Is This Climate Change Like Any Other?

As mentioned in the introduction, the Earth has warmed by an average of 1.5 degrees compared to the period 1850-1900. However, it has been demonstrated that, throughout history, the average surface temperature of the globe has varied primarily based on the concentration of greenhouse gases in the atmosphere.

This discovery is owed to an unexpected source: the controversial act of putting ice in whiskey. In 1965, glaciologist Claude Lorius, then on an expedition to Adélie Land, had the intuition that air bubbles trapped in ice could reflect the composition of the past atmosphere. This led to the discovery of the correlation between climate temperature evolution and greenhouse gas concentration.

The graph above clearly illustrates the almost perfect correlation between temperature and CO₂ concentration over the past 800,000 years. It also shows that CO₂ concentration fluctuates between 180 and 300 ppm. Moreover, there has never been a change as sudden and rapid as the current one, where we have reached 415 ppm. The consequences on temperature evolution remain uncertain and not yet fully perceptible, especially since the concentration of greenhouse gases in the atmosphere continues to increase exponentially.

Finally, during the last glaciation, the CO₂ concentration was about 180 ppm, and the temperature was approximately 5 degrees lower than today.

Currently, we have solid scientific evidence linking greenhouse gas concentration to climate evolution. But how do these gases accumulate, and what are their natural degradation dynamics?

Causes of Climate Change

As previously discussed, greenhouse gas concentrations have increased exponentially since 1850, leading to positive radiative forcing and rising average temperatures. Let’s now examine the causes of this evolution.

Since 1850, humanity has discovered and exploited new, incredibly dense energy sources, particularly coal, oil, and natural gas. These energy sources, known as fossil fuels due to their formation process, today account for 80% of total energy consumption.

Energy is primarily generated by burning these resources. However, this combustion emits CO₂, an extremely stable greenhouse gas that accumulates in the atmosphere. About 60% of emitted CO₂ will be degraded by the biosphere within 100 years, while 20% will persist for over 1,000 years. The energy sector alone accounts for approximately 75% of global greenhouse gas emissions.

Additionally, human population growth has led to intensified agriculture. This is reflected in both:

• An increase in cultivated land (from 1 billion hectares to 4.5 billion hectares).
• Improved agricultural yields (e.g., global wheat yield has increased from about 1 ton to nearly 4 tons per hectare).

However, the decomposition of organic matter from agricultural activities emits another greenhouse gas: methane (CH₄), which has 28 times the global warming potential of CO₂, even though it degrades within a decade.

Furthermore, increased yields rely heavily on fertilizers, which generate nitrous oxide (N₂O), a gas 273 times more powerful than CO₂, with an atmospheric lifespan of 121 years. Agriculture accounts for 12% of global greenhouse gas emissions, making it the second-largest sector after energy.

Thus, the cause of climate change is solely linked to human activities.

If human activities are responsible for the rapid increase in greenhouse gases, nature has mechanisms to absorb part of them. Let’s analyze these processes to better understand why they are no longer sufficient to balance the climate system.

The Natural Degradation of Greenhouse Gases

Not all greenhouse gases behave the same way in the atmosphere. Some, like water vapor, are quickly regulated by natural cycles, while others, like CO₂ and methane, have very different lifespans and impacts. Let’s detail their behavior to better understand their future impact on the radiation balance.

🌊 The Water Cycle

Unlike other greenhouse gases, water vapor does not accumulate permanently in the atmosphere. Water droplets condense and return to liquid form when clouds reach high altitudes.

However, climate change disrupts this cycle. Higher temperatures result in more water vapor in the air, which can amplify warming, dry out soils, and cause more intense rainfall, leading to surface runoff on already dry land. We will explore these phenomena in detail when discussing extreme weather events in the section on climate change consequences.

🌱 The Carbon Cycle

The natural reduction of CO₂ relies on several biogeochemical processes integrated into the carbon cycle. These mechanisms allow terrestrial and oceanic ecosystems to naturally capture and store CO₂, forming the so-called carbon sinks:

1️⃣ Photosynthesis

• Forests, grasslands, and crops capture carbon and store it in their biomass.
• Oceans capture CO₂ through phytoplankton, which plays a key role in its regulation.

2️⃣ Carbon storage in soils

• Some of the carbon captured by plants returns to the soil as organic matter (dead leaves, humus).
• This carbon can be stored for centuries or transformed into coal, oil, or natural gas over millions of years.

3️⃣ CO₂ absorption by oceans

• 30% of human-emitted CO₂ is absorbed by oceans.
• However, this acidifies the oceans, threatening marine life and disrupting the calcification of marine organisms.

4️⃣ Natural CO₂ mineralization

• Certain rocks react with CO₂ to form carbonates (e.g., limestone), locking carbon away in the long term.

🔄 The Methane Cycle (CH₄)

Unlike CO₂, which can persist for centuries, methane (CH₄) is a more reactive gas that is naturally removed through chemical and biological processes:

1️⃣ Oxidation of methane by hydroxyl radicals (OH·)

• The primary natural degradation mechanism for methane is its reaction with hydroxyl radicals (OH·) in the troposphere.
• This reaction triggers an oxidation chain that gradually converts methane into CO₂ and water vapor over about 10 years.

2️⃣ Oxidation in the stratosphere

• A small portion of methane reaches the stratosphere, where it reacts with ozone (O₃) and chlorine (Cl).
• This reaction contributes to water vapor formation in the stratosphere, which itself has a greenhouse effect.

3️⃣ Biological consumption by methanotrophic bacteria

• Some bacteria in soils and oceans absorb and consume methane before it enters the atmosphere.
• These bacteria act as a natural filter, particularly in wetlands, rice fields, and peatlands.

📌 Current problem: Despite these degradation mechanisms, human methane emissions are now too high. As a result, CH₄ accumulates, reinforcing the greenhouse effect and contributing to global warming.

🔄 The Nitrous Oxide Cycle (N₂O)

Nitrous oxide (N₂O) is an extremely potent greenhouse gas (273 times more than CO₂) and is particularly stable in the atmosphere.

1️⃣ What happens to N₂O in the atmosphere?

• Unlike methane, which is quickly oxidized in the troposphere, N₂O is extremely stable and can persist for over 120 years.
• It slowly rises to the stratosphere, where it is mainly destroyed by UV radiation.

2️⃣ Degradation of N₂O in the stratosphere

• At altitudes above 20 km, UV radiation breaks down N₂O.
• This reaction produces nitrogen oxides (NO, NO₂), which react with ozone (O₃) and accelerate ozone depletion (similar to CFCs).

3️⃣ Absorption in soils and oceans

• A small portion of N₂O is consumed by bacteria in soils and oceans through denitrification.
• Some bacteria transform N₂O into nitrogen gas (N₂), which is harmless to the atmosphere.

📌 Current problem: The intensification of agriculture and the massive use of nitrogen fertilizers have led to rapid, uncontrolled N₂O emissions, accumulating in the atmosphere and contributing to climate change.

🔥 Uncontrolled Accumulation

In conclusion, the rapid accumulation of greenhouse gases due to human activities has led to an unprecedented rise in global temperatures. Despite natural degradation processes, our current emission rates far exceed what the Earth can absorb. This imbalance is driving climate change, with projected temperature increases ranging from +1.8°C to +5.7°C by 2100. Understanding these dynamics is crucial for addressing the challenges ahead. »

In the next article, we will analyze the consequences of this warming.