The 1.5°C target – an obituary?

„The truth is that we have failed to avoid an overshooting above 1.5°C in the next few years,“ UN Secretary-General António Guterres recently admitted ahead of the COP30 UN-Climate Change Conference. The target set ten years ago in the Paris Climate Agreement was already very ambitious at that time and has become increasingly implausible over the years due to inadequate societal-political development and rapid warming. However, the 1.5°C limit has not yet actually been exceeded. And doesn't the mark of “well below 2°C, preferably 1.5°C” allow the 1.5°C limit to be exceeded at least temporarily? So, is it too early for an obituary?

Let's approach this question from a physical perspective. Firstly, we need to clarify how close we are to reach 1.5°C of global warming today. In the last assessment report of the Intergovernmental Panel on Climate Change (IPCC), it was expected that this limit would be exceeded in the early 2030s. This was based on the 20-year average temperature increase, a value that, strictly speaking, could only be determined ten years later. Now, however, global warming in 2023 has already scratched the 1.5°C mark and has likely exceeded it in 2024 with 1.55°C (±0.13°C). By the way, the uncertainty range is not primarily based on current measurement uncertainties, but on the pre-industrial level, which is estimated from the sparse data for the period 1850 to 1900.

It is not without reason that the IPCC uses a long-term average, as the values for individual years are strongly influenced by natural random fluctuations. For example, 2023 marked the end of a longer phase of La Niña conditions in the Pacific, and the subsequent El Niño event caused a strong temperature spike in 2024 in particular, which has since subsided again as expected. Now, cooler waters in the tropical Pacific point once more towards La Niña, with 2025 expected to come in below the 1.5°C mark. And yet: the high temperatures of recent years no longer fit with the old rule of thumb of 0.2°C warming per decade and suggest a current increase of 0.25°C to 0.3°C per decade.

In addition to the continuing emission of additional greenhouse gases, which are mainly responsible for the warming, this acceleration is probably also due to a decreasing Earth's albedo: an increasing proportion of the incoming sunlight is absorbed by the Earth instead of being reflected, which, in addition to the loss of bright ice and snow areas, is mainly due to a decrease in cloud cover. This in turn can be partly attributed to a decrease in air pollution, as fewer suspended particles (aerosols) serve as condensation nuclei for cloud droplets. At the same time, the decline in cloud cover suggests that greenhouse gas-induced warming itself could be at work in the form of an amplifying feedback loop, possibly due to changes in large-scale wind patterns.

Even if the latest development is something of a surprise, it is not completely incompatible with climate models and their forecasts. Rather, it helps to reduce the unfortunately still considerable uncertainties regarding the reaction of the climate system to greenhouse gases. Models with moderate to stronger warming now fit the observations better than those with milder warming.

So, what does the rapid warming mean for the 1.5°C target? The longer-term overshoot cannot just be determined with a ten-year delay, as the natural fluctuations can be factored out reasonably well. The result: we are currently at around 1.4°C warming. At the current rate of warming, we would therefore exceed the 1.5°C limit by 2028/29, slightly earlier than expected in the last IPCC report. Adding the deplorable climate policy developments to the picture, the 1.5°C target begins to disappear into thin air.

Finally, two thought experiments to better understand the situation. What would happen if we could freeze greenhouse gas concentrations (and other factors) at today's levels? We would only be allowed to emit as much CO2 as the natural carbon sinks (ocean and land) can absorb, which would roughly correspond to an immediate halving and a subsequent further reduction of emissions to close to zero. Unfortunately, the temperature would not stabilize immediately but rise by about another half a degree due to the waning cooling effect of the ocean, eventually exceeding the “well below 2.0°C” target. In reality, the goal must therefore be to bring greenhouse gas concentrations well below today's levels.

Second experiment: What if we could immediately reduce greenhouse gas emissions to zero? In fact, CO2 concentrations would instantly begin to fall gradually due to the natural sinks. For a while, however, this would only cool down the temperature to about the same extent as the cooling effect of the ocean wanes. As a result, the surface temperature would remain at today's level for quite some time and then decline only gradually. In this scenario, we would therefore just about meet the 1.5°C target without exceeding it. In other words, the total amount of carbon we can release without exceeding 1.5°C is now almost depleted.

So, in purely physical terms, the 1.5°C target has not yet been broken. At the same time, scenarios that could keep the temperature “well below 2°C” are increasingly implausible. Strictly speaking, it is therefore still too early for an obituary for the 1.5°C target. However, it is almost certainly not a waste of time to write it and have it ready in the drawer.