Exploring Future Emissions Pathways to Meet Climate Targets

In a recent paper, @piersforster et al updated the remaining carbon budget to limit warming to 1.5C or 2C. In a new post at The Climate Brink, we explore what these mean for future emissions pathways: https://t.co/ESNkZk3b5C pic.twitter.com/RZuLFIGwBD

— Zeke Hausfather (@hausfath) June 26, 2023

Here we find that to meet our climate targets in the absence of net-negative global emissions, we would have to reach zero emissions in:
▪ 1.5C with a 66% chance – 2030
▪ 1.5C with a 50% chance – 2035
▪ 2C with a 66% chance – 2069
▪ 2C with a 50% chance – 2079

— Zeke Hausfather (@hausfath) June 26, 2023

Its clear that global emissions will not get to zero in 2030 or 2035, which means we will end up in a world of overshoot, where the only way to get global temperatures back down to 1.5C is by removing more CO2 from the atmosphere than we emit.

— Zeke Hausfather (@hausfath) June 26, 2023

However, even these overshoot scenarios quickly become implausible in the amount of negative emissions required if global emissions do not rapidly decrease and reach net zero by the 2050s (as in most IPCC WG3 1.5C scenarios).

— Zeke Hausfather (@hausfath) June 26, 2023

While we are already quite close to 1.5C today – global warming to-date has been around 1.3C – there is still a ways to go until we get close to 2C. This means that the remaining carbon budget and required net-zero dates are correspondingly larger and further out.

— Zeke Hausfather (@hausfath) June 26, 2023

Its easier to imagine a world of net-zero by 2069 with no large-scale net-negative emissions. At the same time, if we continue to emit CO2 at current levels for another decade or so, achieving well-below 2C (~66% chance) will be as challenging as avoiding 1.5C is today.

— Zeke Hausfather (@hausfath) June 26, 2023

Updates to the carbon budget happen due to:
▪ Additional years of emissions
▪ Refinements to historical emissions and temperature estimate
▪ Refinements to models and estimated climate sensitivity
▪ Changing assumptions about future non-CO2 GHGs and aerosols

— Zeke Hausfather (@hausfath) June 26, 2023

The figure below shows the old simplified emissions pathways associated with the AR6 WG1 report (dashed lines) compared to the new ones from Forster et al. In all cases, the net-zero date is sooner and slope of the curve is steeper than in the AR6 budgets. pic.twitter.com/l6jP6Hb4s6

— Zeke Hausfather (@hausfath) June 26, 2023