Climate Change

Climate change refers to significant and lasting alterations in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It encompasses changes in temperature, precipitation, wind patterns, and other elements of the Earth’s climate system. These changes may be driven by natural processes or human activities that modify the composition of the atmosphere and alter the planet’s radiation balance. Climate is often described as “average weather”, and the standard climatological period for such averages is typically 30 years.

The Climate System and Forcing

The climate system is composed of the atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere, all of which interact dynamically over time. The evolution of climate results both from internal dynamics (natural processes within the system) and from external forcings that influence it.External forcings include natural factors, such as volcanic eruptions and solar variations, and anthropogenic factors, such as the emission of greenhouse gases and aerosols. Solar radiation acts as the primary source of energy driving the climate system. When any of these forcings change, they disrupt the balance between incoming and outgoing radiation, causing climatic variations.

Radiation Balance of the Earth

The Earth’s radiation balance is central to understanding climate behaviour. The Sun emits shortwave radiation, part of which reaches the Earth’s surface, while the planet emits longwave (infrared) radiation back into space. Three fundamental processes can modify this balance:

1. Changing incoming solar radiation – through variations in the Earth’s orbit or solar output.
2. Altering reflectivity (albedo) – due to changes in cloud cover, atmospheric aerosols, or surface characteristics such as vegetation and ice.
3. Changing outgoing longwave radiation – by modifying concentrations of greenhouse gases.

Approximately 1,370 watts per square metre (W/m²) of solar energy reaches the top of the Earth’s atmosphere when facing the Sun directly. Averaged across the entire planet, this value is about 342.5 W/m². Around 30% of incoming sunlight is reflected back into space; roughly two-thirds of this reflectivity results from clouds and atmospheric aerosols, while the remainder is due to reflective surfaces such as snow, ice, and deserts. The energy absorbed by the Earth and atmosphere totals about 240 W/m², which must be radiated back to space to maintain equilibrium.

Volcanic Eruptions and Aerosols

Volcanic activity is a significant natural factor influencing the climate system. During major eruptions, vast quantities of ash and sulphate aerosols are injected into the upper atmosphere. These particles reflect sunlight, leading to a temporary cooling effect. While typical aerosols are removed from the atmosphere within weeks, volcanic aerosols that reach the stratosphere may persist for one to two years, lowering global surface temperatures by up to 0.5°C. Historical examples include the 1991 eruption of Mount Pinatubo, which led to a measurable short-term decline in global temperatures.

Longwave Emissions and Surface Temperature

The Earth emits longwave radiation to balance the absorbed solar energy. Theoretically, to radiate 240 W/m², a body would need a temperature of around –19°C, which corresponds to the temperature found approximately 5 km above the Earth’s surface. However, the global mean surface temperature is much higher, about 14°C, owing to the natural greenhouse effect that traps part of the outgoing longwave radiation.

The Natural Greenhouse Effect

The greenhouse effect occurs when certain gases in the atmosphere act as a blanket, absorbing and re-emitting infrared radiation. The most significant greenhouse gases are water vapour and carbon dioxide (CO₂), while nitrogen and oxygen do not contribute to this effect.Clouds also play a dual role: they both reflect solar radiation (cooling) and trap outgoing infrared radiation (warming). On average, the cooling effect dominates, although locally, clouds can cause warming, as seen on cloudy nights when heat loss is reduced.

Human Influence on the Greenhouse Effect

Since the Industrial Revolution, human activities have intensified the greenhouse effect by increasing the concentration of greenhouse gases in the atmosphere. The amount of carbon dioxide has risen by about 35% due to the burning of fossil fuels and deforestation. Other human-produced gases, such as methane (CH₄) and nitrous oxide (N₂O), further contribute to this warming. These alterations have changed the atmospheric composition, leading to measurable increases in global temperature and altering weather patterns worldwide.

Transportation of Energy in the Climate System

The uneven distribution of solar energy, with more radiation received in the tropics than at higher latitudes, creates a natural energy imbalance. This imbalance drives atmospheric and oceanic circulations that redistribute heat across the planet.Energy is also transported through the evaporation and condensation cycle. When water evaporates, latent heat is absorbed, and when it condenses in clouds, that energy is released, fuelling atmospheric circulation. Winds, in turn, drive ocean currents, which further transfer heat, affecting regional and global climates.

The Role of Earth’s Rotation

The rotation of the Earth shapes global wind patterns, causing them to flow predominantly in an east-west direction. These movements form the trade winds, westerlies, and polar easterlies. In mid-latitudes, migrating low- and high-pressure systems, along with associated cold and warm fronts, act as mechanisms for transporting heat towards the poles. Such interactions between rotation, pressure, and heat transfer produce the complex weather systems observed globally.

Feedback Mechanisms in Climate Change

Feedback mechanisms determine how the climate responds to external forcings. Positive feedbacks amplify changes, while negative feedbacks counteract them. A prominent example is the ice–albedo feedback. When greenhouse gases cause warming, ice and snow melt, exposing darker surfaces such as oceans and land. These darker areas absorb more solar radiation, causing further warming and more melting. This self-reinforcing cycle intensifies the initial change.
Other feedbacks include the water vapour feedback, where warming increases evaporation, leading to more water vapour (itself a greenhouse gas), thereby enhancing warming. Understanding and quantifying these mechanisms are central to modern climate science research.

Climate Prediction versus Weather Forecasting

Although climate and weather are closely connected, they differ fundamentally in predictability and timescale. Weather describes short-term atmospheric conditions and is inherently chaotic, making it unpredictable beyond a few days. Climate, in contrast, concerns long-term averages and trends, which can be modelled with greater reliability.
This distinction explains how scientists can project climate changes decades ahead even though short-term weather forecasts remain limited. Climate models rely on physical laws and long-term boundary conditions, such as greenhouse gas concentrations and solar radiation, rather than specific initial states of the atmosphere.
An analogy often used is that, while one cannot predict the exact age at which an individual will die, one can estimate the average life expectancy for a population with high confidence. Similarly, a single cold winter does not disprove global warming, as climate refers to long-term averages, not short-term anomalies.