Dangerous overshoot of 1.5°C warming threshold must be limited, Imperial scientist warns in UN report

Global temperatures are set to cross 1.5°C above pre-industrial levels within years, leading to an escalation of climate impacts, the UN Environment Programme has warned.

In a landmark report, co-authored by Imperial College London’s Professor Joeri Rogelj, scientists concluded that years of inaction and delay mean that the Paris Agreement’s critical 1.5°C warming threshold will be crossed, likely within years, and that the world must return below it as quickly as possible to minimise the consequences.

The report, Limiting Overshoot, warns that climate impacts, both sudden and progressive, will intensify with every fraction of a degree above 1.5°C, and that remaining above the threshold increases the likelihood of crossing irreversible tipping points including the destabilisation of major ice sheets, degradation of the Amazon rainforest and disruption of the Atlantic Meridional Overturning Circulation, which gives north-western Europe its mild climate.

Above 1.5°C, more extreme weather, ecosystem loss, and the submersion of some island states and low-lying coastal cities is anticipated, with severe damage also expected to human health, food security, water supplies, infrastructure, and economies.

Greenhouse gas emissions, primarily from the burning of fossil fuels, have already pushed global temperatures to around 1.4°C above pre-industrial levels, leading to intense and prolonged heatwaves, droughts, floods and storms around the world and an acceleration of biodiversity loss, glacial melt and mass coral bleaching.  

A Grantham Institute study cited in the report found that human-caused climate change tripled the number of heat-related deaths arising from last summer’s European heatwave, while in May and June this year, around 2,700 people are estimated to have died in England and Wales due to a series of unprecedented heatwaves.

Professor Rogelj, Director of Research at Imperial College London’s Grantham Institute – Climate Change and the Environment and Professor of Climate Science & Policy at the Centre for Environmental Policy, was lead author on the UNEP report’s opening chapter, Entering an era of 1.5°C overshoot, which finds that national climate pledges are currently “nowhere near sufficient” to prevent overshoot and that implementation of existing pledges is weak.

“Overshooting 1.5°C is a painful reminder that failing to cut greenhouse gas emissions has consequences we can now no longer avoid, only limit,” says Professor Rogelj. “Our response requires mitigation and adaptation to move together: near-term emissions reductions limit overshoot and reduce harm, but weak mitigation will push communities and ecosystems beyond their ability to adapt.”

Overshoot, peak and decline

The report finds that an ‘overshoot, peak and decline’ pathway is now the best remaining option. This would minimise peak temperature rise through immediate and sustained cuts to greenhouse gas emissions and bring down temperatures through long-term net-negative emissions. It also requires adaptation actions to reduce vulnerability to the current, expected and unexpected impacts on societies, communities and economies.

Carefully governed Carbon Dioxide Removal – the process of removing carbon from the air and storing it through afforestation or other means – will be an essential part of the mix, the report concludes, but it warns that bringing temperatures back down will be complex and uncertain.

Some impacts will be irreversible on human timescales regardless of how quickly temperatures decline. While reversing global warming can slow future sea-level rise, for example, sea levels will remain elevated above present-day levels for thousands of years, and glacier loss will continue for centuries after temperatures are stabilised.

The most optimistic scenario would see temperature rise peak at 1.7 to 1.8°C above pre-industrial levels, though most other scenarios predict higher peaks. Strong political will and multilateralism, effective policies, inclusive governance, and finance are all important enabling conditions for action – and equity and justice must underpin all responses, the authors say.

“There are no good outcomes if we remain above 1.5°C,” says Inger Andersen, Executive Director of UNEP. “We can – and must – get on the right path by cutting greenhouse gas emissions, strengthening resilience and adaptation, and ensuring exceedance of 1.5°C is as small and short as possible.”

António Guterres, Secretary-General of the United Nations described the report as a stark warning that the world is failing on the commitment made in the Paris Agreement to avoid the worst climate devastation.  

"We must make the overshoot as small and short as possible – limiting how high temperatures rise and how long they remain above 1.5°C,” he said. “That demands an overshoot of ambition: accelerating the phaseout of fossil fuels and the renewables revolution; slashing methane pollution; and protecting land, forest and oceans.”