The NBER Summer Institute hosted a panel discussion of Brexit on Tuesday. Richard Baldwin, Thomas Sampson, Helene Rey, and Anil Kashyap spoke about the consequences of Brexit for the European project, trade policy, macroeconomic growth, and London as a financial hub. I won’t try to summarize the discussion. The NBER should post video of the panel soon, and you can also learn their views from Baldwin’s twitter feed, Kashyap’s 538 piece, and Sampson’s CEP chapters.

I want to highlight three parameters that are key to forecasting Brexit’s economic consequences. They are (1) the size of the non-tariff barriers eliminated by the EU as a customs union, (2) the elasticity of real income with respect to trade, and (3) the strength of agglomeration economies in finance.

Non-tariff barriers are important because rich countries’ import tariffs are quite low. The two potential UK trading regimes people are most frequently discussing are a “Norway option” and a “WTO option“. Under the Norway option, the UK would have tariff-free access to EU markets via the EEA, but face non-tariff barriers due to leaving the customs union (e.g. rules of origin requirements and anti-dumping duties). Under the WTO option, the UK would face the EU’s MFN tariff schedule (only a few percentage points, on average) and a much wider array of non-tariff barriers due to the EU being far ahead of the WTO in reducing and/or harmonizing behind-the-border barriers and regulations.

Non-tariff barriers presumably don’t look like the iceberg trade costs frequently employed in quantitative trade models. You can find some estimates of these meaures, but they don’t seem to receive academic attention proportionate to their relevance for policy concerns like Brexit.

In contrast, the second key parameter, the elasticity of real income with respect to trade, has received considerable academic attention. However, there is not yet consensus regarding its value. In their CEP chapter, Dhingra, Ottaviano, Sampson and van Reenen review different paths one might take.

Using the class of quantitative trade models that yield the ACR formula, they estimate Brexit losses on the order of 1.3% to 2.6%. Given that the United Kingdom’s total gains from trade (relative to autarky) range from 3% to 23% in Table 4.1 of Costinot and Rodriguez-Clare’s Handbook chapter, this methodology necessarily produces numbers of this magnitude.

An alternative approach is to use reduced-form estimates of how trade changes income, presumably on the premise that there are important channels (such as dynamic effects) that are omitted from the standard quantitative models. Ed Prescott, for example, holds this view. Using Jim Feyrer’s air-vs-sea paper, which estimates that the elasticity of income with respect to trade is about one half, the CEP team infers that Brexit would reduce UK income by 6.3% to 9.5%.

The effect of trade on income is obviously important, and I expect that trade economists will always be investigating this question. At the moment, plausible predictions of Brexit-induced trade losses range widely, from 1% to 10%.

The third parameter of interest is the strength of agglomeration economies in finance. Brexit is going to reduce the role of London as a financial center, as EU-specific activities migrate to the continent. The question is whether non-EU-specific activities will follow. How complementary are these different types of financial services and how large are the relevant scale economies? I think this is an open question in urban economics, in the sense that we don’t entirely understand why the US financial sector is so concentrated in New York City. Suddenly, this has become a crucial question for London in the context of Brexit. Anil Kashyap has stressed that the financial services industry generates 11% of UK tax revenue, so strong agglomeration economies that imply an unraveling of the City of London would mean a big budget problem for the UK government.

The volatility and uncertainty of the unfolding political process makes forecasting Brexit’s consequences virtually impossible, but these are three parameters that are important to thinking about the relevant economic mechanisms.