Just three months from now, in the final days before Christmas, we will know — for better or worse — what happened at Copenhagen. Looking back at a year that was described to me, as it opened, as “arguably the most important in human history”, we will know whether the world has matched up to it.

So now, as the world enters the end game with a chain of summits this week, experts are beginning to work out what an agreement deal might look like, and whether that would be sufficient.

First up is a report from a well regarded Anglo-Indian team from the HSBC bank, based at its Climate Change Centre of Excellence in Bangalore. The HSBC team has little time for the so far ineffectual U.N. negotiations — the report quotes a description of them as “an aquarium full of hamsters connected to rudimentary motors” producing “a lot of frantic running, a lot of sweat and heat but, in the end, very little light.”

And, recalling how the negotiations at Kyoto almost failed twelve years ago, the document cheerfully stresses that “Copenhagen should be much more difficult … as far more countries are involved and much more is at stake.”

Yet, perhaps surprisingly, the report — entitled The Emperor, the Nightingale and the Phoenix (all will be explained later) — concludes that “a framework deal will be done.” The authors base their “cautious optimism” on two factors, both of which have some validity.

One is that almost all governments sincerely want a deal, however much they may disagree on the details. And the other is that the build up to the conference has already created “significant policy momentum” and “accelerating political, business and social action.” Together, the authors say, these have led to “significant convergence” which is taking the world “close to a reasonable deal in Copenhagen that could be deepened later.”

What could that look like? First, says the report, there should be agreement to limit global warming to 2 degrees centigrade, with a long-term commitment by industrialized countries to reduce emissions by at least 80 percent by 2050. That much is already virtually secured: agreement along those lines was reached at the G8 and Major Economies Forum summits in Italy last July.

Somewhat harder, but still relatively straightforward, would be its insistence on an agreement that total global emissions should be cut by 50 percent by mid century — implying, as it does, that developing countries will, in time, have to reduce their emissions. They refused to endorse the figure in L’Aquila but did, nevertheless, agree that their pollution should “peak as soon as possible.”

Next, the report recommends that all countries should commit to “low-carbon growth paths”, with developing countries agreeing to a framework of the cumbersomely entitled Nationally Appropriate Mitigation Actions (NAMAs). Again that should be possible: the main question would be whether the NAMAs are to be voluntary or compulsory.

There are also recommendations about protecting forests, aiding technology transfer, establishing carbon markets, and addressing emissions from aviation and shipping and trade. But the really controversial stuff concerns the mid-term targets that rich countries should adopt and the amount of finance they will provide for developing ones.

The report suggests that a realistic target would be a reduction of 15-20 percent on 1990 emission levels by 2020, which falls far short of the 25-40 percent the scientific consensus says will be necessary. And it suggests starting with an $8-12 billion package for 2010-2012, much less than developing countries want.

Current targets, it explains, work out on average at an 11 percent reduction in emissions, and it believes that the 15-20 percent level “could be achieved with an injection of political will.” And the financial figure represents roughly a doubling of present commitments, though the report believes that it will end up towards the lower end of its forecast range.

The big question, the team admits, is whether developing countries will accept such relatively unambitious undertakings from industrialized ones “on the basis that something is better than nothing.”

And that is where the emperor, the nightingale and the phoenix come in, all appropriately taken from the writings of Copenhagen’s favorite author, Hans Christian Andersen. The emperor, as in the one that has no clothes, represents failure in Kyoto, with global warming heading towards an apocalyptic five degree centigrade rise. The nightingale, from the story of one that saves the life of another emperor, envisages complete disaster being averted through implementing present best practices — but still likely resulting in a destructive temperature increase of around three degrees.

Only the phoenix scenario, where the world is reborn as leaders and citizens agree to take climate change seriously and implement “the toughest scientific recommendations,” offers a good chance of meeting the vital two degree target.

The HSBC team reckons that its proposed deal would put the world into a “nightingale” scenario, providing the opportunity to move on to tougher measures. “It is important” it says “to be realistic about what the negotiations can deliver.”

In just three months we will know how prescient, or otherwise, it has been.