a) There is a general trade, and sometimes prominent as in case of UK, Canada, and Europe. When the tariff rates are showing an upward trend, the trade/GDP ratio is either declining or getting stagnant. Similarly, when it's showing a downward trend, the trade/GDP ratio is showing increasing.
b) The era of the GREAT DEPRESSION was a steep rise in the tariff rates throughout the world. This had actually worsened the situation. All countries were trying to protect their own indigenous industries. But, this situation had backfired.
Whenever, there is uncertainty in the market, or there is some kind of recession, countries throughout the world tend to increase the tariffs to protect their home based industries from foreign competition. However, this situation leads to a deepening of recession as demand gets constrained and hence there is a general phenomenon of overproduction. However, the correct approach should be the one which stimulates demand. This may be in the form of lowering the tariffs which will in turn, lead to availability of goods at lower prices in the market and hence will keep the produced stock of goods getting consumed. Coupled with increased government spending, this will help the world come out of recession.