Two other things are also unclear. First, how Vladimir Putin will respond. The Russian president offered the people a bargain: accept a hard man in the Kremlin in return for rising living standards. That deal will be broken in 2015, and there is no guarantee that it will encourage the Kremlin to take a softer line over Ukraine. On the contrary, a failing economy could spur Putin into acts of nationalist defiance. That would not just intensify the recession; it would also have knock-on effects for Russia's neighbours and for the eurozone.
The second unknown is whether Russia will be a special case. The fear is that it will set off a chain reaction across other emerging markets that have attracted the copious amounts of footloose capital generated by the quantitative-easing (money-creation) programmes of the world's central banks.
Turkey and Indonesia are two big countries to look out for.
Oil
In the northern summer of 2014, a barrel of Brent crude was changing hands at US$115 ($148) a barrel. By Christmas it could be obtained for barely half that price. The big drop in the oil price is positive for global growth: it puts more spending power in the hands of consumers and it cuts costs for businesses.
The link between the cost of crude and the world economy is well established: the long booms of 1948-73 and the 15-year period that preceded the great recession of 2008-09 were both built on cheap oil. The four recessions of the postwar era (1974-75, 1981-82, 1990-91 and 2008-09) have all been associated with rising oil prices.
Trevor Greetham, director of asset allocation at Fidelity Solutions, says: "A low oil price is a stimulus for consumers. Global growth should pick up over 2015 and there are as yet few signs of the kind of inflation that would necessitate meaningful monetary tightening."
But there is a caveat. Greetham says the plunging oil price could prompt "credit stress". This would affect governments, such as Russia, Venezuela and Iran, that can only balance their books if the oil price is at US$100 a barrel or more. And it would affect the shale gas sector in the US, where much of the investment has been financed by high-yielding but risky junk bonds. As the Bank of England points out in its recent Financial Stability Review: "As US oil and gas exploration firms account for 13 per cent of outstanding debt in US high-yield bond markets, an increase in the perceived or realised credit risk in this sector could lead to sales by investors and potentially illiquidity in the broader high-yield market". In other words, shale could be the next sub-prime.
China
China will be crucial to the performance of the global economy in 2015. Depending on the yardstick used, it is now the world's biggest economy. It is also, according to Kenneth Culkier of the Economist magazine, a net exporter of foreign direct investment. China could soon join the select club of countries with a reserve currency.
But 2014 has been an uneasy year, as Beijing has tried to mop up the credit excesses left behind after the growth-at-all-costs approach adopted during the deep downturn of late 2008. Policymakers have been running a tight ship and the constraints on credit have started to bite. Growth will be lower in 2015: the question is how much lower.
A marked slowdown would affect the rest of the world in two big ways. First, exports to China would weaken. This would affect countries such as Germany, which sell the machine tools needed for China's industrial expansion, and those, such as Australia, that provide China with its raw materials. A sluggish Chinese economy in 2015 will compound a low oil price.
Second, China will export deflation to the rest of the world. The prices of goods leaving China are already falling and that trend will continue. The US and Europe will be flooded with cheap Chinese goods, driving down inflation. In the case of the eurozone, it may result in deflation. Central banks, faced with inflation being well below target, will be cautious about raising interest rates.
United States
Next year will be hugely significant for chairwoman Janet Yellen and her colleagues at the Federal Reserve, and for global markets. A focus for investors in the new year will be the timing of the first rise in interest rates.
Rates have been in a record low range of between zero and 0.25 per cent since December 2008, but the economy has been gaining momentum in recent months. The Fed has already called time on its US$4.5 trillion bond-buying programme, completing its final purchases in October. Winding the clock back to May 2013, then chairman Ben Bernanke triggered a so-called "taper tantrum" when he suggested the Fed might start slowing the rate of its bond-buying sooner than markets were expecting. Investors - hooked on ultra-loose monetary policy since the crisis fully erupted in 2008 - took fright and triggered a fresh wave of volatility.
Given we're talking about the world's largest economy, speculation on the first rate rise will have repercussions around the world. Investors will scrutinise Fed statements for any change in tone.
Expect market volatility when the central bank drops its cautious tone as it paves the way for the first rate rise since the GFC.
Eurozone
The eurozone is the crisis that keeps on giving, and there is every reason to believe this will remain the case in 2015. Mario Draghi, the eloquent president of the European Central Bank, lifted the single currency bloc out of the worst phase of the crisis in the summer of 2012 simply by saying that he would do "whatever it takes" to save the euro. But he now faces one of his biggest challenges yet.
In 2014, the story in the eurozone was one of a recovery that failed to get off the ground and of the mounting threat of deflation. Neither of those problems has gone away, with growth of just 0.2 per cent in the third quarter of 2014 and an annual inflation rate of 0.3 per cent at last count in November. Greece and Spain are already stuck in a deflationary rut and there is concern that a dangerous deflationary spiral will spread to the rest of the region. The fear is that as prices continue to fall, businesses and consumers will delay spending plans as they expect prices to fall further. With a backdrop of weak growth, low oil prices and lack of inflationary pressures, the ECB's battle against deflation will continue well into 2015.
Measures announced in 2014 - including charging banks to park cash with the central bank in a bid to encourage more lending - have failed to provide a silver bullet. The bank has one weapon left up its sleeve: full-blown quantitative easing.
So far the eurozone's policymakers have failed to take the plunge with QE, largely as a result of forceful opposition from Germany. But 2015 could be the year to abandon the hints and throw the kitchen sink at the problem. More weak data from the eurozone will make investors nervy. Failure to press the QE button in the face of weakness could trigger outright panic. Observer
