Once global warming was exclusively a naturally-occurring process.
The Earth’s atmosphere is made up of gases, some of which absorb infrared radiation. This means they retain heat from the sun, forming a thermal layer around the planet.
Without this so-called greenhouse effect, temperatures would be around -18 degrees Celsius, so there would be no — or little — life on Earth.
But the warming process has been destabilized, as levels of key greenhouse gases — carbon dioxide, methane and nitrous oxide — have reached their highest in 800,000 years.
In the last 130 years the global average surface temperature has risen by 0.85 degrees Celsius. The current trend could lead to a four degree rise after another 80 years.
Scientists agree that humans are influencing the climate system through:
– Fossil fuel burning; around 2,000 billion tonnes of CO2 have been emitted since the start of the Industrial Revolution in 1750
– Intensive agriculture; methane produced by the sector accounts for 10-12 percent of global greenhouse gas emissions — 40 percent of which comes from livestock alone
– And deforestation; which destroys CO2-absorbing vegetation
Recent climate changes are already impacting humans and natural systems worldwide, with an increase in phenomena like:
– worsening drought
– flooding
– rising seas as glaciers melt and sea water expands
– ocean acidification
– and the extinction of species
United Nations’ climate experts warn of devastating and irreversible consequences if global warming is not quickly and sharply limited.
Sources: IPCC, NOAA, NASA
MARIAN HENBEST, STÉPHANE KOGUC / AFP VIDEOGRAPHICS / AFP