, while the link to emissions over shorter periods or in the years 2020 or 2050 is generally weaker. However, cumulative targets appear to conflict with the concept of a ‘floor’ in emissions caused by sectors such as food production. Here, we show that the introduction of emissions floors does not reduce the importance of cumulative emissions, but may make some warming targets unachievable. For pathways that give a most likely warming up to about 4°C, cumulative emissions from pre-industrial times to year 2200 correlate strongly with most likely resultant peak warming regardless of the shape of emissions floors used, providing a more natural long-term policy horizon than 2050 or 2100. The maximum rate of CO2-induced warming, which will affect the feasibility and cost of adapting to climate change, is not determined by cumulative emissions but is tightly aligned with peak rates of emissions. Hence, cumulative carbon emissions to 2200 and peak emission rates could provide a clear and simple framework for CO2 mitigation policy