NCERT Solutions for Class 9th Science Chapter 13 Chapter opener — Think It Over

Book page 252 Updated on2026-09-08

Q1.

How does the warming of Arabian Sea water affect the southwest monsoon in India?

Answer

Warmer sea water evaporates faster, so more water vapour is pushed into the monsoon winds — and the monsoon becomes stronger but far more erratic.

Warmer Arabian Sea surface → more evaporation
More evaporation → more water vapour carried by the southwest monsoon
More vapour + rising air → heavier bursts of rain in some regions
Same total moisture, unevenly delivered → dry spells and drought elsewhere

Why it happens: the rate at which water evaporates from a sea surface rises steeply with temperature, and warm air can also hold more water vapour before it condenses. So a warmer Arabian Sea loads the monsoon winds with extra moisture. When that moist air is forced to rise — over the Western Ghats, or in a low-pressure system — it cools, condenses and dumps the moisture in short, violent spells rather than in steady rain. The chapter states this outcome directly: fluctuations in the southwest monsoon give variability in rainfall, bringing floods to some regions of India while leaving others in drought.

Note: this is a good example of the chapter's main idea. The change begins in the hydrosphere (sea temperature), travels through the atmosphere (monsoon winds), and lands in the biosphere (crops and farmers). One sphere is never disturbed alone.

Q2.

If a large forest is cleared, how can that affect the flow of a river in that area?

Answer

The river becomes flashy — it floods heavily during rain and shrinks or dries up between rains — and it carries much more silt.

Why it happens: a forest works like a sponge placed on a slope. Three things change when it is removed.

Infiltration collapses. Leaf litter and root channels normally let rain soak into the soil. On bare ground the same rain runs straight off the surface. The chapter calls this "more run off … and less infiltration".
Groundwater recharge falls. The water that used to seep down and feed the river slowly, all through the dry months, is no longer stored. So the river's lean-season flow drops.
Soil erosion rises. "Without tree roots to hold the soil together, soil erosion could increase." The eroded soil ends up as silt in the river bed, which raises the bed and makes flooding worse.

There is also an effect on how much rain falls at all: clearing forests means decreased photosynthesis and reduced transpiration, which can lead to decline in the local rainfall.

Tip: remember the pattern as high peak, low base. The same annual rainfall is delivered to the river as sharp floods instead of a steady supply.

Q3.

What might happen to coastal cities in India if glaciers and polar ice keep melting faster?

Answer

Sea level rises, and low-lying coastal cities such as Mumbai and Chennai face flooding, salt water entering their groundwater, and loss of land.

Rise in atmospheric temperature → melting of glaciers and polar ice (cryosphere)
Melt water flows into the ocean → volume of ocean water increases
Increased volume → sea level rises
Higher sea level + storm surge → flooding of low-lying coastal land

Why it happens: ice locked up on land is water that has been taken out of the ocean. When it melts, that water is returned, so the ocean gains volume and the sea stands higher against the coast. India is especially exposed because a large share of its population, ports and industry sit within a few metres of sea level. The chapter says this "can raise sea levels that may threaten coastal cities", and adds the further consequence — the "ecosystems within the biosphere" are disturbed "by causing a habitat loss", which for a coast means mangroves and fisheries.

Did you know? Melting sea ice (ice already floating in the ocean) does not by itself raise sea level much, because floating ice already displaces its own weight of water. It is the ice sitting on land — Himalayan glaciers, the Greenland and Antarctic ice sheets — that adds new water to the ocean.

Q4.

How would increasing carbon dioxide levels in the atmosphere affect the ocean plankton?

Answer

The extra CO₂ dissolves in sea water and makes it more acidic, which harms plankton — especially the tiny organisms that build shells of calcium carbonate.

More CO₂ in air → more CO₂ dissolves in the ocean
Dissolved CO₂ + water → carbonic acid → more H⁺ ions
More H⁺ → sea water becomes more acidic
Acidic water attacks carbonate shells → shell-building plankton weaken

Why it matters so much: the chapter states that "excess atmospheric CO₂ increases ocean absorption, making sea water more acidic. This could threaten tiny plankton and coral reefs, disrupting the marine ecosystems." Plankton are not a minor detail of the ocean — phytoplankton carry out an enormous share of the world's photosynthesis, they are the base of every marine food chain, and their shells carry carbon down to the sea floor when they die. Damage the plankton and you damage fisheries, the oxygen supply and the ocean's ability to store carbon, all at once.

A second squeeze: the chapter also notes that "warmer ocean water reduces the ocean's capacity to absorb CO₂". So as the sea warms, it takes up less CO₂ — leaving more in the air, which warms it further. That is a positive feedback: the change feeds itself.

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