5.2 Cuts in emissions

But exactly how dramatically do carbon dioxide emissions need to fall to meet the Paris Agreement targets and limit the increase in global mean surface temperature to well below 2 °C, with the aim of 1.5 °C? The IPCC have stated that ‘without immediate and deep emissions reductions across all sectors, limiting global warming to 1.5 °C is beyond reach’ (IPCC, 2022), and they have produced a set of projections to show just how immediate and deep these reductions need to be (see Figure 12).

Figure 12 IPCC projections for future carbon dioxide and methane reductions, as well as the total greenhouse gas emission reductions necessary to limit global temperature rise to (a) 1.5 °C and (b) below 2 °C, by 2100 (adapted from IPCC, 2023). The dashed lines show emissions projections based on policies as of 2020 (i.e. in a ‘business as usual’ scenario).

Carbon dioxide emissions need to fall rapidly to limit warming to 1.5 °C by 2100 (Figure 12(a)), and even to keep below 2 °C warming (Figure 12(b)). For 1.5 °C, this means reaching net zero by around 2050 and then falling further below net zero (meaning that more carbon dioxide would need to be absorbed or captured from the atmosphere than is emitted). Even the lesser target of keeping below 2 °C means reaching net zero just 20 years later, in 2070.

Figure 12 also shows the methane reductions needed to contribute to the overall reduction in greenhouse gases. This reduction is less drastic than for carbon dioxide but is likely to be more challenging to achieve. About 40% of methane emissions are associated with food production; reducing these will be difficult because it will require lifestyle changes, with a move away from meat (particularly lamb and beef) and dairy products to a more plant-based diet. Fortunately, some methane emissions are easier to tackle because they are directly associated with fossil fuel use – as this falls, so will methane emissions.

Box 2 Net zero emissions

As discussed earlier, net zero emissions are core to international climate change agreements and many national policies. But what does net zero mean in practice?

It is fairly straightforward to define in one sentence: the point at which greenhouse gas emissions from human activity are balanced by greenhouse gases removed from the atmosphere by human intervention.

This balance cannot be achieved without a large reduction in greenhouse gas emissions from human activity, and this will need to be a focus to achieve net zero.

Solutions to reduce emissions fall into several categories:

increased efficiency – using technologies that use energy and resources more efficiently, so that waste is reduced
reducing and stopping deforestation
a ‘transition away from fossil fuels’ – rapidly increasing the use of renewable energy sources such as wind and solar power, together with modest increases in hydro power and nuclear electricity generation
behavioural changes – millions of individuals making small but significant changes, such as moving to a more plant-based diet, using public rather than private transport, avoiding flying and buying more refurbished or second-hand goods rather than new ones
carbon capture, utilisation and storage (CCUS) – capturing carbon dioxide from power stations and industrial processes before it enters the atmosphere, and either using it or burying it deep underground.

Although the definition of net zero refers to all greenhouse gases, in practice the removal of these from the atmosphere centres around carbon dioxide. Methods for increasing this side of the net zero balance include:

planting new woodlands and forests
restoring natural landscapes, such as wetlands
enhancing other natural processes (known as carbon geoengineering)

Some overall removal of carbon dioxide from the atmosphere will be necessary to offset those greenhouse gas emissions that are harder to deal with. These include methane emissions from agriculture and carbon dioxide associated with the production of cement. The need to combat these emissions helps to explain why, in Figure 12, projected overall CO₂ emissions fall below net zero to become negative.

As described in Section 2, once carbon dioxide has been emitted into the atmosphere, some of it will stay there for a century or more. So what will happen to the concentration of carbon dioxide in the atmosphere if the world can move towards net zero carbon emissions?

Figure 13(a) shows three possible scenarios for future global CO₂ emissions reductions. The historical emissions (the black line on the graph) are from fossil fuel combustion and exclude those from land use change and deforestation (as these data are less certain than those for emissions from energy consumption). This accounts for the approximate 5 Gt difference in 2020 in Figure 13(a) compared to Figure 12. Figure 13(b) shows the consequences for the concentration of CO₂ in the atmosphere of the three emission pathways (i.e. future projections of the Keeling Curve).

Figure 13 (a) Three possible future scenarios of global CO₂ emission reductions (based on the energy sector); (b) their projected effect on the future concentration of CO₂ in the atmosphere and the resulting rise in global mean surface temperature by 2100 (historical CO₂ emissions and CO2 concentration data: Climate Interactive, 2023; 1.5 °C and 2 °C emissions pathways: IPCC, 2023; CO₂ concentration projections: author’s estimates using the C-ROADS climate change policy simulator)

The three scenarios shown in Figure 13 are described below.

Constant CO₂ emissions of 37 Gt per year
If global emissions could be held at their 2022 level for the remainder of the century (the red line in Figure 13(a)) then the Keeling Curve – atmospheric CO₂ concentrations – would continue to rise at its 2022 rate (the red line in Figure 13(b)). By 2100, the atmospheric CO₂ concentration would have increased by 50% from its 2022 value, reaching 630 ppm. The global mean surface temperature would then be around 3 °C above its pre-industrial level, and still rising.
Limiting global mean surface temperature rise to 2 °C
The orange line in Figure 13(a) shows a possible emissions trajectory to achieve this. In this scenario, CO₂ emissions would need to fall rapidly, down to one-sixth of their 2022 value by 2055 and reaching net zero by 2070. The Keeling Curve, shown by the orange line in Figure 13(b), would flatten out, reaching about 470 ppm by 2100. The global mean surface temperature rise would stabilise at approximately 2 °C above pre-industrial levels.
Limiting global mean surface temperature rise to 1.5 °C
This will require immediate drastic cuts in emissions, as shown by the green line in Figure 13(a). Global CO₂ emissions will need to be reduced by a factor of six by 2040 and down to net zero around 2050. The Keeling Curve, shown by the green line in Figure 13(b), would peak at around 435 ppm in about 2035 and then fall slowly to about 405 ppm by 2100. The global mean surface temperature rise would stabilise at approximately 1.5 °C above pre-industrial levels. The effects of this are likely to be long-lasting and the consequent environmental benefits are likely to be experienced well into the 22nd century.

How likely are these different scenarios? As discussed in the previous section, many countries have committed to net zero carbon emission policies. If these ambitions are realised, and more countries follow suit, then a future that limits warming to 2 °C or lower could be on track. These drastic cuts in emissions, including ‘negative emissions’, will need to happen across all sectors, but one of the most important (since it is intrinsic to our daily lives) and largest (in terms of CO₂ emissions) is the energy sector. Although this – and in particular the technology solutions – will be the focus of the next section, actions to reduce emissions are still vital across all sectors, including agriculture.

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