Global warming and the carbon cycle

Factual Questions
1

A question for any climatologists out there:

It seems that warming the planet significantly through burning fossil fuels should be much more difficult than it might seem on the surface. I would think that putting a bit more CO2 in the atmosphere would be fantastic for green plants. Two reasons for this. One, I guess you could call plants “carbon preditors”. The live by pulling CO2 from the air. Also, even slight increases in the growing season would have much larger effects on things like trees. It would seem that they expend quite a lot of energy regrowing their leaves each year. It seems reasonable that they spend quite alot of the growing season storing enough energy to do the same next season. Adding just a few weeks to the growing season should add quite large percent to amount of surplus energy that a tree could use for growth (and hence putting more carbon away long term). So, it would seem that it might take considerably more carbon exhaust (per year) to get really large green house effects.

Also, it would seem that creating an ice age would be easier to do. By slightly elevating the available carbon for a short time (a century is a short time to the planet), one would effectively spend the time increasing the population/size of the “carbon preditors”. If we ran out of fossil fuels or suddenly decided to stop using them over a time span of just a couple of years, it seems possible to wind up with a CO2 depletion problem shortly after. And the nifty thing about ice ages is that the presence of the ground ice nicely keeps things that way just by reflecting so much sunlight back to space.

Anyway, by looking at these two ideas it would seem that it would be easier to create an ice age by suddenly withdrawing the use of fossil fuels than to really create a major heat problem by using them. (especially since the increase in our use of fossil fuels happened relatively gradually compared to my proposed removal of them).

This is to be read as a question to the knowledgible here, not a prediction by me as to what is really gonna happen. Where are the problems in this reasoning? Are there any good supporting facts for it? (Perhaps, we have found that in the past ice ages are usually preceded by brief very warm periods, or something similar?)

2

IANAClimatologist

I think it’s presumptuous to assume that more CO[sub]2[/sub] will automatically be better for plants:

  1. Not all plants have CO[sub]2[/sub] as their limiting nutrient. Many lack sufficient phosphorous, nitrogen, iron, and various other nutrients to effectively take up more carbon.
  2. Not all plants are effected equally by CO[sub]2[/sub]. C3 plants are effected differently from C4 plants.
  3. Changes in temperature will not necessarily result in a lengthening of the growing season. That will depend greatly on local conditions, such as changes in rainfall patterns.

Also, plants don’t really put carbon away for the long term. Once the plant dies and decomposes or burns in a forest fire, much of the carbon is released. Plants are considered a short-term carbon sink. Real long-term carbon sinks include the deep ocean (thousands of years) and rocks. Growing lots of plants will not effectively sequester carbon.

Considerably more than what?

Ironically, global warming might accomplish exactly that. Very brief version: increasing temperatures increase melted water from the Arctic ice cap and glaciers. Increased fresh water in the Norwegian Sea stratifies that area, preventing Norwegian Sea water from sinking to the ocean bottom, backing up and slowing down (perhaps even stopping?) the Gulf Stream. Changed ocean circulation prevents export of warm tropical waters in the Atlantic to the poles, sending the North Atlantic region into a colder climate.

I’m not following you. What’s taking CO[sub]2[/sub] out of the atmosphere so fast that we’ll wind up with a depletion problem?

Will it be enough? It wasn’t enough to maintain the last ice age. In fact, geologically speaking, ice ages seem to come and go rather frequently.

3

Unfortunately you get me. IANA Climatologist or biologist, but here’s what occurs to me. Yes, many trees do have to regrow leaves every year; but they also dispose of their leaves every year which returns carbon to the environment. I heard once that making new forests might actually put more C02 into the atmosphere because of the decomposing matter. (But from my non-scientific POV, I’d like to see more forests.) Another problem is that great areas of forest, particularly in South America and Africa, are being cut down and burned to make way for methane-producing bovines. Perhaps your model would work if we’d stop cutting down the rain forests/air cleaners. (Oh, IIRC we get most of our O[sub]2[/sub] from plankton.)

Another thing I heard is that global warming might trigger an ice age. Going on memory here… The ocean currents are like gigantic heat pumps that move warm water to the poles and cold water to the equator. If the ice caps melt, the theory goes, the pump will stop and (somehow – remember I’m not a scientist) trigger global cooling.

4

See what happens when I post at work? A cow-orker comes by and by the time I can finish the post and hit the enter key, someone else posts the same thing.

5

Oops! Sorry, I missed this question at the end. :slight_smile:

IIRC, the periods between the recent ice ages (“interglacials”) are relatively warm… maybe not as warm as the Carboniferous, but certainly warmer than the ice ages, I think the present is not unusually warm for an interglacial period, but I don’t have any references handy. Can anyone help me out here?

6

Your problem, Johnny L.A. is that you let work interfere with posting. Where are your priorities? :wink:

7

Gah! Another slow typist checking in… IAAClimatologist, but unfortunately am a little short on time today, so I’ll cover a few basic points of your post, scotth

There have been a bunch of experimental studies done to examine the effects of elevated CO2 on plants. There is no across-the-board benefit to raising CO2, as wevets pointed out – it depends upon the type of plant you’re considering, and just how elevated the CO2 is (some plants actually do less well in high CO2 situations, with stalks that become very spindly and weak). Moreover, terrestrial biomass in the form of plants is not a very significant reservoir of CO2 as carbon compared to the ocean or to the earth’s crust – the ocean currently holds about 38,000 billion metric tons (BMT) of carbon, and the crust about 60,000,000 BMT of carbon, compared to about 2,500 BMT in terrestrial biomass (including plants and animals). To really bury carbon so that it stays out of the atmosphere, you need to find ways to store it either underground or in deep sea settings. I know a few people who are working on the engineering of such possibilities, but it’s too soon to know which (if any) of these will not only work, but also be economically feasible.

Scotth mentioned that an abrupt shift away from using fossil fuels might lead to atmospheric cooling and a new ice age, and others commented that the present warming trend might lead to a new ice age instead. Both views have their proponents, but no one need worry about whether it’ll happen in 2005, say. Although there have been some indications that fairly dramatic climatic changes can take place in a matter of decades, there is an inertia to the climate system that would keep prevent something as dramatic as the next ice advance from happening to us in “just a couple of years.” To get large ice sheets to form, climatic conditions have to be warm enough to encourage lots of precipitation, but cool enough so that the precip falls as snow in the winter and doesn’t melt during the summer (you can have 50 feet of snow in the winter, but if it all melts over the summer you won’t build an ice sheet). Even if you have snow persisting year-round, it would take some time – centuries, at a minimum – for the snow to build up, recrystallize into ice, and then start flowing under its own weight. So we would have a fair bit of warning before such an event were to occur.

wevets is correct in saying that the current interglacial is not unusually warm (yet); the last major interglacial (ca. 125,000 years ago) was considerably warmer than now. The Pliocene epoch, just prior to the Pleistocene and onset of the current ice age, is in fact being used as a model for warm climate studies and global warming projections by a colleague of mine who is working in conjunction with the US Geological Survey.

It may come as a surprise, but we actually have not yet figured out exactly why our world swings in and out of ice ages in the first place. The main problem there is the same problem we have with trying to predict exactly how increased CO2 will warm our planet: the climate system includes a number of elements that have non-linear responses to “tweaking” of conditions like the level of atmospheric CO2. Albedo (i.e., reflectivity) is only one negative feedback to warming, and at higher latitudes (>45 deg.) it just doesn’t have enough of an impact to keep an ice age going.

8

Interesting you should mention this. Just this morning on NPR, there was a story about a scientist studying the changing color of the Earth over the past few decades (data from satellite photos). He concluded that his study area (North America) was getting greener and that this is linked to increased CO2 levels (I don’t recall if he went as far to call it “global warming” or not). He thinks that this is not due to “greener” plants or more widespread vegetation, but rather, more leaves per plant and more dense vegetative growth.

Although this seems to be good, he did not say that global warming is good for a few reasons…(1) other species may not react as well; (2) it is not known whether this increased “greening” can be sustained at higher CO2 levels; and (3) warmer climates can result in an increased rate of natural disasters (e.g., hurricanes, etc.).