For some scientists, they are the necessary next stage in averting the existential threat of climate chaos. For others, they should not even be talked about. Carbon Dioxide Removal (CDR) technologies, which provide the means to remove carbon from the atmosphere, are one of the hottest areas of climate research, but also the most controversial. The debate on whether and how to develop a CDR was sparked last month by the release of the final part of the Intergovernmental Panel on Climate Change's (IPCC) Comprehensive Review of Climatology. The report found that ways to capture and store carbon dioxide, while expensive, can play a role in trying to keep global temperatures within safe limits. But scientists and policymakers are divided. Some say that technology must be the immediate priority of research. Others urge caution, warning against trusting in untested technologies before we fully deploy reliable low-carbon technologies like the renewable energy we already have. John Kerry, the US President's special climate envoy, spoke about his concerns. “Some scientists suggest that it's possible that there could be an overshoot [of global temperatures above the 1.5°C threshold above pre-industrial levels that governments are aiming for] and you could go back, so to speak; You have technology and other things that allow you to come back. "The danger with this, which worries me and motivates me the most, is that according to the science and the best scientists in the world, we may be at or beyond several tipping points that we've been warning about for some time," he said. "That's the danger, the irreversibility." The British government's former chief scientific adviser, Sir David King, strongly disagrees. He believes CDR of many kinds will be needed, along with means of climate 'repair' such as refreezing glaciers, as the world is almost certain to exceed the global target limit of 1.5°C above pre-industrial levels.