What’s in a number? The significance of the 1.5°C warming threshold and reporting on its possible breach in popular media

1.5 is an important number in climate science. It is the recommended target, in degrees Celsius, to which global warming should be limited relative to preindustrial levels, according to the Intergovernmental Panel on Climate Change (IPCC), the world’s leading authority on climate science^[1]. Further, the legally binding Paris Agreement tasks its global signatories to limit warming to “well below 2°C above preindustrial levels” and “pursu[e] efforts to limit the temperature increase to 1.5°C above preindustrial levels^[2].”

Warming above 1.5°C, the IPCC notes, could exacerbate various climate impacts, such as increasing the intensity or frequency of extreme weather and climate events, decreasing agricultural productivity and food availability, and increasing heat-related mortality and morbidity^[1]. A 2022 study found that warming of 1.5°C could also trigger several climate “tipping points,” or thresholds beyond which changes in parts of the Earth system become self-perpetuating^[3]. Examples of these particularly sensitive tipping points include the collapse of the Greenland and West Antarctic ice sheets and the die-off of low-latitude coral reefs.

On May 17 2023, the World Meteorological Organization (WMO) announced that there is a 66% chance that the world will experience an average temperature of 1.5°C above preindustrial levels during at least one of the next five years^[4]. These findings were widely covered by popular media outlets, including in articles published by The Guardian, CNN, and BBC, which all reported that this temperature increase would represent a breach of the key IPCC threshold. However, this is missing some important context.

“A single year above 1.5°C does not mean the world has passed that particular warming level”, said Zeke Hausfather, a climate scientist at Berkeley Earth, in a tweet. The reason is that climate is measured over decades, rather than years^[5]. Such nuance was better captured by articles published in Reuters and Axios, which both correctly did not report that these new temperature projections, if realized, would constitute a breach of the threshold.

Global temperature can vary from year to year because of oscillations inherent to the climate system. This variability is caused by naturally occurring processes such as El Niño events, which can increase the global temperature of the planet for several years independent of human activity. Indeed, the WMO notes that a portion of the expected warming over the next five years is due to an emerging El Niño^[4].

What is El Niño and why is it important?

The climate system is replete with naturally occurring cycles that affect global temperature, oceanic circulation, and atmospheric processes, which are commonly referred to as modes of internal climate variability^[6]. The El Niño Southern Oscillation (ENSO), driven by a change in the circulation of the Pacific Ocean and a reorganization of heat exchange between surface and deep waters, is perhaps the most famous example of internal climate variability^[7].

ENSO consists of two phases: El Niño and La Niña. The El Niño phase is characterized by elevated surface-water warming in the eastern Pacific Ocean due to an entrapment of relatively warm waters originating from South America. Because of the vast size of the Pacific Ocean, such warming also temporarily increases global surface temperature (Fig. 1). By contrast, the La Niña phase is characterized by increased heat transfer to deeper waters, and thus relative cooling in surface waters, manifesting as temporary relative global cooling (Fig. 1)^[7].

Figure 1 – Historical change in global surface temperature demarcated by the presence of an El Niño (red), La Niña (blue), or neutral (white) ENSO phase. Note that strong El Niño events contribute to higher average temperatures, whereas strong La Niña events contribute to lower average temperatures. These short term fluctuations add “noise” to the global warming trend. Data are taken from HadCRUT.

These events, along with others, can temporarily alter global surface temperature by about 0.2°C in both directions, complicating efforts to determine when a certain climate threshold is actually crossed^[8]. “Due to year to year variability driven by El Niño and La Niña events, we’d expect to see single years above 1.5°C up to a decade before long-term human-driven warming passes that level”, continued Hausfather.

Our best guess on breaching 1.5°C and what that means

The extent of future global warming will ultimately be determined by current and future emissions decisions made by humans. However, even under the most aggressive emissions reduction scenario, the IPCC expects that the 1.5°C threshold will be crossed at around the middle of the century. Under this optimistic scenario, referred to as Shared Socioeconomic Pathway (SSP) 1-1.9 in IPCC parlance, the large-scale deployment of negative-emissions technologies, such as those that capture carbon dioxide from the atmosphere and store it for long periods of time, will be necessary to return global climate to a state of warming below 1.5°C relative to preindustrial levels by 2100. Because the Paris Agreement explicitly tasks countries with limiting end-century warming to 1.5°C, SSP 1-1.9 would still meet this international target^[2].

The SSP 1-1.9 scenario requires that global emissions reach net zero by the middle of the century. Less ambitious climate scenarios (SSPs 1-2.6, 2-4.5, 3-7.0, and 5-8.5) result in both a faster rate of warming and a higher magnitude of end-century warming (Fig. 2). Under all scenarios, the best estimate for when we will cross the 1.5°C threshold is between 2027 and 2035, according to analyses by the IPCC and Carbon Brief^[1,8,9]. These results are consistent with later work that estimated this threshold would be crossed between 2033 and 2035^[10].

Figure 2 – Range of expected breaching times of the 1.5°C threshold according to analyses by the IPCC^[1,9] and Carbon Brief^[8] (5-95% range). Central estimates are denoted by the black dots. Note that the estimates for all climate scenarios fall between 2027 (very high emissions) and 2035 (aggressive emissions reductions). Source: Carbon Brief.

While we haven’t yet reached 1.5°C of warming, the IPCC notes that contemporary levels of climate change have already had widespread negative consequences for human and natural systems^[9]. “Climate change is already affecting every region on Earth, in multiple ways. The changes we experience will increase with additional warming”, said Panmao Zhai, co-chair of Working Group 1 for the IPCC’s Sixth Assessment Report, in 2021. In fact, that 2022 study on climate tipping points^[3] found that some of these could be triggered by just 1°C of warming, a threshold we’ve already crossed^[10]. This instructs us to not mistake the climate system for a binary one. The impacts of climate change do not suddenly emerge after 1.5°C of warming, but rather become progressively more severe with every increment of additional warming beyond preindustrial levels^[9].

REFERENCES

• [1] IPCC (2018). Special report on global warming of 1.5°C
• [2] UNFCC (2015). The Paris Agreement
• [3] Armstrong McKay et al. (2022). Exceeding 1.5°C global warming could trigger multiple climate tipping points. Science
• [4] WMO (2023). Global annual to decadal climate update (target years: 2023-2027)
• [5] Rogelj (2017). Getting it right matters: Temperature goal interpretations in geoscience research. Geophysical Research Letters
• [6] NAS (2016). Modes and Mechanisms of Internal Variability. In: Frontiers in Decadal Climate Variability: Proceedings of a Workshop
• [7] NOAA (2018). El Niño and La Niña: Frequently asked questions
• [8] Hausfather (2021). Analysis: What the new IPCC report says about when world may pass 1.5C and 2C. Carbon Brief
• [9] IPCC (2021). Sixth Assessment Report
• [10] Diffenbaugh and Barnes (2023). Data-driven predictions of the time remaining until critical global warming thresholds are reached. Proceedings of the National Academy of Sciences

Top image – Chart showing observed monthly temperatures (grey line), estimated human-caused warming (orange), and idealized potential pathways to meeting 1.5°C limit in 2100 (grey, blue and purple). Temperature relative to 1850-1900. Credit: IPCC.