Overshoot likely needed to keep to 1.5C rise by end of century

Could the world still be on track to keep global temperature rise to 1.5C above pre-industrial levels by the end of this century? 

Yes, with ambition it could be. But, increasingly, this almost certainly involves some form of ‘overshoot’ where temperature rise exceeds the 1.5C threshold before coming back down nearer the end of the century. If this happens there are several apposite questions, including: the duration of any overshoot; how far above 1.5C any overshoot could be; and consideration of what the wider environmental impacts could be. 

Any overshoot of 1.5C above pre-industrial levels will place increasing stress on parts of the environment, including the cryosphere, such as here in Antarctica. Picture: Shutterstock.

On Monday 4 April, the IPCC will be publishing the third part of its AR6 assessment – the most complete review of climate science in nearly a decade. Working Group III will present the scientific evidence around mitigation – the considerations about how to avert the worst impacts of climate change by slowing the rate of greenhouse gas emissions, and even removing the greenhouse gases from the atmosphere which are causing the planet’s temperature to rise. 

Dr Andy Wiltshire is the Met Office Head of Earth System and Mitigation Science. He said: “According to the IPCC’s latest assessment, the earth’s temperature is around 1.1C above pre-industrial levels – generally recognised as the period between 1850 and 1900. This really doesn’t give us much headroom to avoid breaking one of the Paris Agreement ambitions of not exceeding 1.5C. 

“At COP26 in Glasgow, last November, countries updated their individual pledges to curb greenhouse gas emissions. When totted up these pledges – known as Nationally Determined Contributions (NDCs) – take us a certain way towards averting the worst impacts of climate change, but analysis shows barely a chance that any of the current climate change scenarios will get us to a point where we can prevent peaking above 1.5C for a period of time without more urgent and immediate action.”  

Andy and his team have been working on an academic paper using scenarios prepared by the IPCC for its SR1.5 report in 2018. Andy added: “Our study reveals that it is extremely difficult to find a scenario consistent with the CoP26 pledges that can keep the world from rising above 1.5C with confidence. In this set of scenarios, meeting the COP26 pledges likely implies an overshoot of 30 to 70 years, reaching an average value of around 1.7C before coming back down to around 1.5C at the end of the century.” 

Although feasible, the overshoot trajectories come with added risks for climate impacts along with the apparent optimism. These include:  

  • Reliance on technologies untested at scale:
    It relies on more aggressive action to tackle atmospheric greenhouse gas concentrations to help bend the curve downwards after the temperature peak. Increasingly this is likely to require the use of technologies to ‘suck’ extra greenhouse gases out of the atmosphere; If the greenhouse gas removal can’t be scaled the world will stay above 1.5C for longer. 
  • Commitment to higher long-term climate risks:
    By the end of the century sea levels will have risen more than without an overshoot because of the period of enhanced warming. Additionally, the Cryosphere – the ice world – will have lost even more volume due to the higher temperatures. Furthermore, some impacts, such as loss of species, will be irreversible. 
  • Exposure to a greater chance of passing key climate tipping points:
    Whilst we don’t know the precise trigger points for large-scale climate tipping points even a temporary incursion above 1.5C will take us closer to thresholds which could initiate tipping points. 

Prof Jason Lowe OBE is the Head of Climate Services at the Met Office Hadley Centre. He said: “Tackling climate change is ultimately about trying to prevent every fraction of a degree rise in the earth’s global temperature, and adapting to the fraction of warming that is unavoidable. Every delay in reducing emissions increases the risks from climate impacts and increases the reliance on technologies for enhanced carbon-dioxide removal that remain unproven at a large-scale. 

“There is reason to be optimistic. Pledges made during CoP26 have further bent the curve downwards and the ratcheting mechanisms under the Paris Agreement are taking effect. “However, we are increasingly looking to a future where we will have to actively remove the greenhouses gases that we currently emit into the atmosphere if we want a 1.5C warming limit.”

The Working Group III report will provide further insight into the range of options available to reduce emissions, including their costs and feasibility at scale.  It will also help us further understand how to develop climate resilient development pathways that consider mitigation, adaptation and other societal development needs in a more holistic way.  

Andy Wiltshire concluded: “Emission reductions are inescapable if we are to avoid the worst impacts from climate change.  Every year delay in reaching peak emissions increasingly commits future generations to a technological solution that is currently untested at the scale that will be required.” 

Met Office science increasingly focuses on making a resilient net zero future a reality. Our expertise can improve the deployment and operation of renewable energy generation. Our ability to simulate the earth system provides tools that can help us better work with the natural system to take up carbon, methane and other greenhouse gases. Our observational datasets can help us monitor progress towards a net zero future, with our inversion-monitoring approach helping to identify emission sources.   

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