World News

Avoiding other plants to take root, in particular ones with deep roots as they would form weak points in the dense felt-like root system grass has. Also ease of inspection.

There's about a millennium of engineering experience in those dikes... and plenty of historical compromises. Like, we knew back in the middle ages that flat profiles secured by grass are the most stable and secure but they require massive amounts of material so it was necessary to use inferior dikes with vertical faces made of wood planks. Most recent notable innovation is sand cores and ditches behind the dike to manage seepage water (behind meaning on the land side, always confuses them tourists), and some minor alterations to geometry to improve the way waves hit it.

We probably knew that sheep were good for dikes before we built them as, at least in principle, dikes are nothing but a warft with a hole in the middle and we've built those since time immemorial.

And in case you're wondering yes we're raising them quite a bit higher in anticipation of sea levels rising. Completely uncontroversial decision, only question was whether to rise the dikes very high, or use the same budget to raise them not as high, but wider, so that they can easily be made even very higher in the future. We went with the latter option, which is kinda optimistic and pessimistic at the same time.

A single study will not tell you everything about everything. No, ruminants will not just magically appear in the landscape, we're living in a causal universe, after all.

Suppose, for the sake of argument, that all ruminants indeed all grazers (also deer, giraffes, whatever) are extinct. Plants will flourish, not being eaten by them, then individual plants, or parts of them (falling leaves etc) will die as part of their normal life/reproduction cycle -- and get eaten by fungi, bacteria, etc. Which will burp CO2 and probably other greenhouse gases.

The condition for nature to produce CO2 are simple: The presence of carbon in a form that can be oxidised, such as sugar and starches of which plants produce plenty, the presence of oxygen, and a critter, any critter, that can do it. Even if it's just a single species, it's going to eat the whole thing and release all the carbon back into the atmosphere. Consuming available energy to reproduce itself is literally what life is all about.

If there's energy around that can be used, nature will use it. Have a look at the most biodiverse and productive ecosystem in the world, the Amazon rain forest: It has very poor soil because as soon as something dies, its remains are recycled by something else. Destroying the Amazon rain forest releases CO2, again planting stuff there re-captures it, but reconstituted forest doesn't continue to sequester carbon indefinitely: Only until it has accumulated the amount of carbon that it needs to sustain itself, after that it's going to be carbon-neutral.

You may be asking "but then how did all that oil and coal end up in the soil": Highly specific circumstances: Plants were producing stuff that critters couldn't eat. But we're currently not in that situation and in fact critters seem to be ludicrously efficient at evolving to break up new compounds. PET was first synthesised 1941, in 2016 scientists found critters which can eat it -- producing CO2 in the process, of course. That's exactly what's going to happen to all that herbivore-free land you envision. If we want to sequester carbon, care has to be taken that nature won't dig it up again.