25 June 2024

1.5 degrees Celsius is here and now

Surface air temperatures, 21 June 2024.
Credit: C3S/ECMWF (pulse.climate.copernicus.eu)
By David Spratt

Has the world already reached a global warming trend of 1.5°C (compared to ~1900 pre-industrial baseline)?

There have been some sharp disagreements between scientists over this question, with former NASA climate science chief James Hansen saying that for all practical purposes the climate system trend is now at the 1.5°C mark, whilst Penn State University’s Michael E Mann and others disagree and say we have up to a decade to go.

In May 2024, Hansen wrote that the 12-month mean global temperature “is still rising at 1.56°C relative to 1880-1920 in the GISS analysis through April. Robert Rohde reports that it is 1.65°C relative to 1850-1900 in the BerkeleyEarth analysis (for the same period).  El Nino/La Nina average global temperature likely is about 1.5°C, suggesting that, for all practical purposes, global temperature has already reached that milestone.”   [El Niño (the warm phase) and La Niña (the cool phase) lead to significant differences from the average ocean temperatures, winds, surface pressure, and rainfall across parts of the tropical Pacific. Neutral conditions are near their long-term average.]

Another pointer is the fact that the 2023 El Nino was moderate rather than extreme, so when the temperature falls back as the El Nino ends, that decrease will not be as sharp as it was, for example, at the end of the 2015-2016 “super El Nino”. The warmest twelve months around the 2023-24 El Nino was about 1.65°C, and the decrease may only be in the range of 0.2-0.25°C. Hence Hansen’s statement: in summary, El Nino peak of ~1.65°C, La Nina trough of ~1.4°C and El Nino/La Nina average of about 1.5°C.

As well, global ocean surface temperatures are continuing on their record breaking ways, as illustrated in Figure 1.  The orange band shows the extraordinary difference between the current year and all previous records. 

Figure 1:
Daily sea-surface temperature, June 2023 to May 2024, from 60°N to 60°S

What happened in the last year

Warming for the 2023 calendar year was ~1.5°C, with September above 1.8°C, the second half of 2023 at 1.67°C; and the global average temperature for the past 12 months (June 2023-May 2024) at 1.63°C above the pre-industrial baseline, according to Copernicus C3S data.

Datasets of global temperatures vary a little depending on method, but two of the most significant are Berkeley Earth which put the 2023 calendar year at 1.54°C above the pre-industrial (1850-1900) level, and Copernicus/ECMWF at 1.48°C.

Berkeley said that “a single year exceeding 1.5°C is a stark warning sign of how close the overall climate system has come to exceeding this Paris Agreement goal. With greenhouse gas emissions continuing to set record highs, it is likely that climate will regularly exceed 1.5°C in the next decade.”

The global average temperature for May 2024 was 1.52°C above the 1850–1900 pre-industrial average, marking the eleventh consecutive month (since July 2023) for which the global average temperature reached or exceeded 1.5°C. June 2024 is very likely to be the warmest June on record. 

And scientists say that there is now a greater than 50% chance that 2024 will be hotter than 2023. One reason is that higher monthly temperatures are persisting (see Figure 2), even though the El Nino phase associated with higher levels of warming had finished by May 2024.  (In April, the Australian Bureau of Meteorology announced that the recent El Niño was officially over because sea surface temperatures in the central Pacific Ocean have “cooled substantially”.) 

One or even two years of 1.5°C does not constitute a trend, which technically can only be seen in retrospect over 20 to 30 years of data, but looking back in 10 years time to see what the trend was in the mid-2020s is less useful that developing an understanding of what is happening right now.

Figure 2: Global temperatures in 2023 and 2024

For some time, Jim Hansen and his colleagues have been saying that warming is accelerating, even before the extraordinary events in the second half of 2023.  This was encapsulated in their chart, reproduced here as Figure 3 (with an earlier version published in June 2023). Their projected peak of the El Nino above 1.6°C was right on the money, with the projected warming range (in yellow) showing a warming trend of 1.5°C between now and 2030. 

Figure 3:
Projected warming by Hansen et al.

By the end of 2023, Hansen’s view was that “the 1.5 degree limit is deader than a doornail” and warns that warming will accelerate to 1.7°C by 2030 and “2°C will be reached by the late 2030s”.

A paper published at the end of 2023 showed a “robust acceleration of Earth system heating observed over the past six decades”, where the “long-term acceleration of Earth warming aligns qualitatively with the rise in carbon dioxide (CO2) concentrations and the decline in aerosol concentration during the same period, but further investigations are necessary to properly attribute these changes”.

Analysis by CarbonBrief concurred that: 

There is increasing evidence of an acceleration in the rate of warming over the past 15 years…  broadly in line with projections from the latest generation of climate models and IPCC Assessment Report AR6…. the warming rate of 0.18°C per decade seen since 1970 has almost doubled to roughly 0.3°C per decade over the past 15 years.

Carbon Brief’s analysis also reveals that the speed up in warming projected in the latest climate models (known as CMIP6) is similar to the acceleration estimated by prominent climate scientist Dr James Hansen and colleagues in their much-discussed 2023 paper in Oxford Open Climate Change. Hansen and colleagues argued that the rate of warming would increase to between 0.27°C and 0.36°C per decade – or a 50-to-100% increase in the warming rate since 1970 – over the next 30 years.

Celeste Saulo, World Meteorological Organization Secretary-General agrees that the rate of human-caused climate change is accelerating, but “we are not there in terms of our scientific understanding of the implications of this acceleration. We don't fully understand how it is going to evolve.” 

Two key indicators are consistent with the observed acceleration in warming:

  • Ocean heat content: 90% of the heat generated by the greenhouse effect warms the oceans (with only 2% to the atmosphere, and the balance melting the polar ice and warming the land). With this great store of heat, it is oceans that drive atmospheric warming. Research published in 2023 showed that the rate of increase in ocean heat content has accelerated over recent decades. Ocean temperatures started spiking in March-April 2023, and global temperatures in June. The heat stored in the world’s oceans increased by the greatest margin ever in 2023, absorbing more heat than in any other year since records began. Associated with the onset of a strong El Niño, the global sea surface temperature was an astounding 0.3°C above 2022 values for the second half of 2023. 
  • Earth’s Energy Imbalance: Earth’s energy imbalance (EEI) is the difference between incoming energy from the sun and the amount of heat radiating from Earth back into space. The CERES project uses satellites to estimate EEI. Their data suggests that EEI has more than doubled since 2000, resulting in an acceleration of global warming’s impact on the Earth system. This was confirmed by new research published in May 2024. If EEI is increasing over time, it should drive an increase in the world’s rate of warming.

The causes of the acceleration include the still rising and record-breaking levels of annual greenhouse gas pollution — with carbon dioxide up an unprecedented 4.7 ppm in the last year — as well as decreasing levels of sulfates which have a temporary cooling impact, and the weakening of various carbon sinks, such as the Arctic and Amazon which have both become net emitters of greenhouse gases. Research released in April 2024 showed that “Recent reductions in aerosol emissions have increased Earth’s energy imbalance”.  

New climate models CMIP6

A  new generation of climate models (CMIP6) project faster warming than previous models, as CarbonBrief noted. At the time of their release, there was concern that some of the models were running “too hot”, and that this implied a level of climate sensitivity — a measure of how much heating is produced by a burst of greenhouse gases or other warming processes —  that was not realistic. But now there is more evidence that indeed sensitivity may be higher, for example,  “Ancient climate analysis suggests CO2 causes more warming than thought”.  

The average of this latest generation of climate models has an expected rate of warming 0.3°C per decade from 2015 through 2050 in the current-policy-type SSP2-4.5 scenario, which may be conservative. In this scenario, the world is around 2.2°C hotter by 2050.

Hansen, on the other hand, points to warming having already reached the 1.5°C trend by the mid-2020s, and with warming of 0.3°C per decade (or even greater), the 2°C trend being breached by around 2040. 

Whilst the 2023-24 warming burst is within the range of uncertainty of CMIP6 models, the global mean sea surface temperatures between 60S and 60N were at the the very top end of projections, and the exceptional sea surface temperatures over the North Atlantic region (0-60N, 0-80W) were likewise outside the model runs.

Another look at the figures

Figure 4 is taken from work by Zeke Hausfather in October 2023, “Global temperatures remain consistent with climate model projections”. It shows projected warming of CMIP6 models (blue shading with model mean as black line) and observations to September 2023 (red line). Since then there have been another six months of temperatures around the October mark. 

Figure 4: CMIP6 climate model projections and warming observations to September 2023

The chart is based on a 1900-2000 baseline — an odd and confusing thing to do — which as you can see shows zero warming around 1970.

In fact, warming between the recognised pre-industrial baseline of 1880-1910 and 1970 (1900-2000 baseline) is approximately 0.25°C. This is taken into account in Figure 5, where a red dashed line has been added to show the 1.5°C level compared to pre-industrial. 

Chart 5: Climate projections and observations compared to 1.5°C level (pre-industrial benchmark)

When that is done, one sees that CMIP6 actually projects a warming trend of 1.5°C level compared to pre-industrial right now! And the observations (red line) spike to 1.8°C in September 2023.  

Now as the 2023-24 El Nino gives way to a  La Nina, global temperature will cool and the next El Nino period will very likely be higher than the 1.65°C observed in 2023-24..  

In fact, says the WMO Global Annual to Decadal Update, "There is a 47% likelihood that the global temperature averaged over the entire five-year 2024-2028 period will exceed 1.5°C above the pre-industrial era”.  

Let’s repeat that: The World Meteorological Organisation says there is a one-in-two chance the current five-year period will exceed 1.5°C.

So is the climate system, for all practical purposes, now close to a 1.5°C trend?  If CMIP6 is to be taken at face value, the answer is yes. And the data now seems consistent with those models. 

Of course there is uncertainty as there is with all statistical work about future projections, but the precautionary principle should apply as there is no reason for delay, but rather the opposite. It is important to understand the “fat-tail” risks and the plausible worst case scenarios and act with speed to make sure they do not come true.