Significant warming could still occur after the world reaches net-zero carbon emissions, warn the authors of a new study.
The study, led by Imperial College London and University of Exeter researchers, suggests that high uncertainty in how the climate system will continue to react to global warming even after emissions are reduced means urgent action is imperative.
The possibility of future climate risks in a net zero world makes the need to limit our initial disturbance to the planet even more imperative. Sofia Palazzo Corner
Published today in Frontiers in Science, the research maps uncertainty in climate modelling beyond net zero, and proposes a framework for better predicting global warming risk in the long term.
The article presents the first comprehensive analysis of the many factors controlling global temperatures after net zero and provides a framework for improving warming predictions. Lead author and research postgrad on the NERC Science and Solutions for a Changing Planet DTP at Imperial, Sofia Palazzo Corner, said: "Our analysis identifies the levers of global warming after net zero, and explains why current estimates are so uncertain.
"The possibility of future climate risks in a net zero world makes the need to limit our initial disturbance to the planet even more imperative. Crucially for policy, a world that expects warming to continue after net zero will have an even smaller carbon budget to keep total warming below 1.5°C."
The levers of global warming
To limit the effects of global warming, CO2 emissions from human activities must reach to 'net zero' – where the amount of CO2 we emit into the atmosphere is equal to the amount we remove from it. To keep within the 1.5°C limit of the 2015 Paris Agreement, this needs to happen as soon as possible.
The scientific community's current best estimate from models is that global warming will stop at net zero, but there are substantial uncertainties.
Professor Joeri Rogelj, from the Centre for Environmental Policy and the Grantham Institute at Imperial, said: "Uncertainties in climate models mean there is a non-negligible chance that global warming will continue after net zero and intensify dangerous climate change. We must take a precautionary approach and drastically reduce emissions now with the goal of achieving net zero CO2 as soon as possible and preventing climate harm to future generations."
Global temperatures are regulated by multiple natural processes and feedbacks in the oceans, land, and atmosphere. CO2 emissions have influenced many of these, triggering long-term changes which could last for centuries after net zero is reached.
Professor Martin Siegert of the University of Exeter said: "The melting of ice in polar regions is one example. As we have observed in the Arctic Ocean, and recently in the Antarctic, a thin layer of floating ice helps reduce global temperatures by reflecting the sun's energy back into space. However, once this ice melts this reflection is replaced by absorption of solar energy, which drives temperatures even higher."
Even current climate models show these processes could cause significant warming after net zero – with an estimated 1 in 6 chance this warming could exceed 15% of total global warming. This means that if global temperatures have risen by 2°C at the point we reach net zero, the final temperature change could be above 2.3°C.
"Warming of this magnitude would worsen major climate risks to communities across the world, and particularly in the most vulnerable regions," added Professor Siegert.
Preventing climate harm to future generations
Despite persistent progress in the field, exploring the full range of climate change risks is challenging. Models are already very expensive to run, and every added process further increases the computational burden.
The international team – which additionally includes researchers from the Lawrence Berkeley National Laboratory, Melbourne University, and the Max Plank Institute – identified 26 distinct processes, of which more than half could drive significant warming.
One example is a decline in land carbon uptake. Plants are important mitigators of global temperatures since they use CO2 during photosynthesis. But other climate processes, such as changing rainfall patterns, droughts, and heatwaves, can reduce the efficacy of this 'carbon sink'.
Palazzo Corner, also from the Centre for Environmental Policy at Imperial, said: "We have drawn on expertise across climate science to build a catalogue of processes that could affect global temperatures in a net zero world, but we need to better understand their potential impact. We propose a set of key research activities to reduce this uncertainty and improve warming predictions as quickly as possible."
Professor Rogelj said: "We are working on building better models, but should not wait until they are perfect before we act. We understand what needs to happen in the next decades and that is to limit the amount of CO2 we put into the atmosphere as much as possible."
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"The Zero Emissions Commitment and climate stabilization," by Palazzo Corner et al. is published in Frontiers in Science.
