Human greenhouse gas emissions are raising temperatures and sea levels, collapsing ice sheets and acidifying oceans. Now, research maps out the range of emissions pathways that can limit these changes.
How much greenhouse gas can be emitted before the Earth changes beyond the natural world’s ability to adapt? One approach to answer this involves looking at the characteristics of the world humans evolved in and establish limits to preserving these ecosystems1. There are many of these ‘planetary boundaries’ proposed, and previous work has mapped out how to stay within them2. However, in reality, many of the targets interact in complex ways — for instance, the amount of carbon dioxide that can be released while staying below a temperature target can be increased if more cooling sulfates are emitted3, but these sulfates can increase acid rain. Now, writing in Nature Climate Change, Gasser and colleagues4 propose a framework to address these interconnections that assesses a range of climate targets and produces a set of compatible emissions pathways with different degrees of climate uncertainty.
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The analysis starts with a large number of temperature trends and calculates the emissions that generate them for each of a range of equally plausible climate states. This impacts-to-causes calculation is a reversal of the normal climate modelling strategy and answers a call for more work on such inversions to find scenarios that meet a given set of requirements, found in ref. 6. This inversion6 was achieved simply by producing a large number of scenarios and running them forwards to sketch out the scenarios that passed the targets. Other previous attempts used simulated observations to guide the emissions trajectory7. Gasser and colleagues4 instead directly reverse engineer the simple climate model Pathfinder with a known climate behaviour8.
For each temperature scenario and climate state, they calculate which planetary boundaries would be overcome according to this pathway. The scenarios are constructed to remain below 2 °C (often lower), and to return to 1.5 °C by the end of 2500. The fates of other planetary boundaries are calculated. Some, such as the rate of sea-level rise and presence of Arctic sea ice, are simply dependent on temperatures, with a little extra uncertainty. Others, such as ocean acidification, depend on carbon dioxide concentration directly, meaning that solar radiation modification provides no respite. The role of carbon dioxide removal, however, is presented more positively; it protects all the global boundaries investigated (although it can be responsible for more localized problems9), and current emissions levels already make it hard to keep the Earth’s systems in their current states without substantial use of it.
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Gasser and colleagues’ findings are likely to further catalyse discussion of the roles of carbon dioxide removal and solar radiation management going forward. However, the less controversial aspects of their work should not be overlooked; it is sometimes useful to run a climate model backwards to see what the limits of emissions are, and even simple models can better represent complexity than just considering global temperatures when talking about protecting the Earth.
Robin Lamboll, Nature Climate Change, 4 November 2025. Press release.
