What is global warming?

Forest fire due to global warming, orange flames behind black trees with words global warming explained written

The IPCC Special Report on Global Warming of 1.5°C (2018) defines warming as ‘an increase in multi-decade global mean surface temperature above pre industrial levels’ and more specifically as ‘an increase in combined surface air and sea surface temperatures averaged over the globe, over a period of 30 years’ [1]. So basically, global warming is a global average increase in temperature both on land and in the oceans. The same IPCC report also states that global temperature has been increasing by 0.2°C every decade and in 2017, human-induced temperature rise reached around 1°C above pre-industrial levels (period 1850-1900). Right now, the average global temperature has been rising more rapidly on land compared to the oceans and it is believed that 20-40% of the global human population has now experienced greater warming of above 1.5°C above pre industrial levels in at least one season.

How does global warming occur

Natural greenhouse effect

All around the Earth is a layer of gases called the atmosphere. This atmosphere plays a very important role in shielding the planet against the harmful rays of the Sun. When solar radiation reaches the Earth, some of it is reflected back into space; the rest of the energy is absorbed by the land and the oceans. Heat that is not absorbed is reflected back from the Earth into space. Certain gases such as carbon dioxide, methane, nitrous oxide, water vapor and tropospheric ozone, are found that in the atmosphere and they trap some of the heat that is reflected back. This process called the greenhouse effect, allows some heat to stay on the Earth and in this way, help to maintain an adequate temperature to sustain life.

Human induced greenhouse effect

Throughout the billions of years that the planet Earth has existed, there has been a lot of temperature shifts from ice cold to very hot. Yet, the planet has always returned back to a more or less stable temperature, a process which took millions of years, thus allowing life to develop and evolve over the different time scales. Today, the temperature on Earth is increasing alarmingly as a result of human activities related majorly to the burning of fossil fuels on the one hand, and mass deforestation on the other hand.

Right now the most dangerous greenhouse gas is carbon dioxide which is released during the burning of fossil fuels in factories and through vehicular combustion; methane is also a powerful greenhouse gas which is emitted from the agricultural sector. As they are released, these gases make their way to the atmosphere, accumulating over time, thus absorbing even more heat radiated from the Earth. This inevitably leads to an increase in energy content on the planet and a rise in temperature.

In contrast, trees can absorb the carbon dioxide released, with terrestrial forests being very important carbon sinks, absorbing some 2.5 billion metric tons of carbon dioxide on Earth according to a NASA-led study [2]. However, the IUCN declares that since the 1960s, more than half of the world’s tropical forests have been destroyed and around 3.7 million hectares of forest in Europe is in a dreadful condition [3]. This clearing of forests to make way for housing and agriculture has led to a drop in the amount of stored carbon dioxide and an increase in atmospheric level.

Past carbon dioxide levels

The IPCC states that between 600 and 400 MYr BP and 200 and 150 MYr BP, there were very high amounts of carbon dioxide on Earth sometimes, exceeding 3000 ppm [4]. Over the years, due to many natural processes, the amount of carbon dioxide eventually decreased and based on geochemical evidence, it is believed that concentrations were as low as less than 300 ppm some 20 Myr BP. By analyzing trapped bubbles in Vostok ice core, carbon dioxide concentrations for the past four glacial/interglacial periods have been measured. Thus, during glacial periods the amount of carbon dioxide was low (less than 180 ppm) and as high as 300 ppm during interglacial periods. In the Holocene minimum some 8 thousand years ago, the amount of carbon dioxide was about 260 ppm increasing to 280 ppm during the pre-industrial period.  

Present carbon dioxide levels

In 2018, the amount of carbon dioxide level on Earth reached a record 407 ppm. It is strongly believed that the level of carbon dioxide that now exists is higher than what has been observed over the past 420, 000 years. Also, there is strong confidence that present day carbon dioxide rise over the past century is faster by an order of magnitude than what has been happening over the past 20,000 years based on the Vostok record and the Antarctic climate. While in the 1960s the rate of increase of atmospheric carbon dioxide was approximately 0.6 ppm per year, today it has almost quadrupled to 2.3 ppm per year. It is worth to note that this rate of increase seems to be about 100 times faster compared to what has been observed since the end of the last ice age.

Global warming in the future

Natural global warming has occurred several times in the past due to natural processes like volcanic eruption or changes in the Earth’s axis of rotation or a slight change in its orbit around the Sun. Yet, global warming this time is leading to a series of changes on the Earth that is affecting human life and activities, especially in the form of global climate change. Many nations around the world are facing issues in the form of drought, sea level rise, increased frequency and intensity of cyclones to name a few which are related to the increase in temperature on the planet. If nothing is done to curb the amount of carbon dioxide being released into the atmosphere, the level may well reach 900 ppm by 2100 or a temperature increase in the range of 3°C and 6°C.


  1. Allen, M.R., O.P. Dube, W. Solecki, F. Aragón-Durand, W. Cramer, S. Humphreys, M. Kainuma, J. Kala, N. Mahowald, Y. Mulugetta, R. Perez, M. Wairiu, and K. Zickfeld, 2018: Framing and Context. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. In Press.
  2. Rasmussen, C. (2014). NASA finds good news on forests and carbon dioxide. [online] Available at https://www.nasa.gov/jpl/nasa-finds-good-news-on-forests-and-carbon-dioxide [Accessed 16/12/2019]
  3. IUCN. (n.d). Deforestation and forest degradation. [online] Available at https://www.iucn.org/resources/issues-briefs/deforestation-and-forest-degradation [Accessed 16/12/2019]
  4. IPCC. (n.d). Working Group I: The Scientific Basis. [online] Available at https://archive.ipcc.ch/ipccreports/tar/wg1/016.htm [Accessed 16/12/2019]

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