The Sahara Desert is roughly the size of the continental USA, mostly uninhabited.
With mega-scale fusion-powered infrastructure it would be possible to pump and desalinate sufficient cubic km of water into the Atlas mountains and generally to irrigate maximally any part of the Sahara.
Emphatically this is a dangerous idea, for many reasons, mostly having to do with the complexity of environmental systems and balances. However, I think the dangers can be mitigated, and recommend a decades long computational modeling study as part of the process of understanding the global system in all its richness.
The water shortfall in the World Ocean would be made up for by simultaneously colonising and terraforming Antarctica, also a fusion-powered mega-scale infrastructure project. The massive amount of fresh water ice on Antactica could be transferred to deserts of the world (including Gobi, Nevada, etc) via the World Ocean, adjusting for balance.
This process could be used to substantially increase the habitable land on the Earth, in the centuries before other world colonisation becomes (if ever) a viable process.
The architectural possibilities of the mega-scale infrastructure projects are immense: Sahara: Given the amount of sunlight received at the equatorial regions, entire 1000 meter high artificial rainforests could be produced from steel framed vegetation scaffolds, or maybe something like a genetically engineered Jack's Beanstalk (bamboo structure towering into the sky, hosting multiple stories of vegetation) Antarctica: Here there is much less sunlight, but tremendous amount of water, and with fusion power would be able to artificially keep things nice temperature. This could be a good region for artificial sea-life ecosystems, lakes, inland seas. Could also use electrically powered artificial light sources for agriculture there.
Space exploration should be mostly a process undertaken by drones for the forseeable future. It's more valuable to take instruments, sensors, comss gear, power generation, and propulsion units into deep space than slowly verturing out with the expensive support infrastructure for human bodies. Human bodies which are designed for the earth specifically, which are expensive to support, and provide diminishing returns in the operations of a spacecraft.
It's possible that by the time colonisation and distance space travel become an issue, the human species will have attained some measure of mastery over its physical manifestations, no longer limited strictly to the bodies we are familiar with. What technologies, whether biological or mechanical(including computational), it's not necessary to speculate on particulars.
A counterpoint: If propulsion system technology will permit it at some point, human space travel may be re-engaged in the following way: The craft will be capable of 1-3 g-force acceleration continuously for weeks at a time, at gross weights exceeding ~50kt. In this way, the zero-g atrophy effects are mitigated. Mastery of the solar system is attained, with routine flights within this region. Flights to Jupiter at 1g constant accel would be days or a week factoring in the decceleration. Flights off the orbital plane would be possible, and the craft could support a comfortable living and working space for a small or large community. I have no idea if such a propulsion system is a future possibility, just exploring the scenario.
Imperative to prioritise preservation of species and ecosystems. Amazon reforestation, Midwestern American reforestation should be prioritised. If low-density, high-technology human development is compatible on the margins or even within such ecosystems it should be considered. Human development patterns should be re-evaluated and broadly compared; scenarios that have already been tried as well as those which are heretofore purely speculative.
Clearly the Earth is finite, and no matter how one wishes to calculate carrying-capacity, the figure should come out to be finite. Humans have an infinite ability to reproduce, so, deep planning must ultimately address responsibility at a practical level only seen thusfar in the government and society of the PRC. Deep discussions of demographic management are ultimately as much the purview of deep strategic negotiations between great powers as are nuclear weapons.
USA: If the parasite (a variable name pending definition) can be removed from its total control over elite power structures, academia, corporate consensus, there is a wonderful libertarian dynamic underneath, which can show that individual sovereignty & liberty is not incompatible with great culture, sophisticated community structures.
I predict art and music will be vastly better 10 years from now as a result of the competition in culture being unleashed by the elimination of the spirit of hegemon. This will be felt around the world in fascinatingly different ways.
It was understood by a few scientists and thinkers already in 1945, that humanity would have to evolve beyond some of its characteristic rituals (war) in order to avoid extinction. This is more widely understood today, but old habits die hard, and genetically hard-wired habits are even tougher to break. The top political professionals in the West make great undulations (thinking in particular of McCain) intending to manifest a newer greater war, to bring the unipolarity/hegemonic moment to an even higher pitched crescendo. It's become apparent that our system doesn't promote good people to the pivotal roles; corruption can be defined as a system that does the opposite. We have that system. The fact that the corruption-in-China narrative was such a staple of the American elites' conversations in the period 2005-2015: Self-congratulatory, oblivious to ones own faults. In this case, that obliviousness proved fatal for the USA as a power project, and indeed poses a danger to humanity inasmuch as -- 'externalities' ! it's big and doesn't exist in isolation.
Some people understood the 1945-conundrum as necessitating the new totalitarian imperative, soft-authoritarianism, world-governance and a homogenizing over-coding. Corporations invested heavily in the intellectual & propaganda structures for this transformation, hoping to lead with proxies in a liquid world Baudrillardian, lots of TV personalities, supermalls, visa-free travel, no borders, hermetic elites in their own world. Very "1984", but with softer surfaces. Left wing: Brussels typifies this, as does Cannes (the film festival). Right-wing: Monsanto, BP, etc...
One key cool thing about the MultiPolar paradigm is that it avoids the dystopian authoritarian whilst also hopefully (with lots of work) avoiding the catastrophic clash of arms. Nation-state sovereignty is again in vogue, white-knighted by Russia. Not only does this prevent the unipolar/hegemonic culmination required for the total global control desired, it establishes a contrasting paradigm centered on difference, variety. ("Difference & Repetition" by Gilles Deleuze comes highly recommended, especially for its anti-Hegelian lines, outlining the theoretical under-pinnings at work here). If we defend the sovereignty of nation-states, and their capacity for truly differentiating themselves from one another culturally and in every way that matters, we will be in a much better position to defend the sovereignty of the individual person, limited only by affect on surroundings.
In practical and prosaic terms, the next few decades will see a resurgance of classic Balance-of-Power politics between a myriad array of powers, great and small. Eventually we will need to move past this phase, as it is unstable, standard pressures of competition and secrecy will tend ultimately to war. Thus it is urgent to establish global diplomatic negotiations at deep and comprehensive levels heretofore unseen. There is so much to discuss, there is so much work to do.
Ultimately global governance will be an affair emerging from these negotations, held between truly free and sovereign nations acting at a high cultural and intellectual level. This is Earth human survival, in general terms. Without the totalitarian Hegelian dystopia crap.
More esoterically, I argue (semi-seriously) that Earth would ultimately (could be millenia, just eventually) be overrun in a very Darwin-awards kind of way by extra-terrestrial forces eventually should totalitarian parasitic (* var left floating for now) forces achieve their goal of static global control, which would have the effect of strangling advancement, even though for some time they would carry the illusion of advancement, believing (wrongly) that certain technological advancements carry weight beyond very limited situations. The theory continues in a kind of evolutionary-sci-fi / new-agey way, the anti-dystopian multiplicative nature of the MultiPolar world is what enables the total species advancement required for planetary species survival in the very long term (millenia). The independently evolving civilisations of India, China, Russia, America, Europe, et cetera (in whatever ways you would wish to partition the universe of human culture) are much more powerful in combination than some kind of global corporate occult supermall as forseen by our PNAC friends. Anyways, that's about as esoteric and frivolous as I can get on the subject. I guess I could admit I have no idea if the kind of world I want to live in is really optimal for galactic survival of our little species project, and that just esthetically and morally I'm much happier in a place where great powerful individual people exist, and I don't see that happening in a place where sovereignty (including wealth) is concentrated into crony structures and hoarded by rotten entities.
I prefer to emphasise esthetics as encompassing morals when considering future possibilities. I place a great deal of value in nature, such that I'd like to see most of the suburban sprawl parkinglot junk of the American Midwest turned back into the uninterrupted forest it once was. I wouldn't want to claim this as a more moral view, it's more a matter of just preference. Individuals preferences vary widely, so issues that affect everyone require a great deal of negotiation in order to please the most possible people. Defines 'civics', civic life, demos. So much of this is not merely moral, but also practical & esthetic.