The Problem With Trend Lines

This is just a quick post to highlight some of the problems with over interpreting trend lines. These problems can occur in various place but I will use the trend lines drawn in the Suffolk Climate Change Partnership’s Action Plan1. I have a strong feeling that the trend line was generated not as part of any real analysis but part of a ‘look we are doing really well at tackling climate change‘ PR exercise by the council. However, such incorrect analysis can fool the public and possibly the council themselves.

The plan states:

Plotting and extrapolating data previously made available by the Department of Energy and Climate Change (DECC, now part of the Department for Business, Energy & Industrial Strategy) for the period 2005 – 2014 it can be shown that we are currently trending around a 32% reduction in absolute emissions in Suffolk between 2010 and 2025, against our target of 35%.

Although this is not what the post is about it is interesting the following note to this graph:

“but estimates that the UK’s carbon footprint has increased by around 10% since 1993, as growth in imported emissions more than offset the reductions in production emissions. The UK is now one of the world’s largest net importers of emissions (both in absolute and per capita terms), with a carbon footprint that is around 80% larger than its production emissions, reflecting the relatively small share of manufacturing in UK GDP.”

So it is doubtful that this graph captures the change in total CO2 emissions.

However, the main point of this post is that it is not possible to predict the future reduction in CO2 emissions from just drawing a trend line. This is obvious when we break down the CO2 emissions into its components (data from UK local authority and regional carbon dioxide emissions national statistics: 2005-20172):

As you can see from the graph the reason for the reduction in CO2 is mainly due to decarbonisation of electricity production. Other sources of CO2 decline slightly or not at all. However, the decarbonisation of electricity production cannot continue indefinitely on the same straight line – even if we assume that it can continue the same downward trend it cannot go below below zero.

The reason for the steep decline in the carbon intensity for electricity is the phasing out of coal based production. However, this steep decline is expected to end by 20183

This leads to the steep decline in CO2 emissions ending after 2025. So if we now put in a smaller slope for electricity after 2018:


When we now look at the total CO2 emissions we see that there is a much bigger gap between the original trend line and what we might reasonably expect in 2025 and 2030.


This post was about the misuse of trend line so I have not gone into many other factors such as electrifying the transport system, decarbonising the heating of and better insulation of houses and workplaces…

All of these could have a significant affect on CO2 emissions. However, if we are going meet our commitments in reducing CO2 emissions the infrastructure needs to be planned for and built now – something that does not seem to be happening at a fast enough pace.


1 Suffolk Climate Change Action Plan, Suffolk Climate Change Partnership,  (http://www.greensuffolk.org/assets/Greenest-County/SCCP/Climate-Change/Suffolk-Climate-Action-Plan-3.pdf)

2 UK local authority and regional carbon dioxide emissions national statistics: 2005-2017, BEIS (https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/812145/2005-17_UK_local_and_regional_CO2_emissions_tables.ods)

3 Grid Carbon Factor grams of CO2e/KWh, ICAX (https://www.icax.co.uk/image_Carbon_Emissions_Intensity.html)

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