Almost a decade after the adoption of the Paris Agreement, the climate movement has focused on keeping the increase in average global surface temperature below the threshold 1.5 °CDespite activism and multilateral negotiations on emissions reductions It is now inevitable that we will not meet this target – a fact that has been made clear by UN Secretary-General António Guterres. The United Nations (UN) has warned that the world will inevitably cross this “red line”, possibly within the next ten years, due to the continued postponement of significant emissions reductions.

What does "crossing the limit" mean in climate jargon?

In common parlance, an overshoot would simply mean exceeding a threshold. However, in the jargon of the Intergovernmental Panel on Climate Change (IPCC), an “overshoot” refers to scenarios that not only exceed a specific average global temperature, but also include the path to returning below 1.5°C until the end of the century.

While this concept of return is proposed, this trajectory is by no means guaranteed. It requires aggressive measures not only to reduce emissions, but also to reduce the amount of heat-trapping gases already present in the atmosphere.

Irreversible risks in case of temporary overshoot

Exceeding the 1.5°C temperature target, even if only temporarily, dramatically increases the risk of serious and sometimes irreversible impacts.

Extreme events, such as deadly heat waves and extreme rainfall, are projected to continue to increase in frequency and intensity during this exceedance. The impact of half a degree of warming on highly sensitive ecosystems is particularly significant. For example, coral reefs, which are among the most biodiverse ecosystems, are projected to decline by more than 99% at 2°C of warming, effectively eliminating these critical ecosystems from the planet. This loss would be irreversible and would have far-reaching consequences for biodiversity and food security.

Another example of an irreversible impact is the increased likelihood of melting ice sheets and glacier loss, which contributes to the accelerating multi-century sea level riseSmall island nations may lose their entire territory as a result.

Exceeding the limit also increases the likelihood of reaching critical tipping points, such as the weakening or collapse of ocean currents, such as the Atlantic Meridional Overturning Circulation (AMOC). Such a collapse would have serious consequences, including changing global weather patterns. Even if temperatures were to later return to below 1.5°C, the effects of exceeding these tipping points would be irreversible.

The need for negative emissions and insufficient measures

Getting temperatures back below 1.5°C is incredibly difficult. It is not enough to just reduce emissions to zero; the world must transition to net negative emissions, which requires extensive removal of carbon dioxide (CDR) already present in the atmosphere.

The scale of the effort required is enormous. The IPCC’s 2023 Synthesis Report states that to reduce temperatures by 0.1°C beyond the threshold, net negative emissions of 220 gigatonnes of CO2 would be needed. For comparison, total fossil fuel emissions from the three largest emitters (China, the US and India) were around 20 GtCO2 in 2024. While CDR includes both conventional methods such as afforestation and new technologies (e.g. direct air capture), the world currently removes only 2 GtCO2 per year, with most of this dependent on forestry, which is vulnerable to drought and wildfires.

We are at this point in large part because of political leaders who have not yet taken sufficient action, and because of the fossil fuel industry that is blocking action. Only about a third of the parties to the Paris Agreement submitted their updated Nationally Determined Contributions (NDCs) by the deadline last September. The plans that have been submitted, even if fully implemented, would limit global warming to 2.3°C to 2.5°C, depending on the fulfillment of conditional pledges.

While the reality of exceeding the limit brings sadness, the extent and duration of that exceedance is still within our control. It is urgent that any exceedance be minimal and temporaryIf governments meet their medium-term and net-zero emissions commitments, global warming could be halted at 1.9°C by 2100, leaving the 2°C limit achievable.

Exceeding 1.5°C acts like a debt note: the irreversible impacts are the interest we will have to pay, and our ability to return to the safe limit depends on the immediate deployment of expensive and demanding technologies that we have not been able to implement so far. JRi