What does 350 mean?
The emission of carbon dioxide (CO2) from the burning of fossil fuels is leading to a noticeable change in the earth’s atmosphere. As the atmosphere changes, it traps more heat, increasing surface temperatures on Earth, leading to the destabilization of natural systems and toward massive climatic change.
The number 350 comes from an important scientific study describing the safe upper limit on how much of our atmosphere can be made up of CO2. Scientists place that limit at 350 parts per million (ppm) . We are currently at just over 400ppm and rising. Prior to the industrial use of fossil fuels, the world sat at about 280ppm.
Returning atmospheric carbon to as close to 350ppm as possible is our best bet of preserving a climate like the one humanity and the rest of life on earth is adapted to . And even though we’ve surpassed that limit, we can still get back below it if we act now to cut emissions as rapidly as possible .
This means the climate problem is a fossil fuel problem. There is no sustainable way to burn coal, oil, and gas without adding to the amount of CO2 in the atmosphere. We need to manage the de-growth of the fossil fuel sector in order to ensure that humanity leaves the world's remaining coal, oil, and natural gas reserves in the ground where they belong.
Here at Toronto350, we’ve launched a number of campaigns to do just that.
How does 350ppm relate to the 1.5-2˚C temperature limit that governments have agreed to?
At the Paris climate summit in December 2015, governments agreed to limit globally averaged temperature rise to “well below” 2˚C with an aspirational target of 1.5˚C.
There’s a more important difference between temperature targets like these and the 350ppm target. Limiting the atmospheric concentration of carbon to 350ppm seeks to preserve a climate similar to the one we know. But allowing temperatures to rise by close to two degrees brings us into an unknown world—one that we hope human communities and non-human life on Earth can adapt to. In other words, the temperature targets governments agreed to do not stop climate change from happening. The world has warmed by just ~0.8˚C since pre-industrial times, and that has been enough to kick-start unsettling changes like the melting of ice in the Arctic, Antarctic, and Greenland, and more extreme weather events every year.
So why did governments decide to choose the riskier limit of “well below” 2˚C instead of 350ppm? It’s meant to buy enough time to make sure that taking action on climate change won’t disrupt company profits and endless economic growth.
There are some dangerous thresholds in the climate system and the truth is we don’t know at what temperature they get crossed and trigger destabilizing feedback loops like massive arctic methane release. What we do know is that 2°C should be the upper limit of destabilization. All of our economic and political activity should be working right now to ensure that we do not surpass that limit, but unfortunately our political and economic elites seem intent on taking us well past it. The only thing that can stop that is a strong climate movement—the kind we’re helping to build at Toronto350.
What is a Carbon Budget?
A carbon budget is a relatively new way of understanding the task ahead of us. Carbon budgets help us to better conceptualize two major challenges.
First, carbon budgets tell us the total amount of greenhouse gases that can be emitted into the atmosphere to give us a chance of keeping temperature rise below a particular level. And if we know our carbon budget, we can estimate how much time is left before it’s used up. The budgets are measured either in billions of tons of carbon (GtC) or billions of tons of carbon dioxide (GtCO2) .
Usually, carbon budgets tell us how much more we can emit if our goal is to prevent warming of more than 2˚C. There are a few carbon budget estimates out there .
Comparison of Prominent Carbon Budgets
|Source||Temp. Target||Likelihood of Temp. Target||Budget Remaining (GtC)||
Years until budget spent
Friedlingstein et al. (2014)
|Below 2˚C||>66%||314 from 2015||30 years|
|Global Carbon Tracker (2013)||Below 2˚C||80%||245 between 2013-2049 20 after 2050||20 years|
|Rogelj et al. (2016)||Below 2˚C||66%||161 - 338 from 2015||15 to 30 years|
|McKibben (cited in Hansen et al. (2013))||Below 2˚C||80%||128 from 2013||10 years|
|Hansen et al. (2013)||1˚C||N/A||130 from 2013||10 years|
The table above features several prominent carbon budgets. It shows the temperatures each one aims to limit warming to, the likelihood of achieving that target, the amount remaining in the carbon budget for that target, and the approximate time until that budget is spent at the current rate of annual emissions of around 11GtC (See endnotes - for more details).
The carbon budget that gives the best chance (80%) of keeping temperatures below that level is 128 GtC starting from 2013 . The problem is that even if emission rates stay where they are now , that budget will be spent in about a decade.
Second, carbon budgets not only help us calculate how little we can still emit; they also let us compare that amount with how much carbon is in the ground. And what we learn is that there’s just way too much of it: there’s six times more carbon in the world’s fossil fuel reserves than the budget allows! 
For just a better than even chance of staying under 2˚C, the world needs to leave 82% of its coal reserves, 49% of its natural gas reserves, and 33% of its oil reserves buried. In Canada alone, that will mean keeping an astonishing 85% of tar sands reserves and 99% of tar sands resources in the earth .
Because of the limited carbon budget, there’s no reason for fossil fuel companies to be discovering and developing any more resources. But that’s exactly what they’re doing. And there’s a reason for that: their stock value is calculated based on the amount of resources they hold and can sell. But that’s the wrong way to look at things. Once we get serious about climate change, those fossil fuel companies will be holding assets no one wants, and their fossil fuel resources will become worthless. In other words, they’re massively overvalued.
Toronto350’s campaigns are aimed at keeping coal, oil, and gas in the ground. We’re trying to stop tar sands pipelines from coming through Ontario and helping to divest major pension funds and university endowments from fossil fuel holdings.
 For an accessible look at the science behind the 350ppm target, see James Hansen’s Storms of My Grandchildren: The Truth About the Coming Climate Catastrophe and Our Last Chance to Save Humanity (Bloomsbury USA, 2010).
The original scientific study is James Hansen et al., “Target Atmospheric CO2: Where Should Humanity Aim?” The Open Atmospheric Science Journal, 2008, 2: 217-231. Available at http://pubs.giss.nasa.gov/docs/2008/2008_Hansen_etal_1.pdf
 Hansen et al. (2008), 217.
 Hansen et al., “Assessing ‘Dangerous Climate Change’: Required Reduction of Carbon Emissions to Protect Young People, Future Generations and Nature.” PLOS One, 2013, 8(12): 1-26, 10. Available at http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0081648
 Multiply budgets in GtC by 3.664 to convert to GtCO2.
 Probably the most widely cited budget comes from the Intergovernmental Panel on Climate Change (IPCC). The IPCC suggests that if we want a better than two-thirds chance of keeping temperature rise below 2˚C, the total allowable amount of carbon that can be emitted from pre-industrial times is approximately 1,000 GtC (3,670 GtCO2). By 2014, we’d emitted over half of that—roughly 545 GtC (2,000 GtCO2). (See IPCC, Summary for Policymakers, in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge University Press: New York, 2013), 27; and Friedlingstein et al., “Persistent growth of CO2 emissions and implications for reaching climate targets.” Nature Geoscience, 2014, 7: 709-715, 712.)
But that IPCC budget does not include the effect of non-CO2 forcings. In the carbon budget of Friedlingstein et al., non-CO2 forcings are included (making the total budget 872 GtC instead of 1,000 GtC). Because they subtract from that figure the amount already spent by 2014 (i.e., 873 GtC (3,200 GtCO2)-546 GtC (2,000 GtCO2)), they find a remaining budget of just 327 GtC (1,200 GtCO2) at the beginning of 2015. If expended at an expected 10.98 GtC (40.3 GtCO2) per year beginning at 2015, approximately 30 years are left before that budget is spent. (See Friedlingstein et al., 711-712.) Rogelj et al., meanwhile, offer a range of 161-338 GtC from 2015, expended in approximately 15-30 years (See “Differences between carbon budget estimates unravelled,” Nature Climate Change 6 (2016): 251).
When the IPCC budget includes the effect of non-CO2 forcings, it falls to 790 GtC (2,900 GtCO2) since pre-industrial times. The remaining budget from 2011 would thus fall to 275 GtC (1,009 GtCO2). (See IPCC, Summary for Policymakers, Working Group I, 27.)
There are still other estimates, however. A two-degree rise is itself dangerously high, and a mere two-thirds chance of limiting it to even that is hardly reassuring. More stringent budgets result from either lowering the allowable temperature rise or increasing the probability of limiting it to 2˚C. The two most stringent limits are given by climatologist James Hansen et al. and by 350.org founder Bill McKibben (see below).
 Hansen et al.: 2013, 21. The 128 GtC figure updates the budget described by Bill McKibben in a widely read article: “Global Warming’s Terrifying New Math,” Rolling Stone, July 29, 2012, http://www.rollingstone.com/politics/news/global-warmings-terrifying-new-math-20120719.
Hansen et al. (2013) place the remaining budget around 130 GtC in 2013. Due to different assumptions about the possibilities of reducing non-CO2 GHGs and of increasing atmospheric carbon removal, McKibben’s and Hansen et al.’s budgets are similar but yield different results. Hansen et al.’s 130 GtC budget leads to 1˚C warming while McKibben’s 128 GtC leads to an 80% chance of keeping warming below 2˚C. See Hansen et al. 2013, 21.
Carbon Tracker has a higher budget for an 80% chance at 2˚C: 245 GtC (900 GtCO2) between 2013-2049 with an additional 20 GtC (75 GtCO2) between 2050-2100. Carbon Tracker’s estimate assumes high atmospheric aerosol content (and thus more sunlight reflected back to space) and reductions in non-GHG forcings. (See Carbon Tracker & the Grantham Research Institute, “Unburnable Carbon 2013: Wasted capital and stranded assets,” 2013, 11.)
 Global emissions for 2015 are estimated to be around 11 GtC—the highest ever. Friedlingstein et al., 711.
 The total amount of carbon that would be released if we burned the world’s fossil fuel reserves is 790 GtC (2,900 GtCO2). Note: this is just the amount that would result from burning fossil fuel reserves. The amount that would be emitted if we burned the world’s resources is 2,997 GtC (11,000 GtCO2). See note  for the difference between reserves and resources. For reserve and resource estimates, see Christophe McGlade & Paul Ekins, “The geographical distribution of fossil fuels unused when limiting global warming to 2˚C.” Nature, 2015, 517: 187-190, 188.
 Reserves refers to the fossil fuels that would be recoverable using current and future technology without consideration of economic conditions. Resources are a subset of reserves, referring to fossil fuels that are economically recoverable only under current economic conditions and with current technology. See McGlade & Ekins, 190.
For some accessible summaries of the McGlade & Ekins paper, the study behind these figures, see Carol Linnitt, “Development of Oilsands Incompatible with 2C Global Warming Limit: New Study,” DeSmog Canada, January 7, 2015, http://www.desmog.ca/2015/01/07/development-oilsands-incompatible-2c-global-warming-limit-new-study; and Damian Carrington, “Leave fossil fuels buried to prevent climate change, study urges,” Guardian, January 7, 2015, http://www.theguardian.com/environment/2015/jan/07/much-worlds-fossil-fuel-reserve-must-stay-buried-prevent-climate-change-study-says
The full range for the likelihood of staying under 2˚C in the model used for McGlade and Ekins’ study is 44-68%. See IPCC, Assessing Transformation Pathways in Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate. Cambridge University Press: New York, NY, 2014, 431 (Table 6.3).