The following topics are very important for UPSC CSE.
Biodiversity and Climate Change
Biodiversity is the variability of life on Earth. It is a measure of variation at the ecosystem level. There are 4 types of biodiversities-

India has 2.4% of the total land area of the world, with 7-8% of all of the world's species distributed among 10 bio-geographic zones. India is also among the 17 megadiverse countries in the world.
Biodiversity offers a range of natural services like ecosystem services (protection of water resources, soil formation, pollution breakdown and absorption, recovery from unpredictable events, contribution to climate stability, maintenance of ecosystems), social benefits (research, education & monitoring, recreation & tourism and cultural values), and biological resources (food, medicinal resources & pharmaceutical drugs, breeding stocks, population reservoirs, future resources, diversity in genes, species and ecosystems).
Despite so many benefits of biodiversity, it is getting depleted at a rapid pace due to human activity. It is also facilitating massive extinction of species.
A major cause of biodiversity depletion is habitat destruction which is caused by deforestation, overpopulation, and global warming. It is estimated by some experts that 30% of species will be extinct by 2050.
The government of India has launched 5 schemes for the protection of biodiversity-
Climate Change
Climate change is a phenomenon that alters the composition of the atmosphere due to human activity over a period of time. It results in global warming due to an increase in the level of greenhouse gases caused by human activity.
Other factors that are responsible for global warming are deforestation, pollution, variation in the sun's energy received by the earth, changes in earth's orbit.
The consequences of climate change are sinking of islands, an increase in forest fires, and glacial retreat.
In order to mitigate climate change, the government of India has passed certain acts-
Some important international conventions on climate change are as follows-
It is very important to remember the UPSC CSE syllabus or at least keep it handy while studying. In this way, you can focus all your efforts on what needs to be studied.