If anyone is trying to say that the extra CO2 in the atmosphere is from 'natural' sources, they would first have to explain what those natural sources are that are putting so much extra CO2 into the air all of a sudden (so far a 40% increase in CO2), when they haven't ever done it before, when volcanoes haven't increased and deforestation is occurring.
Then they would have to explain where all the additional CO2 being emitted from the burning of fossil fuels is going (31 billion tonnes just in the last year) and why the oceans are not even more acidic than they are as a result - because that's the only other sink that could absorb the extra - and already accounts for about half the extra CO2 going into the air. And there is a limit to how much extra CO2 the oceans can absorb in a given time.
Anyone can find numerous articles on denier websites that argue:
a) the earth is not warming - even though it clearly is - and the ice is melting by magic, warm seasons are getting longer by magic
b) the greenhouse effect is not real (and the earth, rather than being frozen, is at a liveable temperature by magic)
c) it's all uncertain - even though the certainty has been calculated and is well contained
d) the last decade has not warmed - or it's cooled - even though the temperature trend is up, the past decade has been hotter than the one before, which was hotter than the one before that etc, and last year was the equal hottest year on record
e) that China and India building a large number of dirty power stations is not a good explanation for aerosol cooling, even though aerosols have long been known to limit warming and it has been studied and shown to have an effect.
f) it's the sun - even though it's not. The sun's impact is measured and calculated and does not account for the warming.
Some people just don't want to face facts. I don't know why that is - it's a complete mystery to most rational people. They spend their time scouring sites set up for the purpose of disinformation. A true sceptic would look at all the information available - they would look at the entire picture and not limit themselves to small bits of erroneous and often conflicting disinformation to clog up science forums.
The science is in - the greenhouse effect is real and is why we are alive today. Greenhouse gases are building up in the air because we are burning fossil fuels and cutting down trees. The extra greenhouse gases warm the world and are leading to dangerous warming at a pace faster than we can adapt to. If we don't cut emissions enough over the next couple of decades, seas will rise, there will be more droughts, fires, floods and other events that will severely disrupt society and cause immense and irreparable harm - with these impacts lasting for generations to come.
We have the solutions. It's past time to be arguing the science. We just need to get on with fixing it.
