How scared should Europe be of a stoppage in Russian natural gas supplies this winter? That depends on whether countries are willing to sacrifice for one another and unify in their response.

As Russian President Vladimir Putin basked in applause at a Serbian military parade last week, he sent out a clear warning to the European Union: If Ukraine — which has had no natural gas supplies from Russia since June — should siphon gas from transit pipelines to heat its people, he won’t make up the difference but will let the EU go short.

When Russia and Ukraine squared off in a similar gas-price dispute in 2009, Putin went further: He cut all gas transit to the EU via Ukraine for about two weeks, forcing Slovakia to declare a state of emergency and Bulgaria to shut down many of its factories. Even Germany had to scramble to make up for lost supplies. So although talks aimed at resolving the energy dispute between Ukraine and Russia resumed on Tuesday, Putin must be taken seriously.

As a whole, Europe is not as vulnerable as it was five years ago. The biggest single change is the construction of Nord Stream, a twin pipeline project to connect Russia with Germany across the Baltic Sea, completed in 2012. Nord Stream has a capacity to supply 55 billion cubic meters of gas per year and, as a result, the percentage of Russian gas exports to Europe that transit through Ukraine has fallen from about 80 percent to just under 50 percent.

So whereas the stoppage of Russian gas supplies via Ukraine affected 18 countries in 2009, “stress tests” published by the EU last week suggest a six-month transit disruption this winter would severely impact only eight countries. In addition, the 2009 shock prompted the EU to make a few changes of its own. A new 105-km pipeline connecting the Slovakian and Hungarian gas grids should be fully operational by Jan. 1, for example. Meanwhile, underground gas storage vaults across the EU have been filled to 90 percent of capacity, in anticipation of trouble.

The Balkans, Hungary and Poland would come under stress, however. (Finland and the Baltic states might also be severely affected if Russia stopped supplies through pipelines outside Ukraine in a direct attack on those countries.)

So long as EU members respond by spreading the pain and helping each other out, lost supplies would average 10 percent to 60 percent, from Greece to Poland, according to the EU stress tests.

On the other hand, if countries stop supplying others with gas as soon as they run out of surplus themselves, some will be hit much harder. Bosnia and Macedonia would lose 100 percent of their gas supplies, while Bulgaria and Serbia would lose 60 percent to 80 percent. So the severity of the impact would depend on just how willing countries were to work with and sacrifice for each other, making this a big test of EU “solidarity.” Nothing would please Putin more than to see the EU fail that test and so disillusion new members and applicants from the ex-communist bloc in Central Europe — including his hosts last week in Serbia.

It’s hard to know what Putin will do. The 2009 disruption worked out well for him in one sense, encouraging Germany to go forward with Nord Stream, despite protests from Poland and the Baltic States. Building that pipeline weakened Ukraine’s leverage in pricing disputes. Another disruption would harden the determination of countries in Southeastern Europe to build another Russian-backed pipeline, called South Stream, this time crossing the Black Sea to emerge in Bulgaria and deliver gas to Italy, Austria and points between via Serbia, Hungary and Slovenia.

With South Stream in place, Russia would barely need Ukraine’s transit network at all, eliminating Ukraine’s leverage in pricing disputes because it would no longer be able to tap into transit gas for the EU when cut off.

Loss of the Russia-EU gas transit business would also blow a vast hole in Ukraine’s national budget, because it would no longer collect the associated fees. So for Putin, who is doing his best to bankrupt and destabilize Ukraine, there’s a temptation.

The 2009 gas crisis, however, also encouraged the EU to look for ways to create an effective market for gas across Europe, with enough success to develop a limited spot market at supply hubs that has forced Russia’s Gazprom to lower its prices. A true single gas market is still a long way off, but another cutoff might harden resolve, damaging Gazprom in its most important market. At the same time, falling oil prices are already blowing a hole in the Russian budget, so losing gas sales to Europe — which make up 40 percent of Gazprom’s revenues — would hurt.

Putin has shown little sign that he cares about the economic costs of his Ukrainian policies until now, but he may decide making Ukrainians freeze just isn’t worth the cost.

Marc Champion (mchampion7@bloomberg.net) is a London-based contributor to Bloomberg View.

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