Forests, woods, groves, copses... all of these woodland areas take centre stage on the International Day of Forests on March 21st. The day was proclaimed by the United Nations in 2012. With its annually changing themes, it is all about raising awareness of forests and their diverse uses. This year the theme is "Forests & Energy."

Forests are the basis for many types of energy. When used as a fuel, wood can come in handy in solid, gas, and liquid form. Woodfuels such as solid firewood and charcoal, wood gas, and liquid bio-oil or bio-ethanol provide one in three households worldwide with energy. That equals 2.4 billion people globally. Woodfuels' primary domestic uses are cooking, heating and boiling water.

Good old forests and the fuels they generate are ancient, yet sustainable sources of energy. They account for approximately 40 percent of our global renewable energies. In contrast to fossil fuels, the emissions that are set free into the atmosphere when burning woodfuels are later removed again when trees are planted, because they make use of and store CO2.

About 50 percent of wood is used for energy purposes each year, mostly just burnt for fuel. This percentage rises to 60 percent in Asia and 90 percent in Africa. But the smoke causes problems, particularly for people's health. 4.3 million people die every year because of indoor air pollution. That is more than deaths from malaria, tuberculosis and HIV combined.

Woods are an important source of employment. In developing countries, around 883 million people base their livelihoods fully or partly on forests. With new, advanced biofuels picking up, this number could increase - also in developed countries.

The popularity of pellets and briquettes has significantly increased over the years - in 2015, 28 million tons were produced. They are not only smaller and easier to handle, they can also be produced from other feedstock, such as grasses or waste-sawdust. Another benefit of the energy carrier: compared to liquid or gas from fossil fuels, pellets are safer to transport.

Wood is only good for heat? Not necessarily. With our planet heating up, trees can also act as coolers. In overheated cities, trees can cool their environment down by up to eight degrees Celsius (14 degrees Fahrenheit). As a consequence, cooler city air requires less energy for air conditioning in buildings. The trees are working their magic again.

The rainforest plays a key role in sequestering CO2. The key Amazon rainforest is under threat from deforestation as trees are felled and areas cleared for farming, mining or construction projects. Scientists warn that these forests must be saved if the world's climate is to be protected.