Mitigation: reducing greenhouse gas emissions and capturing carbon

Overview

Two broad routes to mitigation, how the energy supply can be decarbonised and case for switching from coal to gas.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is mitigation?

   Model answer

   Mitigation means action taken to tackle the cause of climate change: reducing the amount of greenhouse gas in the atmosphere, and so limiting how much warming eventually occurs. It works either by cutting emissions at source or by removing greenhouse gases and storing them. Carbon capture and the reduction of greenhouse gas emissions are the two headline components. Its defining characteristic is that it acts on the driver rather than on the consequences, so its benefits are global and shared regardless of where the action is taken, and they appear only after a long delay because of the inertia in the climate system - which is precisely why it is politically difficult to secure.

2. 2

   What are the two broad routes to mitigation?

   Model answer

   Reducing the sources and enhancing the sinks. Reducing sources means emitting less: decarbonising electricity generation, using energy more efficiently, electrifying heating and transport, changing industrial processes, cutting methane leakage, and altering agricultural practice and diets. Enhancing sinks means increasing the removal of carbon dioxide already in the atmosphere and storing it securely: afforestation and reforestation, restoring peatland, mangrove and saltmarsh, building soil carbon, and engineered removal such as capturing carbon dioxide from bioenergy plants or directly from the air. Reducing sources is much the larger and more reliable of the two, because sinks are limited in capacity, compete for land and can be reversed by fire, felling or drought.

3. 3

   Explain how the energy supply can be decarbonised, and discuss the difficulties.

   Model answer

   By replacing the combustion of coal, oil and gas with sources that emit little or no carbon dioxide: wind both onshore and offshore, solar photovoltaic, hydroelectricity, geothermal, tidal and wave, biomass, and nuclear fission. The costs of onshore wind and solar have fallen dramatically and they are now the cheapest source of new electricity in many markets, which has made rapid deployment possible. The difficulties are that wind and solar are intermittent and variable, so a system relying on them needs storage, interconnection, demand management or firm backup capacity; that they are land- and material-intensive and attract local opposition; that hydro and nuclear involve very large capital costs and long lead times, with nuclear also raising waste and public acceptance issues; and that electricity is only part of energy use, with heat, heavy industry, shipping and aviation much harder to decarbonise.

4. 4

   How does reducing energy demand contribute to mitigation, and what are the main measures?

   Model answer

   Every unit of energy not used is a unit that does not have to be generated, so demand reduction avoids emissions immediately and reduces the amount of low-carbon supply that has to be built. Measures include insulating walls, lofts and windows and building to high standards for new construction; efficiency standards for appliances, lighting, motors and vehicles; combined heat and power and district heating that use waste heat instead of discarding it; recovering and recycling materials, since recycled aluminium and steel need a fraction of the energy of primary production; and shifting travel from car and plane to rail, bus, cycling and walking through planning and pricing. Efficiency is generally the cheapest form of mitigation, often paying for itself, though savings can be partly offset if people respond to lower running costs by consuming more.

5. 5

   Why does electrifying heating and transport only reduce emissions under one condition?

   Model answer

   Because an electric vehicle or a heat pump does not generate emissions itself; it moves the emissions to wherever the electricity is made. If the grid runs on coal, the emissions are merely relocated and, for some uses, may not fall much at all. The condition is that the electricity supply must be decarbonised, and the two changes have to proceed together. Where the grid is largely low-carbon, the gain is substantial and increases automatically over the life of the vehicle or heat pump as the grid cleans further, and the efficiency of the technologies helps in any case - an electric motor converts far more of its input energy into motion than an internal combustion engine, and a heat pump delivers several units of heat per unit of electricity.

6. 6

   What is the case for switching from coal to gas, and what is the caveat?

   Model answer

   Burning natural gas releases roughly half the carbon dioxide of coal for the same amount of electricity generated, because methane has a higher hydrogen to carbon ratio and gas plant is more efficient; it also removes the sulphur dioxide, particulates and mercury that make coal so damaging to health. Gas plant is quick to build and flexible, so it can balance intermittent renewables. The caveats are that it is still a fossil fuel, so it cannot be an end point, only a transition; that building new gas infrastructure risks locking in emissions for decades of asset life; and, crucially, that methane leaking unburned from wells, pipelines and processing is itself a powerful greenhouse gas, so a leakage rate of only a few per cent erodes much of the advantage over coal.

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