LowKey wrote:Either or both.
Okay, let me dig up some papers. I am going to be mostly without internet access from this evening until Sunday night, so it may be a while.
Assuptions drawn from data of past periods on how differing factors individually and combined affected the overall past climate to help create the model used. Better?
Sorry, I wasn't trying to be pedantic, but it makes a difference. Certainly, our understanding of different processes is based in no small part on on data taken from a relatively small slice of time (past few hundred years). But in order to overcome that, we supplement the observational data with proxy data and physical theory-based modeling. There's a very strong misconception that amounts to "the idea of global warming is based on people extrapolating a short term trend forward" and this is really, really wrong. So I try to correct it when I can.
Okay, as simply as I can put it:
1- Absent humanity's contribution to GHG, what would a chart showing global temperature change from the start of the last/present ice age to the start of the next one (or the period of highest global temperature after this ice ages end) in (let's say) 100 year increments look like? If global climate changes are cyclic, shouldn't we view (at least initially) the whole cycle before trying to isolate smaller portions of if?
1a- Add in humanity's GHG and same question as above.
I'm not sure if I'm understanding this correctly or not, but if I am, then what you're asking is basically:
In the absence of human activity, what would temperature look like over the next glaciation cycle? It would be a continuation of the orbitally-paced glaciation cycling seen in the ice core record, e.g.:
Top is the past ~5 million years, deep ocean temperature proxy record from benthic d18O stacks. Bottom is the ice core record, which is what you want to focus on.
How have we changed that pattern? For one, we've greatly altered the amount of GHGs in the atmosphere:
Here's where it gets complicated. Because we have the choice to take a number of different emissions paths, the future is not set in stone. If a global virus eliminated all humans tomorrow, GHG levels and the ensuing temperature change would look very, very different than if we choose to burn all the fossil fuels, and this would in turn be very, very different than if we choose to stabilize our emissions at a given level. So now we're into different scenarios.
If humans were wiped out tomorrow, our emissions would essentially fall to zero. The ocean and biosphere would slowly scrub our GHGs back down to preindustrial levels:
The red line is if we froze CO2 at present levels (i.e. our emissions decrease to the point where they are balanced by uptake from carbon sinks). The blue and grey lines are different model responses to cutting CO2 emissions to zero (Matthews and Weaver, 2010
If, on the other hand, we burn all of the fossil fuels, it would take millennia for this to happen:
This is the amount of time it would take for the atmosphere to recover from two different pulses of carbon: top is business as usual and bottom is all of the fossil fuel (Archer, 2009
We have raised CO2 levels well beyond what they've done naturally for the past 800,000 years, but we really haven't raised them by a huge amount yet
. We're at ~400ppm. We could increase that to ~800ppm, and carbon cycle feedbacks could take us even higher. That's an enormous change.
Alternatively, we could reign in emissions and gradually level off.
Red is an extreme high end scenario of fossil fuel use, blue is an emissions reductions pathway (Meinshausen et al., 2011
The problem in terms of mass extinctions isn't that what we've done so far is so horrific, it's that we still can do so much more. During the Last Glacial Maximum (what many people refer to as the last Ice Age), globally averaged temps were only about 4-6°C colder (Schneider von Deimling et al., 2006
; Annan and Hargreaves, 2012
). But sea levels were hundreds of feet
lower. A vast ice sheet stretched down to the upper Mid-Atlantic states, and was ~ miles thick in parts of Northern Canada. Vegetation, precipitation, and wildlife patterns were very different. A few degrees of globally-averaged temperature change is enormous.
The End-Permian mass extinction, the worst mass extinction event in the history of the planet, was precipitated by a greenhouse warming of ~8°C (Joachimski et al. 2012
). (To be clear, there were likely other contributing factors, and we're not as vulnerable to a carbon pulse as the End Permian was due to our ocean carbonate buffer, but it's worth keeping in mind for perspective.)
And of course, the rate of change matters as much, if not more.
2- Would the "max" temeprature of the planet at the "high point" of the cycle (from one ice age to another) be the same, lower, or higher without GHG caused by mankind?
For this interglacial, we would be very slowly cooling in the absence of humans, so lower. But there have been other interglacials in the past that were a little warmer than present (by about 1°C or so, see above).
What would the temperature be both with and without our GHG?
I think I've answered this?
How about the low temperature at the bottom (coldest point) in the cycle?
Our best estimate of the cooling at the Last Glacial Maximum is ~4-6°C colder globally-averaged than preindustrial temperatures. So in that neighborhood. Some glacial maxima would have been a little colder, some warmer.