350 ppm, 1ºC, and 100% by 2030

350 ppm, 1ºC, and 100% by 2030

The Climate Science Behind the Targets

By Howie Hawkins

May 12, 2019

What follows is a summary of the climate science that indicates why we are at the brink of dangerous climate change and what we must do about it.

Inaction on climate change over the last decade has put us right at the precipice of dangerous climate change. The climate emergency we face is much more severe than the stronger storms and heat waves we are already experiencing. Escalating at an accelerating rate, the climate crisis means we will see as we approach 2050 mass extinctions and collapsing land and ocean ecosystems, agricultural crises and food shortages, economic contraction and increasing poverty, hundreds of millions of climate refugees, and escalating social conflicts and resource wars

Kevin Anderson, a leading climate scientist at the Tyndall Centre for Climate Change Research in the UK warned in 2009 that we face a climate holocaust: “If you have got a population of nine billion by 2050 and you hit 4°C, 5°C or 6°C, you might have half a billion people surviving.” By 2100, ocean heating and acidification could so reduce ocean phytoplankton, the source of two-thirds of atmospheric oxygen, that it may result in the suffocation of animal life on Earth.

When Bill McKibben and others formed in 2008, they based their goal of getting atmospheric CO2 equivalents back below 350 parts per million (ppm) on the latest science. As McKibben noted at the time, the emerging international goal of 450 ppm was arbitrary. It had evolved from climate modeling begun in the 1970sto determine what would happen if we doubled preindustrial atmospheric carbon from 275 ppm to 550 ppm. While early modeling indicated 550 ppm would cause a 2ºC rise, by 2008 the models said CO2 would have to be limited to 450 ppm to have a chance of limiting the rise to 2ºC. In 2009, the US and other G8 nations adopted the 450 ppm and 2ºC targets, which was affirmed by the world’s nations at the Cancun climate conference in 2010. That goal was adopted because – well, that is what had been modeled. It was like searching under a street light for the keys you dropped at night because you can see the pavement there instead of searching down the street where you actually dropped the keys.

The best climate science already indicated that 450 ppm would lead to dangerous climate change. McKibben cited a talk by NASA climate scientist James Hansen to the American Geophysical Union in December 2007. Hansen said that we have to get CO2 back below 350 ppm. In 2007, CO2 was already 383 ppm. Last month (March 2019) the average was 412 ppm.

Over the last decade, Hansen and colleagues have expanded upon that 350 ppm conclusion in a series of scientific papers drawing on paleoclimate records and updated climate modeling. The papers also warned of tipping points that trigger amplifying positive feedbacks, such as arctic sea ice melt reducing the reflection by ice of solar radiation back into space and resulting in more heat and more ice melt. Once these tipping points initiate such reinforcing cycles of heating, it may be impossible to reverse global warming before we face climate catastrophe.

In December 2008, Hansen et al. published “Target Atmospheric CO2: Where Should Humanity Aim?” in which they made the case for a target of 350 ppm or lower. Reaching that target would require phasing out all fossil fuels by around 2030 and also drawing CO2 out of the atmosphere and into the biosphere through massive afforestation, habitat restoration, and conversion to organic agriculture to rebuild carbon-sequestering living soils.

In September 2009, Kevin Anderson and Alice Larkin (née Bowes) of the Tyndall Centre for Climate Change Research in the U.K. presented findings to the 4 Degrees and Beyond International Climate Conference in Oxford that were consistent with Hansen’s. Their findings published in 2011 in “Beyond ‘dangerous’ climate change: emission scenarios for a new world” saidthat to have a 50% chance of hitting the 2ºC target—and to take account of the cumulative emissions and technological capacity differences between rich and poor countries—the rich countries needed immediately begin cutting their greenhouse gas emissions by 8-10% per year, which would zero out their emissions in the early 2020s.

In December 2009, the German Advisory Council on Global Change (WBGU) also reached similar conclusions. In “Solving the Climate Dilemma: The Budget Approach,” the WBGU concluded that in order to have a two in three chance of keeping global warming to less than the 2°C, atmospheric CO2 must be reduced into the range of 300 to 350 ppm, which is still about 10-25% above the pre-industrial levels of 275-285 ppm. To reach this goal, the cumulative CO2 emissions budget for the global economy needed to be capped at 750 gigatons carbon (GtCO2) through 2050. When this global carbon budget was divided into national carbon budgets on an equal per-capita basis, the United States had only 6 years at its then current emissions rate before it had spent its share of the budget. Needless to say, the U.S. blew past that budget in the mid-2010s.

In December 2013, Hansen and colleagues published “Assessing ‘Dangerous Climate Change’: Required Reduction of Carbon Emissions to Protect Young People, Future Generations and Nature.” They showed how 450 ppm and 2ºC would lead to 3-4ºC due to climate system inertia and tipping points that would trigger reinforcing warming cycles. They described how 3-4ºC catastrophic would be for Earth’s ecosystems and human societies. They made the case for staying within the temperature range of the current interglacial Holocene period of the last 12,000 years in which agriculture developed. The upper limit of the Holocene is 350 ppm and 1ºC. By cutting CO2 emissions by 6% per year to net zero between 2013 and 2030, and beginning to draw down carbon with afforestation, habitat restoration, and improved agricultural practices, they said it was possible to peak the temperature rise at 1.1ºC and reduce CO2 to below 350 ppm by the end of the century. They noted that their analysis indicated that delaying a 6% per year carbon reduction program until 2020 would keep the climate in the dangerous territory above 350 ppm until 2300. Starting in 2020 would mean cuts of 15% per year to achieve 350 ppm by 2100.

Carbon Budget

Putting these climate science findings in terms of a global carbon budget, McKibben wrote in his influential August 2012 Rolling Stonearticle, “Global Warming’s Terrifying New Math,” that the carbon budget for a 4 in 5 chance of hitting the 2ºC target was 565 GtCO2 (gigatons CO2), which was only about 20% of proven coal, oil, and gas reserves.

Drawing on datapresented in detail in Oil Change International’s Sky’s Limit report, McKibben updated these numbers in September 2016 in “Recalculating the Climate Math.” McKibben said that to have a 2 in 3 chance of hitting the 2ºC target, the world could only release 800 more GtCO2. To have a 50% chance of hitting the December 2015 Paris Climate Agreement’s aspirational goal of 1.5ºC, the remaining carbon budget was 353 GtCO2.The thenoperating coal, oil, and gas mines and wells would lead to the release of 942 GtCO2.

The takeaway is that the world has to not only stop opening up new hydrocarbon mines and wells, it hasto shut down operating mines and wells before they extract less than 40% of the coal, oil, and gas in them in order to have a 50% chance of meeting the 1.5ºC goal.

Since McKibben wrote that article in 2016, the world has emitted 109 of the 353 GtCO2 carbon budget for a fifty-fifty chance of capping global warming at 1.5ºC.At last year’s emissions rate of 37.1 GtCO2, the remaining 244 GtCO2 carbon budget will be used up in 2025.

The science reviewed here calls for much steeper declines in carbon emissions than the October 2018 “Special Report: Global Warming of 1.5ºC” by the UN’s International Panel on Climate Change (IPCC). That report called for CO2 emission reductions of 45% from 2010 levels by 2030 and 100% by 2050. That 45% by 2030 target is the source of the media headlines that the world has 12 years left to stop global warming. It must be recognized, however, that the IPCC reports require approval before release of all the members of the United Nations. That means approval by the world’s biggest petrostates and carbon emitters, including the U.S., Russia, China, India, and Saudi Arabia and the other gulf oil states, who have repeatedly had the scientist writers’ conclusions modified. Consequently, the IPCC reports have consistently underestimated the pace of global warming and understated the measures needed to stem it.

The 1.5ºC Special Report is a case in point. It expanded the remaining carbon budgetat current emissions rates to 2028 from 2021, which was the date in the 2013 5thIPCC Assessment Report. The 1.5ºC Special Report was able to expand the carbon budget by assuming that “climate unicorns,” undeveloped and unproven negative carbon emissions technologies, would be operating full scale by mid-century.

Carbon Drawdown

While the IPCC reports are to be faulted for assuming unproven negative emissions technologies will come on line mid-century to save us from climate catastrophe, even a rapid 10-year zeroing out of carbon emissions will not leave atmospheric carbon in the safe zone. Hansen and other climate scientists have been emphasizing for a decade now the need to also draw carbon out of the atmosphere to recover a safe climate.

Hansen’s most recent paper last December, “Climate Change in a Nutshell: The Gathering Storm,” hopefully revises his timeline for carbon reductions to accommodate the passage of time since his earlier warnings. The Nutshell paper said with 6% per year CO2 emissions reductions starting in 2021 combined with a recent higher estimate of the amount of carbon that can be sequestered in the biosphere through afforestation, habitat restoration, and organic agriculture, it might still be possible to “return global temperature close to the Holocene range by the end of this century.” Hansen cites a study estimating that these natural carbon sequestration methods could reduce atmospheric carbon by 150 GtC over the course of this century, which is 50% higher than the 100 GtC capacity for biospheric sequestration that he and his colleagues estimated in 2017 in “Young People's Burden: Requirement of Negative CO2 Emissions. ”Their analysis assumed that with a 6% per year carbon emissions reduction starting in 2010 [!],it would require drawing down153 GtC from the atmosphere by 2100 to meet the 350 ppm target. That carbon drawdown is about equal to the maximum estimated capacity of natural biospheric carbon sequestration methods that Hansen cited in the Nutshell paper a year later. That projection assumes we started cutting emissions in 2010 and that atmospheric carbon isat about50% of what they really are today.

Consequently, the Young People’s Burden paper concludedthat even withan immediate crash program to rapidly phase outfossil fuel emissions and maximize afforestation and regenerative agriculture globally, “continued high fossil fuel emissions today placea burden on young people to undertake massive technological CO2 extraction.” The paper went on to review how technically infeasible and expensive these so-called geoengineering proposals are.Bioenergy with CarbonCapture and Sequestration (BECCS)—the negative emissions technology almost all of the IPCC models for keeping temperature rise to 2ºC depend upon—would require about one-third of the world’s arable land or about half the land area of the United States. BECCS would crowd out food production and impose the other massive environmental costs of industrialized agriculture.

If we are to meet the 350 ppm and 1ºC target by 2100 at this late starting date, the ecosocialist environmental scientists David Schwartzman and Peter Schwartzman argue that biosphericcarbon sequestration will have to be augmented by geospheric carbon sequestration. They call for a solar-powered industrial acceleration of weathering that fixes carbon in the Earth’s crust in the natural geological carbon cycle. Water and CO2 react chemically with rocks to create carbonates that fix carbon in the Earth’s crust for geological time scales.The technical feasibility of accelerating this process industrially was recently demonstrated in Iceland where CO2 dissolved into water was injected into a basaltic rock formation and 95% of the CO2 turned into carbonate within the following two years.

The Schwartzmans note that solar-powered industrial geospheric sequestration will need to continue well beyond2100 because about half of the carbon dioxide emitted by human activities since the fossil fuel age began has gone into the ocean and life forms. While oceans are being acidified by the absorption of atmospheric carbon, oceans also emit carbon dioxide like a carbonated drinks do, especially as they warm. When life forms die their decomposition emits CO2 and methane. The industrial sequestration will be needed because natural carbon sinks in the biosphere will have been filled to capacity by afforestation, wetland and other habitat restoration, and organic agriculture.

The Schwartzmans estimate in The Earth Is Not for Sale: A Path Ouof Fossil Capitalism to the Other World That Is Still Possible (pp.103-107) that it will take on the order of 25 trillion watts of global energy annually to power industrial carbon sequestration in Earth’s crust as well as to end global energy poverty and provide every person on Earth decent standard of living, as climate justice demands. Currentannual global energy production is 18 trillion watts, but the Schwartzmans argue that 25 trillion watts of renewable energy production is technologically feasible. They were the first compute in 2011 how an initial investment non-renewable energy can create a self-reproducing and exponentially-growing 100% wind/solar renewable energy system in about 25 years under the conservative assumptions of using existing renewable technology and of modest rates of energy investment in renewables rates (1% of non-renewables and 10% of renewables during the transition). In a 2016 follow-up study, they find that this 100% global clean energy system can be built with an initial energy investment of just 20% of proven conventional oil reserves. The wells for that oil are already producing. No fracking, arctic, or deep ocean wells are necessary. Oil is chosen because it releases less greenhouse gas than coal and natural gas over their life cycles.


This review of what the climate science says and the climate action it indicates can be summarized in the following conclusions:

  • To avert dangerous climate change that will collapse ecosystems and crash human societies, we must recover the global temperature range in which agriculture and human civilization developed over the 12,000 years since the last Ice Age.
  • Restoring that temperature range means bringing the long-term rise in global temperatures back below 1ºC above the pre fossil fuel age temperature range. Global temperatures have been 1°C and higher since 2015.
  • To get back below 1ºC by the end of this century, the world has to reduce atmospheric CO2 to below 350 ppm by the end of the century.
  • Meeting these targets—350 ppm and 1ºC—means an emergency mobilization for 100% clean energy system and net-zero carbon emissions by 2030 in the rich developed states.
  • Meeting these targets—350 ppm and 1ºC—also means drawing on the order of 150 GtC (billion tons of carbon) out of the atmosphere through massive afforestation, habitat restoration, and soil-building organic agriculture.
  • At this late date, sufficient carbon drawdown may also require a solar-powered industrial acceleration of the natural weathering in Earth’s carbon cycle that converts atmospheric carbon into stable carbonates in rocks in Earth’s crust.

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