If there were no fossil fuels, human life in this world would be significantly different. The earth contains fossil fuels such as coal, petroleum and natural gas. They create electrical power, heat houses, power vehicles, and transform the environment.
Deficiency of fossil fuels would cause the world population to be much less than what it is today. Availability of food depends upon the quantity of fixable nitrogen in the soil. Nature offers most of this nitrogen by means of the action of fungi that survive on the roots of some vegetation, such as soy beans and clover. Humans contribute fixable nitrogen to the soil by manufacturing fertilizers from the Haber Bosch process which makes use of natural gas. Without natural gas to operate the Haber Bosch process, the availability of fixable nitrogen, and thus the amount of food, would be much lower, and the world population would be roughly half of what it is now.
There would be no communism in case fossil fuels were absent. The Industrial Revolution, which started in England in the 1800s, was driven by coal. Without coal, no large factories would have existed in the 19th century. Without these factories, there would not have been the huge migration of employees from the countryside into the sordid situations in London that motivated Karl Marx to create the Communist Manifesto in 1848. Coal mining in England in the 19th century was possibly the most significant period of geologic education in history and without it, the understanding of the history of the world would have been considerably deferred.
There would be no air conditioners without fossil fuels. Air conditioners are powered by electricity, and the overwhelming majority of the electrical power is produced by burning coal or natural gas. Without these fossil fuels, humans still would have discovered electricity from other sources, but these sources would supply significantly less electrical energy. Thus, people would be a lot more particular regarding its use and running air conditioners would not seem sensible.