A temperature rise that could cause irreversible and potentially catastrophic damage to human civilization is practically inevitable, according to rising chatter among experts in the lead up to a year of key negotiations on a new climate change global accord.

World leaders have voluntarily committed to limit warming by the end of the century to 2 degrees Celsius (3.6 degrees Fahrenheit) above the pre-industrial level, a threshold beyond which, scientists argue, severe drought, rising seas and supercharged storms as well as food and water security become routine challenges.

Given the world's historic emissions combined with a continued reliance on fossil fuels to power humanity for the foreseeable future, limiting the increase to 2 degrees Celsius is all but impossible, according to David Victor, a professor of international relations and an expert on climate change policy at the University of California, San Diego.

"There is no scenario by which any accord that's realistic on this planet is going to get us to 2 degrees because the trajectory on emissions right now is way above 2 degrees," he told NBC News.

In recent months, this sentiment has been echoed and reinforced by scientists and policy analysts throughout the climate and environmental communities. And there's debate over whether the 2-degree target should be ditched altogether in favor of a strategy that focuses on attainable, albeit lofty goals built from the ground up.

"The 2-degree target is a great idea," Granger Morgan, a professor of engineering and public policy at Carnegie Mellon University in Pittsburgh, told NBC News. "But we have been so slow in doing anything much about controlling emissions that the accumulative effects are building up on us and … I just do not see the political will to limit emissions to the degree that will be needed to stay below 2 degrees."

Nevertheless, he added, the world urgently needs to get serious about tackling climate change "because otherwise it is going to be much worse."

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The bathtub is full

To understand the challenge, Morgan said to imagine the atmosphere as a bathtub with a giant faucet and a tiny drain hole. Carbon dioxide, a ubiquitous greenhouse gas, currently gushes out of the faucet and is filling the tub to capacity. Carbon dioxide resides in the atmosphere for about 100 years, meaning only a tiny bit escapes through the drain, he explained.

"If you want that level in the bathtub to stabilize, you've got to close the faucet way down because the rate at which the stuff goes out the drain is so slow," he said.

The atmospheric persistence of carbon dioxide is a sharp contrast to more visible air pollutants such as sulfur dioxide, which drain out of the atmosphere within weeks — stop polluting and within a tangible timeframe the issue is visibly resolved.

With climate change, "if you finally get to the point of saying, 'oh my heavens I guess we need to go do something about it,' you are stuck because the stuff you already put in the atmosphere is going to still be there for a long, long time."

Given the carbon already poured into the atmosphere combined with projections of what will be produced in the coming years to power humanity, "warming close to 1.5 degrees C above pre-industrial levels is locked-in to the Earth's atmospheric system," the World Bank warned in a new report released Sunday.

The worst projected changes from warming — a 70 percent decrease in soybean yields in Brazil, water and food insecurity in the Middle East, and amplified warming from thawing permafrost in Russia, for example — "could still be avoided by holding the world below 2 degrees C," the report added. "But, this will require substantial technological, economic, institutional and behavioral change."

"We do know that we need very, very aggressive action if we are going to avoid the worst of climate change impacts."

To stay below 2 degrees C of warming, the world can emit no more than 1,000 gigatons of additional carbon by 2100, according a new report from the United Nations Environment Program. To avoid exceeding that budget, global emissions should be no more than 44 gigatons of carbon a year by 2020, with an aim of even lower emissions after that point. The world currently emits 54 gigatons a year and emissions are growing. On the current path, emissions will reach 87 gigatons annually by the middle of this century.

"There is a huge gap in terms of the pathway that we need to be on for the 2-degree target," Kelly Levin, a climate policy expert at the World Resources Institute, a Washington-based think tank, told NBC News. "But it is still possible if we adopt aggressive measures that rapidly reduce emissions. We haven't closed that window yet."

Ditch the target?

The 2-degree goal provides the international community something to aim for that stresses the importance of avoiding dangerous climate change, noted Morgan. "But we've also got to be realistic," he added.

According to Victor, international climate negotiators should ditch the 2-degree target — or any temperature target — and focus instead on what individual countries can realistically do to rein in greenhouse gas emissions. "See what that adds up to and you turn that into some practical milestones that stretch them a bit further," he said.

In fact, that approach is starting to unfold with recent joint pledges by the U.S. and China to ramp up efforts to reduce greenhouse gas emissions: By 2025, the U.S. aims to cut emissions between 26 percent and 28 percent below 2005 levels; China pledged to peak in emissions by 2030.

The pledges were made in the run-up to a climate conference in Lima next month, which will lay the foundation for a climate accord to be reached in Paris in December 2015.

Given such momentum, "one has to hold out hope," Levin said. "On the other hand, we do know that we need very, very aggressive action if we are going to avoid the worst of climate change impacts. We can't lull ourselves into a false sense of security."