Climate change is driven by a combination of human-caused (anthropogenic) and natural factors. Since the mid-20th century, the human influence has far outweighed natural drivers. Here are the main reasons:

1. Greenhouse gas (GHG) emissions  
  • Carbon dioxide (CO2): Burning coal, oil and gas for electricity, transport and industry; cement manufacture; deforestation (which also releases CO2 and reduces CO2 uptake by trees).  
  • Methane (CH4): Leaks from oil and gas systems; rice paddies; livestock digestion; landfills; wetlands.  
  • Nitrous oxide (N2O): Agricultural soils (especially from synthetic fertilizers), biomass burning, industrial processes.  
  • Fluorinated gases (HFCs, PFCs, SF6): Used in refrigeration, air conditioning and some industrial processes; very potent and long-lived.

2. Land-use changes and deforestation  
  • Clearing forests for agriculture, logging or urban development cuts a major carbon sink and often involves burning or decomposing wood, releasing CO2.  
  • Converting grasslands or wetlands alters local albedo (reflectivity) and soil carbon storage.

3. Aerosols and air pollutants  
  • Sulfate aerosols (from coal burning) reflect sunlight and can have a temporary cooling effect, but also harm human health.  
  • Black carbon (soot) absorbs sunlight, warms the atmosphere and when deposited on snow/ice accelerates melting.  
  • Changes in other particulates and ozone in the lower atmosphere (tropospheric ozone) also affect radiative balance.

4. Natural factors (smaller role in recent warming)  
  • Solar variability: The Sun’s output fluctuates slightly over decades, but since the 1970s these changes have been too small to explain observed warming.  
  • Volcanic eruptions: Inject ash and sulfur compounds into the stratosphere, temporarily cooling Earth for up to a few years.  
  • Internal climate variability: Ocean cycles (El Niño/La Niño, Atlantic Multidecadal Oscillation) shift heat between ocean and atmosphere but don’t change the long-term warming trend.

5. Feedback mechanisms (amplify initial warming)  
  • Water-vapor feedback: Warmer air holds more water vapor (itself a greenhouse gas), causing further warming.  
  • Ice-albedo feedback: Melting snow and ice expose darker land or ocean, absorb more heat and accelerate melt.  
  • Permafrost thaw: Releases CO2 and methane stored in frozen soils.  
  • Cloud changes: Warming can alter cloud cover and type, with complex effects on warming or cooling.

Net effect: Since the Industrial Revolution, rising concentrations of GHGs have increased Earth’s radiative forcing—the balance between incoming solar energy and outgoing infrared radiation—leading to a global average temperature increase of about 1.2 °C (2.2 °F) above pre-industrial levels. This energy imbalance drives shifts in weather patterns, ocean circulation, ice cover, and sea level, with wide-ranging impacts on ecosystems, agriculture and human societies.
