HOW DO WE KNOW IT'S HAPPENING
The climate of the Earth is always changing. In the past it has altered as a result of natural causes (think oceans, atmosphere, orbits and volcanoes). Nowadays the term “climate change” is generally used when referring to changes in our climate that have occurred over the last 100 years or so.
The climate changes we've seen over recent years and those that are predicted over the remainder of this century are thought to be mainly as a result of the influence of human behaviour rather than due to natural changes.
In order to curb the dangers of climate change it is first necessary to understand how we as a species accelerate this change, what effects it has on the planet, and what actions we can take to reduce the strain on Earth's resources.
For more than 200 years we have been burning increasing amounts of fossil fuels like coal, oil and gas, primarily to generate heat, power and light. However, what we didn’t know until relatively recently was that along the way we have been creating a bit of a problem. This is because fossil fuels contain carbon and when we burn them carbon dioxide (CO2) is released. Too much CO2 is not so good for the climate.
THE IPCC REPORTS
The Met Office website lists seven sources of evidence for climate change:
Changes in nature
Sea level rises
And thousands of scientists around the world have collated their research into the Inter-Governmental Reports on Climate Change.
The Intergovernmental Panel on Climate Change (IPCC) issued a fifth assessment report (AR5) on global warming in 2014, in which some 95% of all scientists agree that warming of the planet since the 1950's has been predominantly human-induced. The latest report reinstates that there is 'unequivocal' evidence of warming of the atmosphere and oceans, losses of snow and sea ice cover, and increases in greenhouse gas concentrations unprecedented over timescales from decades to millennia. The report also concludes that climate models have improved since the fourth assessment report (AR4) in 2007, better reconstructing observed changes in temperature during the 20th Century and thus likely to yield more accurate projections of changes in the Earth's climate system during the 21st Century.
Some key points from the latest edition of the six-yearly report:
The last 30-year period (1983-2012) was likely the warmest across the Northern Hemisphere for 1400 years.
Globally, there has been a successive increase in the average Earth surface temperature over the last three decades, all of which were warmer than any preceeding decades since 1850.
Between 1880-2012, an estimated global temperature rise of 0.85˚C is deduced from average land and ocean surface temperature data.
Present-day atmospheric concentrations of carbon dioxide (CO2), methane and nitrous oxide have reached such high levels not seen in over 800,000 years.
Modelled sensitivity of the climate system to a doubling of atmospheric CO2 concentration has been revised in the range of 1.5-4.5˚C.
Over 90% of the additional retained energy during the period 1971-2000 in the Earth's system is thought to have been absorbed by the oceans and this is expressed in terms of ocean warming.
http://www.ipcc.ch/report/ar5/wg1/#.UkxFpYaTiqk - IPCC Fifth Assessment Report (AR5) - Full Report
http://www.ipcc.ch/news_and_events/docs/ar5/ar5_wg1_headlines.pdf - Headline Statements from the Summary for Policymakers
http://www.bbc.co.uk/news/science-environment-24282150 - Key findings of the report from the BBC