Climate Change- 2 | Environment for UPSC CSE PDF Download

Climate Forcings

Climate forcings are external or internal factors that change the Earth's energy balance and thereby drive changes in global or regional climate. Forcings may be positive (leading to net warming) or negative (leading to net cooling). Examples include greenhouse gases, atmospheric aerosols, volcanic eruptions and changes in incoming solar radiation.

• Positive forcings increase the net incoming energy to the climate system and tend to warm the planet; an important example is increased concentrations of greenhouse gases such as carbon dioxide (CO₂).
• Negative forcings reduce net incoming energy and tend to cool the planet; examples include reflective aerosols and the short-term effects of large volcanic eruptions.
• Atmospheric aerosols are tiny particles suspended in air and include volcanic ash, soot from combustion, mineral dust and particles formed from gaseous pollutants. Their climate effects vary by composition, size and altitude.
• Dark, carbon-rich particles such as soot (black carbon) absorb sunlight and warm the atmosphere and can accelerate snow and ice melt when deposited on surfaces.
• Sulphate-rich aerosols produced from high-sulphur coal or oil combustion scatter incoming sunlight back to space and produce a net cooling effect. Similarly, volcanic sulphate aerosols can cool global temperatures for a year or more until they settle out.

Altering the energy balance: Radiative forcing

Radiative forcing is the standard metric used to estimate the power of a process to change the climate: it is the change in the Earth's energy balance (incoming minus outgoing radiation) measured at the top of the atmosphere, typically expressed in watts per square metre (W m⁻²). A positive radiative forcing causes net warming; a negative radiative forcing causes net cooling. Some forcings are well quantified (for example, CO₂), while others - notably the effects of certain aerosols - remain uncertain in magnitude and sign.

Natural forcings

• Natural forcings include changes in solar output, long-term variations in Earth's orbit (Milankovitch cycles), and volcanic eruptions that inject particles and gases into the stratosphere.
• Since the start of the industrial era, solar changes have produced only a small increase in energy reaching the Earth; this small change cannot account for the observed recent warming.
• Large volcanic eruptions can produce substantial, but short-lived, negative forcing by injecting sulphate aerosols into the stratosphere; the cooling typically lasts from months to a few years.

Human-induced forcings

• Human activities are a major source of climate forcings through the emission of greenhouse gases, the release of aerosols, and land-surface changes such as deforestation and urbanisation.
• Greenhouse gases (GHGs) - especially CO₂ emitted from fossil fuel combustion - are the principal positive forcing agents. CO₂ from fossil fuels is presently the largest single climate forcing agent and is estimated to account for more than half of the total positive forcing since 1750.
• Aerosols from human activities are added to the atmosphere through combustion and industrial processes. These aerosols can directly scatter or absorb sunlight and can indirectly alter cloud properties and lifetimes. Overall, many anthropogenic aerosols produce a net cooling (negative forcing), but effects are regionally heterogeneous and carry large uncertainties.

Estimating the climate effect of individual greenhouse gases

The climatic effect of any greenhouse gas depends on three primary properties. These determine how much warming a given emission will cause:

• Concentration (abundance) - the amount of that gas present in the atmosphere, commonly measured in parts per million (ppm), parts per billion (ppb) or parts per trillion (ppt). Global mixing ensures that concentrations are approximately uniform worldwide for long-lived gases.
• Atmospheric lifetime - how long, on average, a molecule of the gas remains in the atmosphere before it is removed by chemical reactions or deposition. Lifetimes range from a few years to many centuries.
• Radiative efficiency (how strongly the gas absorbs energy) - how effectively a molecule absorbs outgoing terrestrial radiation; gases that absorb more energy per molecule cause greater warming for the same mass emitted.

Global Warming Potential (GWP)

The Global Warming Potential (GWP) is a standard index that combines atmospheric lifetime and radiative efficiency to compare the total warming effect of a mass of a greenhouse gas to that of the same mass of carbon dioxide over a specified time horizon (commonly 100 years). Carbon dioxide is the reference gas and has a GWP of 1 by definition.

• Gases with higher GWP values trap more energy per unit mass than gases with lower GWP values over the chosen time period.
• GWP is time-horizon dependent: short-lived gases may have a high near-term GWP but a lower 100-year GWP because they are removed from the atmosphere relatively quickly.

GWP values and atmospheric lifetimes (illustrative)

• Carbon dioxide (CO₂) - GWP = 1 (baseline).
• Methane (CH₄) - 100-year GWP ≈ 21; average atmospheric lifetime ≈ 12 years. On a mass basis, methane absorbs substantially more energy than CO₂ over its atmospheric lifetime, but it is shorter-lived.
• Nitrous oxide (N₂O) - 100-year GWP ≈ 310; average atmospheric lifetime ≈ 120 years.
• Chemicals such as chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF₆) are classed as high-GWP gases because they trap substantially more heat per unit mass than CO₂ and often have long lifetimes.
• Values above are representative and have been used in many assessments; the Intergovernmental Panel on Climate Change (IPCC) provides updated numerical estimates as scientific understanding improves.

Aerosols: direct and indirect climate effects

• Direct effect: aerosols scatter and/or absorb incoming solar radiation; scattering aerosols (for example, sulphates) increase planetary albedo and cool the surface, while absorbing aerosols (black carbon) warm the atmosphere by absorbing sunlight.
• Indirect effect: aerosols act as cloud condensation nuclei and influence cloud droplet number, cloud albedo and cloud lifetime; these changes affect how clouds reflect sunlight and how much longwave radiation they trap.
• The net climatic effect of aerosols is generally cooling at the global scale, but impacts vary by aerosol type, altitude and region. Aerosols also complicate detection of the full greenhouse warming because their cooling partially offsets greenhouse forcing.
• Volcanic eruptions produce natural aerosols that can cool the climate for a year or more until aerosol particles settle out; large eruptions can produce measurable short-term global cooling.

Receding glaciers - a symptom of global climate change

Glacier retreat is a clear and visible indicator of a warming climate. Observations show substantial shrinkage of glaciers around the world over the past century and a half. For example, in Glacier National Park (United States) there were 147 named glaciers about 150 years ago; today only 37 remain and scientists projected that the small remaining glaciers could disappear in coming decades. Glaciers in the Himalayas, Alps and polar and high-mountain regions are retreating year after year.

There are roughly 160,000 glaciers in polar regions and high mountain environments combined. Modern satellite remote sensing and aerial surveys allow scientists to monitor glacier area, volume and mass balance much more rapidly and consistently than was possible with field studies alone.

Impacts of glacial retreat

• Glaciers act as seasonal water stores; their retreat threatens the reliability of water supplies in regions that depend on glacier melt during dry seasons. The retreat of glaciers in the Andes and the Himalayas will affect water availability for agriculture, drinking water and hydropower.
• Changes in temperature and precipitation alter a glacier's mass balance (the difference between accumulation and melting), driving retreat when losses exceed gains.
• The Himalayas and other central Asian mountain chains support large glaciated regions that feed major river systems. A reduction in glacial mass would have major consequences for countries such as Mongolia, western China, Pakistan and Afghanistan, and for downstream water users.
• Glacial loss can increase the risk of glacial lake outburst floods, change seasonal river flow patterns and reduce dry-season base flows with socio-economic consequences for agriculture and energy production.
• Global warming poses serious risks to economies and ecosystems; poorer and low-lying countries are often least able to cope with the combined impacts of changing water availability, more extreme weather, and rising sea levels.

Summary

Climate forcings - whether natural or human-induced - determine how the Earth's energy balance changes and, hence, how climate responds. Greenhouse gases provide the principal positive forcing leading to long-term warming; aerosols generally produce cooling but add large uncertainty. The Global Warming Potential (GWP) is a useful metric to compare gases by combining lifetime and radiative efficiency. Physical consequences such as glacier retreat are already evident and carry significant hydrological, ecological and socio-economic implications. Understanding forcings, their magnitudes and interactions is essential for effective climate mitigation and adaptation policy.