Reaching the Paris Agreement’s 1.5°C limit: are we there yet?

With average global warming in 2024 coming in at or above 1.5°C, and with a run of 12 months to June 2024 where monthly average warming was at or above 1.5°C, serious questions are being raised about whether or not the longer-term 1.5°C limit set out in the Paris Agreement has been reached.

Given the Paris Agreement limit is an average measure over 20 years or longer, working out if we have reached that limit from recent records also means looking at least a decade ahead at the likely warming from current trends in greenhouse gas emissions and policy settings.

Two papers published today look at these questions from different perspectives.

First year above 1.5°C and the Paris Agreement limit

The first by Bevacqua and colleagues asks what a year where average annual global warming is at or above 1.5°C tells us about the timing of reaching or exceeding the Paris Agreement’s longer-term 1.5°C limit. The 2024 calendar year was the first such year.

The paper’s conclusion is quite straightforward – unless emissions are reduced very quickly then there is a very high likelihood that the calendar year 2024 is within the first 20-year period where average global mean warming reaches the Paris Agreement’s 1.5°C limit. The higher emissions are in the next decade; the higher the probability of this.

What this paper also shows is that rapid emission reductions aligned with 1.5°C pathways quickly reduces the risk of annual mean warming exceeding 1.5°C again after the first single year exceeding 1.5°C. In other words, mitigation pathways that have been established to be feasible can still be achieved and can limit the risk of breaching the Paris Agreement limit quite quickly.

What do 12 consecutive months at or above 1.5°C mean?

The second paper by Cannon takes a different approach and asks a related question – what does the observation of 12 consecutive months with global mean surface temperature at 1.5°C or above mean for the likelihood of exceeding the Paris Agreement’s 1.5°C limit?  The 12 months prior to and including June 2024 were at least 1.5°C above preindustrial in two temperature records.

The bottom-line result is that a consecutive 12-month period with monthly warming at or above 1.5°C could indicates a 60 to 80% probability that the long-term global average annual warming has already reached or exceeded 1.5°C, even taking into account the extreme 2023/2024 El Niño. It is important to highlight however such 12-month period has occurred in some global temperature datasets, but not in others. In fact, the Cannon study only relies on the two warmest records available.

There are other important caveats to both papers, both of which ultimately depend on the extent to which CMIP6 models capture all climate system processes and projections over the next decade or so and have considered changes in human induced greenhouse gas emissions and aerosols and other factors since 2015. CMIP6 models use historical forcing up to 2015. If greenhouse gas concentrations, aerosol emissions and other factors are different after 2015, which is probably the case, further uncertainty is introduced.

Examples of this include the Tonga volcanic eruption, changes in greenhouse gases and aerosols which could affect the warming rate such as the reduction in shipping sulphur emissions, changes in the earth's albedo due to for example reduction in snow and ice cover and/or cloud feedbacks and of course natural variability in particular the extreme El Nino of 2023/24. Cannon was only able to consider a few of these factors.

So, are we there yet? Have we reached the Paris Agreement’s 1.5°C limit based on these papers?

The short answer is we’re not there yet, but we are getting very close. Every year without reducing emissions will bring us closer.

There is an enormous amount of work going on trying to understand the causes of the recent extreme warming anomalies, and concern is increasing that we may be seeing the early signs of a cloud feedback effect, but this is not certain.

It is no small irony that these two important papers have been published on the day in which countries have been set a deadline to submit updated Nationally Determined Contributions aligned with the 1.5°C goal in the Paris Agreement.

These papers are a very serious wakeup call that the world is rapidly approaching a drastic overshoot of the Paris 1.5°C limit with all of its adverse consequences unless urgent action is taken to reduce emissions. Very few countries have actually put forward 1.5°C-aligned pathways as of the 10th February 2025, yet we know that doing so is feasible and beneficial for most countries, is very urgent and that reducing emissions will reduce the rate of warming significantly.