The COP21 Sustainable Innovation Forum, organized by the French government, will be in Paris from Nov. 30 to Dec. 11. (DOMINIQUE FAGET/AFP/Getty Images)

With less than a month to go until the all important U.N. climate change conference in Paris, yet another key report has reinforced how off target the world is from the goal of limiting warming to 2 degrees Celsius above pre-industrial levels. And it finds that 1.5 degrees Celsius, a target embraced by many developing nations, would require even “earlier and much stronger action.”

The United Nations Environment Programme’s sixth “Emissions Gap” report, released Friday, provides an overview of the “intended nationally determined contributions” (or INDCs) that nations have proposed leading into Paris. Last week the United Nations Framework Convention on Climate Change released a similar inquiry that suggested that current pledges could possibly keep the world to 2.7 degrees Celsius, but UNEP is not so optimistic. It says 3 to 3.5 degrees Celsius can be expected, with a two-thirds probability, if all the pledges are implemented, including those that are “contingent” on funding and other actions.

A key problem identified in the UNEP report is that the Paris agreement is not expected to take effect until 2020 — and by then, significant time will have been lost. And the inexorable math of the carbon budget does not forgive delays. Rather, the more you emit, the less you have remaining to emit at any (reasonable) time in the future, due to the long residence time of carbon dioxide in the atmosphere.

This inevitably means that waiting to cut emissions, or putting cuts off into the future, makes any target harder to achieve.

“The report confirms unequivocally that current mitigation pledges by more than 140 nations are taking the planet to a warming trajectory well above 2C; and emphasizes the need for a negotiation process that will review those and future pledges with the aim to increase their level of ambition,” says Pep Canadell, executive director of the Global Carbon Project and a researcher at the Commonwealth Scientific and Industrial Organisation (CSIRO) in Australia.

Let’s get into the math a little, to see why this is so.

First, UNEP finds that in 2014 the world emitted 52.7 billion metric tons, or gigatons, of carbon dioxide equivalents — a total that includes carbon dioxide from all sources (fossil fuels, deforestation and land use change, and more) but also other greenhouse gases, like methane and nitrous oxide. Rather startlingly, there is actually a very large uncertainty range around even this present day number, from 47.9 to 57.5. So annual emissions could actually be a lot higher, even now.

To have a two-thirds or better chance of staying below 2C, and assuming action begins in 2020, UNEP finds that this annual number needs to be down to 42 gigatons of carbon dioxide equivalents in 2030. For 1.5 degrees C, it needs to be down to 39.

However, the current pledges, including those that are conditional, only get the world to 54 gigatons of carbon dioxide equivalents  in 2030 — not much different from where we are now. It’s not that the pledges achieve nothing — they actually cut annual emissions by 4 to 6 gigatons, relative to where they might otherwise be in 2030, as the world population and its energy demand continue to grow.

But the pledges are still not enough and leave behind an “emissions gap” of 12 gigatons of carbon dioxide equivalents per year that new policies will have to close if we want to keep within the 2 degree target, at least with a two-thirds or greater probability.

But actually, it’s tougher than this. The UNEP report admits that even 42 gigatons of carbon dioxide equivalents, in 2030, won’t be enough unless still further steps also occur down the road. We will not only have to bring emissions to absolute zero at some point in the 2060s or 2070s, but furthermore, will likely have to implement “negative emissions” technologies that will actually pull carbon dioxide out of the air again. Or as the report puts it:

All scenarios analyzing 2 °C pathways that follow the Cancun pledges until 2020 and with a least-cost starting point in 2020, require strong reductions after 2020. They also rely on so-called “negative emission technologies” such as bioenergy combined with carbon capture and storage.

UNEP’s chief scientist Jacqueline McGlade confirmed by email that the 42 gigatons in 2030 scenario is one of these — it assumes no major actions taken before the year 2020, and therefore does indeed rely on negative emissions later in the century.

It’s obvious how planting a large number of trees pulls carbon out of the atmosphere — bioenergy combined with carbon capture and storage, or BECCS, is not so dissimilar. It means burning trees or plants to get energy (which is carbon neutral if these trees or plants then grow back again), but simultaneously sequestering all of the carbon that results from that burning in the ground (which gets you from carbon neutral to carbon negative).

However, there are significant objections to BECCS at the scale that might be required to really make a dent in an atmosphere filled with as much carbon as we will have put there by the late 21st century.

One involves the amount of land that would be required. “Some institutions have called for producing 20 percent of human energy needs from bioenergy of all sorts by 2050,” writes Tim Searchinger, a senior fellow at the World Resources Institute. “That would require an amount of biomass equal to all the plants harvested annually across the entire world today: all the crops, crop residues, wood and grasses eaten by livestock. The world does not have the room.”

“The feasibility of large scale deployment of negative emission technologies is still a contentious issue,” admits the UNEP report. Needless to say, any 1.5 degree C scenarios will also require negative emissions.

The role of negative emissions in many scenarios for achieving 2C was recently criticized by Kevin Anderson, a researcher with the Tyndall Center for Climate Change Research at the University of Manchester, in an essay in Nature Geoscience. Referring to a scenarios database kept by the U.N.’s Intergovernmental Panel on Climate Change, Anderson wrote that:

Of the 400 scenarios that have a 50% or better chance of no more than 2°C warming (with three scenarios removed >due to incomplete data), 344 assume the successful and large-scale uptake of negative-emission technologies.

Anderson then performed an analysis suggesting that instead of assuming negative emissions, quite sharp CO2 cuts are needed — and fast — to actually stay with the carbon budget.

“As scientists, we must now leverage the clarity gained by the budget concept to combat the almost global-scale cognitive dissonance in acknowledging its quantitative implications,” Anderson wrote. “Yet, so far, we simply have not been prepared to accept the revolutionary implications of our own findings, and even when we do are reluctant to voice such thoughts openly.”

Not everyone agrees with Anderson, but all parties seem to concur that the current Paris pledges aren’t enough — they’ll have to be tightened further, and more ways of limiting carbon will have to be found. The UNEP report singles out a particularly prominent one — restoring forests in developing countries could close the 2030 emissions gap by as much as 9 gigatons of carbon dioxide equivalents, at least theoretically (though real world constraints would surely lessen these gains significantly). The report also says that ambitious actions by cities and subnational actors can also shave off another gigaton or two per year.

In the end, then, what’s really coming into focus just before Paris is just how much farther the world has to go to cut its emissions — and how little time remains for that to happen.