What is global warming?

Global warming is the current increase in temperature of the Earth's surface (both land and water) as well as it's atmosphere. Average temperatures around the world have risen by 0.75°C (1.4°F) over the last 100 years about two thirds of this increase has occurred since 1975.1 2 In the past, when the Earth experienced increases in temperature it was the result of natural causes but today it is being caused by the accumulation of greenhouse gases in the atmosphere produced by human activities.3

The natural greenhouse effect maintains the Earth's temperature at a safe level making it possible for humans and many other lifeforms to exist.4 However, since the Industrial Revolution human activities have significantly enhanced the greenhouse effect causing the Earth's average temperature to rise by almost 1°C. This is creating the global warming we see today. To put this increase in perspective it is important to understand that during the last ice age, a period of massive climate change, the average temperature change around the globe was only about 5°C.5 6

A long series of scientific research and international studies has shown, with more than 90% certainty, that this increase in overall temperatures is due to the greenhouse gases produced by humans.7 Activities such as deforestation and the burning of fossil fuels are the main sources of these emissions. These findings are recognized by the national science academies of all the major industrialized countries.8

Global warming is affecting many places around the world. It is accelerating the melting of ice sheets, permafrost and glaciers which is causing average sea levels to rise.9 10 It is also changing precipitation and weather patterns in many different places, making some places dryer, with more intense periods of drought and at the same time making other places wetter, with stronger storms and increased flooding.11 12 These changes have affected both nature as well as human society and will continue to have increasingly worse effects if greenhouse gas emissions continue to grow at the same pace as today.

What causes global warming?

The cause of global warming is the increasing quantity of greenhouse gases in the our atmosphere produced by human activities, like the burning of fossil fuels or deforestation. These activities produce large amounts of greenhouse gas emissions which is causing global warming.7 Greenhouse gases trap heat in the Earth's atmosphere to keep the planet warm enough to sustain life, this process is called the greenhouse effect.3 It is a natural process and without these gases, the Earth would be too cold for humans, plants and other creatures to live.

The natural greenhouse effect exists due to the balance of the major types of greenhouse gases. However, when abnormally high levels of these gases accumulate in the air, more heat starts getting trapped and leads to the enhancement of the greenhouse effect. Human-caused emissions have been increasing greenhouse levels which is raising worldwide temperatures and driving global warming.3

Greenhouse gas emissions and the enhanced greenhouse effect

Greenhouse gases are produced both naturally and through human activities. Unfortunately, greenhouse gases generated by human activities are being added to the atmosphere at a much faster rate than any natural process can remove them.

Global levels of greenhouse gases have increased dramatically since the dawn of the Industrial Revolution in the 1750s.7 Only a small group of human activities are causing the concentration of the main greenhouse gases (carbon dioxide, methane, nitrous oxide and fluorinated gases) to rise:

• The majority of man-made carbon dioxide emissions is from the burning of fossil fuels such as coal and oil so that humans can power various vehicles, machinery, keep warm and create electricity. Other important sources come from land-use changes (ex: deforestation) and industry (ex: cement production).13
• Methane is created by humans during fossil fuel production and use, livestock and rice farming, as well as landfills.14
• Nitrous oxide emissions are mainly caused by the use of synthetic fertilizers for agriculture, fossil fuel combustion and livestock manure management.15
• Fluorinated gases are used mainly in refrigeration, cooling and manufacturing applications.16

Deforestation

Deforestation has become a massive undertaking by humans and transforming forests into farms has a significant number of impacts as far as greenhouse gas emissions are concerned. For centuries, people have burned and cut down forests to clear land for agriculture. This has a double effect on the atmosphere both emiting carbon dioxide into the atmosphere and simultaneously reducing the number of trees that can remove carbon dioxide from the air.

When forested land is cleared, soil disturbance and increased rates of decomposition in converted soils both create carbon dioxide emissions.17 This also increases soil erosion and nutrient leaching which can further reduces the area's ability to act as a carbon sink.

What are the effects of global warming?

Global warming is damaging the Earth's climate as well as the physical environment. One of the most visible effects of global warming can be seen in the Arctic as glaciers, permafrost and sea ice are melting rapidly. Global warming is harming the environment in several ways including:

• Desertification
• Increased melting of snow and ice
• Sea level rise
• Stronger hurricanes and cyclones

Desertification

Increasing temperatures around the world are making arid and semi-arid areas even more dry than before. Current research is also showing that the water cycle is changing and rainfall patterns are shifting to make areas that are already dry even drier. This is causing water shortages and an intense amount of distress to the over 2.5 million people in dry regions which are degrading into desert.18 This process is called desertification.

Increased melting of snow and ice

Around the world, snow and ice is melting at a much faster pace than in the past. This has been seen in the Alps, Himalayas, Andes, Rockies, Alaska and Africa but is particularly true at the Earth's poles.

Perennial ice cover in the Arctic is melting at the rate of 11.5% per decade and the thickness of the Arctic ice has decreased by 48% since the 1960s.19 During the past 30 years, more than a million square miles of sea ice has vanished, an area equivalent to the size of Norway, Denmark and Sweden combined.20 The continent of Antarctica has been losing more than 100 cubic kilometers (24 cubic miles) of ice per year since 2002.21 Since 2010, the Antarctic ice melt rate has doubled.22

Sea level rise

The Earth's sea level has risen by 21 cm (8 inches) since 1880.23 The rate of rise is accelerating and is now at a pace that has not been seen for at least 5000 years.24 Global warming has caused this by affecting the oceans in two ways: warmer average temperatures cause ocean waters to expand (thermal expansion) and the accelerated melting of ice and glaciers increase the amount of water in the oceans.

Stronger hurricanes and cyclones

Tropical cyclone activity has seen an obvious upswing trend since the early 1970s.25 Interestingly, this matches directly with an observed rise in the oceans' temperature over the same period of time. Since then, the Power Dissipation Index which measures the destructive power of tropical cyclones has increased in the Pacific by 35% and in the Atlantic it has nearly doubled.26 Global warming also increases the frequency of strong cyclones. Every 1 degree C increase in sea surface temperature results in a 31% increase in the global frequency of category 4 and 5 storms.27

• 1. "Global temperatures." U.K. Met Office. http://www.metoffice.gov.uk/climate-change/guide/science/monitoring/global (accessed August 13, 2014).
• 2. Hansen, J., R. Ruedy, M. Sato, and K. Lo. "Global Surface Temperature Change." Reviews of Geophysics 48, no. 4 (2010): RG4004.
• 3. a. b. c. Le Treut, H., R. Somerville, U. Cubasch, Y. Ding, C. Mauritzen, A. Mokssit, T. Peterson and M. Prather. Historical Overview of Climate Change. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2007.
• 4. U.K. Met Office. Warming: A guide to climate change. Exeter, U.K.: Met Office Hadley Centre, 2011.
• 5. Hansen, J., and M. Sato. Paleoclimate Implications for Human-Made Climate Change. In: Climate change inferences from paleoclimate and regional aspects. Wien: Springer, 2012.
• 6. Shakun, Jeremy D., and Anders E. Carlson. "A global perspective on Last Glacial Maximum to Holocene climate change." Quaternary Science Reviews 29, no. 15-16 (2010): 1801-1816.
• 7. a. b. c. IPCC. Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2007.
• 8. "Global response to climate change." The Royal Society. https://royalsociety.org/policy/publications/2005/global-response-climate-change/ (accessed August 13, 2014).
• 9. U.K. Met Office. Evidence: The state of the climate. Exeter, U.K.: Met Office Hadley Centre, 2010.
• 10. National Research Council. Ecological impacts of climate change. Washington, D.C.: National Academies Press, 2008.
• 11. The World Bank. World Development Report 2010: Development and climate change. Washington, DC: World Bank and Oxford University Press, 2010.
• 12. Allison, I.. The Copenhagen diagnosis updating the world on the latest climate science. Sydney: UNSW Climate Change Research Centre, 2009.
• 13. Le Quéré, C., A. K. Jain, M. R. Raupach, J. Schwinger, S. Sitch, B. D. Stocker, N. Viovy, S. Zaehle, C. Huntingford, P. Friedlingstein, R. J. Andres, T. Boden, C. Jourdain, T. Conway, R. A. Houghton, J. I. House, G. Marland, G. P. Peters, G. Van Der Werf, A. Ahlström, R. M. Andrew, L. Bopp, J. G. Canadell, E. Kato, P. Ciais, S. C. Doney, C. Enright, N. Zeng, R. F. Keeling, K. Klein Goldewijk, S. Levis, P. Levy, M. Lomas, and B. Poulter. "The global carbon budget 1959–2011." Earth System Science Data Discussions 5, no. 2 (2012): 1107-1157.
• 14. Bousquet, P., S. C. Tyler, P. Peylin, G. R. Van Der Werf, C. Prigent, D. A. Hauglustaine, E. J. Dlugokencky, J. B. Miller, P. Ciais, J. White, L. P. Steele, M. Schmidt, M. Ramonet, F. Papa, J. Lathière, R. L. Langenfelds, C. Carouge, and E.-G. Brunke. "Contribution of anthropogenic and natural sources to atmospheric methane variability." Nature 443, no. 7110 (2006): 439-443.
• 15. Denman, K.L., G. Brasseur, A. Chidthaisong, P. Ciais, P.M. Cox, R.E. Dickinson, D. Hauglustaine, C. Heinze, E. Holland, D. Jacob, U. Lohmann, S Ramachandran, P.L. da Silva Dias, S.C. Wofsy and X. Zhang. Couplings Between Changes in the Climate System and Biogeochemistry. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2007.
• 16. Montzka, S.A., S. Reimann, A. Engel, K. Krüger, S. O’Doherty, and W.T. Sturges. Ozone-Depleting Substances (ODSs) and Related Chemicals, Chapter 1 in Scientific Assessment of Ozone Depletion: 2010, Global Ozone Research and Monitoring Project–Report No. 52, 516 pp., World Meteorological Organization, Geneva, Switzerland, 2011.
• 17. Houghton, R. A.. "How well do we know the flux of CO2 from land-use change?." Tellus B 62, no. 5 (2010): 337-351.
• 18. DARA. Climate vulnerability monitor 2010 the state of the climate crisis.. Madrid, Spain: DARA/Climate Vulnerable Forum, 2010.
• 19. Comiso, J.C., and D.K. Hall. "Climate trends in the Arctic as observed from space." WIREs Climate Change (2014): 5:389–409.
• 20. "The Consequences of Global Warming On Glaciers and Sea Levels." Natural Resources Defense Council. http://www.nrdc.org/globalwarming/fcons/fcons4.asp (accessed August 13, 2014).
• 21. NASA's Jet Propulsion Laboratory / California Institute of Technology. "Key Indicators." Global Climate Change. http://climate.nasa.gov/key_indicators/#landIce (accessed August 16, 2014).
• 22. McMillan, M., A. Shepherd, A. Sundal, K. Briggs, A. Muir, A. Ridout, A. Hogg, and D. Wingham. "Increased ice losses from Antarctica detected by CryoSat-2." Geophys. Res. Lett. (2014): 41, 3899–3905.
• 23. Church, John A., and Neil J. White. "Sea-Level Rise from the Late 19th to the Early 21st Century." Surveys in Geophysics 32, no. 4-5 (2011): 585-602.
• 24. Hunter, John. "Estimating sea-level extremes under conditions of uncertain sea-level rise." Climatic Change 99, no. 3-4 (2010): 331-350.
• 25. Yu, Jia-Yuh, and Ping-Gin Chiu. "Contrasting Various Metrics for Measuring Tropical Cyclone Activity." Terrestrial, Atmospheric and Oceanic Sciences 23, no. 3 (2011): 303.
• 26. Emanuel, Kerry. "Environmental Factors Affecting Tropical Cyclone Power Dissipation." Journal of Climate 20, no. 22 (2007): 5497.
• 27. Elsner, James B., James P. Kossin, and Thomas H. Jagger. "The increasing intensity of the strongest tropical cyclones." Nature 455, no. 7209 (2008): 92-95.