Previously, I described how Andrew Weaver used different estimates of how sensitive the Earth’s climate is to greenhouse gasses to determine how many total emissions humanity can have without causing more than 2Â°C of warming. The 2Â°C figure is commonly cited as the level of warming that is unambiguously ‘dangerous’ – either because of the harm it would do directly or because warming to that point would kick off positive feedbacks that would then make the planet hotter still.
A new site simplifies this analysis, arguing only that: “If we are to limit global warming caused by carbon dioxide emissions to less than 2Â°C, widely regarded as necessary to avoid dangerous climate change, we need to limit total cumulative emissions to less (possibly much less) than” one trillion tonnes of carbon (equivalent to 3.67 trillion tonnes of CO2). This is probably too high an estimate, given that the IPCC estimates climate sensitivity to be between 3.6Â°C and 4.5Â°C. At the low end, that means we need to cap total emissions below 0.661 trillion tonnes of carbon; at the high end, the limit would be 0.484 trillion tonnes. The website estimates that our emissions to date are around 0.555 trillion tonnes.
In the event that actual climate sensitivity is a high but possible 8Â°C, cumulative emissions of just 0.163 trillion tonnes of carbon would be enough to produce 2Â°C of warming.
Still, ‘trillionth tonne’ is an accessible concept and it is interesting to watch the numbers update in real time. One especially interesting figure is this one: “We would not release the trillionth tonne if emissions were to start falling immediately and indefinitely at…” At present, their estimate is about 2.1% per year. A higher rate of reduction is necessary if the trillion tonne figure proves overly high.