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Why do we look at high warming levels when assessing UK climate risk?

In October 2020, we co-organised a conference on climate risks and adaptation in the UK which considered ‘higher levels’ of global warming. The CCC’s Adaptation Committee bases its assessments of climate risks and adaptation for the UK Government on two levels of warming; ‘2°C’ and ‘4°C’. But what do these mean, and why do we use them? The CCC’s Head of Adaptation, Kathryn Brown, explains.

Global warming levels, like 2°C and 4°C, describe a range of possible changes in the global average temperature over the course of this century. They are effectively shorthand for ‘limiting global temperatures to below 2°C above pre-industrial levels by 2100’ and ‘global temperatures reaching around 4°C above pre-industrial levels between 2080 and 2100’.[1] We consider these two levels as a sensible range of likely futures, based on current efforts to reduce greenhouse gas emissions to limit future climate change.

Governments around the world set out their pledges to reduce greenhouse gas emissions in what are known as ‘Nationally Determined Contributions’ (NDCs). Under current levels of ambition consistent with current NDCs, an analysis tool based on a simple climate model called the ‘Climate Action Tracker (CAT)’ estimates that global temperatures would end up somewhere between 2.3°C and 3.5°C by 2100, with a 90% chance of exceeding 2°C. In a future that is consistent with actual emissions reductions being implemented today, the CAT gives a higher upper level of global warming of 4.1°C by 2100. Other studies that try to estimate this change also suggest a wide range of outcomes, including the possibility of higher levels of warming than predicted by the CAT. This is a topic of ongoing research.

The hope is that countries will continue to ‘ratchet up’ their pledges to reduce emissions in line with the Paris Agreement, and seek to limit global temperature increase to less than 2°C by 2100. The best-case scenario is that if, globally, emissions start to decline sharply in the next few years, and reach global ‘Net Zero’ in the second half of this century, there is a 50% chance of limiting global temperature increase to 1.5°C. The UK’s 2050 Net Zero target represents, if met, its proportionate contribution to this global mitigation effort. However, it is the global effort that will determine global climate change impacts, including for the UK.

For UK adaptation planning, we therefore consider risks for a 2°C rise in global temperature as our minimum global warming level[2], and 4°C as a likely upper range. Our job is to then consider the potential impacts for hazards like flooding, drought, fire, and heatwaves, and give advice to the Government on how well it is planning for these risks through its domestic adaptation policies.

The risks (and opportunities) are set out every five years in the UK Climate Change Risk Assessment (CCRA). The next Evidence Report which underpins the CCRA, due in 2021, is being coordinated by our Adaptation Committee. Our last assessment from 2016 is available here. We also assess progress in adapting to climate change in England every two years. To do that, we look at the quality of plans based on whether there is any evidence of preparations for the minimum 2°C warming level, and consideration of the 4°C level. We don’t necessarily expect measures to be put in place wholesale for a 4°C world now. But thinking still needs to be done today about what is possible in terms of adaptation in that world, and when difficult decisions would need to be taken to start putting measures in place. Our latest report to Parliament shows that we are not yet even prepared for what is; the 2°C scenario, let alone what could be; the 4°C scenario.

Beyond the 2/4°C range, we are also paying particular attention to low likelihood, high impact events in our work to assess climate risk as part of the CCRA3 Evidence Report. These include abrupt or very fast rates of climate change, and irreversible changes, some of which could then accelerate global warming further. Such events are not confined to a 4°C world. We are seeking to gather as much information as possible about more extreme climate change, including these sorts of changes as well as impacts at higher levels of warming.

Thinking about climate change at high levels of warming, like 4°C, is an emotive issue. In my experience, people can be turned off climate change adaptation completely due to this focus on something they are working so desperately to avoid. It can seem very defeatist to think about a world where we fail to reduce greenhouse gas emissions enough. At the same time though, planning for climate risk is the same as planning for any other risk; we have to think about the full range of possibilities – Covid-19 has taught us that in spades. In doing so, we are helping to see how the important things that people care about can be protected and sustained in a range of futures, whether that is nature, business sustainability, or safe and reliable infrastructure. Fundamentally, adaptation is about the messy business of trying to work out how we can live better given our changing climate, and that is hugely motivating. It deserves much more attention, much greater investment, and much greater urgency.

[1] Global temperature is already around 1.2°C warmer than pre-industrial levels today.

[2] There is very little evidence for what impacts will look like in the UK for 1.5°C, and adapting to a 2°C world will also enable adaptation for 1.5°C.