G20 summit: When is it, what will happen, and what will Trump do this time?

With Angela Merkel determined to put climate change, mass migration and free trade at the heart of the agenda, she is engineering a clash with Donald Trump

Previous G20s have been characterised by testy meetings between Putin and Obama. This year's will be the first public meeting between Putin and President Obama's successor, Donald Trump (Pablo Martinez Monsivais/AP)

What is the G20?

An annual meeting of the leaders of the world’s largest economies - just about. Spain is a ‘permanent invitee’ without being an actual member. There are only, in fact, nineteen members. The twentieth is the European Union, which partially explains the absence of other major economies, like the Netherlands.

Alongside the likes of Britain, France, Germany, China and the US, are Turkey, Argentina, South Africa, Indonesia Saudi Arabia and others.

It is a larger and more significant meeting than the G8, which occurred earlier this year, not least as Russia is currently suspended from the G8 over its intervention in Crimea.

Between them, the membership accounts for 85 per cent of the world’s GDP and two thirds of its population.

When it was first established, in Berlin in 1999, it was for finance ministers and central bank governors.

But the collapse of Lehman Brothers in 2008 was deemed severe enough for leaders to attend themselves. Now, they always do.

What’s on the agenda this time?

The G20 take place in Hamburg, Germany this Friday and Saturday. The summit was always likely to have been dominated by climate change, but will be even moreso since Donald Trump removed the United States from the Paris Agreement. It will also be the first public meeting between Donald Trump and Vladimir Putin since Trump became President.

There are likely to be testy public photocalls and private meetings between Trump and German Chancellor Angela Merkel too, after Ms Merkel said the US “could no longer be relied upon” after Trump’s withdrawal from the Paris Agreement.

Ms Merkel has said she will make climate change, free trade and the management of mass migration the major themes of the summit, setting her, and potentially the whole of the EU up for a major clash with President Trump. On climate change the two are poles apart. On free trade Trump’s economic nationalism is distinctly distant from the European position. On mass migration, Merkel has welcomed more than a million migrants to Germany, primarily from Isis warzones, while in the US, Trump has sought to impose a ban on all Muslim immigration.

So what actually happens?

Bilaterals between major countries are arguably more significant than the set piece events. There are no formal votes on any issues. Events tend to be dominated by the political man of the hour. In 2009, when the G20 came to London, Gordon Brown steered a secured an agreement over a more than one trillion dollar injection of stimulus into the global economy. Barack Obama regularly bent the G20 to his will over climate change. This time, it may be up to Angela Merkel and Emmanuel Macron to do the same.