Let’s say, for the sake of argument, that these conditions are true:
[ul]
[li]The climate sensitivity of CO[sub]2[/sub] is 3[sup]o[/sup] C. That is, a doubling of the CO[sub]2[/sub] concentration in the atmosphere to 560 ppm will lead to a 3 degree increase in average temperatures relative to a pre-industrialization average.[/li][li]The CO[sub]2[/sub] concentration in the atmosphere increases over the next 80 years to asymptotically approach 560 ppm. In the year 2100, it is very near 560, and plateaus at that point. It does not meaningfully increase further, nor does it fall. CO[sub]2[/sub] sinks and sources are in balance.[sup]*[/sup][/li][/ul]
How long does it take the climate of the Earth to stabilize at 3[sup]o[/sup] over the pre-industrial average? A decade? A century? Longer?
[sup]*[/sup] Would rather not get into a rathole about feedbacks positive or negative. Let’s assume that sinks and sources are balanced and non-CO[sub]2[/sub] greenhouse gases are stable as well.
The short answer is, like any discussion of climate change - there’s only one way to find out.
Otherwise, everyone has a different computer model that gives a different answer. My totally unscientific guess is - why would there be significant lag? Other than melting glaciers, or water evaporating, what process would slow heating down, considering that temperatures can vary more than that seasonally, or even daily? The sun heats the earth, if the temperature does not escape as IR or convert into other forms (i.e. melting ice, converting water to vapour) then the overall temperature goes up until it reaches a new equilibrium.
There’s a school of thought that suggests with more heat comes more water vapour, and clouds being shiny white, more sunlight reflected back into space, so heating effects are not so great but the earth gets wetter from all the clouds producing rain. However, that concept is a job for climate modellers to analyze. (And of course, more rain and CO2 means more stuff growing and capturing carbon… perhaps)
I think the main cause of the lag is oceans - they have enormous heat capacity, and consequently there is a significant lag to them reaching homeostasis with an atmostphere with a higher concentration of greenhouse gases.
I get your point that no one is going to know with certainty, but that doesn’t mean there isn’t a reasonable range of estimates based on good science. So feel free to understand my question as: what is the current state of the science here?