Analysis: Why the UK’s CO2 emissions have fallen 38% since 1990

§ The big picture

UK emissions have declined from around 600m tonnes of CO2 (MtCO2) in 1990 to 367MtCO2 in 2017. If underlying factors driving emissions had not changed, Carbon Brief’s analysis shows that a growing population and a constant electricity generation mix would have led to emissions increasing by around 25% compared to 1990 levels.

Instead, emissions actually fell by 38% to 367MtCO2, as shown in the black area in the figure below. Each coloured wedge in the figure shows one factor contributing to this decline.

As the chart shows, no single factor was responsible for more than around a third of the total reduction in the UK’s CO2. Overall, emissions in 2017 were 51% lower than they would have been without these changes

Decreases in CO2 emissions from electricity production is one of the main drivers economy-wide (dark blue wedge), accounting for around 36% of the total emissions reduction in 2017. This was driven primarily by the transition away from coal and towards gas and renewable generation.

Lower non-electric energy use in the industrial and residential sectors has been another major factor (yellow wedge), responsible for 31% of the emission reduction in 2017. Savings in industry was the largest part of this.

§ Analysing emission reductions

To assess the causes of the UK’s falling CO2 output, actual emissions in each sector have to be compared to alternative “business-as-usual” scenarios. These show what might have happened if the factors behind CO2 reductions had not changed.

To do this, Carbon Brief’s analysis fixes energy use and emissions at 1990 per-capita levels. This shows what emissions would have been across various sectors of the economy if conditions had stayed the way they were in 1990.

In cases where factors increased faster than fixed 1990 per-capita values – as with electricity use – the business-as-usual scenario begins when emissions start to fall.

Each sector of the economy that emits CO2 from energy is explored in more detail in the sections below. These include: electricity generation, transport and fuel use by buildings and industry.

It is important to note that this analysis only considers CO2 from energy and land-use changes and does not include other greenhouse gases. CO2 emissions from energy comprise upwards of 80% of total greenhouse gas emissions and most of the UK’s non-CO2 emissions have been declining in recent years in official inventories.

§ Electricity generation

Around 20% of the UK’s CO2 emissions in 2017 came from burning coal, oil and gas to produce electricity. This is down from 34% back in 1990.

Coal’s share of this mix has fallen precipitously since 1990, down from 67% of total generation to only 5% today. Oil used for electricity generation has also declined, down from 11% of generation in 1990 to less than 1% today. These have largely been replaced by gas, wind and bioenergy.

The figure below shows the amount of electricity generated by each fuel between 1990 and 2017. It also includes a “business-as-usual” scenario, shown in dashed lines for each fuel. In this scenario, the electricity generation mix remains frozen at 1990 levels.

Electricity use in this scenario tracks observed demand between 1990 and 2005 – a period when electricity use was increasing rapidly – but remains at fixed per-capita 2005 levels rather than declining thereafter.

This gives a sense of what the UK power sector would have looked like without the factors that have driven major shifts in recent years. Coal would have remained king, with gas, wind, bioenergy and solar all remaining negligible. Nuclear generation would have remained largely the same, with somewhat higher output in recent years.

The figure below compares the “business as usual” electricity use scenario (dashed lines) to actual electricity use (solid lines) across industrial, commercial and residential sectors. Here the commercial sector includes businesses as well as public administration, transport and agricultural electricity use.

The decline in electricity use is primarily concentrated in industrial and residential sectors, both of which have demand around 25% lower in 2017 than if it had remained at 2005 per-capita levels.

Electricity-use reductions in the residential sectors reflect a combination of more efficient lightbulbs and appliances and, for electrically-heated properties, better insulation.

Industrial sector electricity use reductions also come from a wide combination of factors, including increased manufacturing efficiency and a shift in the makeup of the UK economy away from heavy industry towards advanced manufacturing and services. The 2007-8 financial crisis also played a role in reducing industrial electricity use, though demand has continued to decline even as the economy has recovered.

These “business-as-usual” scenarios for electricity generation and consumption provide a way to calculate contributions to the UK’s declining CO2 emissions. The figure below shows actual electric sector CO2 emissions in black, along with the additional emissions that would have occurred if not for the rise of gas, wind, solar, bioenergy (blue wedges) and the reduction in electricity use (purple).

Without the effects of gas, renewables and reduced electricity use, CO2 emissions from the electricity sector could have been expected to continue increasing in line with past trends. By 2017, electricity sector CO2 emissions would then have been almost four times higher than they are today.

The transition to renewable energy – wind, bioenergy and solar – is the largest driver, collectively responsible for 37% of electricity sector emissions reductions in 2017. Wind is the largest portion of this, at 20%, with bioenergy at 12% and solar at 5%.

UK and international emissions accounting practice counts bioenergy as zero-carbon at the point of use, a convention followed in this article. Upstream emissions due to harvesting, drying, storing and transporting imported bioenergy, as well as from associated changes in land use and forest carbon stocks are, in theory, accounted for in the country of origin.

These emissions can be large and in some scenarios could offset or even outweigh the climate benefit of replacing coal with biomass-fired power. Note, however, that only around a quarter of the bioenergy used in the UK is imported.

Lower electricity use is currently the second largest driver of CO2 reductions in the power sector after renewable energy, accounting for around 33% of the reduction in 2017.

Coal-to-gas switching starting with the 1990s “dash for gas” is the third largest driver, responsible for 29% of the reduction in 2017. This shift is responsible for the largest cumulative amount of avoided emissions between 1990 and today.

However, this source of emissions savings has been largely exhausted, with coal generation now below 5% of the UK total. This means new gas generation today would be more likely to increase rather than decrease emissions, because there is little coal remaining to be displaced and the alternatives are largely low-carbon sources, such as renewables and nuclear.

It is worth noting that this analysis of electricity sector emissions is incomplete, as it does not consider methane leakage during gas extraction and transport. However, official inventories suggest that methane emissions from gas production have decreased in the UK since 1990 despite a large expansion in the use of gas for electricity generation.

§ Transportation

Transport was responsible for around 34% of the UK’s CO2 emissions in 2017. This share has increased from 21% in 1990 as other sectors of the economy have cut their emissions while transport CO2 output has barely changed.

This sector includes cars and taxis, heavy goods vehicles, vans, buses and coaches, motorcycles and mopeds, rail transport, domestic aviation and shipping and other mobile emission sources. It does not include the UK’s share of international aviation and shipping, as discussed below.

Transport is a particularly challenging sector for emission reductions, as cost-effective electric alternatives to petrol and diesel vehicles are only beginning to emerge. Most of the reductions that have happened in the transportation sector have been driven by more efficient vehicles, with some additional reductions in recent years due to reduced miles driven and the blending of biofuels into petrol and diesel.

The figure below shows changes in vehicle miles travelled and vehicle fuel economy since 1990. The black line is the “business-as-usual” scenario, where vehicle fuel economy remained at 1990 levels and vehicle miles traveled remained at 2007 per-capita levels rather than declining over the past decade.

The number of miles driven in the UK has increased from around 4,500 miles per person per year in 1990 to a peak of 5,100 in 2007. It dropped down to around 4,700 during the aftermath of the financial crisis before recovering to 4,900 by 2017.

Motor vehicles have also become more fuel efficient in recent years in response to regulatory standards. Fuel economy of the average motor vehicle increased from around 33 miles per gallon in 1990 to 41 miles per gallon in 2017.

The combined effect of these three factors is shown in the figure below, where the black area is actual emissions and the three wedges reflect the emission reductions from reduced miles driven per person, increased fuel economy and other transport emissions – comprised of rail, domestic shipping and aviation.

Note that other transport 2017 values are estimated by proportionally attributing overall transport sector CO2 reductions between 2016 and 2017 to the category, as data on CO2 emissions for other transport categories for 2017 were not available at time of publication.

Reductions from “Other transport” are due to emissions from rail, domestic aviation and domestic shipping. Emissions from rail and domestic shipping have remained relatively flat since 1990, while CO2 from domestic aviation has declined by around 40% since 2005 – though the cause of these reductions is unclear. Emissions from military domestic aviation and shipping are also down by around 60% since 1990.

The UK inventory of CO2 emissions from transportation is incomplete, as both emissions from international aviation and shipping are not directly included in the UK’s domestic carbon budget. However, UK emission targets do consider the national contribution to international aviation emissions.

§ Business, residential and other CO2 sources

Energy used by businesses and homes, excluding electricity generation, comes in the form of on-site fuel combustion. For homes, this is mainly gas and oil used for space and water heating. For business, a number of different processes burn fuels that release CO2.

Additional emission sources are public-sector buildings and infrastructure, agriculture and sites such as oil refineries. Land-use emissions have been negative in the UK since 1990, meaning more carbon is absorbed by soils and plants than is released. The size of this carbon sink has grown in recent years.

The figure below shows CO2 emissions from these other sectors between 1990 and 2017. This includes both industrial and commercial entities (light blue line), homes (dark blue), the public sector and agriculture (yellow), land use (green) and other energy supply emissions (brown).

A “business-as-usual” scenario is shown in dashed lines where emissions remain at constant per-capita 1990 levels, or at constant per-capita levels of the year in which emissions began to decline.

Reduced non-electric CO2 emissions from business and industry are the main factor driving declines, falling 42% below 1990 levels and 50% below a “business as usual” scenario where per-capita business and industry energy use remained at 1990 levels.

According to BEIS, the largest factors behind declining business and industrial energy use include improvements in energy efficiency and switching to lower-carbon fuels. The economic recession associated with the 2007-8 financial crisis also notably lowered emissions, though they have remained low since the economy recovered.

About 25% of the reduction in business and industrial emissions over the past decade is attributable to structural movement towards a less carbon-intensive mix of industrial output, with some energy-intensive industry moving to other countries and UK industry increasingly focusing on higher-value manufacturing with lower energy intensity.

Homes have also seen meaningful declines in emissions, due mostly to energy efficiency, though warmer weather may have also played a role in reducing the need for space heating.

Public sector and agricultural CO2 emissions have also declined modestly since the late 1990s, and other energy supply-related emissions – including from refineries and the manufacture of solid fuels – have declined since around 2005.

The figure below shows the combined CO2 reductions from each of these sectors. Total non-electric non-transportation CO2 emissions are shown in black, while the reductions in each sector are shown by the colored wedges.

§ Emissions embodied in trade

The standard territorial accounting of greenhouse gas emissions excludes lifecycle emissions from bioenergy grown overseas, the UK’s share of international aviation and shipping, plus the CO2 generated when making goods that are imported into the UK.

This last factor is  a particularly important to note factor, as the past few decades have seen a dramatic rise in exports of consumer goods from countries such as China, which has a very coal-intensive electricity generation mix.

A number of different research groups provide estimates of the CO2 embodied in imported goods. These estimates can be used to calculate the “consumption footprint” of the UK including CO2 embodied in imported goods and excluding CO2 embedded in goods exported out of the country. [Note that consumer goods are only one part of imported emissions.]

The figure below shows the UK’s territorial CO2 emissions in dark blue, the CO2 emissions “business-as-usual” counterfactual developed in this analysis in yellow and an estimate of the UK consumption CO2 emissions from the Global Carbon Project in light blue. Each line is shown as the change since 1990 to allow easy comparison (as consumption CO2 emissions were already higher than domestic emissions in 1990).

The chart above shows that domestic emissions reductions between 1990 and 2007 were largely cancelled out by increases in imported CO2. However, over the past decade, both domestic and consumption emissions have fallen by similar amounts.

Since the mid-2000s, decreases in the “emissions intensity” of industry and a reduction inflattening of demand for energy have helped drive consumption emissions down at a rate similar to domestic emissions, according to research by Dr Anne Owen at the University of Leeds. Offshoring of production played a role in driving up consumption emissions after 1990, but is not as large a factor recent years.

§ Conclusions

Many factors have come together to drive the UK’s CO2 emissions down since 1990. The largest contributors have been reductions in industrial and business energy (dark yellow wedge in the chart, below), reduced electricity use (purple), increased use of renewable energy (light blue) and the replacement of coal with gas (dark blue). However, no single factor was responsible for the majority of the decline, as the chart below shows.

Emissions continued to fall as the UK recovered from the financial crisis and its associated recession a decade ago, suggesting that this was not a major cause of current emission reductions.

While this analysis focuses on CO2, overall UK greenhouse gas emissions – measured in units of CO2 equivalent – are down 43% since 1990, compared to a reduction of 38% from CO2 alone.

Domestic emissions in the UK have declined in the past 30 years faster than nearly any other country on Earth. Nevertheless, additional rapid reductions will be needed to meet the UK’s legally-binding climate goals – and even faster cuts will be required once the country aims for net-zero emissions in line with the Paris Agreement.

While it is useful to understand the factors behind CO2 reductions to date, additional government policy will need to play a role in driving the deep reductions required to help avoid potentially dangerous warming. The UK’s emission reductions to date have been concentrated in electricity and industrial sectors, while future deep decarbonisation will require large reductions in emissions in more difficult areas, such as transport and farming.

§ Methodology

The python code used to attribute declines in UK CO2 emissions is available in a Jupyter Notebook on GitHub. Spreadsheets are also available with all the underlying data in the same repository.

Electricity

Electricity use by sector and fuel are taken from the UK Department for Business, Energy and Industrial Strategy (BEIS) Energy Trends: electricity and historical electricity data. Total electricity sector CO2 emissions and emissions by fuel were obtained from the BEIS final UK greenhouse gas emissions national statistics: 1990-2016 and provisional 2017 numbers.

The “business-as-usual” (BAU) scenario created by Carbon Brief assumes that the grid mix (percent of electricity generated by each fuel) remained constant at 1990 levels.

Per-capita electricity consumption is assumed to stay constant at 2005 levels after 2005, with observed values used prior to this; this date was chosen rather than 1990 as per-capita electricity consumption increased between 1990 and 2005. This provides a conservative estimate of a counterfactual BAU by assuming that a growing population is the only driver of increased electricity use.

Population data used to calculate per-capita usage was obtained from the UK Office for National Statistics (ONS) population estimates.

The contribution of reduced electricity use to CO2 reductions was calculated by multiplying the difference in actual and BAU scenario electricity generation by month by the average BAU scenario grid mix emission factor for that month.

To calculate the relative contribution of wind, solar, bioenergy, nuclear and gas to CO2 reductions, their additional power output is assumed to proportionally displace the energy sources that declined relative to the BAU scenario. In other words, if coal and oil generation have decreased relative to the BAU scenario, new generation is assumed to replace each proportionate to their decline.

Transport

Data on vehicle miles travelled per month is from the UK Department for Transport roads and traffic data page. Vehicle fuel consumption and transportation greenhouse gas emissions are taken from the department’s energy and environment page.

Average vehicle fuel economy was calculated by dividing the total monthly vehicle fuel consumption (both petrol and diesel) by the total miles driven by all vehicles.

A BAU scenario was created by assuming vehicle miles travelled per capita remained constant at 2007 levels (between 1990 and 2007 per-capita vehicle miles travelled were increasing). Vehicle fuel economy was assumed to remain fixed at 1990 levels. CO2 emissions from non-vehicle transportation modes were assumed to remain constant at 1990 per-capita levels. CO2 reductions were calculated based on the difference between the BAU scenario and actual behavior.

Business, residential and other CO2 sources

Non-electric CO2 emissions from business, residential, public buildings and agriculture, land use and other sources are taken from the BEIS final UK greenhouse gas emissions national statistics: 1990-2016 and provisional 2017 numbers. A BAU scenario was created by fixing non-electric CO2 emissions for each sector at 1990 per-capita levels, or at the year at which per-capita emissions started declining if they increased after 1990. The year chosen as the start of the BAU scenario was 1990 for business, 1998 for residential, 1997 for public buildings and agriculture, 1990 for land use and 2005 for other sources. CO2 reductions were calculated by comparing the BAU scenario and actual emissions for each sector.