Climate and energy

1. What is energy

The word energy derives from the Ancient Greek and was probably first introduced by Aristotle in the 4th century BC. The term identified a qualitative philosophical concept, to indicate a state of mind rather than a physical quantity.

Vincenzo Balzani, an Italian chemist who is hemeritus professor at the University of Bologna, once stated:

If you want to abash someone ask him: “what is energy”? Everybody thinks of knowing it, however, nobody will give you a clear and unique answer.
We know that energy is mandatory for every process, and that energy appear in different forms (chemical, electro-magnetical, electrical, thermal, mechanical, nuclear). Reversible conversion process exists between the form (but not to nuclear energy). Energy conversion is the only way to “produce” energy, and during conversion the “quantity” is saved, while the “quality” degrades.
Energy is causing wars and is the real “power” of the world.
We know that is such to produce changes (material or immaterial).
We know everything of “energy”, but we cannot define it.
It is a quantity of universal nature that appears in material and immaterial forms, and that cannot be “reduced” to a more elemental form.

Today, energy is defined in physics as the quantitative property that must be transferred to a body or physical system to perform work on the body, or to heat it. Simply put, energy is the ability to do work. It is measured in joule, which is the energy transferred to an object by the work of moving it a distance of one meter against a force of one newton.

Energy, water and food are considered primary resources for ensuring human well-being. In particular, energy is essential to life. The sun, directly or indirectly, is the source of all the energy available on Earth and used by people, animals, plants, and microorganisms. This energy may come directly, such as in the form of radiation and photosynthesis, or indirectly, for example in the form of fossil fuels, which long ago trapped the energy of the sun that is released when they are burned.The management of energy by humans impact Earth's systems in ways that may be not completely understood, therefore energy production are to be planned carefully.

To emphasise the importance of energy, we may provide examples of energy consumption for producing essential goods for the human life:

    The production of 1 kilogram of meat on average requires 7 liters of oil;

    We may desalinate 1000 liters of seawater with 3 liters of oil;

    Producing a ton of aluminum requires about 5 tons of oil;

    Producing a car requires about 3 tons of oil, that is, about 25% of energy consumed by a car in its lifetime;

    Energy production accounts for about 75% of global greenhouse gas emissions, that is mainly obtained by burning fossil fuels, with a clear impact on climate change and public heath.

2. Forms of energy and conversion

There are six forms of energy (Figure 1): thermal, chemical, electrical, electromagnetic (light), kinetic (mechanical), nuclear. Chemical energy is the one that is stored by fossil fuels. Electrical can be considered a subsystem of electromagnetic. Antrophogenic and natural conversion processes allow to transform energy from one form to another.

Figure 1. Forms of energy and conversion. Natural processes are indicated in red. Black processes are artificial.

We as human are capable of performing all the transformation processes indicated in Figure 1. Energy transformation is regulated by the energy conservation law, that is, the first principle of thermodynamic: the total energy of a isolated systems remains constant. It can only be converted among the six forms. Therefore, the expression "energy production" is formally wrong. Any process is a energy conversion.

However, conversion implies a degradation of energy. Namely, energy is conserved after transformation but its capacity to produce work decreases. In fact, the second law of thermodynamics deals with energy transformations and the natural tendency to move towards more degraded forms, which cannot be used any longer. This does not mean that the total quantity of energy present in the universe is decreasing, rather that its ability to work is. The thermodynamic function assessing the degree of energy dispersion is called “entropy” . Entropy in the universe tends to increase to reach a state of balance in which the total degradation of energy is achieved corresponding to the complete inability to work. Fortunately, biological systems are open systems, which thanks to the incoming of energy from outside, restore the positive global energetic balance.

A second interesting classification of energy divides it in two categories: primary energy and secondary energy. Primary energy consists of unconverted or original fuels. Secondary energy includes resources that have been converted or stored. For example, primary energy sources include petroleum, natural gas, coal, biomass, flowing water, wind, and solar radiation. Those are the fuels that can be mined, reaped, extracted, harvested, or harnessed directly. Secondary energy cannot be harnessed directly from nature; rather, secondary energy is energy that has already been converted. For example, electricity cannot be mined or harvested, though it is available in quick bursts on occasion from lightning. It is generated as a secondary form from primary fuels, like natural gas. Primary energy can be non-renewable or renewable. Renewable energy is collected from renewable resources that are naturally replenished on a human timescale. It includes sources such as sunlight, wind, rain, tides, waves, and geothermal heat. Renewable energy stands in contrast to fossil fuels, which are being used far more quickly than they are being replenished. Although most renewable energy sources are sustainable, some are not. For example, some biomass sources are considered unsustainable at current rates of exploitation.

Primary energy can be broadly classified into three parts (see Figure 2):

  • Crude oil, hard coal, natural gases, Nuclear energy etc;
  • Waste;
  • Wind, geothermal, biomass, hydroelectricity, wave and tidal energy etc..

Note that hydroelectricity is meant to indicate energy contained in flowing water (water power) and therefore it is primary. Primary energy is measured in TOE (tons of oil equivalent).

Figure 2. Primary and secondary energy. Source: Watt Watchers of Texas.

3. Global energy budget and greenhouse gases emissions

Energy production is the driver of most of the global emissions of greenhouse gases. In fact, recent assessments indicate the following estimates (see also Figure 3):

  • Energy (electricity, heat and transport): 73.2%;
  • Direct Industrial Processes: 5.2%;
  • Waste: 3.2%;
  • Agriculture, Forestry and Land Use: 18.4%;

Figure 3. Greenhouse gases emissions by sector.

Most of the energy is still produced by burning fossil fuels. Renewables are increasing but they still remain a limited energy source (Figure 4 and 5).

Figure 4. World primary energy use by fuel type.

Figure 5. World energy balance.

Half of global energy produced by China, the United States and the Arab states of the Persian Gulf. The Gulf States and Russia export most of their production, largely to the European Union and China where not enough energy is produced to satisfy demand. Energy production increases slowly, except for solar and wind energy which grows more than 20% per year. Renewable energy is emerging as a valuable opportunity to decrease global emissions for energy production, but in turn it entails a significant environmental impact. For instance, wind farms are responsible for noise and visual pollution; solar farms occupy a significant fraction of land and hydropower has an impact on the water cycle and land occupation, besides implying social risks.

Energy is processed to make it suitable for consumption by end users. The supply chain between production and final consumption involves many conversions and much trade and transport among countries, causing a loss of one quarter of energy before it is consumed. It is interesting to note that the global final energy use is given by the figures that follow (source: International Energy Agency, 2017):

  • Residential: 21%;
  • Transport: 29%;
  • Industry: 29%;
  • Other: 21%.

Figure 6 shows a picture of the progress from 2000 to 2020 of global energy use by fuel type:

Figure 6. Progress from 2000 to 2020 of global energy use by fuel type.

Figure 7 shows the total energy consumption by source in 2012.

Figure 7. Total world energy consumption by source in 2012. By Delphi234 - Own work, CC0,

Energy consumption per person in North America is very high while in developing countries it is low and more renewable. There was a significant decline in energy usage worldwide caused by the COVID-19 pandemic, notably in the iron and steel industry as demand for new construction shrank. To reach levels similar to that in 2019, there would need to be an increase in the global demand for manufactured goods by the iron and steel industry (for more information see the page "World energy supply and consumption" in Wikipedia.

Furthermore, it is significant to note that energy production and consumption is enormously differentiated with respect to several regions of the world therefore highlighting a significant disparity.

4. Renewable energy

Renewable energy will play a key role in the decarbonisation of our energy systems in the coming decades. Renewable energy is collected from resources that are naturally replenished on a human timescale, and therefore does not include fossil fuels, which are being used far more quickly than they are being replenished. It includes sources such as solar energy, wind, rain, tides, waves, and geothermal heat. Although most renewable energy sources are sustainable, some are not. For example, some biomass sources are considered unsustainable at current rates of exploitation.

About 20% of humans' global energy consumption is covered by renewable energy, including almost 30% of electricity. Most countries are expanding their sources of renewable energy (see Figure 8).

Figure 8. Renewable energy capacity additions in 2020 expanded by more than 45% from 2019, including 90% more new wind power (green) and a 23% expansion of new solar photovoltaic installations (yellow). From

Figure 9 shows the total energy production by countries, highlighting a still heterogeneous distribution.

Figure 9. Total energy production by countries. The left vertical axis refers to the whole world, while the right vertical axis refers to the individual countries. Yellow is China; brown is Russia; light blue is Africa; blue is United States; green is Europe; dark green is Central and South America. By Delphi234 - U.S. Energy Information Administration (EIA)website, Public Domain,

Most of renewable electric energy is generate by hydropower (see Figure 10).

Figure 10. Renewable energy generation by source. From

Hydropower is an essential resource for its flexibility, as generation can be controlled in time and therefore is an ideal integration of other energy sources that are less flexible. Furthermore, hydropower can be easily stored and pumped back in the reservoir, therefore giving further opportunities for enhancing flexibility. However, hydropower is generating concerns related to its sustainability, as reservoirs are subjected to siltation with limited options for recovery. Furthermore, hydropower is generating a significant environmental impact, including effects on groundwater resources and local and global climate. Finally, hydropower is entailing risks for the communities exposed to floods induced by dam break and mismanagement of the storage capacity.

Figure 11. The Three Gorges Dam on the Yangtze River in China.

Renewable energy potential is characterised by a heterogeneous distribution (see Figure 12 and 13).

Figure 12. Wind energy generation by region over time.

Figure 12. Global map of wind power density potential.

The same heterogeneity is observed for solar energy (Figure 14).

Figure 14. Global map of horizontal irradiation.

Heterogeneity create disparities in countries' accessibility to renewable resources.

4. The water-food-energy nexus and climate change

Water, energy and food are essential to human development for reasons that we all know very well. Food and water are essential for life; access to water is necessary for human sanitation. Water and energy are needed for producing and providing food. Energy needs to be produced in a sustainable manner and hydropower is a significant source of energy at the global level. Water and energy are difficult to store and move, while food can be transferred. The above - not exhaustive - examples clarify that there are numerous feedbacks among water, food, energy and humans. To emphasise suck links and retrofits the idea came forward of using a nexus approach, therefore defining the "water-food-energy nexus".

The idea of a nexus approach to water, energy and food security (WEF nexus) was first presented at the Conference "The Water, Energy and Food Security Nexus - Solutions for the Green Economy", held in Bonn during 16 – 18 November 2011 (see here the background paper of the conference by H. Hoff). A Nexus approach focuses on the complex and dynamic interrelationships between water, energy and food by looking at the impacts that a decision in one sector has on the full plurality of involved sectors. The nexus approach anticipates potential trade-offs and synergies, therefore allowing one to prioritise response options that are viable across different sectors. A nexus approach integrates management and governance across sectors and scales. A nexus approach can support the transition to different forms of economies which aims, among other things, at resource use efficiency and greater policy coherence. Given the increasing interconnectedness across sectors and in space and time, a reduction of negative economic, social and environmental externalities can increase overall resource use efficiency, provide additional benefits and secure the human rights to water and food. The nexus approach focuses on environment, economy and stakeholder dialogue (see Figure 6).

Figure 6. The FAO approach to the water-food-energy nexus (from The Water-Energy-Food Nexus - A new approach in support of food security and sustainable agriculture, Food and Agriculture Organization of the United Nations Rome, 2014, retrieved from

For the reasons outlined above, the WEF nexus is extremely relevant to sustainable development. We present here below some figures related to sustainability of the uses of water, energy and food.

4.1. Sustainability of energy production and use

Energy development is the field of activities focused on obtaining energy from natural resources. These activities include production of conventional, alternative and renewable sources of energy, and the recovery and reuse of energy that would otherwise be wasted.

Last modified on March 16, 2022