No prize for figuring out that it is the Australians who are being whacked around the ears by currency changes which are eating into profits and American coal exporters who are celebrating.

In time, and if the currency trend continues, then the US could reclaim its position as a leading supplier of coal to Asia and Europe with the dollar making it coal ultra-competitive, just as the home market becomes less attractive.

Whether this ever becomes a problem for Australia and other current leaders in the coal export trade will depend on demand and price. Right now, it’s not an issue because prices are fabulous and coal’s great competitor, nuclear, has been sin-binned for a few years.

Times change, as we have seen over the past five years as coal prices have soared, especially premium coking material, triggering a mine-expansion stampede and a flood of corporate activity around the world.

Arch Coal’s $3.4 billion acquisition of International Coal Group is the latest big deal, following hard on the heels of Rio Tinto’s $3 billion Riversdale raid and Alpha consuming Massey Energy.

Similar scrambling for market share is seen during every coal-price boom, and until now not many critics have found much to criticise – but that might be changing for three reasons.

Firstly, the price being paid by Arch has earned a few negative comments in the US along the lines of it being too high when basic investment fundamentals are applied.

Secondly, an important factor in the US deals is currency and the fact that US coal exports have suddenly become very competitive.

Thirdly, US coal companies are being driven into the export market by the sudden arrival of a serious price competitor, shale gas.

When combined, those three factors have the potential to spell trouble for everyone in the coal business, especially if a fourth factors comes into play, and that is a theoretical decline in Chinese coal demand as internal transport bottlenecks in that country are fixed.

More on China’s domestic coal sector in a future Hogsback rant, for now, let’s focus on the three mentioned, starting with criticism of the price paid by Arch for ICG.

The negative view is contained in a small commentary on the influential financial news website owned by Thomsonreuters, breakingviews.com. According to the contributor, Christopher Swann, the Arch price could signal a bubble in coal values – and investors seem to agree with Arch’s share price sliding.

Arch, according to Swann, is paying a considerable premium for ICG ... “and though investors belatedly bid down the shares of Arch, they have yet to fully recognise the extent of value destruction”

Strong words, based largely on the skinny corporate synergy benefits of $80 million and the price being at the upper end of other recent coal deals.

The second factor, more exports, looks to be a winner for Arch thanks to it gaining exposure to Appalachian metallurgical and thermal coal ideally placed for export, and with the US dollar enabling ultra-competitive pricing.

It is the dollar which is the first driver behind the export push which last year lifted US coal exports to 62 million tonnes, or 5.7% of total production, the highest level since 1998 – but it is shale gas, the third factor, which has US coal companies keener than ever to expand their exports.

Tighter government regulations on coal-fired electricity plants, combined with the shale-gas boom in the US, is encouraging power companies to switch from coal to gas with their expansion plans.

In other words, growth in the domestic thermal coal market is slowing in the US thanks to tougher regulations, and tougher fuel competition.

Are these factors signs of a coal bubble getting ready to burst? Not yet, is the view of Hogsback, but they are straws in the wind and a sign that no one should take the current high prices for metallurgical or thermal coal for granted.

Why? Because there are too many variables at work and so much depends on the ability of China to continue its breakneck growth.

Enjoy the boom, but never forget what comes next!