Craig Lindsay, of Totara Park, asks :-

What would be the atmospheric pressure on us if we were on the bottom of the Mariana Trench and there was no sea, just that deep canyon.

Lisa Murray, a meteorologist with Met Service, responded.

The Mariana Trench has a maximum recorded depth (the Challenger Deep) of 10.9km. As any diver knows, 10m of sea water is equivalent to atmospheric pressure so with water present the pressure at the bottom is about 1000 times atmospheric pressure.

On average, a column of air one square metre in cross-section, measured from sea level to the top of the Earth's atmosphere, has a mass of about 10,300 kilograms and a weight of 101,000 newtons (weight = mass times the acceleration due to gravity = m x 9.8 metres per second per second). Pascal was a French mathematician in early hydrostatics. Our modern unit of pressure is 1 Pascal (Pa) = 1 Newton per metre squared. So atmospheric pressure is about 1010hPa, (check your weather map), where h is our symbol for hecto which is a decimal unit prefix in the metric system denoting a factor of one hundred.

Interestingly, if we assume an average person has a horizontal cross-section of 0.1m, this means you are weighed down by 1000kg, or a metric tonne, of air! The reason it doesn’t feel like you’re carrying a small car around all day is because the pressure exerted by the air acts in all directions, not just down, so the upwards component balance out the downwards component. So pressure is the same for a surface regardless of which way it is orientated. You can easily check this by noticing that the pressure recorded by a barometer does not change if you turn it on its side!

Now, if by some terrible turn of events all the ocean was removed, what would be the pressure exerted on anyone who were to stand at the bottom of this newly revealed chasm? Some of that atmosphere would ‘fall into’ the trench. The pressure at the normal sea level would be reduced to three-quarters of that when there was an ocean, because of the reduction of air above. The pressure at the bottom of the trench would be this atmospheric pressure plus the extra 11km of air. Interestingly, gravity actually decreases linearly as you head inside the planet, but even at the bottom of the Mariana Trench you are only 0.17% of the way to the centre of the earth, so the drop in gravity won’t affect our result significantly. The pressure at the bottom of the trench would be more like 2600hPa, or only 2.5 times the current pressure.

However, if the oceans didn’t just disappear but evaporated, now we have a lot more atmosphere to play with! The density of sea water is 1030kg/m3, but the density of water vapour at 107C is 0.58kg/m3. So your oceans are now taking up almost 2000 times more space! So, 1,347,000,000 cubic kms of ocean would extend our atmosphere an extra 4500km or so out into space. That would take it far beyond the protection of our ionosphere, and most of this would disappear into space, blown away by the solar wind.

As for what could cause this. Runaway global warming is one possibility, but even if we burnt all of the available carbon buried in the earth global tempertures of 100C are a stretch! But humans can’t live in conditions where the dew point temperature is above body temperature (37C), so we would never last that long! A massive meteorite strike would boil some of the ocean, but not all of it. The sun running out of its hydrogen, and moving into a helium fusing phase could also lead to these temperatures but aren’t expected for another 5 billion years!

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