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We are now at the stage in history where only the most ardent flat-earther or conspiracy theorist can deny human influence over the climate in recent centuries.
There are many individual strands of evidence that are compelling in themselves, but when put together make up a completely damning picture which points only to one thing: man made climate change is here, it’s real, and it’s happening faster than was thought possible only a few years ago.
The driver, of course, is the consistent rise in greenhouse gases, predominantly CO2, consistent with what one would expect from with huge levels of fossil fuel combustion. Levels have been climbing rapidly (see graph below) and are now at levels last seen 15 million years ago.
Though it doesn’t happen immediately, atmospheric temperatures increase after CO2 is emitted, as has happened in the past. The graph below shows how temperatures have risen in the last half-century.
But rising temperatures are only one way that we are influencing the planet. In 2010 the National Oceanic and Atmospheric Administration (NOAA) released a report highlighting the state of the warming climate. Over 300 scientists from 48 countries contributed to the report, which highlighted ten ways in which the environment is changing due to rising temperatures, including receding glacier cover and sea ice levels, and increasing air temperature, sea temperature, and humidity.
Even the deniers can no longer deny climate change. The Berkeley Earth Project was set up to reassess evidence of the world's warming. It was supported and funded by climate change sceptics (including the Koch Brothers). In October 2011 they released their findings that, instead of contradicting existing measurements, correlated with them extremely closely, supporting the climate science consensus. http://www.bbc.co.uk/news/science-environment-15373071 Whether this puts an end to the sceptics once and for all remains to be seen.
Quick question and answer: how do we know humans are causing climate change?
Could the rise in atmospheric carbon be coming from somewhere else?
How do we know that the extra CO2 in the atmosphere is warming the planet through the greenhouse effect?
(The above Q&A was taken from Skeptical Science, where you can read more about the evidence)
Sources and further reading
Last Time Carbon Dioxide Levels Were This High: 15 Million Years Ago, Scientists Report
Climate change: Key Indicators
Does CO2 always correlate with temperature (and if not, why not?)
NOAA: Past Decade Warmest on Record According to Scientists in 48 Countries
BBC News - Global warming 'confirmed' by independent study