globalchange Google Group

Re: Hansen on runaway warming

2010-01-05T13:06:27Z

Can you cite something, anything, that claims that Earth protected
Venus from icy asteroids?
You can find discussions and explanations of the number of craters on
Venus, but I can't find any that mention the protective effect of the
Earth. Seems an odd omission if there is a protective effect.

Re: Hansen on runaway warming

2010-01-04T18:06:18Z

Well, I can't prove that it happened. Nor AFAIK can anyone.
But I saw an estimate of 5000 times the amount of water on Earth,
formed in the asteroid belt. So we ca let a few of the passing MLB
comets crash into Mars, the Sun, Jupiter, and off to space and still
have enough left over to fill the Earth. Mars is smaller than Earth,

Re: Hansen on runaway warming

2010-01-04T17:49:42Z

Well, he would say that wouldn't he? :-)
[link]
I presume you are referring to this:
============================== =========
Absence of Water Vapor
The clouds contain little water vapor, and there is little evidence
for water in any form on Venus. It is speculated that the absence of

Re: [Global Change: 3411] Re: Hansen on runaway warming

2010-01-04T16:23:43Z

The Earth has a much bigger "gravity well" than surface area. If a comet
or other body comes close, then the Earth's gravity will much more
likely deflect it into a different orbit than there be an impact. This
different orbit might be an Earth impacting one, or also might be a
Venus impacting one, but is more likely going to have multiple close

Re: Hansen on runaway warming

2010-01-04T15:03:58Z

I guess that might make it plausible.
But the references you cite say nothing about Venus being protected.
Hansen cites hydrogen isotope evidence from the current atmosphere of
Venus that it once had lots of water:
[link]

Re: Hansen on runaway warming

2010-01-04T12:36:19Z

Yes, as the major axis increases due to the gravitational pull of
Jupiter, the minor axis decreases until it reaches 1AU, then crash!
The major axis rather than the minor will increase because that is
when the asteroid is on the corresponding part of the ellipse that it
is nearest Jupiter, and so Jupiter has a greater gravitational

Re: Hansen on runaway warming

2010-01-04T11:00:23Z

...
In the chapter 10 'Venus syndrome' of his book 'Storms of my
Grandchildren',
Hansen says similar thing.
But, as far as I understand, he does not properly formulate here
what is the condition for runaway greenhouse effect, or 'Venus
syndrome'.
What he shows with good scientific support is that the climate system

Re: Hansen on runaway warming

2010-01-04T09:17:38Z

I think that we need to be careful when drawing conclusion from past
climate. At least for the last million years we've got very reliable
measurements for CO2 concentrations, but before that we are dealing
with proxy evidence. In addition, the overall forcing presumably
matters, not just the CO2 related forcing, and with regards to

Re: Hansen on runaway warming

2010-01-04T09:01:24Z

I was looking at the forcing multiplier, as if it could happen without
the concurrent temperature increase. That's very unreasonable for a
smooth temperature forcing function, and not so unreasonable with a
tipping point, where suddenly cloud cover or relative moisture
changes.
Without tipping point like changes, I suppose a lot less than 4 W/m2

Re: [Global Change: 3401] Re: Hansen on runaway warming

2010-01-04T04:11:22Z

I don't know much about planetary evolution, but I've read enough to
know that there are some pretty counterintuitive (to me) things going
on, mostly due to the amazingly long time scales involved.
In this case it doesn't seem too implausible that if the comets are
slowly nudged into increasingly elliptical orbits, they will almost

Re: Hansen on runaway warming

2010-01-03T23:47:16Z

Earth would not act as a backstop to regular comets coming from the
the Oort and Kuiper belts, but if the Main (asteroid) Belt Comets,
with circular orbit gradually became more elliptical they would crash
with Earth first, and not become elliptical enough to crash with
Venus. These are asteroids that formed beyond the snowline. Of course

Re: Hansen on runaway warming

2010-01-03T23:56:38Z

What is described by Jewitt is main-belt comets, but these are not the
main comets, and Jewitt estimates there may be only 150. They are in
the main asteroid belt. What is being proposed is that the water on
the Earth came from icy asteroids which formed further than the main
asteroid belt from the Sun and closer to Jupiter. They arn't there now

Re: [Global Change: 3396] Re: Hansen on runaway warming

2010-01-04T01:07:31Z

Thanks for the ref. I wondered if his name would come up, as he has said
similar things on RC.
GCMs can achieve runaway warming - in fact some informal conversations
suggest to me that this is rather more common than you might imagine
based on reading the literature - but this is generally attributed to

Re: [Global Change: 3392] Re: Hansen on runaway warming

2010-01-04T00:58:39Z

Perhaps my previous message was not as clear as it could have been. On
an initial doubling of CO2, the imbalance is 4W/m^2 before there are any
other changes. As the atmosphere warms, the water vapour increases but
the radiative imbalance *decreases* anyway, because the warmer
atmosphere is closer to radiative equilibrium (ie outgoing LW radiation

Re: Hansen on runaway warming

2010-01-04T00:46:50Z

Go back 500 million years, and solar luminosity is only about 7% lower
than now but CO2 was 20 times higher. I have trouble believing that
the small increases in solar luminosity could have left us so
precariously balanced on the edge of a knife that we'd just now be on
the brink of a Venus-like runaway. I'd also add that many points in