Mike Callaghan and Stephen Grenville recently reminded us of the debate around Australia's potential future global GDP ranking, and what that implies for our place at the G20 table in 2050. As Melissa Conley Tyler noted in October last year, this kind of debate is valuable if it leads to improved public policy that help sustain Australia as a 'top 20 country'.

But what about long-term relevance of the G20 itself? In the most recent Lowy Institute G20 Monitor, I suggested that the G20 walks a tightrope of relevancy. The G20 remains the premier forum for international economic cooperation, and is coming off a successful 2014 Australian presidency where it reminded the international community that it can focus on collective solutions to major economic challenges. However, in which there is disillusionment with the slow progress the forum has made in recent years on key global governance issues, the G20 needs to keep showing its relevance or alternative forums will be sought out. So it is important that Turkey's 2015 G20 presidency is a success. This is more than just as a matter of Turkish national pride.

The first discussions by finance ministers under the Turkish presidency, held in early February in Istanbul, were underwhelming. Despite statements from Canadian Finance Minister Joe Olivier that global growth needs a kickstart and from Christine Lagarde that this year has the potential to be a special one for collective action, the Istanbul meetings were most notable for a lack of consensus. Even after a marathon communique drafting session that lasted more than 24 hours, countries could not agree on how to spur growth and were reluctant to commit to the Turkish hosts' plans for binding investment targets.

In the areas where agreement was possible, results were lacklustre. The G20 needs to ensure that the call for countries to each implement only the key parts of the Brisbane Action Plan does not become a step back from the G20's main achievement of 2014. The Turkish presidency rightly pointed to the IMF Executive Board's decision to establish a new $100 million Catastrophe Containment and Relief Fund, which will assist West African countries most affected by the Ebola epidemic, as the communique's clearest and most concrete outcome. While this is commendable, the G20 missed the opportunity to address the larger underlying problems affecting global health governance (a subject I will return to in a future piece). There were also some relatively routine advances in lower-key items on investment, tax and financial regulation, although these were balanced against the lack of progress on IMF reform, something Mike Callaghan and I predicted ahead of the meetings.

In all, the four-page communique read as light on substance, and it is difficult to point to headline areas where the agenda has substantially progressed since Brisbane. There is a clear disconnect between the communique and a vision that will allow the G20 to achieve all of the Turkish presidency's three I's (investment, implementation and inclusiveness). The overall impression from Istanbul is that the G20 recognises the challenges but has run out of ideas for how to collectively address them. Attention is already starting to divert to 2016 and China's impending presidency.

As Mike Callaghan noted, economic size isn't everything. And nor is G20 membership, if the forum itself continues to drift.