Carbon budgets: how hard is the Paris Agreement now?

By Dr Robin Lamboll: find this and more content from Robin on his Science Is Shiny blog

Our paper reported only 6 years (one mostly gone) of current emissions to stay below 1.5C. What does that mean for the Paris agreement?

In the Paris Agreement, governments agreed to “holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change”. How much can we emit while still keeping to this?

When we release carbon dioxide, it traps heat in our atmosphere. The amount of warming is roughly proportional to the cumulative emissions of carbon dioxide – on timescales of decades, it doesn’t matter when the carbon was emitted. So while people like to think in terms of net zero dates, really what matters is the journey to get there – cut emissions more now and you have longer, but delay and the problem gets much harder. This gives rise to a concept known as the “remaining carbon budget” – the total amount of carbon we can still emit while still having a given probability of staying below a certain temperature. The Paris agreement doesn’t define those probabilities very well, but we typically interpret the “pursuing efforts” goal as a 50% chance of staying below 1.5C, and the “well below 2C” commitment to a higher probability.

Calculating that remaining carbon budget is complicated by releasing other greenhouse gases, such as methane, and the possibility of unexpected behaviour of our planet as it warms. In this work, we look at many different possible ways the budget could be calculated to work out how large it is and how confident we can be in it. The results we find is that the most widely-cited value used for the 50% chance of 1.5C budget from 2020 has now halved, due to continued emissions as well as small updates to our understanding of atmospheric physics. This budget from 2023, 250 gigatonnes of CO₂, is roughly equal to six years of current emissions. If we planned to descend to net zero linearly, this would give us 12 years, although in practice most plausible routes to net zero don’t look like this. If we only try to keep to a 90% chance of staying below 2C, we instead have 500 GtCO₂, about 13 years of current emissions.

The largest update comes from our assessment of the cooling effects of smog. Currently, burning fuels emits both carbon, which stays in our atmosphere for centuries, and also smog particles called aerosols, which typically only persist for days to weeks. While this is bad for our health and causes acid rain, it temporarily cools the planet. As we transition away from fossil fuels, we reduce this short-term cooling effect but keep the CO₂ warming. We now think that the cooling effect is a little stronger than we used to, so we will warm up more as it stops. The temperature change from this update is very small – around 0.05C – but with current temperatures already above 1.2C, this makes a big difference to the budgets The 1.5C budget is so small now that fairly minor changes to our understanding of the earth can result in large changes to the estimate, making it hard to use politically.

Changes to the 1.5C budget: AR6 WG1 is the previous budget report; MAGICC is the climate model used, the AR6 DB is the database of emissions used to estimate non-CO2 warming. FaIR is an alternative climate model. The other changes all relate to technical features in the calculation of the non-CO2 warming and ensure better alignment with the definition of peak temperature.

We also investigate how new results in earth science might change our budget estimates. We find that the largest “known unknown” is the behaviour of the earth as we approach net zero. Our best estimate is that warming from CO₂ approximately stops in the year of net zero, and continues fairly linearly until this point. However there is considerable uncertainty about this. It’s possible that the earth will continue warming as a result of effects such as melting ice, methane release and changes in ocean circulation, meaning we continue warming after we reach net zero. It’s also possible that the increase in natural carbon sinks will increase with carbon concentration, meaning we substantially cool down, and may start cooling a little before we reach net zero. At the moment, these earth feedbacks are drowned out by the effects of rising human emissions, so we don’t know whether heating or cooling dominates until we cut emissions significantly. While this is probably not enough to put us over 1.5C if we stop emitting today, it’s already possible. Keeping below 1.5C would require both extreme political effort and good luck in terms of the earth’s response. However we can be pretty sure that we won’t exceed 2C of warming even with over a decade’s emissions, so the weaker part of the Paris agreement is still very much plausible.

The overall message of the paper is that cutting carbon emissions now is essential to keep the Paris agreement alive. The budget for 1.5C is very small and now hard to use politically, though below,  2C is still to play for.