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Estimating the Income Shock From a Commodity Price Move

A country exports 100 billion dollars of goods a year. Iron ore makes up 40 percent of exports. The ore price rises 20 percent and stays there, with volumes unchanged. How much does export income change, and what might it mean for the currency?

Compute the ore exports: 40 percent of 100 billion is 40 billion. A 20 percent price rise adds 8 billion dollars a year, with other exports unchanged. Total exports rise to 108 billion, a gain of 8 percent.

Put it in proportion. If the economy's GDP is 1,500 billion dollars, the gain is about 0.53 percent of GDP. That is a meaningful boost to national income, flowing to miners, the government through taxes and royalties, and workers.

What happens next? Higher export earnings increase the supply of foreign currency that exporters must convert into local currency, supporting the exchange rate. Stronger income may encourage investment and raise growth, and with it the odds of tighter policy.

Limits matter. Many mining companies are foreign-owned, so profits flow abroad. Some revenue is hedged forward at older prices. And global risk sentiment, interest rates, and the dollar may overwhelm the effect.

If instead the ore price had fallen 20 percent, the reverse would apply: a loss of 8 billion, a 0.53 percent drag on GDP.

The exercise gives scale to the idea of terms of trade.

Try this: pick a commodity currency and estimate the effect of a 10 percent move in its main export.

Good structure. Two refinements we use when looking at commodity currencies like AUD. First, the price change usually moves the terms of trade before export volumes respond, so the currency often reacts quickly to the price, then more slowly to the trade balance data.

Second, part of the gain goes to foreign shareholders of the mining companies, so the net income effect for the country is smaller than the gross export gain. That is one reason the currency reaction is often smaller than a simple export calculation suggests.

The scale calculation is the most useful part. An 8 billion dollar gain in export income sounds large in isolation, but at about 0.53 percent of GDP, it's a moderate boost rather than a transformation. That helps set expectations for the currency response. A shift of this size might support the currency over months, but it's unlikely to dominate if interest rates, global risk sentiment, or other factors move strongly. Larger commodity shocks, or those affecting a bigger share of exports, have more obvious effects, as seen in oil exporting countries when crude prices move sharply.

I tried to do this kind of estimate on AUDUSD a few years ago and the simple arithmetic did not match what the chart did, which was a useful lesson. Iron ore moved about 18 percent over two months, and by my rough calculation it should have supported the currency by a visible amount. In practice AUDUSD moved less than 2 percent in the same stretch, because the dollar and risk sentiment were pulling the other way.

What helped me was treating the income shock as a slow background bias rather than a trading signal. I keep a note of the commodity trend over the past month, and if it agrees with my technical direction I allow myself slightly larger size, maybe a quarter more. If it disagrees, I do not fight the chart, but I take profit sooner.

The foreign ownership point matters a lot, and so does hedging. A lot of the sales were locked in at older prices, so the cash flow arrived with a delay of several months.

The limits section is important for realistic analysis. If most mining companies are foreign owned, a large share of the extra profits flows abroad as dividends, which reduces the net inflow and the support for the currency. Forward hedging by miners delays the effect, since some revenue is locked in at older prices. Government revenue from royalties and taxes may be saved in a sovereign fund rather than spent, which also affects the domestic impact. Each of these factors means the currency response is usually smaller and slower than the headline export gain suggests.

The link to policy expectations is often how the effect reaches the currency fastest. If a commodity boom raises expected growth and inflation, markets may price in higher interest rates, which supports the currency through the rate channel even before the trade flows build up. The Australian dollar has often shown this pattern with iron ore and other commodities. But the relationship can break down if the central bank signals it will look through the commodity effect, or if global factors dominate. Watching both the commodity price and the rate expectations gives a clearer picture than either alone.

The terms of trade concept, the ratio of export prices to import prices, captures this idea in one measure. When export prices rise relative to import prices, a country's terms of trade improve, and its currency often strengthens over time. Several central banks and statistics agencies publish terms of trade indices, which can be compared with the currency over long periods. Has anyone here used commodity prices or terms of trade in their analysis of currencies like the Australian or Canadian dollar? I'd be interested in how reliable the relationship has been in recent years.