When Will We Reach the 1.5°C Mark with Climate Change?

Mark Altaweel


For years now we have been hearing from scientists and policy makers that we must sustain global warming under the 1.5°C mark if we are to avoid the worst effects of climate change. That is, we need to keep temperatures under this limit using the average global temperature prior to global industrialization. Currently, we are more than 1 °C warmer than pre-industrial periods. The question is now when will we hit the  1.5°C limit and will we even know it when it happens?

The Intergovernmental Panel on Climate Change (IPCC), the United Nations body charged with forecasting future climate change, is currently estimating that there is a 50% chance that we will overshoot 1.5 °C within a decade. The 1.5 °C limit was acknowledged in the 2015 Paris climate agreement, which is legally binding, that countries must limit emissions so we do not breach this point. The limit has long been seen as a safe guard to prevent the Earth from reaching a tipping point where major damage would not only be caused but there could be further accelerating processes that warm the planet further, likely reaching 2°C that would lead to catastrophic global change, including flooding of many coastal cities and likely famine. In other words, this limit is seen as a type of cliff edge that we should not breach.

Black and white graph showing the change in global temperatures against the average from 1951 to 1980.
This graph plots the variation in the Earth’s surface temperature in relation to the average temperatures recorded between 1951 and 1980. Data source: NASA’s Goddard Institute for Space Studies (GISS). Credit: NASA/GISS.

The problem with this limit is we may not know when we actually hit it. Even in the Paris agreement, there is no clearly defined current level we are in, which makes it unclear what breaching 1.5 °C would be. Additionally, it is possible we could breach 1.5 °C temporarily, but then go into a temporary cooling before then climbing back to  1.5 °C. Simply stated, climate does not move smoothly but in a jagged line, where short-term climate fluctuations such as El Niño often intersect with long-term trends such as increasing global temperatures.[1]

The World Meteorological Organization (WMO) is more clear in defining when the 1.5 °C might be breached.[2] The organization indicates that we now have about a 66% chance of going over 1.5 °C for more than a year over the course of the next 5 years. Nevertheless, even this slightly more clear definition of breaching 1.5 °C  only captures short-term fluctuations rather than a long-term trend, given climate is often more clearly understood over decades.

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A view of dead ponderosa trees on a hillside.
Drought and climate change are causing dieback in the forests of the western United States.  Photo: dead ponderosa pines in New Mexico’s Jemez Mountains, caused by a combination of drought stress and bark beetle attacks on vulnerable trees. Photo: USGS, public domain.

The latest IPCC estimates do attempt to address this issue. They have looked at 20-year averages relative to average temperatures in 1850–1900, which is considered the baseline for global climate. Exceeding 1.5 °C can be determined by taking the midpoint of 20-year periods measured. This has the benefit of smoothing longer-term climate fluctuations, but the danger is we may not know we have breached 1.5 °C until about 10 years after breaching the limit. In other words, smoothing long-term climate has the risk of warning us at a point where it is too late. 

Methods needed must be fast enough to have meaningful impact on how we respond, but not falsely claim we have breached the limit when fluctuations in climate might be temporary. In other words, we need measures to be both instantaneous but also capture longer-term trends. There are different statistical smoothing methods as well as approaches that attempt to find key endpoints in long-term linear trends in climate.[3] Some methods attempt to calculate the amount of greenhouse gases to use as a proxy to determine what temperatures are, given the relationship between greenhouse gases and global warming.

Map of ocean heat content in the upper ocean (from the sea surface to a depth of 700 meters, or 2,300 feet) for 2017 relative to the 1993–2017 baseline. Source: NASA.
Map of ocean heat content in the upper ocean (from the sea surface to a depth of 700 meters, or 2,300 feet) for 2017 relative to the 1993–2017 baseline. Source: NASA.

In fact, one problem is we are not clear when exactly we breached previous marks, such as  0.5 °C. Most scientists believe this happened sometime between 1982-1988, but there is disagreement when exactly.[4] 

One new approach that allows both a more instantaneous measure but can also be calculated within long-term metrics that would clearly show if and when we breach 1.5°C would be to blend climate records for the past 10 years with climate model projections in the next 10 years. Then, one could combine both existing data and future projections to then take the average over a 20-year period. This has the benefit of meeting the IPCC definition, which is used in measuring long-term trends, but could provide us sufficient time to know when and if we have breached the  1.5 °C limit. If this was used for our current climate, we are then currently at around 1.26 °C over the 1850-1900 trends.[5]

A gray shaded relief map of California showing in shades of green where climate mismatched forests are in the Sierra Nevada.
Vegetation Climate Mismatch among conifers in the Sierra Nevada, California as a result of climate change. Map: Caitlin Dempsey with data from Hill et. al, 2023.

Whatever measures are used, there will be a need to agree on a set of metrics to use that determine when we breach certain critical levels that we have distinguished as representing thresholds that could prevent the worst of climate change’s effects. Regardless when we hit this level, we must try to limit the effects of greenhouse gases contributing to increasing global temperatures, but we also must be ready for more extreme adaptive measures required. Our timeline when this has to happen will be in large part determined by when we breach the  1.5 °C limit. We have been getting closer to this limit but we must now act fast on agreeing on how to measure this limit as well.


[1]    An article discussing the importance of the 1.5 °C limit can be found here:  Hoegh-Guldberg, O., D. Jacob, M. Taylor, T. Guillén Bolaños, M. Bindi, S. Brown, I. A. Camilloni, et al. “The Human Imperative of Stabilizing Global Climate Change at 1.5°C.” Science 365, no. 6459 (September 20, 2019): eaaw6974. https://doi.org/10.1126/science.aaw6974.

[2]    For more on the WMO and climate, including how it has recorded climate change, see:  https://wmo.int/.

[3]    For more on statistical smoothing methods that can be used to measure where we are in long-term climate trends, see:  https://climate.nasa.gov/vital-signs/global-temperature/.

[4]    For more on demonstrating when critical climate thresholds may have been breached statistically, see: https://cds.climate.copernicus.eu/apps/c3s/app-c3s-global-temperature-trend-monitor?month:float=10&year:float=2023.

[5]    An article published in Nature discusses this new method of determining when we might breach the 1.5 limit but allow us to demonstrate the limit is breached using long-term calculations and more instantaneous measures. This could be found here:  Betts, Richard A., Stephen E. Belcher, Leon Hermanson, Albert Klein Tank, Jason A. Lowe, Chris D. Jones, Colin P. Morice, Nick A. Rayner, Adam A. Scaife, and Peter A. Stott. “Approaching 1.5 °C: How Will We Know We’ve Reached This Crucial Warming Mark?” Nature 624, no. 7990 (December 7, 2023): 33–35. https://doi.org/10.1038/d41586-023-03775-z.

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About the author
Mark Altaweel
Mark Altaweel is a Reader in Near Eastern Archaeology at the Institute of Archaeology, University College London, having held previous appointments and joint appointments at the University of Chicago, University of Alaska, and Argonne National Laboratory. Mark has an undergraduate degree in Anthropology and Masters and PhD degrees from the University of Chicago’s Department of Near Eastern Languages and Civilizations.