according to a not-so-serious website called Brain of Brian,
" The earth may spin faster on its axis due to deforestation. Just as a figure skater’s rate of spin increases when the arms are brought in close to the body, the cutting of tall trees may cause our planet to spin dangerously fast. "
so, let’s say, if 1/3 of all the world’s trees were cut down (assuming they weren’t replaced immediately by new ones), would the world spin 1 1/2 times as fast or any faster at all than it does now?
Think about the length and weight of one’s arms as compared to ones body and now think about the length and weight of even millions of trees as compared to the size and weight of the Earth.
Theoretically, yes, the Earth would rotate faster on its axis. However, the increase would not be measurable.
You could cut down ALL the trees and it wouldn’t make much difference. Nothing measurable, at any rate. The skater can spin significantly faster because she is bringing a comparitively large percentage of her mass much closer to her center of mass. The mass of the trees is neglibigle compared to the mass of the Earth, and the decrease in diameter is likewise insignificant.
Hot diggity dog! I just happen to have done this calculation! Here’s the relevant thread:
Figure skaters increase spin rpm by pulling in their arms. How ? [May 2000]
and here’s my post:
http://boards.straightdope.com/sdmb/showthread.php?s=&postid=473040#post473040
[Ralph Wiggum]
What’s a nanosecond?
[/Ralph]
But seriously, a nanosecond is what, .000000001 second?
On behalf of the many Dopers, I am grateful that Archernar does not have a life.
Wind and wintry weather can cause the earth to spin faster or slower.
The effect of cutting down trees would likely be less noticable.
If the earth were to be scaled down to be six feet tall, even the world’s tallest tree (~367 ft. tall) would come out to be about .000631 inches tall in that scale… So you can imagine how little effect it would have, even given the numbers of them.
Yes it would, but damming up rivers slowes the rotation
In response to Munch, yes, a nanosecond is 1x10[sup]-9[/sup] ( 0.000000001 sec). A light-nanosecond is very close to a foot.
If 10 million trees is 36 picoseconds, I’d bet the daily speed-up due to simple erosion of mountain peaks must be on the order of a few femtoseconds (10[sup]-15[/sup] s) at least. I wonder by how much one Krakatoa slows us down?
Yes, but what if a gale-force wintry wind blew over all those trees!!! :eek: :eek:
Why thank y… hey!! Now that was almost three years ago. Who says I don’t have a life now? 
This discussion brings up an interesting side issue, Leap Seconds.
In a nutshell, as a result of tidal friction, the Earth’s rotation has historically been getting steadily slower (by about 1.4 milliseconds per day per century), when you compare “planetary” time with atomic measurement. Leap seconds have recently come about once every few years although the last one was on New Years 1999.
It’s interesting to note also that the rate of leap seconds has slowed significantly of late, as can be seen here. Maybe cutting down all those trees does have a measurable effect! Although you’d have to factor in mining and high-rise construction/demolition as well…
Wouldn’t anything within certain levels of the atmosphere have absolutely no effect on the spin of the earth? I always imagined the atmosphere to be sort of a protective shell for most things of this ilk.
On a related note, how would all of the man made structures affect the rotational spin? Think of all of the TV and radio masts, skyscrapers, duplexes, etc (Even ‘short’ structures like houses are generally 15’ - 20’ tall)… wouldn’t we be creating a net decrease in rotational speed? There may not be as many structures built as trees removed, but AFAIK, the farther something is moved out from the axis, the more profound effect it will have on rotational speed, and many of our structures dwarf any tree.
Well, let’s just say that in theory, moving mass closer to the earth’s center will increase the rate of spin, and cutting down a tree so most of it sits closer to the ground is indeed moving its net mass closer to the ground; you could even calculate this effect, but you’d need a decimal point followed by a whole lot of zeros to write down how much the planet’s spin was increased.
And it would probably turn out to be less than the effect of a good rainstorm (which is also moving mass [water] closer to the center; less mass than a forest, but much more distance).
In other words, in theory, there’s an effect, but in practice, an effect so small that its ridiculous.
Now, now–this board has a longstanding culture of discussing effects that are so small that they’re ridiculous! 
Can cutting down several trees make the earth spin faster?
Yes, just as removing a hair from your arm will allow you to run faster!
I’ve merged the two threads to consolidate the comments (and deleted the posts referring to the now gone thread). If the conversation seems a bit disjointed (more so than is the usual case around here) that’s likely the reason.