For nearly a decade, since the Paris Agreement was adopted by nations around the world, keeping the increase in global average surface temperature below 1.5°C has been the goal, message, and drumbeat of the climate movement. But despite organizing, activism, and multilateral negotiations to cut emissions, it’s now inevitable that we won’t meet that target—a reality brought into stark focus in recent remarks by the UN’s Secretary General António Gutierres.
While for climate scientists and others paying attention, this is by no means a surprise, given our ever-increasing emissions since 2015, this news still comes with a deep sadness. Personally, I am gutted by what this means for even more extreme climate impacts, the enormous scale of action required, and the ever-widening gap between those who are most responsible for climate change, and those experiencing the brunt of the impacts. That we are at this point is entirely the fault of political leaders who have failed thus far to take sufficient action to address the climate crisis, as well as the fossil fuel industry, which has blocked action at every turn to preserve their profits.
This overshoot, like climate change, has not only physical but ethical implications that are multi-generational. Understanding what it means is essential for shaping climate action that centers accountability and justice.
Overshoot means temporarily exceeding 1.5°C—but bringing temperatures back down
In regular conversation, overshoot means exceeding a threshold, whether that be a corner three that misses the basket entirely, or a compulsive shopper spending more than had been budgeted (which I include in passive voice here as purely hypothetical).
But in the lingo of the Intergovernmental Panel on Climate Change (IPCC), overshoot refers to several climate change scenarios that not only exceed a specific global average temperature or concentration of heat-trapping gases in the atmosphere, but also incorporate a pathway to return below 1.5°C by the end of the century. A trajectory to bring temperatures back down is by no means guaranteed unless aggressive action is taken to not just cut emissions but draw down atmospheric heat-trapping gases. These overshoot pathways from the IPCC vary in a number of ways. including the peak temperature reached and the time required to return to below 1.5°C. Although these concepts may seem esoteric, they will have real consequences for how the impacts of climate change will be felt by communities across the world.
Overshoot increases harmful climate impacts, some of which may be irreversible
Overshooting the 1.5°C degree temperature target, even temporarily, means greater risks of severe impacts, some of which may be irreversible.
Extreme events, like deadly heatwaves and extreme precipitation, are projected to continue to increase in frequency and intensity during overshoot. However, these trends are likely reversible and may decrease as global average temperatures do.
In 2018, the IPCC released a report comparing projected climate impacts at both 1.5°C and 2°C of warming, highlighting the huge difference that half a degree makes, and providing important context for how a world in the throes of overshoot might look. These examples illustrate what exactly is at stake, but don’t capture the full risks of overshoot since, on our current emissions trajectory, exceeding 2°C of warming by the end of the century remains possible, if not likely.
For highly sensitive ecosystems like coral reefs, the difference in impacts of a half a degree are particularly stark. Coral reefs, some of the most biodiverse ecosystems on the planet, are projected to decline by at least 70% at current levels of warming, while at 2°C of warming, that decline is projected to be greater than 99%, functionally removing these critical ecosystems from our planet. Such an irreversible loss comes with far-reaching consequences for biodiversity, food security, and livelihoods.
As another example of irreversible impacts, overshoot means that the probability of melting ice sheets and loss of glaciers all increase, contributing to additional multi-century accelerating sea level rise. Small island nations may completely lose their territory to rising seas, along with heritage, traditions, and rituals that many, including myself, take for granted, like the ability to visit the graves of ancestors and loved ones.
Similarly, the weakening or collapse of ocean currents like Atlantic Meridional Overturning Circulation (AMOC), which has a higher likelihood of occurring during overshoot, comes with major implications, including changing global weather patterns and crop viability. Even if temperatures are brought back below 1.5°C, the effect of crossing these tipping points will be irreversible. And the longer temperatures stay above 1.5°C before coming down, the greater the risk of significant irreversible impacts.
Bringing temperatures back down requires action and technology that have yet to materialize
The irreversible and, in some cases, fatal, impacts are only part of why overshoot is so concerning. Once overshoot occurs, bringing temperatures back down is incredibly difficult. To actually reduce temperatures, it won’t be enough to just slash emissions to net zero—we will have to go well beyond that and actually remove CO2 that is already in the atmosphere to get to net negative emissions. To date, humanity has been unable to reduce emissions, enhance natural carbon sinks, or deploy carbon removal technology at anywhere near the scale required.
The IPCC’s definition of overshoot presumes that temperatures will peak and then decline, but two things need to happen to get back down to 1.5°C degrees of warming.
First, global emissions need to drop dramatically and stabilize such that atmospheric concentrations of carbon dioxide and methane aren’t increasing, meaning temperatures aren’t increasing either. Unfortunately, the mechanism countries have adopted to plan and report their efforts to cut emissions, Nationally Determined Contributions (NDCs)—which outline each country’s plans to reduce emissions across their economies—have repeatedly fallen short in their collective ambition and implementation. During the most recent round of NDC submissions, as of September 30, only 64 countries, representing 30% of global emissions, submitted a new plan. (The U.S. submitted its most recent NDC in December of 2024, but will not be sending a delegation to this year’s COP as the Trump administration pulled the U.S. out of the Paris Agreement, is attacking science, and doubling down on fossil fuels.) While additional NDCs are being put forward ahead of the global climate summit COP30, and the UN’s report cautions that conclusions can’t be drawn from such a small subset of plans, this trend highlights the real challenges to achieving the scale of reductions required in overshoot scenarios.
Second, to bring temperatures back down, the world would need to deploy widespread carbon dioxide removal (CDR) and enhance land carbon sinks at a scale that has not been achieved to date. CDR refers to a broad range of approaches from ‘conventional’ strategies like reforestation and soil carbon sequestration to emerging technologies like direct air capture and enhanced weathering. There are major economic and technical feasibility challenges to achieving CDR at this scale, and it comes with significant social and ecological risks.
IPCC’s 2023 synthesis report states that lowering temperatures by 0.1°C following overshoot would require net negative emissions (a feat accomplished by both emission reductions and sufficient deployment of CDR) of 220 Gigatons ofCO2. To put that into perspective, fossil fuel emissions from the three highest emitting countries (China, the United States, and India) in 2024 were roughly 20 GtCO2. This means that, after zeroing out emissions, we would need to remove the equivalent of 11 years of these countries’ emissions to reduce temperatures by a tenth of a degree.
Further, the most recent report from the State of Carbon Dioxide Removal initiative demonstrates that only 2 GtCO2 is currently being removed from the atmosphere, with nearly all of this relying on land use and forestry-related activities. These land-based approaches, however, are vulnerable to escalating climate impacts like wildfire, drought, and extreme heat, which threaten to reverse carbon removal and rerelease CO2 to the atmosphere.
We have opportunities for action, but not without obstacles
Despite the sadness and anger I feel in arriving at this moment, I’m hopeful because our collective choices can still shape the future. Powerful actors like those in the fossil fuel industry and the current administration are relying on vintage climate disinformation to maintain the status quo of escalating emissions and temperatures. But as my colleague Rachel Cleetus writes, the importance of this month’s climate negotiations, in Belém, Brazil, cannot be overstated.
Investments in resilience can still help to protect the most vulnerable, and while temperatures will exceed 1.5°C over the long term, the magnitude and duration of that overshoot is still within our power. Our decisions now will determine how much CDR is required to bring temperatures back down below 1.5°C, and how that CDR is deployed can be still be grounded in principles of equity and justice.
While the reality of overshoot is daunting, we cannot allow it to be paralyzing.