On one hand, it is a very, very round number — obviously just a rough and imperfect estimate of the economic impacts of an enormously complex and often unpredictable jumble of warming Earth climate systems, says abcnews.go.com.The financial cost to the world economy of a warming and melting Arctic, due to human induced greenhouse emissions over the next 40 years, will add up to at least $2.4 trillion, according to a study by the Pew Environment Group.

That’s a 24 followed by 11 zeroes.

To get this figure, Pew researchers accumulated and combined estimates of the monies, both public and private, that will be spent — or lost — as the rising global heat brings drought to fields, new demands on energy plants, migration away from sea shores (prompted by what some studies estimate could be at least a 4-foot rise in sea levels this century), infrastructure and trade losses due to severe flooding as well as insurance losses related to all of these.

“As a first cut, I find the study sort of interesting,” environmental economist Gary Yohe of Wesleyan University, and a lead author of the Intergovernmental Panel on Climate Change, told ABC News. He also pointed out that over the past decade there have been some 300 studies trying to estimate the dollar cost of global warming. (This Pew study treats only the Arctic.)

And, notes Yohe, one survey of all those studies found that 12 percent of them concluded that there would be a positive economic impact of global warming in coming decades.

That still leaves 88 percent of the studies saying warming costs would be in the red. Some of the studies concluded that global warming will be extremely damaging to the world’s economy as mid-century approaches.

The point is, this a complex and necessarily speculative field of study.

Yohe also points out that the $2.4 trillion estimate, spread out over the next 40 years, may end up being but a fraction of 1 percent of what the cumulative global GDP could well be by then.

Estimates about the impact of Arctic warming are incomplete since, as the Pew study authors warn, some of the major Arctic Earth systems that will affect world economy cannot be assessed with exactitude.

These unknowns include:

Just how ocean currents will shift as Earth warms.

How much additional warming of the air may result as giant but unseen natural stores of methane in the frigid seabed’s begin to bubble out as warming currents thaw them.

How the complex yearly give-and-take cycles of carbon dioxide (CO2) by Arctic plants and animals — as they breathe in and out — will alter as average global temperature rises.

In addition, as we’ve been hearing for some years now, studies have shown the Arctic is warming twice as fast as the rest of the world, further complicating the calculating.