What climate for you, your children, your grandchildren?  is the written transcript of the conference held by Mrs. Valérie Masson-Delmotte, paleoclimatologist research director at CEA and co-chair of the IPCC Group 1, in Versailles on June 30, 2019. In fact, the book is divided according to the classic sequence of a conference. The first part is devoted to the speech of the physicist, whose objective is to offer keys to understanding the climate and its evolution, while the second part recounts the discussion with the audience that followed.
Valérie Masson-Delmotte began by stating that the scientific community has reached the conclusion that the climate has been warming since 1850. At the time of the conference, the average temperature in the world is one degree higher than in the pre-industrial era, and one and a half degrees higher in France. While climate change is not new, the current warming is unprecedented over such a short time: glacial and interglacial variations have previously occurred on time scales of about 20,000 to 400,000 years.
Past climate changes can be explained by two phenomena: “the starting point is the change in the distribution of sunshine according to latitudes and seasons on time scales of thousands of years, but the Earth also amplifies these disturbances.” The amplifying nature of the earth is caused by the retroactive loops of climate. For example, as the climate warms, snow and ice stored on the continents drain away rapidly, reducing the reverberation effect of heat and causing sea levels to rise; as the seas warm, they release CO2; in tropical regions, wetlands exhale methane; as the heat increases, the concentration of water vapor in the air increases, and with it the greenhouse effect intensifies. Thus, only five degrees on average at the Earth’s surface separate a mild state from an icy state.
Today, the evolution of the climate is not due to the position of the earth in relation to the sun, nor to solar or volcanic activity: these variations cause changes that are too slow, of the order of 1°c per thousand years. It is due to human activity, which has been emitting greenhouse gases since the 1750’s. These emissions, largely due to the use of fossil fuels, are changing the composition of the atmosphere. Levels of carbon dioxide, methane and nitrous oxide are respectively 1.4, 2.5 and 1.3 times higher than pre-industrial levels. “We are now releasing forty billion tons of carbon dioxide per year.” These gases trap heat on Earth and their effect is amplified by climate feedback loops.
However, as Ms. Masson-Delmotte explains, “natural environments help us limit the extent of climate disruption.” The oceans capture 20-30% of CO2 emissions, at the expense of underwater biodiversity, which suffers from the change in water chemistry. Vegetation and soils also absorb about 30% of emissions due to the fertilizing effect of CO2. “15 to 40% [of emissions in the atmosphere today] will continue to be there and change the climate on a time scale of about two thousand years.”
For heat, the warming of the air, the soil and the melting of the ice represent respectively 2%, 5% and 3% of the additional energy stored. 90% of this energy is absorbed by the oceans. If the oceans help us, they also maintain the warming over time because of marine currents. “We cannot go backwards with global warming.” The warming that is now established at one degree above pre-industrial levels cannot be stopped in the short term, but it can be stabilized.
If we stop emitting greenhouse gases, in the best-case scenario, sea levels will continue to rise for several centuries to reach a rise of 1.5 meters; hot days with record heat will multiply as will torrential rains and storms.
If we continue at the current rate, we will reach +1.5°C in the period 2030-2035. Under these conditions, 100 million people would be affected by rising seas; the number of people with water insecurity would double, agricultural yields of cereals and fishing would decrease; a significant loss of biodiversity would be expected. Also, in a very graphic way, “when we put a ton of CO2 in the atmosphere, it represents at the end of the summer 3 square meters of ice floe less”.
The COP21 promises are insufficient to achieve a stabilization trajectory of global warming: “If they are realized, they imply that global greenhouse gas emissions continue to increase until 2030. Whereas if we want to keep warming well below two degrees, emissions must fall by a quarter by 2030.”
However, Ms. Masson-Delmotte ends on a positive note by reviewing the solutions available to stabilize climate change. This would require decarbonizing energy production and its end uses, which seems feasible with the lowering of prices for renewables and the electrification of uses. It would also mean transforming cities and lifestyles by focusing on synergies and including the whole of society. “If we all act collectively now to reduce our greenhouse gas emissions, we have a chance that developments will continue at the current rate until 2050 and then stabilize.”
Coline Metta-Versmessen, Research Fellow Characterization of interactions between the Effort Sharing Regulation and the EU-ETS market.
 Valérie Masson-Delmotte, Quel Climat pour vous, vos enfants, vos petits-enfants ? Ed. Bayard, 2021, 90 p.