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Let me state upfront, I'm not arguing the existence of climate change, just trying to clarify my own understanding of something that doesn't make sense to me yet. With apologies for the layman's question...

I've seen similar numbers frequently, but let's use this xkcd as a reference. It states that a 4.5 C change in the global climate is the difference between Boston under a half mile of ice, and the temperate zone it is today. Boston's current annual average temp is 10.75 (~15 average high, ~7 average low). Looking for an analog for a city that's about 4.5 C cooler than Boston gives a location like Moncton, New Brunswick. Colder, yes, but not buried under half a mile of ice.

My brain says that a place covered by ice is far colder than current temps minus 4.5 C, and that since the ice sheets covered so much of the earth, I can safely skirt the "climate is global, you're looking at local temps" hangup. So what gives? How can a climate like Boston's cool by "only" 4.5 C and support such enormous glaciers? It would seem that the presence of such enormous ice sheets would imply that the average temperature must have been far colder than that...

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    $\begingroup$ you are mixing apples and oranges. It says the GLOBAL climate changes by 4.5 C. That doesn't mean Boston's climate changes by 4.5 C. Keep in mind that the average global temperature change will not be evenly distributed. You should expect high latitudes and high altitudes to undergo the most temperature change, while the ocean surface and tropical zones will undergo the least temperature change. $\endgroup$ – farrenthorpe Jul 21 '17 at 4:05
  • $\begingroup$ ok...makes sense. could you direct me to any layman-friendly resources to get a better handle on how the climate change might realistically be distributed? From your comment, I'm imagining something along the lines of "70-90 latitudes drop by 15 C, 50-70 latitudes drop by 10 C, 30-50 drop by ..." etc. Is that idea (if not the numbers, which I just made up...) roughly accurate? $\endgroup$ – nwhaught Jul 21 '17 at 4:09
  • $\begingroup$ Ugh, I don't know of any images that show global cooling scenarios. Will warming work for you? You can see this image: ipcc.ch/report/graphics/images/Assessment%20Reports/… RCP 2.6 is "a little warming" and RCP 8.5 is more of a worst case scenario. $\endgroup$ – farrenthorpe Jul 21 '17 at 5:08
  • $\begingroup$ oh, absolutely. I'm using cooling to get my head around the problem, since we actually have the historical data, warming is just turning the problem around, and is what I'm really trying to understand anyway. Thanks very much! $\endgroup$ – nwhaught Jul 21 '17 at 5:09
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    $\begingroup$ Perhaps one factor that's missing is that glaciers flow. For a northeast glaciation, the snow probably starts accumulating in northern Canada, with ice flowing south as it reaches sufficient depth. (Scooping out the Great Lakes, Finger Lakes, and other features on the way.) And of course, since ice & snow are reflective, the temperature on the glacier tends to be much lower than average, leading to further snow accumulation... $\endgroup$ – jamesqf Jul 21 '17 at 6:03
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I was able to dig up this (although without being able to find full sources):

ice age temp map

It shows a pattern where the temperatures are dramatically lower where ice sheets have built up relative to today. Interestingly, a lower overall temperature means weaker mixing across latitudes, so some areas are actually warmer. Boston gets chilled by -20 according to the map.

Also note the altitude effect. If the surface is raised by 1 kilometer, then due to the lapse rate, the temperature at the new surface will drop by 6.5 degrees. This is a very important factor in stabilizing large ice sheets.

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  • $\begingroup$ Very cool. No pun intended. Also, first I'd heard of lapse rate. Thanks for the answer! $\endgroup$ – nwhaught Jul 22 '17 at 4:09

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