As always, here's a place to put your questions, particularly about science.
As it's a new year, and resolutions may be present, questions about getting started exercising (especially for running, but other sports also welcome), or good things to read are also welcome.
06 January 2009
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12 comments:
Robert,
Ok, a runnign question - your favorite MD or DC are short course.
My favorite varies some by where my training is -- hillier courses when in better training, flatter courses now. Also by geography. When I'm running more, I'm ok with going farther to a place to run.
Some favorites:
Greenbelt National Park -- Azalea trail now (1.25 miles), Perimeter Trail in better training (5.5 miles, hillier -- shorter and somewhat flatter is to go out on the Perimeter trail, but come back on the road -- 3.4 miles). Buddy Attick Lake in Greenbelt (1.25 miles), and Lake Artemesia (1.35 miles). All up in or near Greenbelt, where I lived for many years.
In DC, the Tidal Basin is a nice route, but usually too crowded for me. Same problem with the Capital Crescent trail (MD to DC).
Lake Needwood and Little Bennett Park over in Montgomery County have some nice trails that aren't too long. The routes I've taken in Rock Creek Park (where I used to run regularly) are longer and fairly hilly. But very nice. I gather there's a road route through it as well which is shorter and flatter, but still scenic.
I do prefer trails to roads, packed dirt to asphalt, and asphalt over concrete.
I've taken 'short' to mean 1-3 or so miles. Do some laps for a longer workout.
For a guide to running routes all around the area, accurately measured, and with commentary and information about things to do and places to eat, see Nadim Ahmed's http://rundc.com/
I have a question if I may:
Has anybody considered any sort of linkage between continental drift and our most recent ice ages?
TomG
That's what this is here for Tom.
The main answer is, yes. There's a bit of detail that I describe more of in types of ice age about what all we mean in saying 'ice age'.
About 2.5 million years ago, we started having large ice sheets build up, and decay, in the Northern Hemisphere. Before that, there weren't large quantities of ice on land in the Northern Hemisphere.
One of the leading candidates (it's a while since I've checked the status) for why it happened around then, rather than a couple million years earlier or not at all, is a plate tectonic explanation.
About that time (or somewhat, but not drastically, before) South America and North America drifted far enough towards each other to meet up. Prior to that, they were separate, and there was a current flowing between the two from Atlantic to Pacific ocean. The mechanism suggested for this translating to ice ages is that the current, instead of going east-west through the strait, started carrying warm waters north towards Greenland. This warmer water then provided more snow on Greenland.
Tectonics is also at heart of the Antarctic glaciation. As South America and Australia pulled away from Antarctica (25-35 million years ago), it permitted the Antarctic atmospheric circulation to become isolated from the rest of the world (Antarctic circumpolar vortex). When this happened, accumulation rates became very low (no warm snow), but melt rates essentially vanished due to the extreme cold.
Tectonics doesn't, however, help explain the ice age cycling of the last 2.5 million years. It set up conditions for ice ages to cycle, but the cycling itself is due to orbital variations.
I had the impression that the Central American seaway closure as a critical factor had fallen into disfavor of late, replaced by the idea that the carbonate sink has been steadily gaining ground on volcanism over the last 40 my or so. I saw some results (can't remember where) in the last month finding that both the Drake Passage opening and the sink reduction were necessary for extensive Antarctic glaciation.
This is the most recent review article I could find. The key passage:
"While the onset of continental-scale glaciation in the earliest Oligocene (Oi-1 event; ~34 Myr) has long been attributed to the opening of Southern Ocean gateways (Kennett, 1977), recent numerical modeling studies suggest declining atmospheric CO2 was the most important factor in Antarctic
glaciation. As the passages between South America and the Antarctic Peninsula (Drake Passage), and Australia and East Antarctica (Tasmanian Passage) widened and deepened during the late Paleogene and early Neogene, the southern oceans experienced cooling sea surface temperatures by several degrees. Estimates for the opening of Drake Passage range between 40 and 20 Myr, blurring the direct ‘cause and effect’ relationship between the gateways and earliest glaciation.
"To help solve this issue, coupled climate-ice sheet models have simulated the Eocene-Oligocene boundary accounting for decreasing CO2 concentrations and orbital variability (DeConto and Pollard, 2003). Results from the modeling show that tectonically-forced changes in ocean circulation and heat transport have only a small effect on temperature and glacial mass balance in the Antarctic interior. Considering the sensitivity of polar climate to the range of CO2 concentrations predicted to have existed over the Paleogene-Neogene, CO2 likely played a fundamental role in controlling Antarctica’s climate. Modeling also revealed that the timing of glaciation in East Antarctica is sensitive to orbital forcing, mountain uplift, and continental vegetation, but only within a very narrow range of atmospheric CO2 concentrations—around 2.8 times modern levels. Once a CO2 threshold is approached, astronomical forcing triggers the growth of a continental-scale ice sheet within 100 kyr (Fig. 1)."
See also this review article on the Central American seaway closure issue in particular.
There does seem to have been a large geologic event behind the Cenozoic glaciation but, as discussed here, apparently it was the rapid transit and sudden stop of India. The abstract:
"India’s northward flight and collision with Asia was a major driver of global tectonics in the Cenozoic and, we argue, of atmospheric CO2 concentration (pCO2) and thus global climate. Subduction of Tethyan oceanic crust with a carpet of carbonate-rich pelagic sediments deposited during transit beneath the high-productivity equatorial belt resulted in a component flux of CO2delivery to the atmosphere capable to maintain high pCO2 levels and warm climate conditions until the decarbonation factory shut down with the collision of Greater India with Asia at the Early Eocene climatic optimum at ~50 Ma. At about this time, the India continent and the highly weatherable Deccan Traps drifted into the equatorial humid belt where uptake of CO2 by efficient silicate weathering further perturbed the delicate equilibrium between CO2 input to and removal from the atmosphere toward progressively lower pCO2 levels, thus marking the onset of a cooling trend over the Middle and Late Eocene that some suggest triggered the rapid expansion of Antarctic ice sheets at around the Eocene-Oligocene boundary."
Summarizing all of this: It's the CO2, stupid. :)
Thank you for your response.
My interest is mainly how drift would affect the Arctic, but it seems I just gained a lot of neat knowledge about the Anarctic as well thanks to yourself and Steve.
TomG
Steve: Thanks for the update and reference. I knew that there were (always had been) questions about the idea, but not where the current balance of evidence was.
Tom: I think that's one of the most fun parts of science. You start out in one direction, and discover a bunch of other interesting things along the way. For the Arctic specifically, drift has been a matter of opening it up over the last 65 million years (or more). This has meant expanding the Arctic ocean, but I don't recall anybody arriving at major climate changes as a result.
Once we get up to the few million year time scale, we do need to (and folks do) include tectonics. In addition to drift, there are hypotheses (again, I don't know current status) that mountain building, such as in the Himalayas, can be important for climate.
This recent review of Cenozoic Arctic environment has the following interesting passage:
"A change from lower to higher organic carbon sediment is found at ~49Myr ago, when the environment is interpreted to have been characterized by fresh, relatively cool (~10–14C) surface waters. Although a deep-water connection did not exist between the Arctic Ocean basin and the other oceans at this time, the presence of fresh water in the Arctic may have enhanced the initiation of sea ice that increased albedo and contributed to global cooling."
A quick scan through Google Scholar didn't find any obvious development of this idea, which isn't to say it's not there.
The article isn't about geography as such, but the changes get mentioned throughout. As Bob says, the only large change was the opening of the eastern Arctic Ocean (meaning east of Greenland), which was more or less the flip side of the North Atlantic opening. Other than that, the Arctic Ocean seems to have stayed put for on the order of 150 million years.
Tom, maps showing these changes shouldn't be too hard to find on the internet if those are what you're after.
Bob, if you're game for a different topic I'd be really interested to know what you think of this paper by Tsonis. He has some related work, all linked on his home page.
Looks interesting Steve. It'll be a while, but I'll assemble some detailed thought.
Thanks, Bob. I had kept expecting to see some development and/or response to this paper, but so far nothing (that I'm able to see anyway). I just now checked Google Scholar for any citations; the first three were Lindzen, Singer and Monckton, which I think we'd have to call a steep negative gradient. The Lindzen cite is just an op-ed of some sort, and there's nothing of value in the rest. Since Tsonis is a serious scientist and the hypothesis is interesting, the apparent lack of response is a surprise.
I do know there are others to see if there's predictability in these cycles, but as far as I know none have proposed anything like a global solution.
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