James Hansen and the Three Categories of the Runaway Greenhouse: Earth Uninhabitable for Humans at ~5,000 Gigatons Fossil Fuel Burned

Runaway Global Warming

The Arctic’s Contribution to Runaway Global Warming (hypothetical worst-case runaway) . The above image provides a potential worst-case scenario for amplifying Arctic methane feedbacks to human-caused climate change. James Hansen’s research shows that a mini-runaway, pushing global temperatures to 10-12 degrees C above the Holocene, is all but certain under continuous, business as usual, fossil fuel burning through 2100. It is important to note that the current non CO2 forcing is equal to about 1/3 of the total CO2 forcing and that atmospheric methane releases are at 1/20th the level we would see during a runaway of the kind depicted in this image.

( Image source: Arctic News)

Forget for a moment that we can still emit about 530 gigatons of CO2 and still keep human warming in the ‘safe range’ of less than 2 degrees (Celsius) temperature increases this century. Forget for a moment how important to the sustenance of human civilization and the prevention of ever-worsening conditions this strict limit on carbon emissions is. Now think for a moment what will happen if Republicans in Congress and fellow conservatives aligned with fossil fuel companies across the country and around the world get their way.

In the past month, Republicans in the House of Representatives have pushed to increase US coal burning, approve the Tar Sands Keyston XL Pipeline, remove energy efficiency standards, and to slash US government (ARPA -E) R&D funding for new renewable energy technology by 80 percent. Fully 55% of all Republicans in the US Congress deny that human caused warming even exists. And the rest clearly are deluded enough to believe that it represents the climate version of a mild summer storm. Their legislative action over the past month, the past year, and for many years following that has been to enforce US dependence on oil, gas, and coal and to delay, diffuse and deny US access to new energy sources that may effectively serve as their replacements. And the millions of dollars in oil, gas and coal company largess they enjoy in the form of contributions is just the final proof that these members aren’t working for the best interests of the American people. They’re working for the international corporate state called Big Oil (BO).

So let’s consider for a moment what would happen if these BO lackeys in Congress were successful in their efforts to kill off alternative energy, to remove efficiency standards, and to shackle the US Energy future to Tar Sands, Coal, and Fracked Shale Oil and Gas.

What would happen?

It’s difficult to argue, given the current extreme and worsening state of the world’s climate, that blind Republican attempts to enforce dependence on BO would result in much in the way of US prosperity. It, essentially, would turn the entire North American Continent into a giant petro-state. It is possible that, for some years, the US will make some energy independence gains, possibly removing a larger fraction of imports from most states except Canada. But the loss of efficiency standards would do ongoing damage by increasing consumption of high-cost unconventional fuels, which would put a drag on the economy. The even greater drag would come from shackling US and North American economies to ever greater degrees to fuel sources, at best, that create a 6 to 1 energy return, where wind and solar could have supplied between 10 and 20 to 1 at ever-lower costs.

Worse still, is the fact that US and Canadian carbon emissions would go through the roof. Tar Sands, Coal, and Tight Shale Fracking are three horsemen of the apocalypse when it comes to climate change change (the fourth being fossil fuel company greed). Coal has always been the worst emitter. But both Tar Sands and Tight Shale Fracking are not far behind. The Republicans would have us depend on these, arguably vast, unconventional sources to the exclusion of all others. They wouldn’t care one whit about capturing the carbon (costs too much and reduces the energy return on already low energy fuels). And, adding yet one more insult, they allow BO to export the fracking and tar sands technologies to other countries consigning them and the world to similar fates.

Total carbon emissions in 2012 (including non CO2 sources) was 45 gigatons. But on the path Republicans set, this level of emission will look minor. Peak emissions would probably pair with peak human civilization at some time around 2050 near 80-90 gigatons per year. At this point, emissions are put in check by mother nature’s outrage at our insults. By 2050, the ‘burn everything’ strategy put in place by Republicans and enforced by conservatives around the world has resulted in near 600 ppm atmospheric CO2. Life in the oceans is in terminal collapse, major cities and island nations are being devoured by a combination of powerful storms and rising seas. The coastlines, for so long productive, have become unstable. And large regions of once fertile land are now being devoured by deserts. Water stress has caused entire countries to collapse. Mass migrations from both the coastlines and from desertified regions has already set in. Human population peaks at about this time near 10 billion.

But over the next 50 years humans maintain enough vestige of prior fossil-fuel based civilization to keep burning. They expend massive efforts to encircle coastal cities with walls. They try to farm indoors more and more. Miami is placed on giant oil platforms (we have more than enough in surplus) whose bases are driven into the limestone beneath the city. The New Orleans’ tidal wall is heightened to 30 feet. New York and the all important Wall Street is surrounded by increasingly high flood barricades. But the massive storms of this age are freakish, wrecking entire regions and knocking out power for weeks to months. Storms and heatwaves kill millions each year and millions more are rendered homeless. Entire countries collapse for want of food or under a tide of refugees they cannot support.

By 2100, CO2 is at 1000 ppm and global temperatures are 7 degrees (Celsius) hotter. Fossil fuel based industry has emitted about 5,000 gigatons of carbon, enough to set off the stages for a mini runaway global warming scenario (Category 1). Sea levels have risen 12 feet and Earth’s population has been reduced to 6 billion. 40% of ocean species are extinct and 10% of land species have suffered the same fate. Summer time results in the emergence of large heat death zones experiencing wet bulb temperatures in excess of 35 degrees Celsius (hot enough to kill most large mammals, including humans, through heat stress alone). Even if all emissions ceased, global temperatures would still rise to around 12-14 degrees Celsius hotter than the Holocene. There is almost no chance, in this case, for human civilization to survive such an insult for more than another 50-100 years. And the chances for humans, long term, are dire indeed.

But the world’s fossil fuel companies are still around, still clawing coal, fracked oil and gas, tar sands, oil shale, and methane hydrates  from the Earth with whatever new high tech process they’ve invented. These manage to survive for another 50 years or so selling off enough dirty fuels to set world CO2 levels to 1500 ppm. And that’s when game over really sets in for just about everything that can’t run to a high mountain range.

Fossil fuel based industry had managed to survive just long enough to emit more than 8,000 gigatons of carbon into the atmosphere. Just long enough to kill off the rest of us and themselves too.

Three Categories of Runaway Warming

The above scenario isn’t science fiction. According to some hard science done by the world’s top climate scientists, it is entirely likely if Republican burn, baby burn policy and the fossil fuel companies that push it survive in their current forms for much longer.

Hansen’s new paper is a more in depth study of Earth Systems Climate Sensitivity to a given level of CO2 forcing. The study looks, with greater detail, into both how much Earth will warm, long term, given a certain level of CO2 emission and how much of this emission is required to set off one of three categories of a runaway greenhouse.

In a less than ideal scenario, Hansen investigates what will happen if we burn all or nearly all the fossil fuels currently included in the unconventional reserves. All, or nearly all, according to Hansen represents between 5,000 to 10,000 gigatons of carbon equivalent fuels. Chillingly, if we tap the most extreme sources, such as methane hydrates, that number could rocket to 20,000 gigatons or more. So even Hansen’s study isn’t an extreme worst case.

Category 1: The Mini-Runaway

The Hansen paper finds that burning between 3,500 and 6,500 gigatons of carbon based fuels is enough to raise world CO2 levels to between 800 and 1200 parts per million. This level of CO2 would set up climate conditions similar to those experienced during the Paleocene-Eocene Thermal Maximum (PETM) in which temperatures were between 10 and 12 degrees Celsius hotter (average) than today.

Hansen finds that this level renders much of the Earth mostly uninhabitable for humans. Hansen notes:

Earth was 10-12 °C warmer than today in the early Eocene and at the peak of the PETM (Fig. 4). How did mammals survive that warmth? Some mammals have higher internal temperatures than humans and there is evidence of evolution of surface-area-to-mass ratio to aid heat dissipation, e.g., transient dwarfing of mammals (Alroy et al., 2000) and even soil fauna (Smith et al., 2004)during the PETM warming. However, human-made warming will occur in a few centuries, as opposed to several millennia in the PETM, thus providing little opportunity for evolutionary dwarfism to alleviate impacts of global warming. We conclude that the large climate change from burning all fossil fuels would threaten the biological health and survival of humanity, making policies that rely substantially on adaptation inadequate.

It is also worth noting that much of the world’s land masses would experience average summer temperatures above the mammal-killing level of 35 degrees Celsius (wet bulb) in a PETM-like world. The added heat of this regime would swiftly soften and obliterate any ice on the planet. But given the killing heat and a hydrological cycle driving droughts and rainfall events that are 80% more extreme, a rapid sea level rise of 200+ feet would likely come as a harsh afterthought. (To this point, it is worth mentioning that most planetary ice disappears when CO2 levels hit and maintain between 500 and 620 parts per million over a number of centuries).

Nearly all climate scientists agree that a return to PETM conditions and CO2 levels, especially on so short a time-scale would be a mass extinction event on the land and in the ocean. Which is why policies that extend the burning of fossil fuels combine the travesties of ecocide, genocide, and suicide in equal measures and to ever greater degrees as time moves forward.

Category 2: The Moist Stratosphere Runaway

If the fossil fuel companies manage to stick around long enough, they may be able to burn through between 8,000 and 15,000 gigatons of additional carbon-based fuels. Such an event would almost certainly spell the end for human beings and probably most of the complex life on Earth as well.

In such a situation, average global temperatures rise by between 15 and 20 degrees Celsius. A 15 degree Celsius temperature rise increases temperatures over land by around 20 degrees C. This puts Earth’s average land temperatures at around 35 degrees Celsius with average daily (wet bulb) highs in the range that is hot enough to kill humans. The entire Earth, in this case, is an enormous human killing field.

The only regions able to even marginally support human life or agriculture would be the high mountains. But even these regions would be under threat. Global heating of around 15 degrees Celsius or greater would pump ever greater levels of moisture into the stratosphere. The added H20 would substantially degrade stratospheric ozone. The added UV radiation would severely hamper both plant and animal life in the remaining habitable regions. Human food crops are highly sensitive to excess UV radiation. So it is seriously doubtful if humans could continue cultivation even on the Himalayan Plateau during a Category 2 Runaway.

Category 3: Evaporated Oceans, Baked Crusts

Thankfully, even the fossil fuel companies aren’t likely to bring about even the worst of the climate change nightmares — Earth transitioning to a state more like Venus. In order to do that, global heating would have to evaporate all of Earth’s oceans and then bake the remaining carbon out of the Earth’s crust. According to new models constructed by Hansen, such conditions would take between 100 million and one billion years to develop. Hansen’s models also show that climate sensitivity is not enough, at the higher CO2 levels, to finally set off the kind of runaway that would force such catastrophic events to occur.

That said, the first two categories of global warming runaway are well within the reach of current fossil fuel reserves. And the fact that all fossil fuel companies are doing everything they can to burn all the reserves on their books and to find ever greater quantities of these fuels is not at all comforting, especially when they have a number of hired trolls in Congress and elsewhere (Republicans) to do their dirty work for them…


New Hansen Paper on Climate Sensitivity, CO2 and Sea Level Rise

Leave a comment


  1. Sourabh

     /  July 16, 2013

    Hi Robert,

    I haven’t read that paper yet, but did Dr. Hansen include feedback effects of methane and ice albedo into mini-runaway warming calculation? If so, then does 800ppm-1200ppm include effects of non-CO2 sources too?

    If permafrost melts, clatherates melts in arctic, we wouldn’t need 800ppm CO2 to cause runaway effect. So, 2100 is way too optimistic. I thought abrupt climate change might occur as early as 2030s if we continue on Business-as-Usual road.


    • What Hansen shows is that, according to Paleoclimate, we get a 10-12 degree C warming (including methane) at around 800-1200 ppm CO2.

      The current forcing at 400 ppm is enough to push a full warming of 3 degrees Celsius (1.5 equilibrium). By 2030, we don’t hit a runaway, but, under BAU, we hit enough warming/CO2 to make some real rough patches for human civilization and to lock in 2 degree C warming by end of century and 4 degree C long-term.

      Enough forcing for a PETM style runaway is unlikely by 2030, thank goodness. But we can still lock in some pretty horrible impacts.


  2. And these scenarios take into account only what Hansen knows within his area of expertise. What are the global effects of the ongoing 6th mass extinction and loss of all those building blocks of life? Can man live without nature inside a glass bubble of his own making? We are surely conducting an experiment without precedent.


  3. Reblogged this on Collapse of Industrial Civilization and commented:
    And these scenarios take into account only what Hansen knows within his area of expertise. What are the global effects of the ongoing 6th mass extinction and loss of all those building blocks of life? Can man live without nature inside a glass bubble of his own making? We are surely conducting an experiment without precedent.


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