Nearly a decade ago, the world rallied around the Paris Agreement on climate change and the goal of holding global temperature rise to 1.5 degrees C (2.7 degrees F). Since then, the “1.5C goal” has become the world’s North Star for climate action — a critical benchmark against which policies are set and progress is measured. But an alarming wave of recent data underscores just how close we are to surpassing this widely cited threshold.

Atmospheric concentration of carbon dioxide reached its highest level in 2 million years last year. 2024 also marked the first single year in which global average surface temperature rose more than 1.5 degrees C above pre-industrial levels. While the Paris Agreement goal refers to a long-term average, not a single year, scientists warn that we may be at the beginning of a full breach — and with it, increasingly dangerous floods, droughts, fires and other climate impacts.

So what exactly is the 1.5 degrees C goal, how was it set, and what happens if we exceed it? Here’s what to know.

Electric rickshaw on a New Delhi street
An electric rickshaw in New Delhi, India. The country has made significant moves toward electric vehicles in an effort to curb its emissions and reduce air pollution. Photo by Pradeep Gaurs/Shutterstock

Where Did the 1.5 Degrees C Goal Come From?

The Paris Agreement’s 1.5 degrees C temperature goal reflects decades of interaction between climate science and climate politics.

The UN’s 1992 Framework Convention on Climate Change (UNFCCC), the world’s first international treaty to confront the climate crisis, did not specify a temperature limit. A temperature goal emerged later, as scientific assessments, the European Council, and the G8 initially converged on a benchmark of 2 degrees C (3.6 degrees F) above pre-industrial levels. The 2010 Cancun Agreements became the first global agreements to reference 2 degrees C, while also recognizing the potential for a stronger 1.5 degrees C limit. 

In the lead-up to the 2015 Paris Agreement, experts concluded that even 2 degrees C of warming posed severe risks. The Alliance of Small Island States (AOSIS) and Least Developed Countries (LDCs) pushed hard during the agreement’s negotiations for a more stringent limit, resulting in a compromise: Countries agreed to collectively limit warming to “well below 2 degrees C” and “pursue efforts” to limit it to 1.5 degrees C, reflecting growing recognition of the heightened risks from every fraction of a degree of temperature rise.

The Intergovernmental Panel on Climate Change (IPCC), the world’s pre-eminent group of climate scientists, then prepared a report on the 1.5 degrees C goal. They found a stark contrast in climate change impacts under 1.5 vs. 2 degrees C of warming. They also mapped what it would take to stay within the 1.5 degrees C limit.

How Is the 1.5 Degrees C Goal Used? 

Since its adoption in the 2015 Paris Agreement, the 1.5 degrees C goal has served both as a powerful rhetorical symbol and as the basis for concrete technical benchmarks.

Vulnerable countries, NGOs and diplomats alike invoke the 1.5 degrees C goal to highlight what’s at stake in UN climate negotiations and other policy decisions. Climate-vulnerable nations like small island states have long framed 1.5 degrees C as a matter of survival, popularized in the phrase “1.5 to stay alive.” Youth movements and UN leaders, including Secretary-General António Guterres, have repeatedly called for urgent action to keep 1.5 degrees C “within reach.”

Technically, the 1.5 degrees C goal underpins a range of benchmarks and scientific assessments of climate action. It’s used to evaluate the ambition of countries’ national climate commitments (known as “nationally determined contributions” (NDCs)) and corporate emissions-reduction targets. It helps scientists estimate the remaining carbon budget — the amount of CO2 the world can still emit while limiting warming to 1.5 degrees C. The 1.5 degrees C-aligned pathways assessed by the IPCC form the basis for emissions-reduction timelines (such as the widely cited global benchmark of reducing emissions 43% below 2019 levels by 2030), deadlines for reaching net-zero emissions, and sector-specific targets like phasing out coal or replacing fossil-fueled vehicles with electric ones.

These timelines and benchmarks, in turn, underpin accountability tools like the Emissions Gap Report, the Science-Based Targets Initiative and the State of Climate Action report.

A woman grows cabbages in rural Kenya
A woman tends to her cabbages in rural Kenya. Small farmers are often on the front lines of the impacts of climate change. Photo by James Karuga/Shutterstock

How Do We Know When We’ve Exceeded the 1.5 Degrees C Goal?

The Paris Agreement calls for “holding the increase in the global average temperature to well below 2 degrees C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5 degrees C above pre-industrial levels.” To know when we’ve exceeded the threshold, it’s important to define some terms:

  • Global average temperature refers to the “estimated global average of near-surface air temperatures over land and sea ice, and sea surface temperature over ice-free ocean regions.” This estimate comes from temperature measurements from weather stations, ships and buoys around the world, which are then averaged to produce a global figure. (Some regions — including, critically, those over land — tend to warm more than average, while others warm less.)
  • While the Paris Agreement does not define “pre-industrial levels,” scientists like those in the IPCC typically use 1850-1900. This period marks the beginning of reliable global temperature records.
  • Today’s global average temperature is then compared to past conditions. The difference between the two is called the temperature anomaly.

By scientific convention, the Paris goal refers to a sustained temperature anomaly over a period of at least 20 years. This approach smooths out the effects of year-on-year temperature fluctuations due to factors like El Niño/La Niña, volcanic activity and changes in ocean circulation to provide a better estimate of human-caused warming. Scientists typically assess whether the long-term average exceeds 1.5 degrees C at any point during the 21st century, and specifically in 2100. If warming temporarily exceeds the threshold before falling back below it by the end of the century, this is referred to as “overshoot.”

Under this approach, the year in which a given temperature threshold is breached is the middle year of the 20-year period. As WMO put it, “1.5 degrees C of warming would only be confirmed once the observed temperature has reached that level over a 20-year period, 10 years after the year of exceedance. Thus, there would be a 10-year delay in recognizing and reacting to exceedance of the long-term temperature goal.” The scientific community is considering alternative approaches that would require less delay to confirm the year of exceedance.

Has the Earth Already Exceeded 1.5 Degrees C of Warming?

Yes, but it hasn’t yet breached the Paris Agreement’s temperature goal.

2024 was the first full year on record in which the global average temperature was more than 1.5 degrees C above pre-industrial levels, with an estimated anomaly of 1.55 degrees C. This continued an alarming pattern in which the 10 warmest years on record have all occurred since 2015. Prior to 2024, no full year had seen such a large temperature anomaly; though there were shorter periods — days, weeks and months — for which temperatures temporarily exceeded 1.5 degrees C. The WMO forecasts a 70% chance that the next five years will exceed 1.5 degrees C on average.

Importantly, a single hot year does not mean the Paris Agreement temperature goal has been breached or is no longer within reach. The global long-term average temperature anomaly is currently estimated at around 1.34 degrees C to 1.41 degrees C, depending on the method used.

Nevertheless, two recent climate modeling studies suggest that last year’s high temperatures likely mean the world has already entered the 20-year period in which the average temperature anomaly will breach the 1.5 degrees C limit.

What Happens if We Breach 1.5 Degrees C of Warming?

At current levels of 1.34 degrees C-1.41 degrees C of warming, we are already experiencing more frequent and intense heatwaves, wildfires, storms and floods. Food and water security are under growing pressure. Ecosystems like coral reefs are suffering irreversible damage. Surpassing 1.5 degrees C — even temporarily — augments the risks associated with these and other impacts.

According to the IPCC, even warming of 1.5 degrees C would expose nearly 1 billion people to water stress and desertification, cost an estimated $63 billion in adaptation and residual damages to major crops, and put 14% of the world’s species at risk of extinction. About 24% more people would face flooding compared to historical levels, coral reefs would decline by 70-90%, and the distribution of malaria would expand.

These same climate impacts would intensify if we exceed 1.5 degrees of warming. For instance, relative to 1.5 degrees C of warming, 2 degrees C of warming is expected to cause twice as many heat waves in Southern Africa, 1.6 times as much area burnt by wildfires in Mediterranean Europe, and cost $17 billion more globally in residual damage and adaptation for major crops. 2 degrees C of warming would also functionally destroy coral reefs. Extreme heatwaves would become increasingly common, creating dire impacts on human health and productivity, especially in South Asian and African cities.

Climate impacts chart.

Breaching 1.5 degrees C also increases the risk of crossing tipping points — critical thresholds beyond which parts of Earth may undergo abrupt, self-perpetuating and potentially irreversible changes. For example, the Greenland and West Antarctic ice sheets are at risk of collapse, worsening sea-level rise and disrupting ocean currents. Low-latitude coral reefs could die off entirely, a devastating blow for biodiversity and those whose livelihoods depend on fisheries. And boreal permafrost may abruptly thaw, releasing vast quantities of climate-warming methane into the atmosphere.

These systems are at risk of crossing tipping points around 1.5 degrees C. At higher temperatures, additional ice sheets, ocean circulation systems and ecosystems could completely collapse. Many tipping points shift systems from one stable state to another — such as the Amazon rainforest turning to savannah. Once these kinds of shifts occur, it becomes very hard or even impossible to reverse them on human timescales.

The bottom line is that with every additional fraction of a degree of warming, the impacts become harder or even impossible to adapt to. This is what’s known as loss and damage. Losses and damages can be economic, such as shrinking industries and associated loss of jobs and livelihoods. But the toughest are often those that can’t be quantified in economic terms — such as the losing burial grounds, family homes and loved ones.

Is it Still Possible to Hold Warming to 1.5 Degrees C?

In theory, it is still physically possible to avoid spending down the carbon budget required to limit warming to 1.5 degrees C through rapid, sustained emissions reductions. But the plausibility of executing the transformations at the pace and scale required is another matter.

The 2022 IPCC report on climate change mitigation identifies pathways for limiting warming to 1.5 degrees C with no or limited overshoot. They entail “rapid and deep and, in most cases, immediate greenhouse gas reductions in all sectors.” Specifically, greenhouse gas emissions would have peaked before 2025 and would decline by 43% by 2030 (from 2019 levels), and by 60% by 2035. CO2 emissions would reach net zero by around 2050, before exhausting the carbon budget. Concretely, this would involve phasing out unabated fossil fuel consumption, dramatically scaling up zero-carbon power, expanding sustainable transport, electrifying transport and industry, and halting deforestation. Removal of carbon from the atmosphere — by uptake in trees and through technologies — would also need to scale up dramatically.

The problem is that we are not following those pathways. The State of Climate Action report found that of 42 key milestones needed to limit warming to 1.5 degrees C, only one — the share of electric vehicles in light-duty vehicle sales — is currently on track for 2030. Despite significant increases in renewable energy capacity and electric vehicle sales, nearly 80% of global energy still comes from fossil fuels, the world is burning more coal than ever before, and deforestation continues at alarming rates. Coal and gas infrastructure continue to expand.

As a result, the world still emits 42 GtCO2 per year (and an additional 14 GtCO2e of other greenhouse gases). The remaining carbon budget to limit warming to 1.5 degrees C is 130 Gt CO2, which will be consumed in just over three years at current annual rates.

In simple, back-of-the-envelope terms, a linear path from 2024 CO2 emissions to net-zero CO2 emissions that stays within the remaining carbon budget would require cutting emissions by nearly 6 Gt CO2 per year, starting immediately and for the rest of the decade. For comparison, the economic shock during the COVID-19 pandemic caused CO2 emissions to drop by only  2 Gt in 2020, and the decline was temporary.

Achieving sustained emissions cuts of the magnitude needed would require unprecedented political and corporate leadership, backed by citizens, leading to a comprehensive and coordinated structural transformation across sectors and economies. There is scant evidence that such a deep transformation is poised to begin in the near future. One recent study found that while it is still theoretically possible to limit warming to below 1.6 degrees C at around 50% likelihood, the probability drops to between 5% and 45% when considering institutional and other barriers.

All these factors suggest that long-term average temperatures are likely to surpass 1.5 degrees C, at least temporarily. By how much, and for how long, is still very much in play.

Should a New Goal Be Chosen if 1.5 Degrees C Is Surpassed?

As skepticism about the feasibility of the 1.5 degrees C goal has grown, some scientists have proposed refocusing attention on limiting warming to “well below 2 degrees C” or returning warming to 1.5 degrees C after temporarily overshooting it. But adopting new benchmarks consistent with a higher temperature outcome poses another challenge: If derived from the modeling scenarios available today, these benchmarks would have the world reduce emissions more slowly and phase out fossil fuels later than we currently aim to do — an ineffective response to increasingly severe climate impacts. 

At the same time, our rhetoric should acknowledge the evolving reality. If long-term global average temperature rise surpasses 1.5 degrees C, it will no longer make sense to frame action in terms of what’s required “to avoid breaching the 1.5 degrees C limit.” Instead, we can refer to minimizing overshoot, stabilizing at the lowest feasible temperature, and avoiding as much warming as we can to protect people’s lives and economies.

A young girl carries water through an informal settlement in Johannesburg, South Africa.
A young girl carries water through an informal settlement in Johannesburg, South Africa. Many African cities face extreme heat, water scarcity and other impacts of climate change. Photo by Joe Eldridge/Alamy Stock Photo

What Should We Do If the World Surpasses 1.5 Degrees C?

If 1.5 degrees C falls out of reach, the world should still do what it ought to be doing today: rapidly reducing emissions and enhancing removals, while stepping up efforts to build resilience. Because warming will continue to increase until CO2 emissions reach net zero, reducing net emissions will remain critical. And because most adaptation planning scenarios already envision the possibility of exceeding 1.5 degrees C, it is more important to ensure those plans are implemented than to develop new plans for higher-temperature scenarios.

Breaching 1.5 degrees C, however, does augment the scale and pace of action ultimately required, and may introduce new risks and trade-offs. For example, the further we exceed the carbon budget, the more CO2 will need to be removed from the atmosphere, demanding greater investment in CO2 removal. While most adaptation plans already account for higher temperatures, reaching those levels earlier shortens the planning horizon. Failing to keep pace could increase costs, result in inadequate adaptation, or even compound the risk of maladaptation. The world would also need to prepare to address higher levels of loss and damage, consequences of climate change that exceed what people can adapt to. Finally, breaching the established temperature goal may lend salience to controversial solar radiation modification (SRM) approaches that would temporarily cool the planet by reflecting sunlight. While SRM is risky and untested, its relatively low direct cost and potential to stem near-term warming point to the potential for unilateral deployment, which would require more proactive oversight and governance.

And while surpassing 1.5 degrees C is increasingly likely, the extent and duration of warming — and its associated impacts — remain within our control. Limiting peak warming will require deep and sustained emissions cuts, maintaining and enhancing carbon sinks, and strengthening resilience. Achieving these outcomes depends not only on geophysical and technological factors, but also on a step change in political and corporate leadership. Leaders must foster the right economic conditions for clean technologies to take hold, adopt public policies that catalyze investment in emissions reductions and resilience, and ensure people see economic and social benefits from low-carbon development.

Ultimately, every fraction of a degree matters. Decisions made today will continue to shape climate risk for generations to come.