In its article 2, the Paris agreement commits the international community to limit warming relative to the pre-industrial era “well below 2°C” and to “continue action” to reach the target of 1.5° vs. It does not explicitly indicate which indicator to use to judge where we stand in relation to these objectives.
By Christian de Perthuis, Paris Dauphine University – PSL
In January, the World Meteorological Organization (WMO) estimated that the average temperature in 2023 will be at 1.45°C above the pre-industrial era. She announces that 2024 risks being even hotter, due to the extension of the episode El Niño appeared last summer. A warming of 1.5°C could be observed for the first time over a full year. According to the European Copernicus program, this is already the case considering the rolling year from February 2023 to January 2024.
Due to short-term climate variability, it would be incorrect to conclude that global warming has reached 1.5°C. But how are these indicators established and how can we use them to judge our current position with regard to the objectives of the Paris agreement?
How we observe global temperature
The WMO is a United Nations agency, based in Geneva. It consolidates in its annual reports information from 6 organizations which have their own observation systems and manage historical databases on global temperatures.
Three are based in the United States: the public agency NOOA in charge of observing the oceans and the atmosphere, the GISS which depends on NASA and the Berkeley Earth, a non-profit association of scientists. In Japan, the database JRA-55 is managed from the national meteorological service, as is that of HadCRUT5 from Hadley Center UK. Finally, the European program Copernicus takes care of the ERA5 database.
Over the recent period, the dispersion of global average temperature estimates is very low. It increases as we go back in time. In past periods, we had in fact much fewer observations and they had neither the precision nor the reliability of those provided today by satellites. This raises the question of the historical reference to be taken into account to calculate warming relative to the pre-industrial era.
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The IPCC has looked into this question in its various assessment reports. He recommends considering the average temperature estimated over the period 1850-1900 as representative of that of the pre-industrial period. By summarizing existing work, it gives an estimate of the rise in the thermometer between 1850 and 1900 and the recent period (+0.69 °C between 1850-1900 and 1986-2005 according to the 6e report).
The WMO is resuming this work to consolidate the observations provided by the 6 research centers and produce the reference estimate of the level of warming reached each year. It is the latter which appears in the graph below: an average rise in the thermometer estimated at 1.45°C for the year 2023, suggesting the possibility of a warming of 1.5°C in 2024 if the The El Niño episode is not weakening.
Infra-annual averages: 1.5°C reached in 2015 and 2016
In reality, the 1.5°C target had been reached much earlier, if we refer to sub-annual data. According to those reported in the Copernicus program database, the first days experiencing a warming of 1.5 °C were measured for the first time in 2015. In November 2023, Copernicus reported the first daily averages above 2 °C.
Regarding monthly averages, February 2016 was the first month to observe a warming of 1.5°C relative to the pre-industrial era. The rise in the thermometer during this month was also explained by an episode El Niño high intensity. In 2023, monthly averages will exceed 1.5°C every month starting in summer.
However, it would not occur to anyone to assert that we have reached the critical warming threshold of the Paris agreement because of these daily or monthly exceedances. But what if the 1.5°C target is reached for a full year?
The IPCC points out that a year with warming at 1.5°C does not mean that the corresponding target of the Paris Agreement has been reached. He recommends using multi-year averages. In the sixth IPCC report the diagnosis of an observed warming of 1.1°C relative to the pre-industrial era thus concerns the decade 2011-2020. To diagnose whether the 1.5°C target or the 2°C limit has been reached, the IPCC recommends using an indicator spanning two decades.
Where are we in 2024?
The WMO indicates that following the average temperature observed in 2023, warming reaches 1.2°C over the last decade (2014-2023). Following the IPCC recommendations, we will have to wait to know the average warming over the next decade (2024-2033) to be able to assess the state of current warming. The method makes it possible to judge a posteriori whether the temperature targets have been reached or exceeded.
By definition, we do not know the temperatures for the next decade. But we can estimate them based on past developments. Since 1970, the evolution curve of average temperatures has followed a very statistically robust trend: the thermometer has increased by 0.2°C per decade. As long as the growth in the stock of greenhouse gases in the atmosphere is not contained, there is no reason to expect this trend to slow down.
So let’s extend this trend over the next 10 years. Average warming will then amount to 1.4°C over the next decade 2024-2033 (1.2°C + 0.2°C). We can therefore consider that we have reached an average warming of 1.3°C at the beginning of 2024, with an unchanged warming trend over the next ten years.
Of course, this above all shows that the trend must be reversed urgently. If it continues over the next decade, the 1.5°C target will be reached around the middle of the next decade, as shown in the graph. If it continued thereafter, in 2050 we would be halfway between the 1.5°C target and the 2°C limit.
Christian de PerthuisProfessor of economics, founder of the “Climate Economics” chair, Paris Dauphine University – PSL
This article is republished from The Conversation under Creative Commons license. Read theoriginal article.