Climate known: Greenhouse gases are warming the planet

Studies of Earth's past climate tell us that whenever CO2 levels have risen, the planet has warmed. Since the beginning of the industrial age in the 19th century, CO2 levels in the atmosphere have increased from 280 parts per million to 380 ppm. Satellite measurements now show both that less infrared of the specific frequencies absorbed by CO2 and other greenhouses gases is escaping the planet and that more infrared of the same frequencies is being reflected back to Earth's surface. While many factors affect our planet's climate, there is overwhelming evidence that CO2 is the prime cause of its recent warming.

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Climate unknown: How much greenhouse gas to expect

The biggest uncertainty is human. Were we to cut our emissions drastically tomorrow, CO2 levels might not rise much beyond 400 parts per million. But that is improbable: only a few countries - and not the biggest emitters such as China and the US - are promising cuts on anything like the scale needed, and the credibility of such promises is undermined by those same countries building more coal-fired power stations. New Scientist 

Climate known: Other pollutants are cooling the planet

After large volcanic eruptions that pump sulphur dioxide into the atmosphere ... the planet cools for a year or two. But unlike CO2, the influence of SO2 is short-lived ... Sulphur emissions began rising again in 2000, largely as China built more coal-fired power stations. Now China is installing sulphur-scrubbing equipment in those power stations. If SO2 emissions fall, global warming could accelerate. New Scientist 

Climate unknown: How great our cooling effects are

... there is still a great deal of uncertainty about the magnitude of the overall cooling from pollutants such as SO2. The effect is currently trumped by warming due to increasing amounts of CO2, that much is clear. But is this warming the net effect of strong warming partly counteracted by strong cooling? Or is it a result of more modest warming partly counteracted by modest cooling? New Scientist 

Climate known: The planet is going to get a lot hotter

Barring some unexpected catastrophe such as a megavolcano eruption ... the planet is going to warm considerably. But by how much? New Scientist 

Climate unknown: Just how much hotter things will get

One way to get an idea of how complex feedbacks play out in Earth's climate is to use computer models. The other, more trustworthy method is to look at how changes in CO2 have affected past climate, from the recent past to millions of years ago. Both methods suggest doubling CO2 warms the planet by 2°C at least ... Some studies of past climate, however, point to climate sensitivities of 6°C or more. One reason for this discrepancy is that climate models include only "fast" feedbacks, whereas palaeoclimate studies also include longer-term feedbacks such as changes to ice-sheet coverage ... New Scientist 

Climate unknown: How things will change in each region

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Climate known: Sea level is going to rise many metres

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Climate unknown: How quickly sea level will rise

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Climate unknown: How serious the threat to life is

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Climate known: There will be more floods and droughts

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Climate unknown: Will there be more hurricanes?

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Climate unknown: If and when tipping points will come

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The biggest climate change uncertainty of all

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