The US-China agreement - which saw the former pledge to reduce emissions by 26 to 28 per cent of 2005 levels by 2025, and the latter set itself the goal of its emissions peaking by 2030 and then falling - reportedly took Australia by surprise.
According to political commentator Michelle Grattan, Abbott was not forewarned of Wednesday's announcement by Presidents Barack Obama and Xi Jinping.
Instead, he had been "ambushed almost on the eve" of the G20, Grattan wrote on The Conversation website. She also noted the US and China's joint communique had referred to climate change "already harming economies around the world" - effectively countering Abbott's rationale for excluding it from the agenda in Brisbane.
Scientists and environmentalists say the G20 is an important opportunity to build momentum ahead of next year's Paris conference, where it is hoped a new global pact on fighting climate change will be hammered out.
With the European Union agreeing last month to reduce emissions by at least 40 per cent of 1990 levels by 2030, Australia - which has scrapped a carbon price and is trying to reduce renewable energy targets - is looking increasingly out of step with the rest of the developed world.
The Abbott Government's "Direct Action" policy, which will see big polluters paid for reducing emissions, was passed by the Senate last week. But few experts believe it will enable Australia to meet its very modest target of cutting emissions by 5 per cent of 2000 levels by 2020.
Professor Tim Flannery, a leading Australian scientist, wrote that, following the China-US agreement, "it's time for Australia to get serious about one of the greatest challenges facing humanity".
Asked where the deal left Australia, Ross Garnaut, an economics professor and the former federal government's chief adviser on climate change, replied: "Exactly where it was before the US-China announcement - up shit creek."