Major challenges to sustainability, what are they?
Population Rise and longevity:
In the book, “Population: The First Essay” by Thomas Robert Malthus, he said, “The power of population is so superior to the power of the earth to produce subsistence for man, that premature death must in some shape or other visit the human race.” 
Nature runs its own course. It has its own way to control population. Although, I believe what Ester Boserup once said, that population growth also has had a positive impact in our world, it has supplied us labor it has made us feel the need for technological advancements and hence we have grown in terms of technology and we have developed the skills to combat with the growing needs of mankind. 
What can we do: Educate people about human population control i.e. reducing birth rate by adopting birth control via family planning.
Growing energy needs:
What we can do: Educate the mass about technology so that there is social acceptance. For example, nuclear energy should be explained publicly to gain social acceptance. Use alternative energy sources like fuel cell technology, solar energy, geothermal energy, space-based solar power, tidal energy, wave power, wind energy, and nuclear power, to name a few. Really? Are such technologies really our panacea? Read this article about how sustainable are really these so-called sustainable technologies. Use the concept of process integration (also known as heat integration, energy integration and pinch technology) which emphasizes the designing of process to minimize the energy consumption and maximization of heat recovery.
Growing material needs:
Growing material needs correspond to the growing population.
What we can do: Use the concept of 3R and life-cycle analysis of materials, encourage people to participate in the recycling of household materials, develop better technologies in agriculture keeping in mind the safety of humans and the environment.
It is one of the ten threats identified in 2004 by the High Level Threat Panel of the United Nations. The underlying causes for environmental degradation as mentioned in the Union Budget and Economic Survey on India are broadly classified into Social and Economic Factors. 
Population: Increase in population affects the natural resources; there is increase in waste, pollution of land; air and water.
“To know why 1,000 Indian children die of diarrhoeal sickness every day, take a wary stroll along the Ganges in Varanasi. As it enters the city, Hinduism’s sacred river contains 60,000 faecal coliform bacteria per 100 millilitres, 120 times more than is considered safe for bathing. Four miles downstream, with inputs from 24 gushing sewers and 60,000 pilgrim-bathers, the concentration is 3,000 times over the safety limit. In places, the Ganges becomes black and septic. Corpses, of semi-cremated adults or enshrouded babies, drift slowly by.”  Also, an Eco group was formed to clean the Mithi River. 
Poverty: Poor people use more natural resources than the rich since they are more dependent on them. For example, they will burn wood for heat instead of using modern stoves.
Urbanization: Sudden and unplanned expansion of cities has resulted deterioration of air and water quality and has generated lots of waste, polluting the urban cities very quickly. This is due to the migration of poor people from villages to these urban areas who come to seek for opportunities of employment. Human encroachment has also affected the wildlife; we are building our homes while destroying theirs.
Industries: Various industries have contributed to pollution and depletion of natural resources.
Transport: Air, water and road transport has led to pollution.
Agriculture: Agricultural development has a negative effect on the environment, this means the over exploitation of land and water resources and use of fertilizers and pesticides have increased.
What we can do: Eradicate poverty, provide facilities to the rural areas and avoid the exodus, conserve natural resources, adopt green technologies and improve existing technologies and encourage the R&D for the same.
The world has already passed the point of no return for climate change and civilisation as we know it is now unlikely to survive, according to James Lovelock.  By what I understand of what he meant, I agree to the fact that earth has undergone changes without humans in the past, ice-age and warming. Human activities are only making the environment harsher by polluting it and we can solve this problem by embracing green technologies and improve existing ones. For handling toxins in the environment the concept of 3R i.e. Reduce-Reuse-Recycle should be implemented effectively.
What we can do: Conservation of natural resources like forests, rainwater harvesting, and the concept of 3R, the development and implementation of green technologies.
Reliable and Hygienic food supply:
What we can do: Protection of crops, study the effects of climate change and pollution and prepare for the same, genetically modified foods (GM foods) should be given time to cancel out its disadvantages like safety issues and environmental concerns. Bedfordshire County Council’s ‘Recycle Now’  branding is more than ten years old. Before this, Bedfordshire households only recycled and composted 6% of household waste. Since launching the ‘recycle now’ campaign, this figure has risen to 40%, smashing its targets. “Recycling rates are continuously rising. Even in years where new facilities have not been introduced, recycling has continued to rise as people’s awareness increases with strongly branded messaging.” What it actually does is, it recycles waste food into fertilizer by anaerobically digesting it. This means waste food is helping to produce more food. This should be done in other countries too. Another solution to the food crisis would be the idea of a vertical farm, greenhouse stacked over another greenhouse which makes a whole building made of greenhouses stacked over one another. This eats less space and produces more food. Another idea is the idea of Aquaponics. Aquaponic systems are recirculating aquaculture systems that incorporate the production of plants without soil. Recirculating systems are designed to raise large quantities of fish in relatively small volumes of water by treating the water to remove toxic waste products and then reusing it.  Soil inoculants symbiotically fix nitrogen, and hence can help us reduce the use of fertilizers.  Thus, a lot of solutions working together can help us fight this crisis.
Clean water supply:
What we can do: Solar water disinfection, also known as SODIS is a low-cost method of purifying water that can often be implemented with locally available materials. Unlike methods that rely on firewood, it has low impact on the environment. Such technologies for purifying waste water for reuse should be encouraged. Water pollution control should be improved in terms of technology, for more efficient control.
Hitherto unknown diseases and epidemic control:
What we can do: Studies of evolution of disease causing species should be encouraged. Technology to study this should be made better. People should be educated regarding various diseases and ways to prevent and cure them.
- “Population: The First Essay” by Thomas Robert Malthus
- The Department of Gender & Women’s Studies, University of California, Berkeley, Available Online.
- The Economist on December 11, 2008, Available online at: <http://www.economist.com/node/12749787?story_id=12749787>
- DNA, India, Available online at: <http://www.dnaindia.com/mumbai/report_eco-group-formed-to-revive-mithi-river_1220795>
- UN General Assembly Report, Available online at: <http://www.un.org/secureworld/report.pdf>
- The Union Budget and Economic Survey of India, Available online at:<http://indiabudget.nic.in/es98-99/chap1104.pdf>
- The Independent, Available online at:<independent.co.uk>
- Recycle Now, Available at: <http://www.recyclenow.com/>
- Recirculating Aquaculture Tank Production Systems: Aquaponics—Integrating Fish and Plant Culture, Available online at:
- “Symbiotic Nitrogen Fixation” , Panagiota Mylona, Katharina Pawlowski, and Ton Bisseling, Available online at: < http://www.ncbi.nlm.nih.gov/pmc/articles/PMC160880/pdf/070869.pdf>