What is Earth’s normal temperature?
There isn’t one, that’s the thing. It has swung widely, with ice covering the planet for more time than it has been like it is now. We are overdue for another ice age. If one wants to wory about climate change, worry about that. The Sun will cycle down again and earth will get very cold again. We could not produce enough CO2 to stop it. (CO2 has no bearing on temperature, anyway)
Actually, historically, much warmer than now - 8-10 degrees warmer.
It was only after South America broke apart from Antarctica that a polar current could form that kept the cold air there from following sea currents north, so Antarctica shifted from Pacific Northwest weather to Alaska.
And at the same time, the Himalayas rose, pulling CO2 from the atmosphere from their weathering of rock at a much faster rate than the historical norm. Less CO2, lower greenhouse temps.
The end result is a much colder planet. After all, the Ice Ages only started 40 or so million years ago, though there may have been 100K warm/chill cycles before the 100K ice age cycles we see today.
And a lot of global warming alarmists forget we are in a WARM period between ice ages. And at 12K years into a warm period that lasts on average 10-20K years, we could easily slip back into a new ice age.
If you look at just the last 40 million years with ice age cycles, the average warm period is around now’s temperature, with highs and lows around that. But factor in the ice age periods, and we’re unusually warm. It has to be, when you had millennia of mile thick ice sheets in the Northern Hemisphere.
Never mind that.
What's their null hypothesis? I mean, is it that Earth's climate is invariant without human activity?