if temps were .24 degrees cooler than we thought ..... then it follows that the temp rise is .24 greater than we thought ......for the same increase in GHG"s ....... hence sensitivity is greater
Or did i miss something ? ......always happy to be shown the error of my ways !!
Sensitivity is calculated by starting a model in an equilibrium state, which is got by running it for 100 years or so to check that nothing substantial is changing. Then CO2 is instantly increased by four times, and the model is run for another 100 years or so to see how much the temperature increases. Half that increase is the sensitivity.
If the starting equilibrium is not a realistic climate, then there's a problem with the model in general, and fixing it might move the sensitivity up a bit, up a lot, or even down a bit. There's plenty of reasons to think it might be higher, and this points vaguely in that direction too, but its far from conclusive.
The equilibrium state is key. We know temperatures fluctuated significantly over the previous millennia, while CO2 levels remained fairly constant.
What do you mean? If you're talking about the local variations, commonly referred to as the "little ice age" and the "medieval warm period", those were not global, but mostly limited to the northern hemisphere (especially Europe). The global temperature average was quite stable over the past 2,000 years, varying by no more than about +/- 0.3°C over that period (much less than what we've seen since 1850). Quote: "Global average temperatures show that the Medieval Warm Period was not a planet-wide phenomenon, and that the Little Ice Age was not a distinct planet-wide time period but rather the end of a long temperature decline that preceded recent global warming." (See here for source information.)
Global temperatures were in equilibrium (small/slow variations notwithstanding) for thousands of years before the current warming began. Even the temperature increase at the end of the last ice age, 17,000 to 10,000 years ago, while much larger absolutely, was also much slower than what we see today. Global temperatures are increasing at a rate of about 0.3°C per decade right now, faster than at any point during the last warming period.
I mean exactly what I said. Even if the mwp and lia were confined to the northern hemisphere, that is still half of the globe. Surely, that would have some impact! In recent warming, the northern hemisphere is outpacing the southern. Shall we dismiss that as an aberration also?
I prefer data from scientific sources, rather than wikipedia, and acknowledge that there is an uncertainty in proxy data, and choosing the one dataset that best represents your viewpoint, is not the best practice. As can be seen from the graph, the choice of timeframe for preindustrial temperatures can make a large difference, and may include natural causes.
"Please understand that there is not one single correct reconstruction. Most of these reconstructions show a clear Medieval Warm Period and a clear Little Ice Age. Presently, we are interested in the reconstructions prior to about 1950."
http://www.atmo.arizona.edu/students/courselinks/spring17/atmo336s2/lectures/sec5/controversy_spring20.html"There is a great deal of difficulty and uncertainty in trying to reconstruct global average temperature changes based on proxy data. One thing we can say with some certainty is that global average temperatures have fluctuated all throughout Earth's history, including the last 2000 years. Prior to 1900, we can safely say that the changes were not caused by anthropogenic additions of greenhouse gases, but were natural fluctuations."
