First we should turn the Sahara into a lush green forest as an exercise. Terraforming a remote planet will be a million times more difficult.

Oh, wait, we can't even work together as humanity to contain a virus.

(disclaimer: I work in a climate group)

Please do not do this :).

Deserts are great for shedding heat into space (highly reflective in the visible, highly emissive in the infrared, low moisture in the atmosphere).

We had a theory (not yet published, so take it with a grain of salt) that desertification might buffer the radiative forcing effects of CO2. Unfortunately, when we analyzed the last 20 years of satellite data, the opposite is happening--people have been watering the desert like crazy, leading to a net reduction on the order of ~100-200,000km^2.

Better to experiment off earth than on it in such planet changing endeavors. You can afford less attention to (safety)detail in that scenario.

Or maybe not. Turning the Sahara into a forest would have a massive climate impact.

I am not a climate scientist but I’d imagine there would be significant changes to rainfall patterns over most of Europe, Africa, and the Middle East

Edit - sorry, reply was meant to be to parent

There is a nice article about some paper modeling the climate if all the Sahara was covered with solar panels and wind turbines.The temperature on the Sahara would actually increase on average.rain would also

https://arstechnica.com/science/2018/09/carpeting-sahara-wit...

A fair amount of the phosphorus in the Amazon comes from dust storms originating in the Sahara.

[1] https://www.nasa.gov/content/goddard/nasa-satellite-reveals-...

given that the Sahara is only a couple thousand years old does that mean the amazon is just as young?

An interesting article on how people shaped the Amazon.

https://www.theatlantic.com/science/archive/2017/03/its-now-...

The Amazon is much younger; much of what is jungle today wasn't before the Columbian Exchange.

There is a theory that American reforestation was responsible for the Little Ice Age.

Very good question.

Potentially habitable planets close enough to colonize are rare enough that we can’t afford mistakes either way.

There's a theory that the Sahara alternates between desert and green every X thousand years, dramatically altering the Mediterranean, European, and African climates.

And BTW our species is conducting its largest peaceful "mobilization" ever to deal with this virus; it's actually quite inspiring.

> a theory that the Sahara alternates between desert and green every X thousand years, dramatically altering the Mediterranean, European, and African climates.

This is a funny way to phrase the causation. The state of the Sahara is surely not exogenous -- the transition must be caused by the climate. (It can then contribute further, but why attribute the changes to an intermediary effect?)

fewer dissenting voices to deal with on another planet, for the time being

If you have the industrial capacity to handle metallic lead precipitation and strip away 92 times as much atmosphere as the total on planet Earth, dissenting voices are of less relevance than the contents of your spam folder.

Those are not exclusive options.

Mars can retain water vapor just fine. It occasionally snows lightly.

Without a magnetic field (which could be built easier than terraforming), over 100 million years, it can slowly lose atmosphere, but nothing like a relevant rate for humans. The sun is also getting significantly hotter on a similar timescale, which makes habitation on Venus (And Earth, for that matter) significantly harder and dependent on active geoengineering methods.

> cool it down

If you blocked all incoming sunlight, how long would it take for the surface to cool to human-tolerable temperatures?

Or more extreme, how long would it take to cool it down to the freezing point of CO2, then build some sort of maglev launch system and lob solid CO2 to... Mars maybe, that planet could use more atmosphere.

And how much stuff would you need to launch to spin up Venus? Months long nights are not fun. Can you launch stuff to yield more spin, then have it sling around Earth or Mercury, and then have it hit Venus again, resulting in more spin up, i.e. getting twice the rotational acceleration from a launch?

About a hundred years. There’s a really good study on this in the journal of the British interplanetary society, but I’m on mobile right now.

Venus isn't a better option. Mars is ready for human shelters on day one, venus would crush and melt anything we're sending there for hundreds of years. It'd be far easier to warm Mars than cool Venus, as we're discovering on earth, warming is much easier to cause.

For mars, You could crash nitrogenous asteroids into the surface, make GHG production facilities that emit CFCs or HFCs, and hopefully soil carbon gets released in a runaway effect. Thermonuclear weapons over the pole is impractical but a fun idea.

For venus, it's so tough, you'd have to block solar radiation and then fix the massive amounts of carbon with co2 capture... maybe if we develop co2 capture for dirt cheap on earth, we could make sky cities with carbon fixers on venus, after a few hundred years it'd be colder...

Fun to think of, but ultimately we need to terraform earth towards cooling, as our current warming is too successful

> venus would crush and melt anything we're sending there for hundreds of years

The way to settle Venus is in balloons floating in the atmosphere, at altitudes with decent temperature and pressure.

This got me thinking in the other direction..

If I were a very determined, ultimate Bond villain* who set out to destroy the whole universe, what are my options? How bad can I fuck it up for all ? Back-to-back mega nuclear explosions occurring simultaneously on all planets within reach? Could that start chain reactions around our galaxy that'd bring it all down? What else?

* Funnily enough, I just started reading 'From Russia with Love' whose first few chapters deal with the unusually vicious nature of our henchman.

We'd need to replace a lot of hydrogen Venus lost. Also, by being closer to the Sun, I'm not sure we can stop that loss without a parasol.

The good news is that we can use the parasol to funnel hydrogen from the solar wind back to Venus.

For anyone who is wondering:

https://en.wikipedia.org/wiki/Space_sunshade

Mr. Burns, eat your heart out.

For settlement with current technology, Mars is much easier. Maybe in the future once we have better geoengineering we can consider Venus.

I mean, how do you build a habitat capable of keeping humans alive when it’s 400 degrees outside?

Breathable air is a lifting gas in Venus' atmosphere. You build your cities in the sky.

https://en.wikipedia.org/wiki/Colonization_of_Venus

I think you'll also have to deal with the minor inconvenience of acid rains in Venus as well.

At current temperatures it evaporates before it gets to the ground, so who knows when it gets cooler

The acid reaction with the surface may end up freeing lots of hydrogen and making saltwater.

Also don't forget that in 10 million years, one of Mars' moons will crash into the planet, or at least parts of it.

Don't want humans to be around on Mars when that happens.

10 million is too long a time frame on human scale, given that humanity looks so drastically different even compared to 50k years back, and agriculture is also some 14k years old. If humans manage to survive on mars for 10 million years, they would be advanced enough to be out of it.

Don't fear Phobos.

It's small enough that we'll be able to move it (if we haven't mined it apart by then).