Financial assets across the world have taken a beating since the start of the year. Policymakers in emerging markets already know the feeling. Since the long boom in middle-income economies came to an end two or three years ago and waves of turbulence rocked their asset markets, central banks and finance ministries have been trying to work out what to do.

Investors still too often treat emerging economies as though they were an undifferentiated mass. In reality some have more options than others, depending on how policymakers prepared for the inevitable tightening of external financial conditions.

One of the more original contributions to the debate arrived this week from Agustín Carstens, the well-regarded governor of the Banco de Mexico. He suggested central banks look to intervene in bond markets to prevent a credit crunch. The withdrawal of foreign capital has starved many emerging market sovereign debt markets of a ready supply of buyers.

Mr Carstens stopped short of advocating full-blown quantitative easing on the model of the US Federal Reserve and other developed economy central banks. Instead, he suggested that central banks could swap long-dated bonds for short-dated, thus easing liquidity problems further down the yield curve and in effect acting as a market-maker of last resort.

For a relatively stable, functioning economy like Mexico, such a move may make sense, though it carries substantial risks. Mr Carstens accepted that it would be a policy to which he would turn only reluctantly.

For the more typical emerging market where the fiscal solvency of the government is doubtful, the dangers of such a policy almost certainly outweigh the benefits. The problem arises from the aftermath of the flood of capital into emerging markets during the boom that followed the global financial crisis. The inflow was aided, ironically enough, by the QE practised by the Fed, which sent money around the world searching for higher yields. While emerging market governments often avoided racking up big debts themselves, many of their corporates went on a dollar-denominated borrowing spree that left economies vulnerable to currency depreciation.

In this context, undertaking a policy that is likely to be seen as monetary loosening while the Fed is tightening is fraught with the danger of encouraging further devaluation, making the debt problem worse. This is particularly true for those economies like Brazil and South Africa, where high inflation and fiscal laxity have driven down their currencies and questioned the solvency of the government, let alone their corporates.

Ultimately, the credibility of monetary policy needs fiscal solidity as a backstop. The policy outlined by Mr Carstens makes sense only if it involves tiding markets over a bad patch that policymakers, and investors, can safely regard as temporary. That is not, sadly, the position in which many emerging markets find themselves.

It is hard to think of more than a small handful of countries where the idea of the central bank being market-maker of last resort is both practicable and reasonably safe. Emerging markets rode out the global financial crisis in 2008 and 2009 quite well relative to developed economies. They are unlikely to do so again this time.

Mr Carstens’s idea is an interesting intervention in the debate about the function of central banks in the emerging markets. Unfortunately, with the exception of himself, there appear to be few central bank governors for whom it is a wise proposition.