CO₂ and the greenhouse effect
To understand the impact of carbon dioxide on the climate, it is important to understand the greenhouse effect. The Earth receives energy from the Sun. Some of this energy is absorbed by the planet’s surface and later released as infrared radiation.
Greenhouse gases, including carbon dioxide, methane and water vapor, absorb and re-emit some of this outgoing heat. This natural process keeps the Earth warm enough to sustain life. Without the natural greenhouse effect, average temperatures would be far lower.
The problem is therefore not the existence of CO₂ itself, but the rapid increase in its atmospheric concentration. Adding more carbon dioxide strengthens the greenhouse effect and changes the planet’s energy balance.
Why are CO₂ levels increasing?
Human activity is the primary reason for the rapid rise in atmospheric CO₂ concentrations observed since the Industrial Revolution.
The combustion of coal, oil and natural gas releases carbon that had been stored underground for millions of years. Electricity generation, transportation, heating and industrial production are consequently major sources of emissions.
Deforestation also contributes to the problem. Trees naturally absorb carbon dioxide through photosynthesis and store carbon in their biomass. When forests are destroyed or burned, this storage capacity is reduced and some of the accumulated carbon is released back into the atmosphere.
Cement production and several other industrial processes represent additional sources of CO₂ emissions.
Rising temperatures
One of the clearest consequences of increasing greenhouse gas concentrations is global warming. As more heat is retained within the climate system, the average temperature of the Earth’s surface increases.
This does not mean that every region becomes warmer at exactly the same rate or that cold weather disappears. Climate describes long-term patterns rather than individual weather events. Regional temperatures can vary considerably from one year to another even while the global long-term trend moves upward.
Increasing temperatures can have numerous consequences, including more frequent or intense heat extremes in many regions, changes in growing seasons and additional stress on ecosystems.
CO₂ and the oceans
The oceans play an important role in limiting the amount of CO₂ remaining in the atmosphere because they absorb part of human-generated carbon dioxide. But this process also has consequences.
When carbon dioxide dissolves in seawater, it causes chemical reactions that reduce the ocean’s pH. This phenomenon is known as ocean acidification.
Changes in ocean chemistry can particularly affect organisms that build shells or skeletons from calcium carbonate. Because marine ecosystems consist of complex food webs, disturbances affecting certain organisms can potentially have wider ecological consequences.
The oceans also absorb much of the additional heat accumulated in the climate system. As ocean temperatures rise, seawater expands, contributing to rising sea levels.
Melting ice and rising sea levels
Global warming influences glaciers and ice sheets around the world. When land-based ice melts, the resulting water eventually contributes to higher sea levels.
Sea-level rise is also caused by the thermal expansion of seawater as it warms. Even relatively gradual increases can become important for coastal regions because they can increase the risk associated with flooding, storm surges and coastal erosion.
The consequences are particularly significant for low-lying islands, densely populated coastal areas and infrastructure located close to the shoreline.
Changes in extreme weather
Climate change can also influence extreme weather, although the relationship differs depending on the type of event and the region concerned.
Higher temperatures make extreme heat more likely and can increase evaporation, affecting drought conditions in some areas. A warmer atmosphere can also hold more water vapor, which can contribute to heavier rainfall during certain weather events.
The relationship between climate change and storms is more complex. Scientists therefore use climate observations, physical models and statistical attribution methods to determine how human-induced warming changes the probability or intensity of particular types of extreme events.
Effects on ecosystems
Climate conditions strongly influence where plants and animals can survive. As temperatures and rainfall patterns change, species may migrate toward cooler areas, change their seasonal behavior or experience increasing pressure on their habitats.
Some ecosystems are particularly vulnerable because they cannot adapt quickly enough to rapid environmental changes. Coral reefs, polar environments, mountain ecosystems and certain forests are among the environments facing substantial climate-related pressures.
Climate change can also interact with other threats such as pollution, habitat destruction and overexploitation, making the overall impact on biodiversity more complicated.
Why CO₂ remains in the atmosphere for so long
One reason carbon dioxide is particularly important for climate policy is its long-lasting influence. After CO₂ is released, it circulates between the atmosphere, oceans, vegetation and soils through the global carbon cycle.
Different portions of an emission are removed from the atmosphere at different speeds, and some of its climatic influence can persist for very long periods. This means that today’s emissions can affect the climate well into the future.
Climate change is therefore closely related to cumulative CO₂ emissions: the total quantity emitted over time matters, not simply emissions during a single year.
Reducing the impact of CO₂
Limiting future warming requires reducing net greenhouse gas emissions. For carbon dioxide, this involves several complementary approaches.
Replacing fossil fuels with low-carbon energy sources can reduce emissions from electricity and heating. Improving energy efficiency can lower the amount of energy required by buildings, transportation and industry. Electrification can also reduce fossil-fuel consumption when the electricity used is produced with low-carbon sources.
Protecting forests and restoring ecosystems can help preserve natural carbon sinks, while technological solutions such as carbon capture may play a role in sectors where emissions are particularly difficult to eliminate.
A global and long-term challenge
Carbon dioxide is not the only factor influencing the Earth’s climate, but its increasing concentration has a central role in modern climate change. Its effects extend beyond rising temperatures to oceans, ice, sea levels, ecosystems and many aspects of the global climate system.
The scientific challenge is increasingly focused not only on understanding these mechanisms but also on improving projections at regional and local levels. The societal challenge is to determine how rapidly emissions can be reduced while adapting infrastructure, economies and ecosystems to changes that are already occurring.
Because CO₂ can influence the climate for generations, decisions made today will help determine the climatic conditions experienced far into the future.
