Three Paths to Carbon Zero






Many local authorities in the UK have stated their approaches to achieving zero or greatly reduced carbon dioxide output, some over a period of a few years, some over several decades. Two such statements will be examined, and compared with a proposal based on cumulative CO2 output.



The first plan is bold and simple: zero CO2 by 2025. If we take this to mean by the beginning of 2025, and expect action from the start of 2020, then we might envisage a straight line graph descending from the current output level to zero in five years, and so falling by 20% of the present level in each year. This has the virtue of simplicity, but is unlikely to represent reality. If in the first year the plan did meet its target, we could reasonably suppose that the measures taken were the easier ones, and that in each subsequent year the task would become more difficult.  The shape of graph representing a realistic carbon reduction program is likely to be a curve, sloping downward steeply at first, and less steeply in each successive time interval, as the task becomes harder. It may not reach zero in the time frame, but success could be defined by a close approach.



Whatever the shape of the graph, the area under it represents the cumulative CO2 output over the period considered. This is easy to calculate for a straight line graph, being simply the average rate of CO2 output over the period multiplied by the duration. A straight line descent from say 1 MtCO2 per year to zero in five years means a total output of 2.5 MtCO2. A curved descent starting with the same slope but flattening out would represent a higher cumulative output, and one starting with a significantly steeper descent would give a lower total. This is straightforward to estimate if the curve is approximated to an exponential decay.



A second local authority plan is rather less ambitious, and bases itself on the Climate Change Act target, promising a 52% reduction in carbon emissions by 2040 compared with 2017 levels.  This represents a 2.6% reduction per year on the 2017 levels over the period 2020 to 2040, which is just about in line with the rate at which this authority’s emissions were falling before 2017. Assuming a straight line descent from 1 to 0.48 MtCO2 per year gives an average of 0.74 MtCO2 per year, and hence a cumulative 14.8 Mt CO2 emission over the twenty year term alone. 



Cumulative CO2 is central to work done by the Tyndall Centre for Climate Change Research [1]. Researchers have produced an online tool based on carbon budgets derived from the United Nations Paris Agreement, which recommends pathways to carbon neutrality for UK local authorities. The Tyndall Carbon Budget Tool sets the rate of reduction on an exponential decay path which will lead to near zero emissions and also meet the cumulative CO2 target. Rates of reduction are typically in the range 12 to 15% per annum from the previous year, continuing to the end of the century, but giving “near zero” dates around 2040 to 2045. Many local authorities would use their entire carbon budget in seven years at 2017 emission levels.



Returning to the two reduction plans outlined above, both were for local authorities with 2017 outputs of 1 MtCO2 per year, and we can see from Tyndall data that a typical carbon budget would be around 6 MtCO2. The first plan, reducing to zero in five years, would, if successful, come well inside target at 2.5 MtCO2, whereas the second would far exceed the budget at more than 14.8 Mt CO2 .



A discussion of the terms ‘net zero’, ‘zero carbon’ and ‘carbon neutral’ is available at Manchester Policy Blogs: Energy and Environment [2]





References



[1] Tyndall Centre for Climate Change Research, University of Manchester



https://carbonbudget.manchester.ac.uk/



[2] Manchester Policy Blogs: Energy and Environment



http://blog.policy.manchester.ac.uk/energy_environment/2019/04/setting-climate-targets-when-is-net-zero-really-net-zero/

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