The world is experiencing the catastrophic repercussions of global warming, which include intense heat waves, deluges of rain, and catastrophic storms. Since global warming is an inevitable consequence of burning fossil fuels, the only strategy we have to stop it is to bring human activity's CO2 emissions dow n to 'net zero,' or the point at which the amount of CO2 we release into the atmosphere and the amount we take out of it is equal. This is a necessary step to stay inside the 2015 Paris Agreement's 1.5degC limit.
Even though the scientific community believes global warming will halt at net zero, a Frontiers in Science essay raises doubts. "These estimates come with substantial uncertainty, meaning there is a non-negligible chance that global warming will continue after net zero and intensify dangerous climate change," said Prof Joeri Rogelj at Imperial College London and one of the international team of authors. "Worldwide emissions reduction plans overlook this important risk, which should be urgently addressed at COP28."
Report provides a framework for improving warming projections
The article presents the first comprehensive analysis of the many factors controlling global temperatures and provides a framework for improving warming predictions. "Our analysis identifies the levers of global warming after net zero and explains why current estimates are so uncertain," said lead author Sofia Palazzo Corner, also of Imperial College London.
"The potential of future climate risks in a net zero world makes the need to limit our initial disturbance to the planet even more imperative," she added.
"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 1.5degC." Prof Michael Mann at the University of Pennsylvania said that despite its alarming message on the prospect of continued global warming, this study offers hope.
"It reminds us that the obstacles to climate action are neither physical nor technological. At this point, they remain political. And history teaches us that political obstacles can be overcome," he wrote in an editorial accompanying the article, also published in Frontiers in Science.
Multiple natural factors influence Earth's temperatures
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 that could last for centuries after net zero is reached.
"The melting of ice in polar regions is one example," explained Prof Martin Siegert of the University of Exeter, another of the study's authors. "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 into space. However, once this ice melts this reflection is replaced by absorption of solar energy, which drives temperatures even higher."
Present climate models predict processes will generate significant warming post-net zero
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 per cent of total global warming. This means that if global temperatures have risen by 2 degrees C at the point we reach net zero, the final temperature change could be above 2.3 degrees C.
"Warming of this magnitude would worsen major climate risks to communities across the world, and particularly in the most vulnerable regions," said Siegert.
Need for a collaborative effort
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. "We need a collaborative effort between diverse climate experts to develop a suite of scientific tools that allow us to more deeply explore and understand the level of global warming we can expect. Our study takes the first step by mapping all processes that affect global temperatures and estimating their impact across millennia," said Rogelj.
The international team, including 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'.
"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," said Palazzo Corner.