Energy
Energy is a physical quantity defined as the ability to produce work or heat. The total amount of energy in any closed system is conserved, and cannot be created or destroyed. Energy can take many forms, and it may be transformed from one to another. Its two main forms are kinetic energy and potential energy. In practice, any work performed results in an inevitable loss of energy as heat, which means heat can never be totally converted into useful work.
In order for life to exist, it needs a constant source of energy. The Earth's main source of energy is the Sun, which radiates energy by converting hydrogen atoms into helium.[1] The Sun has a limited supply of hydrogen, but it has enough to last for a few billion years.
The Earth's atmosphere keeps heat from completely escaping to space thanks to the greenhouse effect. Since the beginning of the industrial revolution, the increase of greenhouse gases in the atmosphere caused by fossil fuel emissions and farming has exacerbated this effect, leading to climate change, which is having destructive consequences for the environment that will become catastrophic if allowed to continue.
All production involves a consumption of energy, whether by labour power or by technology. The way a society obtains and expends its energy in production constitutes its mode of production.
The current mode of production relies heavily on fossil fuels and will need a deep structural transition to renewable energy sources and probably nuclear power. The way humans obtain food from their diet also relies too much on animal products, which are significantly less energy-efficient sources. Current technology relying on combustion engines will also need to be replaced.