1. Solar radiation consists of a wide range of EM waves. The energy reaching Earth is concentrated mainly in the UV, visible, and infrared (IR) ranges — about 99% of the Sun’s energy falls within these wavelengths.
2. UV radiation (100–400 nm) is mostly absorbed by the ozone layer in the stratosphere. This prevents harmful UV from reaching the surface and also warms the stratosphere.
3. Visible light passes through the atmosphere and reaches the Earth’s surface. It is the energy source for photosynthesis and also warms the land and water.
4. Infrared radiation warms the Earth’s surface. The surface then re-radiates this heat back into the atmosphere.
5. Greenhouse gases — CO₂, CH₄, and water vapour — absorb this re-radiated infrared heat and prevent it from escaping into space. This is the greenhouse effect, which keeps the Earth at a temperature suitable for life.
6. However, excess CO₂ from human activities enhances this effect, causing global warming and climate change.

1. Near the equator, intense solar heating causes warm air to rise rapidly, creating an equatorial low pressure belt. Clouds and heavy rainfall are common here.
2. The rising warm air moves towards the poles at high altitude. It cools and becomes denser, sinking around 30° North and South latitudes, forming the sub-tropical high pressure belts.
3. From these high-pressure regions, air flows back along the surface towards the equator, completing one large circulation loop.
4. Some of the air at 30° also moves poleward along the surface and rises again around 60° North and South, where it meets cold polar air. This creates the sub-polar low pressure belts.
5. At the poles (90° N and S), extremely cold air sinks and creates polar high pressure belts. Air from here flows towards the sub-polar belts.
6. Importantly, the Earth’s rotation deflects all planetary winds — towards the right in the Northern Hemisphere and towards the left in the Southern Hemisphere. This is why winds follow curved paths rather than moving directly from high to low pressure.

1. Planetary winds drag ocean surface water due to friction, setting surface currents in motion.
2. Temperature differences: Warm equatorial water is less dense and flows on the surface towards the poles; cold polar water is denser and sinks, flowing back towards the equator through deeper levels.
3. Salinity differences: Water with higher salinity is denser and sinks, while lower-salinity water stays near the surface.
4. The Earth’s rotation deflects ocean currents, forming large circular patterns called gyres — clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere.
5. Continents further redirect ocean currents by blocking their paths.

Role in climate: Ocean currents transport heat from the warm equator towards the cold poles, reducing extreme temperature differences across the planet. They also support marine ecosystems by transporting nutrients.

1. Evaporation: Water from oceans, rivers, and lakes evaporates due to solar heat and enters the atmosphere as water vapour.
2. Transpiration: Plants release water vapour through their leaves into the atmosphere.
3. Condensation: As water vapour rises and cools, it condenses around dust particles to form clouds and water droplets.
4. Precipitation: Water falls back to the surface as rain, hail, or snow.
5. Run-off and Infiltration: Water flows over land into rivers and oceans (run-off), or seeps through soil into groundwater (infiltration). Water also dissolves and carries minerals from rocks and soil to oceans, supporting marine life.

Impact of Climate Change on the Water Cycle:

• A warmer atmosphere holds more moisture, causing heavier rainfall (intensified monsoons) in some areas and droughts in others.
• Melting glaciers (cryosphere) add more water to rivers and raise sea levels, threatening coastal cities like Mumbai and Chennai (hydrosphere).
• Intense rainfall increases surface run-off, eroding soil (geosphere) and reducing groundwater recharge, making agriculture difficult (biosphere).

1. Fast Carbon Cycle (days to years): Plants absorb atmospheric CO₂ through photosynthesis and convert it to glucose using sunlight. Animals eat plants, using carbon in their bodies. When organisms respire or decompose after death, CO₂ is returned to the atmosphere. Oceans also continuously exchange CO₂ with the atmosphere.
2. Slow Carbon Cycle (millions of years): Dead plants and animals get buried under layers of rock over millions of years and are converted into fossil fuels (coal, oil, gas). These fuels store carbon for extremely long periods.
3. Human Disruption: Burning fossil fuels releases stored carbon back into the atmosphere as CO₂ at a very rapid rate — far faster than the slow cycle. Deforestation reduces the number of trees that can absorb CO₂. Together, these have raised atmospheric CO₂ by about 35% since 1960 (from 315 ppm to about 420 ppm).
4. Consequences: Excess CO₂ intensifies the greenhouse effect, causing global warming, glacial melting, sea level rise, and more extreme weather events in India such as intense monsoons.

Part (b): Causes and effects on India:

• Causes: Burning of fossil fuels (coal, oil, gas) for electricity, transportation, and industry; deforestation which reduces trees that absorb CO₂.
• Effects on India: More intense monsoons as warmer air holds more moisture; threats to agriculture due to changing rainfall patterns; accelerated melting of Himalayan glaciers threatening river water supply; rise in sea levels threatening coastal cities like Mumbai and Chennai; disruption of ocean plankton and coral reef ecosystems.

1. Nitrogen Fixation: Bacteria like Rhizobium (in legume root nodules) and Azotobacter (in soil) convert atmospheric N₂ into ammonia (NH₃). Lightning also fixes small amounts of nitrogen as nitrogen oxides.
2. Nitrification: Nitrosomonas bacteria convert ammonia into nitrite (NO₂⁻); then Nitrobacter bacteria convert nitrite into nitrate (NO₃⁻), which plants can absorb.
3. Assimilation: Plants absorb nitrates from the soil through their roots and build proteins. Animals obtain nitrogen by eating plants or other animals.
4. Ammonification: When plants and animals die or produce waste, decomposers (bacteria and fungi) break down organic matter and release ammonia back into the soil.
5. Denitrification: Pseudomonas bacteria convert some nitrates back into atmospheric N₂ gas, completing the cycle and maintaining nitrogen balance in ecosystems.

If nitrogen were not cycled: Soils would quickly lose nitrates, plants would not be able to make proteins, herbivores and carnivores would starve, and ecosystems would collapse entirely due to the breakdown of the entire food chain.

1. Consumption of Oxygen:
   • Respiration: Plants and animals use O₂ to break down food for energy, releasing CO₂ as a by-product.
   • Combustion: Burning of fossil fuels and wood uses large amounts of O₂ and releases CO₂.
   • Oxide formation: Oxygen combines with metals and minerals in the geosphere.
2. Production of Oxygen:
   • Photosynthesis: Green plants, algae, and phytoplankton use sunlight, water, and CO₂ to produce glucose and release O₂ back into the atmosphere. This is the primary source of atmospheric oxygen.
3. Balance: Under natural conditions, the oxygen consumed by respiration and combustion is roughly balanced by the oxygen produced by photosynthesis. This keeps atmospheric oxygen at a relatively constant 21%.
4. Human Disruption: Increased burning of fossil fuels and deforestation are tipping this balance — more O₂ is consumed and less is produced by photosynthesis, slowly altering the atmospheric composition.

1. Disruption of the Carbon Cycle: Burning fossil fuels and deforestation have raised atmospheric CO₂ by ~35% since 1960. Excess CO₂ intensifies the greenhouse effect, causing global warming and more extreme weather. It also increases ocean acidity, threatening plankton, coral reefs, and marine food chains.
2. Disruption of the Nitrogen Cycle: Overuse of fertilisers adds excess nitrates to water bodies through agricultural runoff, causing eutrophication (algal blooms that deplete oxygen and kill aquatic life), threatening rivers, lakes, and coastal fisheries.
3. Disruption of the Oxygen Cycle: Deforestation reduces photosynthesis, decreasing O₂ production. Loss of forests also reduces transpiration, leading to local rainfall decline, soil erosion, and habitat loss for countless species.
4. Vehicular Emissions: React with sunlight to form ground-level smog and harmful ground-level ozone (not to be confused with the protective stratospheric ozone), making urban air unhealthy.

Measures to Restore Balance: