(a) Hydrogen is the main element of water and is circulated along with water. Water is available from various sources in the form of water vapour. Water evaporates from water bodies, such as oceans, lakes, rivers, and also from living organisms. Animals excrete water as urine and sweat, which eventually evaporates and plants, transpire, thus contributing to the water-vapour content in the atmosphere. Water vapour from the above sources gets condensed at low temperature, in the upper reaches of the atmosphere and form droplets. These droplets get collected to form clouds, which when turning heavy, fall as rain and snow. This process is referred to as precipitation. Thus, water returns to the earth. The water cycle makes fresh water available to the organisms. The water cycle is also meant for the cycling of hydrogen, one of the macronutrients for plants. Plants use’water, mainly as a source of hydrogen for reducing carbon dioxide, during photosynthesis. During the light reaction of photosynthesis, water is split into H+ and OH ions. From these, hydrogen is used for fixing carbon dioxide and oxygen is removed as a by-product.
(b) CO2 and methane absorb the infrared radiations from the earth. They trap the heat within the atmosphere, preventing its escape into space. The carbon dioxide of the atmosphere keeps the earth warm, much like the glass which keeps the greenhouse warm. That is why this effect is called greenhouse effect. Without the greenhouse effect, the earth would be too cold for life. But the increase in the amount of CO2 and methane has led to global warming, i.e., an increase in the average temperature of the earth. Global warming may lead to melting of polar ice caps and ultimately rise in sea level.