Right. This *may* kick-off. But let's try. QUESTION: In how many years will #HS2 have removed more CO2 that it emits? Can we agree on some shared facts? Buckle-in. @AdamWJT @Greens4HS2 @TheGreenParty @GarethDennis @XRebellionUK @Hs2RebelRebel @HS2ltd

Firstly, some points for those new to this. Building a big railway project emits a lot of CO2. But, the idea is that lower-carbon travel like a high-speed train will eventually compensate for that by reducing car and plane travel.

So, how long will that take? Well, initial Government assessments suggested at least 120 years. That is the figure that @stophs2 protesters have been widely quoting in the last few weeks. In fact, many of them think it'll be even longer.

In 2020, Boris weighs in and commissions the Oakervee review to look at in again - and it finds that actually it could be closer to 60 years.

Why the huge variation? Well, pro-HS2 campaigners say the Govt didn't give HS2 a fair-enough evaluation of its carbon benefits. It comes down to complex models done by computers that compare 'sliding doors' moments - the difference between two projected realities.

And that's where this gets messy. Different projections of how we will live our lives, how many Zoom calls we will make, or electric cars we will buy, or flights we will take, will affect how long it takes HS2 to be carbon neutral.

Recently, some in @Greens4HS2 have done analysis suggesting carbon neutrality is more like 28 years. Some even say 5 years. Of course, @stophs2's estimates are going the opposite way. Longer than 120 years.

There are, of course, a whole host of other environmental debates around HS2 (biodiversity impact, woodlands etc).

But if you're an average punter, how do you make sense of the hugely varied estimates on HS2 emissions?

Groups like @stophs2 say that given the scale of the #climatecrisis, it was foolish to launch such a carbon intensive project in the first place, no matter how many years it takes. They point to the fact we only have 10, maybe 20 years, at best to sort the climate crisis out.

Groups like @Greens4HS2 argue that you wouldn't refuse to construct a wind farm, because it initially emits carbon to build it. They say we can't get 'net zero' without HS2, because otherwise we'll just be using too many cars, high-carbon slow trains etc.

A lot of this comes down to the scale of your perspective. Interestingly, @AdamWJT told me that we have to think beyond deadlines for 'net zero'. He says we are going to need a sustainable rail network for generations to come - and we can't think about this in a 10yr scale

The worst part of all of this, is that it is (often) environmentalists arguing with environmentalists. What is undeniably true, is that there are far worse emitters / projects that are getting barely a jot of the attention.

Perhaps the only thing all sides can agree on, is that we need this level of scrutiny on many more projects, if we're going to solve the climate crisis.