The state of the Earth’s climate (how long before the annual-mean warming will beabove 1.5° C?) 

Editorial by Roberto Buizza – Full Professor of Physics at the Scuola Superiore Sant’Anna in Pisa.

Since the beginning of the industrial era, man has been changing the Earth’s climate substantially and extremely quickly, due to the use of fossil fuels: coal, fuel oil and methane gas. The impact is particularly evident in the average global warming, which in September 2023 reached 1.75oC (compared to the average temperature of September in the period 1850-1900), a value never touched before (Figure 1). 2023 could be the warmest year since 1979, as September follows three months, August, July, and June, characterized by record temperatures and an average global warming of 1.5oC.

Figure 1. Global average surface warming (temperature at 2-meters). The figure shows, for each year, the average of the monthly anomalies, where the monthly anomaly is the difference between the average temperature of the month and the average of the months of the period 1850-1900. The two red lines indicate linear growth in two 20-year periods, 1980-2001 and 2002-2023. Their slopes indicate an increase in the global average temperature of 0.11oC every 10 years in the first period, and 0.23oC in the second period. Please note that the anomaly for 2023 is an approximation given that we are at the beginning of October, and has been calculated by averaging the anomalies of the last 12 months available, from October 2022 to September 2023 (Data from Copernicus Climate Change Service, 

The continuous emissions of greenhouse gases and their continuous accumulation in the atmosphere continue to cause not only the warming of the planet, but also the melting of ice, the rise in sea levels, and an increase in the frequency and intensity of extreme phenomena: intense rainfall, heat waves and droughts, windstorms. Events that cause increasingly substantial damage to ecosystems and society, deaths and damages to infrastructure, and a reduction in biodiversity. Damages that affect the poorest countries the most; countries that have limited resources to adapt to the changing climate and to cope with extreme events, and that have contributed the least to the accumulation of greenhouse gases in the atmosphere. Damages that make entire countries unliveable, and cause migration.

Man is the main cause of climate change
There is no doubt that man is the main cause of climate change. This is confirmed by observations, experiments and considerations based on the laws of physics. This statement is confirmed, for example, by the fact that the growth in the concentration of greenhouse gases in the atmosphere since the pre-industrial era is consistent with the combustion of coal, fuel oil and methane carried out during the same period, and with the decrease in the concentration of oxygen in the atmosphere linked to this combustion. It is also confirmed by numerical experiments with Earth system models, which show that the reconstruction of the current climate is possible only if the increase in the concentration of greenhouse gases is considered. 

In terms of CO2 concentration, we have to go back in time to 2.5 million years ago to find values similar to those of the last 5 years, above 400 parts per million (ppm). In terms of changes in CO2 concentration over the last 800,000 years, before 1900 the concentration fluctuated between 200 and 300 ppm, with variations of the order of 100 ppm over thousands or tens of thousands of years. It is only after the continued growth of fossil fuel use that values have risen extremely rapidly, from 320 ppm in 1960 to over 420 ppm in 2023. For methane, in 1985 emissions were about 1,650 ppb (parts per ‘billion’), and in 2021 they reached about 1920 ppb, with a relative growth in the period 1985-2021 even greater than that of CO2, partly because of the increase in the last 20 years of the use of methane instead of coal in combustion processes.
The responsibility for these emissions lies substantially with the developed Western world, which has contributed mostly to the accumulation of greenhouse gases in the atmosphere. Given that warming is the effect of the accumulation of greenhouse gases, if we add up greenhouse gas emissions from 1850 to 2021 we find that the largest contribution came from the United States of America (18% of emissions), the European Union (12%) and China (12%). They are followed by the other countries of the world, with the countries mostly affected by the impacts of change often characterized by minimal contributions.

There is no ‘acceptable’ value of global warming
Science indicates that if we have not seen irreversible changes so far, continued warming could induce them. Components of the Earth system that could suffer them are, for example, the Arctic sea-ice that could melt completely, the Atlantic ocean currents that could change intensity and direction substantially, and the Amazon forest that could transform from a sink to a source of greenhouse gases. If even one of these components of the Earth system underwent irreversible changes, it would cause even more radical changes to Earth’s global climate than has been observed to date. It would cause migration from the countries most affected by the effects of climate change to increase to more substantial levels than today.
Continued increases in emissions are causing accelerating warming, which in turn is causing rising sea levels and melting ice to accelerate. Figure 1 shows that over the period 2002-2023 the global average temperature increased by about 0.23oC every 10 years, more than double its growth of 0.11oC every 10 years between 1980-2001. Since 2006, sea level rise has accelerated by almost a factor of 3 compared to previous decades, from +1.4mm per year to +3.6mm per year. The ice at the poles continues to retreat and reduce in thickness: in the last year the extent of the ice of both polar caps has reached minimum values.

Another effect of climate warming is the increase in the frequency and intensity of extreme events, such as heat waves and droughts, precipitation, and floods. In many regions of the world, now precipitation tends to be increasingly concentrated in shorter periods, during which they are more intense than in the past. The increase in intensity is related to the fact that a warmer atmosphere is able to contain greater amounts of water vapor, and therefore cooled air masses can cause more intense precipitation. A warmer atmosphere is also a more energetic system, and therefore phenomena such as low pressure systems and cyclones can intensify more, and therefore be associated with more intense and destructive winds.

Greenhouse gas emissions must be reduced
In 1960 global CO2 emissions were about 22 Gt CO2-eq (1 Gt is 109 tons) and the concentration of CO2 in the atmosphere was about 318 parts per million (ppm). In 2021 global emissions were 54.6 Gt CO2-eq, and today the concentration of CO2 in the atmosphere has reached 420 ppm. If we add up the annual emissions of greenhouse gases over tens of years, we find that between 1960 and 2021, in 61 years, about 2,370 Gt CO2-eq were released into the atmosphere, and that in the previous 110 years, between 1850 and 1959, about 1,022 Gt CO2-eq were emitted into the atmosphere. Given that global emissions do not decrease, if we continue to emit about 54 Gt CO2-eq per year as in 2021, it will take only 19 years to emit another 1,022 Gt CO2-eq, and only 44 years to emit another 2,370 Gt CO2-eq into the atmosphere.

Quasi-linear projections for the future
There is a quasi-linear relationship between greenhouse gas emissions into the atmosphere, the concentration of CO2 observed in the atmosphere and the average global warming, which indicates that each further injection into the atmosphere of 1000 Gt CO2-eq of greenhouse gases corresponds to an increase in CO2 concentration of about 42 ppm, and a further global average warming of about 0.5oC.

This relationship can be applied to estimate that to limit average annual global warming to below 1.5oC, we have to limit global emissions to less than 600 Gt CO2-eq of greenhouse gases. If in the coming years we continue, as in 2021, to emit about 54 Gt CO2-eq of greenhouse gases, in 11 years we will have emitted an additional 600 Gt CO2-eq of greenhouse gases, and therefore the average annual warming limit 1.5oC will be exceeded in 2032. Note that Europe, a region with an average warming greater than twice the global value (about 0.5oC every 10 years), in 2032 will have reached an average annual warming compared to the pre-industrial period of about 3.0oC (today the average temperature in Europe is already about 2.5oC higher than in the pre-industrial period).

Figure 2. Total greenhouse gas emissions of Italy, expressed as % compared to 1990, when 0.50 Gt CO2-eq were emitted (data from Our World In Data;

The emission reduction targets that Italy should adopt
We must stop denying the anthropogenic origin of global warming, and act to rapidly reduce emissions if we want to avoid disastrous impacts. Impacts that for Italy could translate into more frequent and intense extreme events, an acceleration of the melting of the Alps glaciers with an impact on water supply in the Po’ Valley, periods of drought in the South, and much more substantial immigration from North African countries.

The European Union has adopted as a 2030 target a 55% reduction in emissions compared to the 1990 value (‘Fit-for-55’). The comparison between emissions in 1990 and 2021 (Figure 2) indicates that in this period Italy has reduced emissions by 25%, on average by about 1% per year. From the 2021 level it can be calculated that to achieve a 55% reduction in emissions by 2030, Italy must reduce global greenhouse gas emissions by 5.5% per year, every year from 2021 to 2030. This means accelerating in a substantial way the reduction of the emissions of greenhouse gases.

Another goal discussed at the Conference of the Parties (COP) meetings is to achieve net-zero greenhouse gas emissions by 2050. Suppose Italy meets the EU’s ‘Fit-for-55’ targets, and manages to reduce emissions between 2022 and 2030 by 5.5% per year. A further average annual reduction of 5.5% between 2031 and 2050 would lead Italy to emit in 2050 about 14% of 1990 emissions (0.07 Gt CO2-eq instead of 0.5), while an average annual reduction of 7% per year would lead Italy to emit 9.7% of 1990 emissions in 2050 (0.049 Gt CO2-eq instead of 0.5).

These numbers indicate that Italy must reduce emissions by at least 5.5% per year between 2022 and 2030, and therefore by at least 7% between 2031 and 2050, if it really wants to reach the European Union’s ‘Fit-for-55’ target by 2030, and get ‘close’ to the goal of net-zero emissions by 2050.

Further readings and essential links:

  • Copernicus Climate Change Service:
  • Copernicus Climate Change Service (C3S), 2023: European State of the Climate 2022:
  • European Union, 2021: European Green Deal: Commission proposes transformation of EU economy and society to meet climate ambitions.
  • Intergovermental Panel on Climate Change (IPCC):
  • IPCC, 2021: Summary for Policymakers, Working Group I: The Physical Science Basis.
  • IPCC, 2022a: Summary for Policymakers, Working Group III: Mitigation of Climate Change.
  • IPCC, 2022b: Summary for Policymakers, Working Group II: Impacts, Adaptation, and Vulnerability.
  • National Oceanographic and Atmospheric Administration (NOAA) Global Monitoring Laboratory, Mauna Loa, Hawaii –
  • Our World in Data: