18 October 2023

One swallow doesn’t make a Spring, so do a few super-warm months mean global warming has really hit 1.5°C?

by David Spratt

One swallow doesn’t make a Spring. And a week, a month, or even a year of global warming above 1.5°C does not make that the long-term trend.

In this field, a trend is an average over a longer term, by scientific convention 30 years, though sometimes shorter periods may be used.  From this point of view, a trend can’t be determined till way after the event when the running averages can be calculated. It’s similar for tipping points — you generally can’t say they have been breached till you have the observational evidence some time after the event — and then it is too late.

So a more pertinent question is this: when we look back in five, ten, fifteen years, is it likely that the global warming trend during 2023 and 2024 will be seen to have been 1.5°C or above?

 James Hansen, the former climate chief for NASA — and sometimes affectionately known as the godfather of contemporary climate change research — has a stunning answer. 

We are now moving into a new El Nino period, in which warming is enhanced as some heat is transferred from the oceans to the atmosphere. In such events — in which a single cycle generally runs from one northern hemisphere autumn to the next — warming gradually builds and reaches its peak in the post-Christmas period, as seen in the diagramme below. (There can also be multi-year El Nino events.)

In his most recent monthly e-newsletter, Hansen and his colleagues plot this year’s monthly temperatures against previous recent El Ninos:


Hansen et al. report that “the September global temperature anomaly leaped to more than +1.7°C relative to the 1880-1920 mean, which exceeds the prior warmest September in the period of instrumental data by about +0.5°C”.”   In some databases, the warming is found to be 1.8°C.  

Hansen et al. have been warning for years that we are entering a period of accelerated warming, driven by continuing high human-caused greenhouse gas emissions, combined with less temporary cooling from sulfate aerosols as clean air policies drive down the use of high-sulfate fossil fuels, such as bunker fuel used in commercial shipping. 

Looking at the August figures, they wrote that they “anticipate acceleration of the long-term global warming rate by at least 50%, i.e., to at least 0.27°C/decade, mainly due to reduction of human-made aerosols (fine airborne particles).”

Their projected of accelerated warming is shown as the yellow band in the following diagram, with the estimated el Nino peak as a purple block. The yellow band shows warming of 1.5-1.6°C by 2030.

The last four months — June to September — have been more than 0.4°C warmer than any previous similar period in the instrumental record. They say that:

The average anomaly of the past 4 months (+0.44°C relative to the same months in 2015, the origin year of the 2015-16 El Nino) is probably more important. If this relative anomaly is maintained through this El Nino (through Northern Hemisphere 2024 spring) the peak 12-month mean global warming will reach +1.6-1.7°C relative to 1880-1920.  Decline of global temperature following an El Nino peak is 0.2-0.3°C.

Their conclusion is straight-forward but stunning: "Thus, if this El Nino peak is as high as we project it will be, global temperature will oscillate about the yellow region. The 1.5°C global warming level will have been reached, for all practical purposes” (emphasis added).

If this is the case, "there will be no need to ruminate for 20 years about whether the 1.5°C level has been reached, as IPCC proposes. On the contrary, Earth’s enormous energy imbalance assures that global temperature will be rising still higher for the foreseeable future."

Berkeley Earth says it is likely that 2023 will exceed 1.5°C (see diagramme below).


And the Earth’s Energy Imbalance — an indicator of the level of future warming — is rising rapidly and in an unprecedented manner, which would suggest the rate of warming will increase: 

As to whether the rate of warming is accelerating, most in the scientific community have had a wait-and-see response to Hansen’s analysis. But now some are beginning to agree with him.  

The European Space Agency's Copernicus Climate Change Services says that “satellite measurements of Earth’s energy imbalance — the difference between energy entering the atmosphere from the sun and the amount of heat leaving — show a strong increase in the amount of heat trapped over the past two decades. If Earth’s energy imbalance is increasing over time, it should drive an increase in the world’s rate of warming.”

Prof. Ed Hawkins responded to the September figures with: “Surprising. Astounding. Staggering. Unnerving. Bewildering. Flabbergasting. Disquieting. Gobsmacking. Shocking. Mind boggling.”

“The Earth right now is far warmer than the previously measured record for this time of year. Even with a growing El Niño, the pace and size of the uptick that we've seen this year is pretty shocking,” said Dr Robert Rhode.

And there is another debate about whether the developing El Nino can explain the large jump in warning. Warming normally reaches it peak in the post-Christmas period, so it is somewhat unusual that such leaps in monthly temperatures would be observed early in the El Nino cycle.

Prof. Stefan Rahmstorf says “it is clear that the anomalously high global temperatures of the past four months (above the global warming trend) can’t be explained by El Niño, if we take past El Niño events as examples”.

And Dr Matt Patterson of Oxford concurs: “Can the strong El Nino explain high September global temperatures? My very rudimentary analysis suggests it can only explain a relatively small amount of the 'jump'.”.

Prof. Michael E. Mann who has been insisting that there is no observed acceleration in warming and no poorly-understood factors behind the recent temperature data says that: “If the October global surface temperature anomaly comes in close to the record September value, I'll reconsider the possibility that there is something unusual & not easily explained i.e. in terms of El Nino & natural variability on top of steady, long-term human-caused warming.”