What is global warming? | wwf

What is global warming?

Throughout geological history, the world has experienced cold and warm periods known as ice ages and interglacial periods. These natural changes have largely been due to variations in Earth’s orbit around the Sun, which affects the amount of sunlight reaching Earth’s surface.

However, scientists have found that our planet is now warming at a rapid rate, over a short period of time, playing havoc with our climate system and natural environment.


According to the World Meteorology Organization, the years 2010, 2005 and 1998 ranked as the hottest on record, and the ten warmest years on record have all occurred since 199811.

In the Southern Hemisphere, 2009 was the warmest year since records began in 188012.

The United Nations’ Intergovernmental Panel on Climate Change (IPCC) 2007 report projects global temperature rises by century end of between 1.1°C and 6.4°C13.
A 6.4°C rise will have a devastating impact on our planet and all life.


The science of global warming – the natural greenhouse effect

Our atmosphere naturally allows the Sun’s infra-red radiation to reach Earth’s surface and warm it. Some of the radiation is absorbed and some of it bounces off Earth’s surface and into the atmosphere.

Naturally-occurring greenhouse gases in the atmosphere – carbon dioxide, water vapour, nitrous oxide, methane and other gases – create a blanket-like effect and trap the infra-red radiation. This maintains average temperatures of about 15°C on the Earth’s surface. Increased greenhouse gases would result in more heat being trapped on the planet surface.


The human-induced greenhouse effect and global warming

Scientists have looked at a range of potential causes of current global warming, including the effects of the Sun, volcanoes, water vapour and aerosols.

They have found that an increase in the amount of long-lived greenhouse gases in the Earth’s atmosphere, particularly carbon dioxide, is the major cause of current global warming.

Scientists have discovered that the greater concentration of greenhouse gases in the atmosphere is a result of human activity. Burning fossil fuels (coal and oil) and land clearing (which releases the carbon trapped in soil and vegetation) are rapidly changing the carbon dioxide balance of our atmosphere.

For more information, go to What Causes Global Warming.

An example of the impact of global warming – Arctic ice melting

As the Earth warms, the impacts can fuel each other. This accelerates the process of global warming, causing runaway, irreversible changes.

For instance, polar ice reflects sunlight away from the Earth.
When the ice melts (and Arctic sea ice is already predicted to be mostly gone in the summer months by 2020), more heat will be absorbed. When heat is absorbed, thawing permafrost releases trapped methane, just as drying forests and warming oceans also emit their stores of CO2. All of these changes further increase the damaging impacts of the greenhouse effect.


(1) http://nsidc.org/arcticseaicenews/
(2) http://www.geo.unizh.ch/wgms/mbb/sum08.html
(3) http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-a-palaeoclimatic.html
(4) http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-direct-observations.html
(5) http://www.giss.nasa.gov/research/news/20100121/
(6) http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch5s5-2-2.html
(7) http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-direct-observations.html
(8) http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch10s10-3-6-3.html
(9) http://www.abc.net.au/science/articles/2008/11/11/2415539.htm
(10) http://www.ipcc.ch/publications_and_data/ar4/wg2/en/ch4s4-4-1.html
(11) http://www.wmo.int/pages/mediacentre/press_releases/pr_906_en.html
(12) http://www.giss.nasa.gov/research/news/20100121/
(13) http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-projections-of.html#table-spm-3

Evidence of Global Warming

  • The extent of glaciers and Arctic sea-ice has decreased1 2

  • The warmth of the Northern Hemisphere in the past 50 years is likely the highest in the past 1,300 years3

  • Global warming of 0.7°C and sea-level rise of 17 cm since 19004
     
  • The decade ending 2009 was the warmest on record5

  • Warming of the upper 700 m of ocean6

  • Increase in extremely high temperatures, decrease in extremely low temperatures7

  • More intense cyclones in the North Atlantic since 19708

  • Oceans have become more acidic due to higher CO29
     
  • Shifts in plant and animal locations and behaviour10