IPCC 2021 Summary for Policy Makers¹ :
A frightening synthesis about global warming

What can no longer be ignored or debated²

- The increases in greenhouse gas (GHG) concentration that have been observed since 1850 are primarily caused by human activity.

- Each of the last four decades has been the warmest on record since 1850. The global surface temperature between 2001 and 2020 was 0.99°C higher than between 1850 and 1900. The bigger increases took place over land (1.59°C) rather than in the oceans (0.88°C).

- Human influence is certainly one of the main factors explaining the global retreat of glaciers since the 1990s and the melting of the Arctic ice pack between 2010 and 2019 (about - 40% in September).

- The average sea level has been raising by 1.9 mm per year between 1971 and 2006 ; this expansion is now 3.7 mm per year (average over the period 2006-2018).

- Since the measurements have been taken into account in the last report (2011), the concentrations of carbon, methane and nitrous oxide in the atmosphere have continued to increase. 2019 CO2 concentrations are higher than at any time in the past 2 million years.

The rate of global warming and its associated effects have been unprecedented for, at least, the last 2,000 years.

Five climate scenarios and projections for the end of the century.

The five scenarios modeled by the IPCC take into account a wide range of human and natural factors that influence climate change. The projections are made for the period 2015-2100, with the period 1850-1900 as a point of comparison.

The two most optimistic scenarios assume a very significant decrease in CO2 emissions from 2025, to reach carbon neutrality in 2060 (1st scenario) and 2080 (2nd scenario), and then enter an era of negative emissions. Global surface warming would then be limited to 1.9°C and 2.6°C respectively.

The median scenario corresponds to the current situation : awareness, but inability of the main emitting countries to meet their commitments and targets. A gradual and continuous decrease would only be achieved from 2060 onwards and would result in a 4.5°C increase in 2100 compared to the 1850-1900 baseline.

The two extreme scenarii offer trajectories of increasing CO2 emissions based on the absence of real mechanisms or will to correct. They would lead to a catastrophic warming of +7 to 8.5 °C.

Adaptation is no longer optional.

Whatever the hypothesis, we must keep in mind that, as said above, very long-term mechanisms are already at work.

They will inevitably result in the release of large quantities of carbon that have been naturally stored for thousands of years, thus making the equation more complex.

Similarly, IPCC experts insist on the significant increase of the frequency and amplitude of extreme climatic phenomena that have been documented and analyzed for several decades. The frequency of heavy rain episodes, tropical storms and hurricanes, heat waves (up to 50°C), droughts and major fires... should significantly increase from now until the end of the century. Reliability levels of these prediction range from "medium" to "strong", depending on the latitude.

On the global scale, this fairly unpleasant picture raises questions about the capacity to meet the food needs of the world's population in a few decades, in case of absence of proactive and coordinated adaptive measures.

As far as national and local public authorities are concerned, it raises the key questions of settlement policies and/ or the vulnerability of human activities in coastal and highland areas.