Ok, safety tip, no ateroids landing in the Yellowstone Caldera. I would say land would be best for impact, however the surface of this planet is 2/3 water, odds are against a terrestrial impact.
Can we ignore the consequences of a rising sea level as a result of the melted land ice ?
According to the Earth Impact Effects page, Earth gets hit by a meteor this size every 1.5 million years, so the longer term non-local effects are pretty negligible.
Could we paint a target on the Canadian Shield?
So, is a cooling followed by a warming better or worse than something that starts as a cooling and stays a cooling for awhile (dust)
Also, if it condenses to clouds, doesnt that imply rain in the near future, which causes the water to get “out” so to speak ?
Rain drops and ice crystals may grow better and faster and fall out of the atmosphere faster than supper fine high altitude dust.
And, if there is more than the normal amount of water vapor, doesnt the green house efffect from the vapor where there arent clouds offset the cooling (to some extent) from the places where the extra vapor has condensed to form clouds?
I am still of the mind that water vapor is going to return to a normal equilibrium condition significantly faster than dust. But thats just my semi-technical opinion.
As a Canadian who’s fond of the Canadian Shield, I’d suggest the Siberian taiga instead — but I suppose we already did that once.
As long as it doesn’t hit the Old North Church, Paul Revere will still be able to see the signal. That’s the important thing.
Well, we can’t ignore them, but I think that would be a less dire result than any of the others. And if it hit dead-center on the continent, a lot of that melted land ice would stay where it was in a lake and re-freeze.
What about somewhere on the Great Lakes? That’d at least reduce the tsunami problem other water landings would have. There are many fewer people on the shores of any of the lakes than on, say, the Atlantic, plus at least some of the Great Lakes cities are at a decent elevation above the water level (I don’t think there’s even enough water in Lake Erie to tsunam Cleveland, for instance).
As a resident of Milwaukee, I am a strong opponent of this suggestion. Given that this is a message board for a column that started in a Chicago newspaper, there’s probably going to be a disproportionately strong aversion to it amidst the general SDMB population, too. 
True, but the flip side of Lake Erie not containing much water is that the asteroid has a kinetic energy comparable to that required to vaporize the lake outright. Using the impact effects calculator linked to above, we find that a 2-kilometer-diameter stony meteor impacting Earth at 17 km/s has enough kinetic energy to turn the ~500 cubic kilometers of water in Lake Erie into steam.
Of course, an asteroid impacting the center of Lake Erie wouldn’t be a very efficient way of heating the lake — it’d be far too localized. Rather, the meteor would punch through to the lake bed and create a crater there. Depending on where in the lake the meteor struck, some portion of the water would drain back into the crater (cooling off the crater, but probably being vaporized in the process), and some would drain out into the Niagara River. Niagara Falls would probably dry up for some period of time while the Detroit River refilled the newly enlarged lake basin. Clevelanders probably wouldn’t have to worry about a tsunami in this scenario, it’s true, though if the asteroid landed close enough to the city, they might wish for one to help put out the fires caused by the thermal radiation.
Is the main function of how much water will slow down an impact typical speeds and temperatures more a function of density than anything else? Because at only around 500 feet deep and 1/5 as dense as rock, the Great Lakes would probably not provide hardly any stopping power before the bolide hits rock anyway. Could we get the worst of both worlds, a tsunami and a rock impact?
In terms of minimizing the effect of a tsunami, how about the North Sea? It’s isolated enough that the blast might be far enough to not do much primary damage yet those areas that are close enough to experience the tsunami have rugged coastlines that rise quickly.
ETA and it’s also shallow enough to not produce a devastating continent-wide tsunami.
Hmmm, looking at it some more it looks like I assumed the southern Norwegian Sea was part of the North Sea (the central north sea is too close to the west Hanseatic coast/low countries which do not have a steep slope). Does the space where the two seas meet have a space that is far enough from land yet shallow enough to not make a huge tsunami?
I thought this was always the issue with a meteor this big.
Even if it hits water it is still hitting land.
The rock in the OP is 1.6 kilometers wide. If we assume an ocean depth of 10,000 feet (it averages 12,000 feet but close enough and a round number) then the rock itself is about half the depth of the ocean.
Travelling at 17 km/s (again about average for a meteor) it will hit the bottom of the ocean in 0.18 seconds…call it 0.2 seconds because it slows down some on the way in.
In short, it is going to pile drive right into the earth under the sea and toss all that crud up along with the water. Doubtless it would be less earth than would be tossed in a land strike but still…lots and lots are going to go into the atmosphere. Add in all the water to boot.
Overall guessing a land impact is “better” but only in a relative sense.
How much warning do we have? Would we have time to build a mile-wide trampoline and bounce that sucker back into space?
Good point. And isn’t the crust thinner under the oceans? How big a rock would we need to punch into the mantle?
Guys, I am trying really, **really **hard not to make a Mariana Trench joke right now.
Maryanna Trench? I dated her.
Once, in the 1960s, for twenty minutes.
How big would the tsunamis from this kind of impact be?
I wonder how loud it would be.
No; we are talking about clouds that will stay there for many years, perhaps decades ( the estimate I’ve heard for the Dinosaur Killer ). The upper atmosphere and the lower don’t mix much, which is why getting large amounts of water vapor or dust up there does damage for so long.
We are talking about a hemispherical or even worldwide overcast here.
[Dirty Harry]
Loud enough to blow your head clean off.
[/Dirty Harry]
Or more seriously, we are talking about the kinds of forces that kill you instantly; too loud to hear because you’d be too dead.