Global temperature is likely to hit a new record high before 2025, and there is a slight chance of a year 1.5°C warmer than preindustrial levels by then, according to a new decadal outlook released by the UK Met Office on Thursday.
Issued each year since 2014, these outlooks are based on long-range forecast models that incorporate slow-changing ocean temperatures as well as other factors—including the relentless increase in greenhouse gases from fossil fuel use.
The most recent forecast was based on ocean temperatures for October 2019, along with similar observations of temperature and salinity below the ocean surface, atmospheric winds, temperatures and surface pressure. Going forward, the model calculates global temperature based on the evolving state of the atmosphere and ocean, together with changes in energy flowing into and out of the atmosphere as a result of greenhouse gases, aerosols (both volcanic and anthropogenic), and solar variability.
Though tagged as decadal, the UK Met Office’s outlooks extend out to five years. This is the first time in the history of the forecasts that a new global record high has been deemed likely.
The year 2019 was the second warmest in records going back to 1880, according to both NOAA and NASA. Over the past decade, successive global temperature records were set in the NOAA database in 2010, 2014, 2015, and 2016.
Warning: 1.5°C is dead ahead
Ominously, the Met Office outlook finds a slight chance (about 10%) that global average temperature will hit 1.5°C (2.7°F) above preindustrial levels for at least one year. Scientists and activists have increasingly emphasized the importance of the 1.5°C threshold, which was highlighted in a special report from the Intergovernmental Panel on Climate Change in 2018.
Most analyses consider the threshold as being met when global temperature is at or above 1.5°C when averaged over at least a few years (e.g., a decade), in order to smooth out year-to-year variations. The IPCC’s 2018 report is based on 30-year averaging. By this definition, meeting the threshold in a single year would not mean “we’re there”. However, if the current rate of increase persists, the decadal average could hit the 1.5°C point sometime in the 2030s. The 30-year average could also hit the threshold at around the same time, although it would take another 15 years for it to be assessed as such.
The IPCC report found that to have a two-thirds chance of avoiding a sustained rise of 1.5°C, carbon dioxide emissions would need to drop by roughly 60% by the year 2030. Global emissions were nearly flat from 2014 to 2016 but rose more than 1% in 2017.
Statistics from the Global Carbon Project indicate that emissions rose 2.7% in 2018 and 0.6% in 2019, according to Our World in Data. As each year goes by without sustained emission cuts, the task of attaining a 60% drop by 2030 becomes increasingly Herculean.
El Niño and global record highs
Within the long-term human-caused warming trend, record-warm years are most likely to be triggered by El Niño events. During El Niño, temperatures warm across a large stretch of the equatorial Pacific Ocean, and large amounts of heat are transferred from the ocean to the atmosphere.
The rise in global air temperature can be enhanced or muted during long periods dominated by El Niño or La Niña, respectively. Global air temperatures rose less dramatically from the late 1990s to early 2010s, then more sharply since then, as the planet shifted from a mode favoring La Niña (a negative phase of the Pacific Decadal Oscillation) to one favoring El Niño (a positive PDO phase). Throughout these periods, the Earth system as a whole continues to gain heat from the nonstop increase in greenhouse gas concentrations. More than 90% of that heat goes into the oceans, so a small change in that percentage—such as heat released during El Niño or retained during La Niña—can have big implications for the atmosphere in a given year.
By breaking out the rise in global temperature separately for El Niño, La Niña, and neutral years—as shown in the video embedded above—it’s easy to see that the influence of longer-term warming transcends the ups and downs caused by the El Niño–Southern Oscillation.
The regional picture
When examined in map form, the five-year outlook produced in late 2014 for the period 2015-2019 captured the global warmth well, especially across land areas. However, it underestimated the extent of cold in the far North Atlantic and parts of the Southern Ocean.
For 2020-2024, according to the Met Office, “spatial patterns suggest enhanced warming is likely over much of the globe, especially over land and at high northern latitudes.” They noted that “uncertainties in the forecast are considerable…Most regions are expected to be warmer than the average of 1981 to 2010, but regional cooling is possible – especially over parts of the Southern Ocean.”
Another noteworthy aspect of the forecast: “Current relatively cool conditions in the north Atlantic sub-polar gyre are predicted to warm, with potentially important climate impacts over Europe, America and Africa.” Among other things, warmer temperatures in the subpolar gyre tend to be associated with increased hurricane activity in the Atlantic, based on analyses conducted by Phil Klotzbach and colleagues at Colorado State University.
