Climate scientist Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research (PIK), speaks at the German CCC congress, on the 31th of December 2020, on climate tipping points.
Politicians, economists and even some natural scientists have tended to assume that tipping points in the Earth system — such as the loss of the Amazon rainforest or the West Antarctic ice sheet — are of low probability and little understood. Yet evidence is mounting that these events could be more likely than was thought, have high impacts and are interconnected across different biophysical systems, potentially committing the world to long-term irreversible changes.
This talk summarizes evidence on the threat of exceeding tipping points and explores the effects of such large-scale changes, how quickly they might unfold and whether we still have any control over them. The Intergovernmental Panel on Climate Change (IPCC) introduced the idea of tipping points two decades ago. At that time, these ‘large-scale discontinuities’ in the climate system were considered likely only if global warming exceeded 5 °C above pre-industrial levels.
Information summarized in the two most recent IPCC Special Reports (published in 2018 and in September 2019) suggests that tipping points could be exceeded even between 1 and 2 °C of warming (see ‘Too close for comfort’). If current national pledges to reduce greenhouse-gas emissions are implemented — and that’s a big ‘if’ — they are likely to result in at least 3 °C of global warming. This is despite the goal of the 2015 Paris agreement to limit warming to well below 2 °C.
Some economists, assuming that climate tipping points are of very low probability (even if they would be catastrophic), have suggested that 3 °C warming is optimal from a cost–benefit perspective. However, if tipping points are looking more likely, then the ‘optimal policy’ recommendation of simple cost–benefit climate-economy models aligns with those of the recent IPCC report. In other words, warming must be limited to 1.5 °C.
This requires an emergency response. In our view, the clearest emergency would be if we were approaching a global cascade of tipping points that led to a new, less habitable, ‘hothouse’ climate state. Interactions could happen through ocean and atmospheric circulation or through feedbacks that increase greenhouse-gas levels and global temperature. Alternatively, strong cloud feedbacks could cause a global tipping point.
This talk will give an overview on how these different scenarios might play out, how likely they are, and what we need to do to avoid them.
Congress site, with talk download links https://media.ccc.de/v/rc3-11361-climate_tipping_points