Global changes

Climate refers to the average weather conditions over long periods (temperature, rainfall, winds, etc.). It differs from day-to-day weather and also includes extreme events (storms, heatwaves, etc.) as well as the interactions between:

• air (atmosphere),
• land (lithosphere),
• water (hydrosphere),
• ice (cryosphere),
• living organisms (biosphere).

Climate change therefore describes a gradual alteration of these parameters over the long term.

This transformation is largely the result of increased greenhouse gases linked to human activities such as industry, transport, and intensive agriculture.

The greenhouse effect is a natural phenomenon: certain gases present in the atmosphere (CO₂, CH₄, water vapour, etc.) trap part of the Earth’s heat, maintaining an average temperature of 15 °C at the planet’s surface.

Without the greenhouse effect, the temperature would be around -18 °C, as on the Moon. In other words, life on Earth as we know it today could never have developed.

But since the Industrial Era, greenhouse gas emissions have increased sharply, causing global warming. This is referred to as radiative forcing: the Earth traps more heat, which intensifies climate disruption.

The ocean absorbs around a quarter of the CO₂ present in the atmosphere. Once dissolved in the water, this gas alters its chemical balance, lowering its pH. This phenomenon is called acidification.

With a pH above 7, the ocean is considered basic. However, since the start of industrialisation, the average pH of surface ocean waters has dropped from 8.2 to 8.1. This is why we speak of acidification.

Although it seems slight, a decrease of 0.1 pH units actually corresponds to an increase of about 30% in ocean acidity since the pre-industrial era. This is explained by the fact that the pH scale is logarithmic: each small change represents a significant variation in acidity.

In the long term, ocean acidification threatens calcifying organisms such as corals, shellfish, and plankton, which require calcium carbonate to develop. Moreover, these species are essential to the balance of marine ecosystems… and to our food supply!

Glaciers, sea ice, snow: the cryosphere is melting at a concerning rate. This is one of the most obvious consequences of global warming.

However, these ice masses play a key role by reflecting the Sun’s rays thanks to their white colour (albedo effect). When they melt, the Earth’s surface absorbs more heat, which further accelerates global warming.

Furthermore, some ice also traps methane. As it melts, it releases this potent greenhouse gas, worsening the climate disruption.

Since 1900, the oceans have risen by more than 20 cm. Two factors explain this rise:

• water expands as it warms,

• melting glaciers add fresh water to the oceans.

Some coastal areas are already facing more frequent flooding, erosion, and salinisation of their freshwater resources.