A heat engine is a device that repeatedly converts thermal energy into kinetic energy. It needs the difference of temperature and a working substance with a high rate of expansion to do useful work for us. It can be done in two ways: utilizing the change of volume of the working substance that hot and cold reservoirs make alternately expand or compress, and utilizing convection that the change of density resulting from the change of volume causes. A heat engine is not confined to an artificial one. The Earth also does useful work for living systems as a heat engine. The heat engine whose hot reservoir is solar radiation heat maintains the circulation of air and water, while the heat engine whose hot reservoir is geothermal heat maintains the circulation of mantle and mineral nutrition.

The Copernican heliocentric model was not the victory of science over the religious superstition prevailing in the Middle Age. In fact his system was neither simpler nor more accurate than Ptolemy's geocentric model. Copernicus nonetheless proposed the heliocentric model and it was accepted by not a few astronomers, because Neoplatonism that worshipped the Sun was in fashion in those days. The climate background of the sun worship is the modern Little Ice Age, since people tend to accept the common sense of geocentric cosmology in warm periods, while it is doubted in cool periods.

About one-third of the world's land surface is arid or semi-arid. The Earth's landmasses are losing 24 billion tons of topsoil and making 12 million hectares of the Earth useless for cultivation every year. The desertification is, however, not necessarily anthropogenic. So, first, we must elucidate what are the natural conditions of the desert. Then we can identify what are the artificial causes of desertification.

Today, mainstream scientists recognize global warming as a challenge for our civilization, while skepticism persists. There are various types of skepticism, but we can classify them into three categories: skepticism about the trend of global warming, its anthropogenic causes, and the assessments of its impacts. Here, I would like to examine these three issues about global warming: facts, causes, and assessments. I will then consider how we should deal with global warming.

One of the most popular environmental problems is the global warming caused by the greenhouse effect, though most of the people are not sure what bad effect it will have on us.