Exploration Class 9 Science Chapter 13: Earth as a System: Energy, Matter, and Life — NCERT Solutions

Explanation: Take India's yearly electricity demand as about 1.6 x 10¹² kWh. Assume the ground receives roughly 5 kWh per m² each day, which is about 1,825 kWh per m² in a year, and take a panel efficiency of about 15-20%, giving nearly 300 kWh of electricity per m² per year. Dividing the total demand by this output gives an area of roughly 5,000 km². Since the Thar desert covers about 2,00,000 km², the panels would need only a few percent of it, so covering part of a desert with solar panels is a feasible way to meet the demand.

Explanation: Albedo is the fraction of incoming solar radiation that a surface reflects. Light, pale surfaces reflect more and so have a higher albedo, while dark surfaces absorb most of the sunlight and have a low albedo. Light-coloured soil is fairly reflective (about 0.30-0.40), black soil is dark and absorbs most radiation (about 0.05-0.15), and deep ocean water also absorbs most of the light falling on it, giving a low albedo (about 0.05-0.10). This is why dark surfaces heat up far more than light ones under the same Sun.

Explanation: The Earth is made of interacting spheres. The geosphere is the solid rock and soil, seen as the boulders and mountains; the hydrosphere is all the liquid water, seen as the lake; the cryosphere is the frozen water, seen as the snow and ice on the peaks; the atmosphere is the envelope of gases, seen as the sky and clouds; and the biosphere is all living organisms, seen as the shepherd, his sheep and the grass. The single scene shows all five spheres meeting in one place, showing how closely they interact.

Explanation: A cool mountain breeze reduces the air temperature around the fields. cooler air slows down the loss of water by evaporation from the soil surface and by transpiration from leaves, so the soil stays moist for longer and the plants do not wilt or suffer heat stress. The lower temperature also reduces the activity of many pests and can help set fruit and grain, so the moderated microclimate is favourable for healthy crop growth.

Explanation: Trees and their roots intercept rain and hold the soil, letting water soak in and slowly recharge groundwater that keeps a river flowing all year. When the forest is cleared, rain hits the bare ground and runs off quickly instead of soaking in, so during the monsoon the river swells rapidly and may flood, carrying large amounts of eroded soil that silt up the channel. In the dry season there is little stored groundwater to feed the river, so its flow drops sharply or it may even dry up. This shows how the biosphere, geosphere and hydrosphere are linked.

Explanation: Energy equals power multiplied by time. The insolation of 1 kW per m² falling on an area of 1 m² gives a power of 1 kW, and over a time of 1 hour the energy received is 1 kW x 1 h = 1 kilowatt-hour (kWh). Converting to joules, 1 kWh = 1000 W x 3600 s = 3.6 x 10⁶ J, i.e. 3.6 MJ.

Explanation: The Earth's surface, warmed by sunlight, gives off heat as long-wave infrared radiation. Greenhouse gases absorb some of this outgoing radiation and re-emit part of it back towards the surface. In the simulation, increasing the greenhouse gas concentration traps more infrared radiation, so less heat escapes to space and the surface temperature settles at a higher value; reducing the concentration lets more heat escape and the surface cools. So higher greenhouse gas concentration means a warmer planet - a direct relationship.

Explanation: The atmosphere holds only about 1% of global carbon, yet it is the pool through which carbon constantly moves between the other spheres - plants take CO2 from the air in photosynthesis, and respiration, decay and burning return it. Because this reservoir is so small compared with the oceans and living matter, adding even a modest amount of carbon (for example by burning fossil fuels) changes its concentration by a large percentage. Since atmospheric CO2 controls the greenhouse effect, this small but sensitive pool has an outsized influence on global temperature and climate, and is easily disturbed by human activity.

Explanation: On any of these planets the Sun heats the equator more than the poles, so warm air rises at the equator and sinks near the poles, setting up large-scale planetary winds. On the all-ocean planet the water heats and cools slowly and evenly, so the winds are steady, regular and humid with few local contrasts. On the all-land planet the surface heats and cools quickly, giving big day-night and seasonal temperature swings and therefore strong, variable, dry winds. Earth has both land and sea, so on top of the planetary winds their different heating rates create extra wind systems such as monsoons and land and sea breezes.

Explanation: Water near the equator is heated strongly by the Sun and moves poleward as a warm surface current. As it travels it gives up heat to the cooler air, so it gradually cools; evaporation makes it saltier, and the cooling and higher salinity make it denser until it eventually sinks at high latitudes, driving the ocean's thermohaline circulation. Along the way it carries and releases equatorial heat to the regions it passes, keeping their coasts warmer and often wetter than they would otherwise be. In this way ocean currents redistribute heat from the equator towards the poles and moderate climates.

Explanation: When atmospheric carbon dioxide rises, more of it dissolves into the ocean. On one hand, phytoplankton use CO2 for photosynthesis, so extra CO2 can help some of them grow faster. On the other hand, dissolved CO2 forms carbonic acid and lowers the water's pH, a process called ocean acidification. Acidic water dissolves and weakens the calcium carbonate shells and skeletons of shell-forming plankton, so their populations fall. Since plankton form the base of the ocean food chain, such changes can disturb the whole marine ecosystem.

Explanation: Sunlight warms the Earth's surface, and the warm surface then gives off this energy as long-wave infrared (heat) radiation directed to the atmosphere and out to space. This outgoing radiation is the main way the planet loses the heat it gains from the Sun. The process is important because it balances the incoming solar radiation, so the Earth neither keeps heating up nor cools down without limit, keeping the average temperature steady and within the range that supports life.

Explanation: Human activities add greenhouse gases mainly by burning fossil fuels (coal, petrol, diesel) for electricity, transport and industry, which releases carbon dioxide; by cutting down forests, which removes trees that absorb CO2; by rearing cattle and growing paddy, which release methane; and by industries, cement making and rotting waste in landfills. To reduce my own emissions I can switch off unused lights and appliances and use energy-efficient LED devices, walk, cycle or take public transport instead of private vehicles, plant and protect trees, and follow reduce-reuse-recycle to cut waste. Choosing renewable energy and avoiding the burning of rubbish also help.

Explanation: Biogeochemical cycles such as the carbon, nitrogen, oxygen and water cycles continuously recycle nutrients between the living and non-living parts of the Earth, so organisms get a fresh supply of the materials they need. If they stopped, these materials would get locked up and could not be reused. For example, if the carbon cycle stopped, plants would run out of CO2 for photosynthesis, so they and the animals depending on them would die; if the nitrogen cycle stopped, plants could not make proteins and would fail to grow; and if the water cycle stopped, there would be no rain or fresh water, causing drought. Dead matter would also not decompose to return nutrients to the soil, so food chains would collapse and life on Earth could not survive.

Explanation: The greenhouse effect, the main driver of Earth's warming, involves solar radiation being absorbed by the surface and then re-emitted as infrared radiation, which is then trapped by atmospheric greenhouse gases.

Explanation: Biogeochemical cycles are the pathways by which essential elements like carbon and nitrogen are circulated between the biotic (living) and abiotic (non-living) components of an ecosystem, ensuring their continuous availability.