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Why is achieving net zero necessary?

“Human activities, principally through emissions of greenhouse gases, have unequivocally caused global warming.” This quote is not from Greta Thunberg or Al Gore: it is the conclusion of the most recent Intergovernmental Panel on Climate Change (IPCC) assessment of the science related to climate change – an exercise that took four years, considered over 14,000 scientific publications, was reviewed by 1,891 experts and finally approved by 195 countries.

Despite this well-established consensus, climate change is still sometimes incorrectly and misleadingly portrayed as being subject to debate, for example by giving people who do not accept the evidence for the link between human activity and climate change as much airtime as climate scientists.

We need to limit global warming to avoid dangerous impacts

Climate change brings with it some very serious risks, including to human health, food production, livelihoods, culture and ways of life, homes and infrastructure, and ecosystems. It also heightens the risk of abrupt or irreversible changes to physical systems such as the Atlantic Meridional Overturning Circulation (AMOC), a system of ocean currents that affects our climate in the UK, and of triggering dangerous feedback loops, such as the release of methane trapped in permafrost which would lock us into further warming. What is more, these risks are not distributed equally: some people and places face particularly acute risks.

The science is very clear: the level of risk escalates with every increment of global warming. Even at just 1.5°C above the pre-industrial average some impacts are set to become widespread and persistent. At higher levels of warming, impacts will further intensify.

Meeting the temperature goals depends on how fast we reduce emissions

Under the United Nations Framework Convention on Climate Change (UNFCCC), countries come together with the objective of preventing dangerous human-caused interference with the climate system. The Paris Agreement, agreed under the UNFCCC, is a legally binding international treaty with an overarching goal to hold “the increase in the global average temperature to well below 2°C above pre-industrial levels” and pursue efforts “to limit the temperature increase to 1.5°C”. The 1.5°C and 2°C figures are not arbitrary: they are informed by scientific understanding of the levels of risk associated with different levels of warming and indicate where a ‘defence line’ should be drawn.

To meet the Paris targets, we need to stop adding greenhouse gases to the atmosphere, carbon dioxide in particular. Only when we reach ‘net zero’ will the risks stop growing; this is the point at which emissions are reduced as much as possible and remaining emissions are compensated for by actively removing carbon dioxide from the atmosphere.

Scientists build ‘scenarios’, or emission reduction ‘pathways’, to understand how quickly we need to reduce emissions to cap temperature rise at 1.5°C. This is what underpins the Paris Agreement’s aim to “reach global peaking of greenhouse gas emissions as soon as possible” and “to undertake rapid reductions thereafter”, ultimately achieving a balance between human-caused emissions and removals of greenhouse gases from the atmosphere in the second half of this century.

The UK’s own net zero target, to bring all territorial greenhouse gas emissions to net zero by 2050, was based on the scientific advice of the independent Climate Change Committee and represents an “appropriate contribution” from the UK to meeting the Paris Agreement goal.

We will have to live with some climate impacts but can reduce further risk by moving away from fossil fuels

Because our emissions increase carbon dioxide in the atmosphere for thousands of years, historic emissions have already locked in some climate impacts. This means that even if we reached net zero tomorrow, we would still have to adapt to impacts already committed to. Nevertheless, when net zero greenhouse gas emissions are achieved, scientists predict that global temperature increases will peak and then slowly decline, over a period of hundreds of years. In the meantime, climate impacts — such as sea level rise from melting ice sheets — may be slowed down, but will continue to occur for many centuries.

Some argue that since some impacts of climate change (like sea level rise) cannot be stopped immediately, we should not bother reducing emissions and instead should focus on adaptation. The problem with this perspective is that it ignores the fact that by continuing to emit greenhouse gases, impacts will become more and more extreme and the cost of adapting also increasingly greater. At a certain point the impacts may become so extreme that it is not physically possible to adapt. For example, water scarcity could mean people can no longer continue living in certain places. Reducing emissions will not only slow down these impacts: it will also limit their ultimate magnitude. Furthermore, it will reduce the risk of triggering reinforcing feedback loops, the consequences of which could be very serious indeed.

It is clear that there is not a choice between mitigation and adaptation: we must do both. To avoid dangerous levels of climate change we must move away from burning fossil fuels and bring down emissions to net zero as quickly as possible. At the same time, it is essential that we protect our societies by adapting to the warming and impacts that are already locked in because of emissions from the past.

Authors and contacts

This background briefing was written by Joeri Rogelj, Imperial College London, with editing and review by Caterina Brandmayr, Georgina Kyriacou and Esin Serin.

It was produced as part of a UK-focused ‘myth-busting’ project between the LSE and Imperial College London Grantham Institutes. The series of ten explainers will be published as a single volume in spring 2024. The project is designed to deepen understanding of climate change action among current and prospective decision makers, the policy community and the public in the UK in the run-up to the 2024 general election.

Media enquiries: grantham.media@imperial.ac.uk
Policy enquiries: c.brandmayr@imperial.ac.uk