Climate overshoot: What happens when the world gets too hot for too long?

Letting global warming run past the 1.5 degree Celsius target will hurt people far more than a world that stays below that line.

Researchers at several institutions now argue that this so-called climate overshoot will reshape risks for societies long before temperatures drift back down.

The concern is not only how high temperatures climb but also how long they stay elevated, which could lock in new patterns of disaster.

What climate overshoot means

The work was led by Andrew Kruczkiewicz, a researcher at Columbia Climate School in New York. His research focuses on using weather and climate information to support humanitarian decisions and early warnings for vulnerable communities.

Scientists use the term climate overshoot, a period when global warming rises above a target then later falls, to describe this scenario.

“Climate overshoot is no longer a distant possibility,” said Kruczkiewicz. His team investigated how overshoot could disrupt food, health, and migration systems.

The researchers noted that decision makers still lack solid evidence on who will be most affected and how quickly risks will rise.

They are calling for better data, models, and policy tools so that humanitarian organizations are not guessing in the dark when overshoot arrives.

How hot and how long

Recent monitoring shows that from 2015 to 2024 the world was about 1.24 degrees Celsius warmer than in the late 1800s. The analysis estimates that human driven warming is increasing by about 0.27 degrees Celsius each decade.

Scientists describe the remaining carbon budget, the total emissions allowed if we want a fair chance of staying near 1.5 degrees, as very small.

One recent estimate puts it at about 130 billion tons of carbon dioxide starting in 2025 – roughly three years of current emissions.

A major United Nations climate science report finds that overshoot entails added impacts and risks. It notes that some of those impacts are irreversible, and that the risks grow with the size and length of the overshoot.

A special IPCC assessment on 1.5 degrees found that most modeled pathways include a period of overshoot before temperatures are brought back down. 

The extra time spent above the target raises the odds of crossing tipping points in ice sheets, ecosystems, and social systems.

Climate overshoot and tipping points

Overshoot will not feel the same everywhere, because some regions are already warming much faster than the global average. A recent WMO update concludes that one year in the next five is likely to exceed 1.5 degrees above preindustrial levels.

Places that are heating quickly, such as parts of the Arctic and Southern Africa, also tend to have communities with higher social and economic vulnerability. 

In such regions, overshoot can turn repeated floods, droughts, or heatwaves into overlapping crises that stretch local coping systems to the breaking point.

Because of local climate feedbacks, a small rise in global temperature can trigger much larger shifts in rainfall, snow cover, or sea ice in particular places. 

That amplification means overshoot could push some cities or rural areas over thresholds for growing crops, building on coastlines, or staying outdoors during hot seasons.

The humanitarian sector, the network of agencies that respond to crises, will face rising demand in the places where resources are already stretched thin. 

Kruczkiewicz and colleagues emphasized that aid operations, early warning systems, and infrastructure planning need to account for overshoot hotspots rather than focusing on averages.

Protection may come too late

Many systems, from coral reefs to power grids, need years or decades to adjust to new climate conditions.

If overshoot causes temperatures or extreme events to intensify faster than those systems can adapt, more damage occurs before protective measures are in place.

Researchers talk about adaptation limits, levels of change beyond which local measures cannot prevent serious harm.

During overshoot, those limits may be crossed more often, such as when repeated heatwaves make outdoor labor unsafe for much of the year.

Time lags also matter, because building flood defenses, redesigning housing, or moving key services away from floodplains can take decades. 

If overshoot arrives sooner than expected, some cities could be finishing flood defenses even as flooding or rain overwhelms areas that were never protected.

The dangers of uncertainty

Most overshoot pathways assume heavy use of carbon dioxide removal, large scale methods that pull CO2 out of the air later in the century. 

These range from planting vast areas of forests to industrial machines that capture carbon, each with its costs, land demands, and political tradeoffs.

Scientists do not know how ecosystems, oceans, and social systems will respond if large amounts of carbon are removed and temperatures fall after overshoot. 

That uncertainty makes it risky to treat overshoot as a simple pause on the way to safety. Recovery paths with sudden cooling or renewed warming could create new waves of stress for societies.

Preparing for climate overshoot

The new work encourages planners to consider peak temperature, overshoot duration, locations of impact, timing of arrival, and the limits of adaptation. 

Different combinations of those factors could lead to different humanitarian needs, from long term migration support to sudden spikes in emergency shelter.

At the same time, the Columbia team stresses that human outcomes depend on whether overshoot is small and brief or deep and prolonged. 

Stronger commitments, data on vulnerable populations, and links between climate research and humanitarian planning can reduce suffering even if the planet exceeds 1.5 degrees.

The study is published in the journal PNAS Nexus.

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