Maharashtra Board Class 11 Geography Chapter 5 Global Climate Change PDF Download

Global Climate Change

Can You Tell?

The graph in fig. 5.1 shows the difference between the global average temperature of the twentieth century and global monthly temperatures from 1985 to 2015. Answer the questions given below:

1) In which year is the difference the least?

2) What is the difference between the mean temperatures of the twentieth century and temperature in 2015?

3) Why do the temperatures differ in different months?

Geographical Explanation

We see that the differences in 2015 are above the rest of the given years. This shows that the average temperature of the Earth is rising. Scientists around the world have collected more than a century's temperature records. These analyses all point to a rise of close to 0.8° C in the average surface air temperature of the Earth over the last century.

Do You Know?

How do scientists calculate the average earth temperature?

To get a complete picture of Earth's temperature, scientists combine measurements from the air above land and the ocean surface collected by ships, buoys and sometimes satellites, too.

The temperature at each land and ocean station is compared daily to find out what is 'normal' for that location and time, typically the long-term average over a 30-year period. The differences are called 'anomalies' and they help scientists to evaluate how temperature is changing over time. A 'positive' anomaly means the temperature is warmer than the long-term average; a 'negative' anomaly means its cooler. Daily anomalies are averaged together over a whole month. These are, in turn, used to work out temperature anomalies from season-to-season and year-to-year.

Let us have a look at the surface temperatures of a few planets in the following table.

This table shows that the Earth's surface temperature makes it suitable for life to exist.

Teacher's Note

Temperature is changing all around us. In India, we can see that summers are becoming hotter each year than before.

Exam Trick

Remember: Earth's temperature of 14°C is perfect for life. Like Goldilocks - not too hot, not too cold, just right!

Points to Remember

Earth's average temperature is around 14°C.
Temperature has increased by 0.8° C in the last century.
Scientists measure temperature using ships, buoys and satellites.
Anomalies help scientists understand how temperature is changing.
Venus and Mercury are too hot for life to exist.

Try This

Look at the following graph. Answer the following question.

1) Which of these gases has the highest contribution?

2) Which of these gases come from natural and man-made sources?

3) Which activities are responsible for their emission?

4) Out of these, whose emission can be controlled by humans?

Geographical Explanation

The average temperature on the surface of the Earth depends on a number of factors. These include the time of day, the time of year, and where the temperatures measurements are being taken.

The average temperature of the Earth is around 14°C. As given in the earlier graph, this average temperature has increased by 0.8° C. The average surface temperature of the earth is increasing and is likely to increase. It has been found that the impact of gases such as carbon dioxide, methane, etc. has been enormous. It has led to increase in the heat holding capacity of the atmosphere which in turn increases the temperature. This phenomenon is called global warming.

Is This Small Rise A Big Concern?

It may sound like a 0.8° C is not such a big number, but the impact of this rise appears to be phenomenal.

Let's Look At The Effects Of Global Warming

1) Heat Waves: Increases in heat holding capacity of the atmosphere increases the temperature especially during summers. Summers can become even more intense or even deadlier and could bring hot spells. Heat waves of Chicago (1995) and Paris (2003) led to deaths of hundreds of people.

2) Heat Islands: Global warming serves as a base from which heat waves become much worse – especially in big cities where the heat island effect comes into play. Urban areas heat up more readily because of paved surfaces and concrete constructions as compared to a field or forest areas. This phenomenon may further increase the temperature of the earth.

3) Increase In Sea Level: Look at the graph in fig. 5.2 and answer the following questions:

i) What does the graph show?

ii) In which year is the change around 225 mm?

iii) What conclusions can you draw by seeing the graph?

iv) What correlation can be seen in this graph and the graph of rising temperatures?

Geographical Explanation

The graph shows increase in global sea level from 1880 till present. With continued global warming, sea levels are likely to rise. Global sea level has been rising over the past century. As seen in the graph, global sea level is around 50 mm more than that in 1990s. Sea level continues to rise at a rate of about 3 mm/per year. This global sea level rise is apparently being caused because of increased melting of ice such as glaciers and ice sheets.

Higher sea levels may lead to deadlier cyclones and also frequent flooding of coastal areas. Many islands are also at risk of getting submerged. It also means loss of habitat for fishes, birds and plants.

In India too, sea levels are changing at different rates along the coast as per the studies carried out by the Indian National Centre for Ocean Information Services. It projects sea level rise by 9 to 90 cm between 1990 and 2100. This may result in saline coastal groundwater endangered wetlands and flooding of cities and coastal communities around Kutch in Gujarat, Mumbai parts of Konkan coast and southern Kerala. The deltas of the Ganga, Krishna, Godavari, Kaveri and Mahanadi on the East coast are threatened too.

Do You Know?

What is the difference between global and local sea level? These are two different measurements. Sea level rise at specific locations may be more or less than the global average due to local factors like subsidence of land, flood control in upper part of the river, regional ocean currents, etc.

Sea level is measured using tidal data from stations with the help of satellite and laser altimeters around the world. Tide stations tell about local levels. Satellite data provides us with average height of the whole ocean. Together, both of these help us to know how our ocean levels are changing over time.

4) Melting Of Glaciers At High Altitudes And Snow In Polar Areas: Melting of ice is a very natural process. But when glaciers and icebergs melt at an alarming rate, it is a cause of concern. Evident impacts of global warming are retreating glaciers. Studies using satellite data from 1989 till present show that glaciers have retreated. See fig 5.3 A and B. The satellite images show the Gangotri Glacier, situated in the Uttarkashi District of Garhwal Himalayas. Compare these images. Gangotri glacier is one of the largest in the Himalayas. Over the last 25 years, Gangotri glacier has retreated more than 850 meters, with a recession of 76 meters from 1996 to 1999 alone. You can notice the change in between the years 1984 and 2018. This is an indicator of glacial retreat. This means that there is lesser ice formation each year than its rate of melting. This retreat is around 22 m per year.

Many scientists regard receding glaciers as a symptom of global climate change. Similar observations have also been made in the glaciers near Mt. Kilimanjaro in Africa. In general glaciers in polar areas have also been reported to have melting at faster rates. Similarly, glaciers all across the Alps are retreating and disappearing every year.

Teacher's Note

In India, the Gangotri Glacier is melting very fast. Our Himalayan glaciers are like the water bank for India's rivers.

Exam Trick

Remember: Glaciers melting = Less water = Less ice. Like ice cream melting in summer heat, but it's happening everywhere on Earth now!

Points to Remember

Sea level is rising at 3 mm per year.
Glaciers are retreating faster than ice is forming.
Gangotri Glacier has retreated 850 meters in 25 years.
Rising sea levels can flood coastal cities.
Melting glaciers are a sign of global warming.

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