One of the most important ways is surely the temperature data itself. It's highly manipulated in various ways, which is a very ponderous question all by itself, because why exactly would there be a need to adjust temperature data? What exactly are you doing to it, and why? The data is the data. In what sense can we apply "adjustments" to raw data without essentially corrupting it?
Then there is the question of revising older temperatures to establish trends that do not really exist. From the article above:
When future generations look back on the global-warming scare of the past 30 years, nothing will shock them more than the extent to which the official temperature records – on which the entire panic ultimately rested – were systematically “adjusted” to show the Earth as having warmed much more than the actual data justified.Check out these animated comparisons of historical data over the last 100 years or so: raw, unadjusted data overlaid with the adjusted data. Note that in every case, the raw historical data was adjusted down, but less so as it moved towards today, and so it turned what was a somewhat chaotic, real-world overall cooling trend into a nice smooth warming trend.
Two weeks ago, under the headline “How we are being tricked by flawed data on global warming”, I wrote about Paul Homewood, who, on his Notalotofpeopleknowthat blog, had checked the published temperature graphs for three weather stations in Paraguay against the temperatures that had originally been recorded. In each instance, the actual trend of 60 years of data had been dramatically reversed, so that a cooling trend was changed to one that showed a marked warming.
This was only the latest of many examples of a practice long recognised by expert observers around the world – one that raises an ever larger question mark over the entire official surface-temperature record.
When the temperature data is inconvenient, and you as a climate scientist need evidence of a warming trend to justify your existence and compete for the buckets of cash handed out by governments and NGOs to climate scientists who can show evidence of warming, well, what do you do? You adjust it.
Presto! What was a cooling trend instantly becomes a warming trend instead. All due to the magic of "adjustments" to raw data. Again: why does it need adjusting in the first place? It is by nature empirical data, recorded by the instruments available at the time. It is what it is. The practice of adjusting it brings up obvious credibility questions, as demonstrated by those animated gifs at the link above.
If this practice is widespread, then we know that most or all of the evidence establishing warming trends is essentially garbage. Which means the conclusion is garbage. And so are all the speeches, the carbon cap-and-trade legislation, the carbon financial markets, the carbon "offsets" and all the shady businesses spawned by them, etc.
I think we can all figure it out from here.