What past Brexit information have they got to say it will be armageddon?
They have the past info on climate change and are projecting a trend. They have the past info on being in the EU and are projecting a trend. We can disagree with their figures but at least they have something to go on. they don;t have anything to project a trend on Brexit. They are "guessing."
You are repeating the "when tariffs are imposed" when you nor they know what tariffs will (if any) be imposed, nor what the government will do to alter things. Do you, I or they know what tariffs and schedules the UK submitted to the WTO in July? Nope.
Yes they know what the EUs tariffs for things outside the EU are so they can take them BUT they do not know what any counter measures the government will take to reduce or nullify those tariff costs.
They are speculating on WTO "maximums" and EU actuals. They have no idea what counter measures will be deployed. Pure speculation presented as "facts."
Nope. Climate scientists have no back information on what happens when the temperature rises by 0.5C for the next ten years from what it currently is in Baffin Island. But they do know what happens when ice melts elsewhere and how water flows are affected elsewhere and on and on. Repeat 10,000 times for 10,000 different areas of the world. Amalgamate and you have an estimate for the future. So they estimate what will happen in the future and you buy low-carbon electricity.
If we go for Brexageddon Max (better than "Crash Brexit"?) then we'll go onto WTO terms. Under those, the EU will, under WTO rules, either impose their current tariff on vehicle imports from the UK or it will have to remove tariffs for all imports into the EU of vehicles from every country in the world with whom they have no FTA. It's a pretty slam dunk bet (given how protectionist you complain the EU is) that they'll go for the former. So you can predict, with a certain level of certainty that there'll be x% tariffs on cars. Economists know what happens when tariffs of x% are put on cars from places where it's happened. Do you disagree with that assessment of vehicle trade? Fine. Revise the model to what you think will happen.
Repeat that 10,000 times on 10,000 other market sectors or products and you have a reasonable view of what will happen to the economy as a whole. Maybe individual items will be wrong but the overall number will be a reasonable estimate of what's likely to happen. (That also explains why the estimates have error ranges - to take into account the fact that they are estimates).
And your response this time is "It's pure speculation presented as facts". Good.
Vin
PS I know this is boring but unfortunately, the Brexit bunch like to sweep details under the carpet rather than dealing with them - and the devil's in the tedious details of how WTO actually works rather than in the fantasies of the ERG. Sorry. I'll stop now.