The Paris Agreement temperature goal is to keep global warming ‘well below 2°C’, and pursue efforts to limit it to 1.5°C. This goal provides clear guidance that the maximum level of warming should be as far below 2°C as possible, and that limiting it to 1.5°C would significantly reduce risks and impacts^[1],[2]. As the global average temperature has already warmed by 1°C since 1850, limiting warming to 1.5°C equates to only a further 0.5°C of warming.

To stop global warming at any level, total CO₂ emissions must be kept to within a specific carbon budget. Staying within a limited carbon budget can only be achieved if annual global CO₂ emissions are eventually reduced to net-zero levels – balancing sources and removal of emissions from the atmosphere.

To limit warming to 1.5°C, we need to reach net zero carbon emissions globally by the middle of this century; although if achieved, this still delivers only around a 50% chance that warming will not exceed 1.5°C^[3]. Limiting warming to 1.5°C will only be possible if global emissions peak within the next few years, and then start to decline rapidly, halving by 2030. To achieve this, we must transform all sectors of our economy and create a zero-carbon society^[3],[4],[5].

We already have the tools to implement most of the solutions. They include shifting away from fossil fuels and to renewables and other zero-carbon energy sources; cutting emissions from agriculture, including by enabling changes towards more plant-based diets; massively increasing energy efficiency measures to eliminate wasted energy; changing farming practices and land use; decarbonising industrial processes; creating zero carbon transport and heating systems; and embracing a ‘circular economy' of recycling and reusing.

Greenhouse gas emissions from human activities have already increased the surface temperature of the planet by about 1°C since around 1850^[1]. To avoid warming beyond 1.5°C – the lower end of the temperature target in the Paris Agreement – global emissions need to reach their peak immediately and then fall drastically within the next decade. If emissions stay at current levels, or continue to grow if we do not take action, global warming is likely to exceed 1.5°C between 2030 and 2050.

The impacts of 1°C of warming are already being felt across the globe and will become more severe with additional warming^[1]. Even with 1.5°C of warming, climate change will have dangerous effects, such as severe heatwaves that could affect more than 1 billion people once every five years on average^[2].

If warming goes beyond 1.5°C, humans face much more dangerous potential impacts. At 2°C of warming, 1.7 billion more people would be exposed to severe heatwaves at least once every five years than at 1.5°C^[2]. In addition, sea levels could rise by up to 43cm by 2100^[3], increasing the risks of flooding in coastal regions and cities in the UK and globally^[3].

If emissions continue to rise, the risk of reaching a level of warming that triggers irreversible effects would also grow. Destabilisation of the Greenland and West Antarctic ice sheets, for example, could eventually raise sea levels by up to 10 m globally, with profound effects for coastal cities and communities.

Halting global warming requires putting a limit on the total amount of CO₂ that human activities can emit, called a ‘carbon budget’. Humans have already emitted about 2,200 billion tonnes of CO₂ since preindustrial times^[1], [4]. We now have a small carbon budget of around 420 billion tonnes of CO₂ left to emit globally from 2018 onwards, if we want a 66% chance of limiting warming to 1.5°C, and 580 billion tonnes CO₂ for a 50% chance^[1], [5].

We currently emit about 42 billion tonnes of CO₂ per year globally^[1]. To stay within the global carbon budget, the world needs to halve its current emissions by 2030 and reduce net global CO₂ emissions to net zero by 2050; a challenge that can only be met if emissions reductions start now. If emissions do not decline rapidly to half their current level by 2030, they must decline faster afterwards and would likely require ‘negative emissions’, which means directly removing CO₂ from the atmosphere. This increases the risk of not stopping warming at the necessary level.

Another reason for acting on climate change immediately is the time it takes to overcome challenges when deploying new technologies and rolling out new policies. We can implement many potential policies for reducing greenhouse gas emissions to bring about substitution of renewables in place of fossil fuels; putting a price on greenhouse gas emissions; and introducing measures that encourage changes in dietary choices, boost energy efficiency and promote electric transport^{[6], [7], [5], [8]}.

These changes will come with new social, economic and environmental benefits. Switching to a more plant-based balanced diet, for example, will have benefits for human health; and more electric transport options will improve air quality. All these measures, however, also come with challenges that will make them more or less likely to succeed. We need to act on climate change immediately to gain the experience of what works and what needs adjusting.

Figure: CO₂ emission reduction pathways for limiting warming to below 1.5°C

Even if we put in place all possible ways to reduce global emissions immediately, it is unlikely we can phase out all the activities that emit CO₂ by the middle of the century; which is necessary to have at least a 50% chance of limiting global warming to 1.5°C^[1],[2],[3]. For some activities, like cement and steel production, aviation and shipping, we may not have the solutions needed to make them fully emissions-free by that time. That means we will need strategies to remove CO₂ from the atmosphere.

This is why we talk about ‘net zero’ rather than just zero emissions. Net zero refers to the idea of balancing emissions from human activities with ways of removing emissions from the atmosphere. A target for achieving net zero emissions clarifies that some ‘residual emissions’ will remain from sectors where it would be impossible to fully phase out their emissions by mid-century. This is either because we lack the solutions to do so, or because their activities by design imply emissions followed by removals, such as in sustainable forestry where felling a tree releases CO₂ and planting a new one works to absorb it again.

These residual emissions would then be removed from the atmosphere to reach net zero; what is called ‘negative emissions’. An example of a technology for achieving negative emissions is Direct Air Capture (DAC), which captures CO₂ from the atmosphere and transforms it to a form that can be used or stored. Planting more trees that soak up the excess CO₂ is another way to remove emissions from the atmosphere, although the effect would only take place years to decades after planting and would be vulnerable to events like forest fires.

The amount of negative emissions we will need depends on the global strategy for tackling climate change that the world’s countries choose to follow. If emissions decrease significantly in the next decade, countries can minimise the amount of CO₂ that would need to be removed in order to meet the Paris Agreement goals^{[4],[5],[2],[3]}.

Relying on negative emissions strategies also comes with risks. Many of the technologies are not yet commercially viable, and their large-scale use could use and their large-scale use could conflict with other goals for sustainable development^[6].

Bioenergy with carbon capture and storage (BECCS), for example, relies on planting large amounts of trees and plants that absorb CO₂ as they grow. The plant material is then burned to produce bioenergy, and the carbon dioxide that is produced is captured and stored instead of released into the atmosphere. However, this could require large amounts of land and create risks for biodiversity conservation, land rights or food production if not managed adequately^[7].

Getting to net zero by 2050 - which is necessary to stay below 1.5°C of global warming - requires very deep and rapid cuts in emissions, in all sectors of the economy.

However, some ‘residual emissions’ will likely remain from sectors where it will be difficult to fully phase out their emissions by mid-century. These residual emissions would need to be removed from the atmosphere to reach net zero; what is called ‘negative emissions’, or ‘greenhouse gas removal’ (GGR). We will need to both reduce emissions as well as remove residual emissions, not one or the other.

Many ways to reduce emissions are less costly and easier to roll out than greenhouse gas removal currently. These solutions include switching away from fossil fuels and towards renewable energy, improving energy efficiency and switching to electric vehicles, public transport and more walking and cycling. Actions to cut emissions at the source of where they come from is the most important thing to tackle climate change. 

However, responsible innovation and deployment take time. In order to have the option of using greenhouse gas removal processes at scale by 2050, work needs to start on developing the technologies and approaches to do it now. These approaches range from planting more trees and restoring soil in order to draw down carbon, to using engineered processes to capture carbon in the atmosphere and then store it underground.

Beyond helping us remove residual emissions from sectors that are challenging to fully reduce emissions for, greenhouse gas removal may also be needed in some countries as part of the global effort to get to net zero. It can help countries for whom reaching net zero emissions proves difficult.

Removals also open the possibility of reaching net negative emissions in the future, which would mean the world collectively taking more emissions out of the atmosphere than are being put in. This could begin to reverse some aspects of climate change, although some changes such as sea level rise would continue for decades to millennia.¹

All these potential uses for greenhouse gas removals are not a substitute for reducing emissions rapidly across all sectors. Many removal technologies are not yet commercially viable and face implementation challenges. At scale they may also conflict with other goals for sustainable development, such as access to water and land.²

However, many greenhouse gas removal methods are currently being tested. Pursuing the development of these methods must happen in an environmentally and socially robust manner, underpinned by appropriate governance and social permissions. 

If you would like to read more about greenhouse gas removals (GGR), you can find more FAQs on the CO2RE website.

References:

[1] IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth 

Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press. In Press. FAQ 5.3, p. 1272. 

2 IPCC, 2022: Summary for Policymakers. In: Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [P.R. Shukla, J. Skea, R. Slade, A. Al Khourdajie, R. van Diemen, D. McCollum, M. Pathak, S. Some, P. Vyas, R. Fradera, M. Belkacemi, A. Hasija, G. Lisboa, S. Luz, J. Malley, (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA. doi: 10.1017/9781009157926.001