Most of them have absolutely no clue, really, mostly willfully. Air temperature records are accurate but can fluctuate due to atmospheric events like volcanic eruptions. The most accurate climate change data we have come from ice core samples and permafrost temperature samples from Alaska and Siberia.
The data, about which there is almost complete scientific consensus, show that both the amount of carbon in the atmosphere and the global temperature are headed steadily upwards (though air temperature always fluctuates within a few degrees). Atmospheric carbon levels are at ~380 parts per million, an unprecedented level which hasn't been seen in about 420,000 years, since the mid-Pliocene. By 2100, if present trends continue, we will have about ~500 parts per million of carbon dioxide, levels not seen in millions of years - perhaps since the time of the dinosaurs and what was essentially a swamp/jungle planet.
In the last decade, Greenland's ice pack melting has doubled its normal rate; by 2080 many predict the Arctic itself will be ice-free during the summer, something not seen since before the last ice age.
Basic data have already shown that climate change is affecting natural selection; many species of amphibians have become profoundly affected by the change in rainfall patterns in tropical areas; some mosquito species in the Eastern US have seen change as well because of changed periods of warmth.
Now it might come as a shock that I don't believe the mainstream climate change hypothesis - I believe we've been changing the climate for *far longer* than the last 200 years of industrialization. It's more likely we were adding greenhouse gases into the atmosphere since we began massive agricultural programs as a species sometime in the last 10,000 years, cutting down forests and planting crops that give off greenhouse gases like methane.
What it seems like is that climate change is inescapable; there are too many people in the US who are willfully blinding themselves to taking any steps to prevent it because they are afraid of changing their own lifestyles to ensure the long-term viability of life on the planet.
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