"Just explain it to me simply ... how much time do we have left?"

This is a question I get asked a lot, and I've found that before long — after explaining the intricacies of the latest IPCC climate science consensus (known as AR5), carbon accounting metrics, "the gigatonne gap," CO2 sequestration, and several other key concepts — my subject's eyes glaze over with either boredom or confusion, or some combination of the two. 

So I've taken to a metaphor that seems to do the trick: the SMS Titanic. Yes, I know this is an ironic choice of metaphor. If carbon emissions continue at their current rate eventually there will be no icebergs left floating near Newfoundland in mid-April as there was on that fateful day in 1912. But it does help when explaining the predicament we now find ourselves in, so please indulge me for a minute.

It's like we're all living on one great big Titanic ship. We've been having a pretty good time so far (at least those staying in first and second class) and our journey has been steady until recently when we've hit some choppy waters. A couple storms have passed by, a polar vortex or two. But we continue playing shuffleboard and dining in the banquet halls, listening to the orchestras playing. Many kindhearted philanthropists work to make the journey better for those in the lower decks by giving to charities that support churches, schools and other important social services. 

Philanthropy vs the climate

Yet we do almost nothing to deal with the fact that the ship is on a direct course with the hidden killer of climate change, which lurks silently in the dark waters below, threatening to take the entire ship down. Instead of slowing down or aggressively investing in preparations that would help us brace for the oncoming impact (less than 3 percent of philanthropy goes to environment/climate) our Titanic steams ahead at full clip, accelerating with each passing day.

The four smoke stacks of the ship represent the four major sources of carbon pollution in the atmosphere, which are creating the current volatility in our climatic systems:

The burning of fossil fuels — coal, oil, natural gas — are the primary drivers, adding about 34 Gt ("gigatonnes" aka 'billion tons') of CO2e globally (CO2e is a combined total of carbon dioxide + other gases including methane, nitrogen and halocarbons converted to CO2 equivalents). This number factors in pollution associated with mining, processing, shipping and burning of fossil fuels, based on a recent report by Ecofys (check out the detailed diagram for more information). Terrestrial sources of CO2 — mostly deforestation, agriculture and livestock — add 10 Gt. A few other sources — industrial pollutants, waste, construction — add another 5 Gt. Taken together we emitted approximately 49 Gt of CO2e pollution for the reference year 2010. 

Our forests and oceans absorb quite a lot of this pollution, but only CO2 pollution (50 percent based on the latest research). This means that in 2010 we added 30 Gt of net CO2e pollution to the atmosphere (19 Gt of CO2 + 11 Gt of other CO2e gases). Every subsequent year, this goes up by about 2 percent (in 2013 it is estimated we emitted about 51+ Gt of pollution).

This number is important because it tells us our "burn rate" — how fast we are approaching the point of no return. According to the latest research, summarized nicely by Climate Nexus (PDF), we have a very strict "carbon budget" which we cannot bust without losing a two-thirds ("likely") chance of staying below the dangerous tipping point of 2C in global temperature rise. That budget is 987 Gt commencing in 2012 (see below); or 925 Gt commencing in 2014.

tracking CO2 emissions

So let's start the clock on Jan. 1, 2014, with 925 Gt left in our budget. That is the amount of pollution we can emit for the entire century.

Based on the world's current trajectory, we will break that budget in 20 years. That means starting in the year 2034, we would not be able to emit a single puff of carbon pollution — not one belch from a dairy cow, not one mile in a gas-driven car, not one acre of land cleared — or we lose our "likely" chance of staying below the deadly 2C threshold. 

Needless to say, no one thinks it's even remotely possible for us to terminate all sources of carbon pollution within the next 20 years, at least not without a total and complete collapse of our modern-day economic system. And no one wants to pass on the enormous risk of a mere 50/50 chance of survival to future generations if we were to continue burning more carbon. So what's our course of action? How do we slow down enough to avoid the looming threat of 925 Gt? 

That is the central question of the ongoing multinational climate negotiations, which will be receiving a shot in the arm this coming September as the U.N. holds its first Climate Leaders Summit, where world governments will (hopefully) come to the table with more ambitious carbon reduction pledges than they've previously been willing to offer.

It's clear to everyone that sometime in the 2017-2019 time frame we must begin slamming on the brakes on the Titanic. Without this rapid course correction, we'll be barreling straight towards 59 Gt by 2020, even with the current increase in the renewable (non-carbon) energy sector. Many experts believe it would be next to impossible to correct our course should we stay on this trajectory. 

The current science tells us that by 2020, we need to be at significantly less than 49 Gt to have a fighting chance of staying below the "likely" 2C zone, according to UNEP, 2013 (PDF). This is what everyone now refers to as "the gigatonne gap," and it is the greatest single challenge that humanity will face in the next 3-5 years:

Emissions must decline each year subsequent to the peak year (2018 or so), Based on the projections shown above (the red line in the diagram) this means 49 Gt by 2020; 38 Gt by 2030; 27 Gt by 2040; 20 Gt by 2050; 14 Gt by 2060; 9 Gt by 2070; 5 Gt by 2080; 3 Gt by 2090; and 1 Gt per year by 2100.

This will be difficult. Like the crazy ski slope at the Sochi Olympics (sorry, I know I'm on a metaphor kick) survival is not certain. But if we can get to the bottom, we will be able to stay under our limit of 925 Gt by the end of the century (assuming oceans and forests remain healthy enough to continue absorbing a large portion of excess CO2).

So how much time do we have left? My answer is about three to five years. That's the amount of time we have before we can safely steer our giant Titanic in a new direction. If we miss that window, another clock begins to tick: the countdown to 925. And it may come sooner than we think.

The opinions expressed by MNN Bloggers and those providing comments are theirs alone, and do not reflect the opinions of MNN.com. While we have reviewed their content to make sure it complies with our Terms and Conditions, MNN is not responsible for the accuracy of any of their information.