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<\/div>Carbon emissions have pushed the world to the brink of 1.5\u00b0C of warming<\/p>\n
Dennis MacDonald\/Shutterstock<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n
It is clear that the world is going to exceed the 1.5\u00b0C target for global warming<\/a>, leading to an increasing focus on plans to cool it down again by removing carbon dioxide from the atmosphere. But there is no guarantee that we will be able to achieve this \u2013 and even if we can, some changes can\u2019t be reversed.<\/p>\n \u201cDeaths are not reversible,\u201d says Joeri Rogelj<\/a> at Imperial College London. The focus needs to be on urgent emissions cuts to limit warming now, he and his colleagues are warning after studying various \u201covershoot\u201d scenarios.<\/p>\n There are at least five big problems with the idea of overshooting climate targets and then cooling the planet back down, according to their study. The first is that many such scenarios give a misleading picture of the uncertainties and risks involved.<\/p>\n For instance, in its last major report, the Intergovernmental Panel on Climate Change (IPCC) looked at an overshoot scenario in which the world reached 1.6\u00b0C above preindustrial levels by around mid-century, just 0.1\u00b0C past the limit in the Paris Agreement. But because of uncertainties in how global temperatures will change in response to a given amount of CO2 in the atmosphere, the level of emissions assumed in this scenario could in fact result in anything up to 3.1\u00b0C of warming.<\/p>\n <\/p>\n \u201cFor the same emission levels, there would be about a one in 10 chance that warming exceeds 2\u00b0C,\u201d says Rogelj. \u201cA one in 10 chance of a potential existential threat is not small.\u201d<\/p>\n The second issue is that there is no guarantee warming will stop<\/a> even if we stop adding CO2 to the atmosphere, so-called net-zero emissions.<\/p>\n For instance, warming could trigger stronger positive feedback effects<\/a> than expected, leading to higher-than-projected emissions of carbon from, say, peat and permafrost, precipitating continued rises in global temperature even after we reach net zero.<\/a><\/p>\n What\u2019s more, achieving net zero requires removing CO2 from the atmosphere, because for some activities such as farming, there may not be any way of reducing their emissions to zero. But there might be no affordable way to remove large enough quantities of CO2<\/a> from the atmosphere to compensate.<\/p>\n That is also the third big problem with overshoot scenarios. Cooling the planet after reaching net zero requires the removal of massive quantities of CO2, above and beyond those required to simply maintain net zero.<\/p>\n Even if the technology could be developed to do this, governments may baulk at the costs of something that, at least in the short term, is pretty much all loss and no gain. \u201cIn most cases the only benefit of carbon dioxide removal is that it removes carbon,\u201d says Rogelj. \u201cBut otherwise it uses energy, it costs money, it requires investment and long-term planning.\u201d<\/p>\n Such carbon removal could also have some disastrous side effects<\/a>. For instance, one idea is to grow energy crops and burn them in power plants<\/a>, capturing the carbon released during combustion and locking it away. But forests are still being cleared to create farmland for growing food and biofuel. Growing crops for carbon capture would make this problem worse and lead to even more habitat and biodiversity loss.<\/p>\n The fourth problem is that even if we do manage to remove enough CO2 to get temperatures back down again, it is going to take decades, says team member Carl-Friedrich Schleussner<\/a> at the International Institute for Applied Systems Analysis in Laxenburg, Austria. That means we are still going to have to adapt to the higher temperatures while they last.<\/p>\n Yet as the last IPCC report pointed out, even adapting to the relatively small changes so far is proving more difficult than<\/a> expected. \u201cWe have an overconfidence in our ability to adapt to [an] overshoot,\u201d says Schleussner.<\/p>\n The fifth issue is that bringing temperatures back down won\u2019t reverse all the changes. If more people die in extreme weather events or from starvation due to crop failures, there is no bringing them back.<\/p>\n Nor it is likely that species that go extinct can be brought back, for all the talk of de-extinction<\/a>. Damaged ecosystems may not be able to recover, at least on human timescales. And higher temperatures, even if eventually reversed, will still lead to higher sea level rises in the following decades and centuries.<\/p>\n The worst-case scenario would be that overshooting triggers a tipping point such as the collapse of the West Antarctic ice sheet<\/a> or the drying up of the Amazon, which will be impossible to reverse for many millennia.<\/p>\n